Patent Description:
The following description relates to vapor cells with reduced scattering cross-sections and their methods of manufacture.

Vapor cells are manufactured by sealing a vapor or gas within an enclosed volume. The vapor or gas is used as a medium to interact with electromagnetic radiation received by the vapor cells. Beams of light, such as generated by lasers, may be directed through the vapor or gas to probe and measure a response of the vapor or gas to the received electromagnetic radiation. In this way, the vapor cells may be used to determine properties of the received electromagnetic radiation and serve as sensors of electromagnetic radiation. However, such measurements may be negatively impacted by nonuniformities of the received electromagnetic radiation in the enclosed volume occupied by the vapor or gas. The measurements may also be negatively impacted by scattering of the received electromagnetic off walls or the body of the vapor cell. Vapor cells and methods of manufacture are desired that avoid or mitigate such negative phenomena.

<CIT> discloses a magnetic field measuring apparatus configured such that a process layer of the magnetic field measuring apparatus has such a structure that includes a first hollow portion and a second hollow portion provided opposed to first hollow portion with a first isolation wall interposed therebetween. Alternatively, a method for manufacturing the magnetic field measuring apparatus includes breaking the first isolation wall after generating alkali metal. <CIT> discloses a production method of a gas cell, the method including: forming a coating layer on a surface of a plate material; assembling a plurality of the plate materials having the coating layer formed thereon so as to form a cell surrounded by the surface having the coating layer formed thereon; and filling the formed cell with an alkali metal gas.

The present invention provides a method of manufacturing a vapor cell according to appended claim <NUM> and a vapor cell according to appended claim <NUM>. In some aspects of what is described here, small, stemless vapor cells are presented that have improved electromagnetic transparency relative to conventional vapor cells. In addition, a uniformity of the electromagnetic field within the vapor cells is orders of magnitude better than that found in standard vapor cell geometries. The vapor cells incorporate the principles of metamaterials to reduce scattering cross-sections for a target electromagnetic radiation to be measured. This reduction can increase the field uniformity, and thus the accuracy of the measurement, while maintaining a structural strength required for the vapor cells. Rydberg atom-based electric field sensors based on such vapor cells have a wide range of possible applications, such as metrological applications. The vapor cells presented herein include sub-wavelength vapor cells that measure a uniform electric field in the region where vapor inside the vapor cells interacts with the test electric field for sensing. Such vapor cells and their uniform electric fields are capable of measuring frequencies over the MHz-THz range, which is important for over the air standards testing.

In some aspects of what is described here, methods of manufacturing are present that can manufacture a vapor cell for Rydberg atom-based electric field sensing. The vapor cell has a low scattering cross-section and a uniform electromagnetic field in the region of the vapor cell where the electromagnetic field is measured. The uniform electromagnetic field is robust to manufacturing variations of the vapor cell due to the accuracy of fabrication in relation to the target field wavelength(s) to be measured, and in many instances, due to the sub-wavelength dimension of the vapor cell. Furthermore, the methods of manufacturing are compatible with mass production. The methods are employed to fabricate vapor cells with metamaterial walls that create a more index-matched vapor cell. The methods may allow a pure alkali gas to be filled into the vapor cell. The methods may also allow vapor cells to be made entirely of glass, or alternatively, of silicon and glass. Other materials are possible. In the methods, laser and deep-reactive ion etching (DRIE) machining tools may be used to accurately form the metamaterial walls and holes therein so that small, sub-wavelength manufacturing variations, which are difficult to avoid, are much less than <NUM>% of the target field wavelength(s) to be measured.

The decrease in the scattering of the electromagnetic waves and increase in the uniformity of the electromagnetic field in the measurement region of the vapor cell makes the vapor cells presented herein ideally suited for metrological measurements. Since the vapor cells are small compared to a wavelength of measured electromagnetic radiation, and furthermore is made entirely of dielectric materials, multiple vapor cells can be used in spatial proximity to one another to sense the electromagnetic field over a region of space. The vapor cells have an order of magnitude improvement in the field uniformity and scattering cross-section at electromagnetic frequencies up to <NUM> when compared to vapor cells without metamaterial walls. The methods of manufacturing can also be used to fabricate vapor cells for higher electromagnetic frequencies.

The metamaterial walls include holes, cavities, and so forth to define shapes and patterns for the walls. The shapes and patterns may be chosen to make the vapor cells structurally sound, e.g., to maintain the high vacuum levels necessary for Rydberg atom-based electric field sensing. By shaping the walls using cavities rather than making them thin, the vapor cell can be made structurally sound. Using the methods, the resulting vapor cells can have longer operational lifetimes since their vacuum permeabilities are decreased relative to those with similar electromagnetic properties, but fabricated to have a reduced wall thickness. The vapor cells are also conducive for coating with anti-relaxation coatings, which are important for applications in magnetometry. Moreover, the vapor cells can be coupled optically over free space or through a waveguide such as an optical fiber. The vapor cell may even be encased in a thin layer of epoxy or parylene to increase its stability in some situations.

The use of Rydberg atoms for electrometry has already led to the most accurate, absolute measurements of high frequency electric fields (HFE) to date. This technology holds considerable promise for advancing the state-of-the-art in a number of antenna measurement applications. However, there are challenges at the extremes of antenna size (both small and large) as well at high power that can be mitigated by what is fundamentally a more electromagnetically transparent and conductor-free HFE probe than anything used at present. Beam-forming antennas at HFE are particularly difficult devices to focus and optimize. The use of a dielectric probe, such as the vapor cells described herein, can drive down the achievable error floor in these measurements while providing a means to drastically reduce costs. Such benefits open up new application spaces. Replacing antennas on satellites and unmanned aerial vehicles with self-calibrated sensors brings significant advantages, especially when considering that the overall detector package can be significantly more compact. By almost completely removing undesirable materials from the probe and tailoring its geometry to enhance desirable electromagnetic properties, new types of measurement can be enabled. One example is the measurement, in situ, of exceptionally large power densities in large-scale ground-based radar systems and air-based electronic warfare equipment, i.e., actively electronically scanned (AESA) radars. Probes that accurately reproduce the incident electromagnetic radiation field and have small scattering cross-sections are particularly important in the near-field regime and can be used to acquire multiple field points simultaneously.

Furthermore, millimeter waves can only be propagated relatively small distances, which presents a challenge for the design of <NUM> telecommunications equipment. The short propagation distance of millimeter waves (e.g., about <NUM>-<NUM> maximum) will dramatically increase the number of cell towers and other equipment required for the implementation of <NUM> networks, escalating the maintenance costs. In contrast, current cell towers provide a range of about <NUM> - <NUM>. Beamforming antennas are expected to play an important role in <NUM> networks and require service and testing. For backhaul systems, <NUM> nodes will need to be densely deployed throughout cities. Ease of service and testing, i.e. maintenance, of a dense network in an urban environment is a key challenge. Additionally, fragmented licensing and standards across the globe for millimeter wave technology could adversely affect <NUM> implementations. Over-the-air (OTA) testing is a key ingredient for the adoption of <NUM> technology and its support of the Internet of Things (IoT). Many of these issues also permeate the military test market as well.

To overcome free space path loss in millimeter wave communications, radar and sensing, antenna arrays are used to achieve higher directionality by forming a steerable beam that points to a target device. Beamforming antennas consist of an array of elements that are used to obtain a directional characteristic, so called multiple-input multiple-output MIMO antennas. The role of MIMO antennas for <NUM> New Radio (NR) technology makes OTA testing for the performance evaluation of radiation patterns essential. The antenna array calibration is critically important because of the strict requirement of antenna array beam steering along with that of sidelobe suppression. Precise phase and amplitude differences among antenna elements need to be calibrated and adjusted to maximize performance. Calibrations and optimization are further complicated by the fact that other system elements such as transceivers will be integrated into the antenna system. Many of the parameters affecting the calibration of the antenna system will change over time as they will be affected by factors such as weather and aging. Multiple field point measurements at the antenna need to be made in order to characterize a millimeter wave beam.

Testing of <NUM> networks will be radically different from present <NUM> (or LTE) networks, as it will expand beyond wireless device calibration, RF parametric testing, and functional testing. The industry will have to assure reliability of the network and within the devices. Military millimeter-wave devices must also meet rigorous testing requirements. In the United States, carriers are required to set industry standards regarding device performance. OTA testing will be critical for the evaluation of systems and components in both mobile and fixed location devices. Since some of this testing will be done in the field and during the research and development phase, an absolute, self-calibrated sensor and standard is important in order to make direct comparisons between tests in these very different environments. Because the transmission distances at millimeter wave frequencies decrease, it is necessary to accurately test under a wide range of conditions and effects, such as free space path loss, atmospheric absorption, scattering due to rain and particulates, line-of-sight obstructions, and so forth. <NUM> networks will service more than increased personal communications and entertainment. Calibration and standards regarding base stations will be critical because <NUM> networks will enable technology such as autonomous vehicles, drones, and industrial equipment that affect public safety. An absolute, self-calibrated sensor system will be a prerequisite for certification.

To enable a self-calibrated sensor, ideal for the applications in the area of metrological measurement, the probe needs to measure as uniform an electromagnetic field as possible in the sensing region, the effect of the vapor cell on the incident field must be known and the scattering cross-section of the probe must be as small as possible. It is also advantageous for vapors (e.g., gaseous atoms) in the vapor cell to have long coherence times. The long coherence times can be disrupted by collisions with a background gas when the atoms are in Rydberg states. Hence, pure alkali-metal vapor cells without buffer gases are important for such applications. For field testing, it is obvious that the probe must be structurally strong. The vapor cells disclosed herein can maintain their structural integrity while minimally perturbing the incident electromagnetic field. In addition, the vapor cells can measure the incident field accurately, be loaded with pure samples of atoms and molecules, and be precisely built en masse so their effect on the target field is well-known.

Now referring to <FIG>, an exploded view is presented, in perspective, of an example vapor cell <NUM> having a dielectric body <NUM> and an optical window <NUM>. <FIG> presents the example vapor cell <NUM> of <FIG>, but in which the optical window <NUM> is bonded to the dielectric body <NUM>. The dielectric body <NUM> may be a substrate defined by opposing planar surfaces, as shown in <FIG>. However, other configurations are possible for the dielectric body <NUM>. Moreover, although <FIG> depict the dielectric body <NUM> as being square-shaped, other shapes are possible. The optical window <NUM> may also be a substrate defined by opposing planar surfaces. However, other configurations are possible for the optical window <NUM>. In general, the optical window <NUM> includes one surface adapted to mate (or bond) against a surface of the dielectric body <NUM>, thereby allowing a seal to form (e.g., via a contact bond).

The dielectric body <NUM> may be formed of a material highly transparent to electric fields (or electromagnetic radiation) measured by the vapor cell <NUM>. The material may be an insulating material having a high resistivity, e.g., ρ > <NUM><NUM> Ω·cm, and may also correspond to a single crystal, a polycrystalline ceramic, or an amorphous glass. For example, the dielectric body <NUM> may be formed of silicon. In another example, the dielectric body <NUM> may be formed of a glass that includes silicon oxide (e.g., SiO<NUM>, SiOx, etc.), such as vitreous silica, a borosilicate glass, or an aluminosilicate glass. In some instances, the material of the dielectric body <NUM> is an oxide material such as magnesium oxide (e.g., MgO), aluminum oxide (e.g., Al<NUM>O<NUM>), silicon dioxide (e.g., SiO<NUM>), titanium dioxide (e.g., TiO<NUM>), zirconium dioxide, (e.g., ZrO<NUM>), yttrium oxide (e.g., Y<NUM>O<NUM>), lanthanum oxide (e.g., La<NUM>O<NUM>), and so forth. The oxide material may be non-stoichiometric (e.g., SiOx), and may also be a combination of one or more binary oxides (e.g., Y:ZrO<NUM>, LaAlO<NUM>, etc.). In other instances, the material of the dielectric body <NUM> is a non-oxide material such as silicon (Si), diamond (C), gallium nitride (GaN), calcium fluoride (CaF), and so forth. In these instances, an adhesion layer may be disposed on the dielectric body <NUM> to define the surface <NUM> of the dielectric body <NUM>. The adhesion layer may be capable of bonding to the non-oxide material of the dielectric body <NUM> while also being capable of forming a contact bond with the optical window <NUM>. For example, the dielectric body <NUM> may be formed of silicon and the example vapor cell <NUM> may include an adhesion layer that includes silicon oxide (e.g., SiO<NUM>, SiOx, etc.) on the dielectric body <NUM>. This adhesion layer defines the surface <NUM> of the dielectric body <NUM> and is capable of forming a contact bond that includes siloxane bonds.

The dielectric body <NUM> includes a surface <NUM> that defines an opening <NUM> to a cavity <NUM> in the dielectric body <NUM>. The surface <NUM> may be a planar surface, as shown in <FIG>, although other surfaces are possible (e.g., curved). The opening <NUM> may be any type of opening that allows access to an internal volume of the cavity <NUM> and may have any shape (e.g., circular, square, hexagonal, oval, etc.). Such access may allow a vapor (or a source of the vapor) to be disposed into the cavity <NUM> during manufacture of the vapor cell <NUM>. The cavity <NUM> extends from the surface <NUM> into the dielectric body <NUM> and stops before extending completely through the dielectric body <NUM>. The cavity <NUM> may have a uniform cross-section along its extension through the dielectric body. However, in some variations, the cross-section of cavity <NUM> may vary along its extension.

The dielectric body <NUM> also includes a plurality of holes <NUM> between the cavity <NUM> and a side <NUM> of the dielectric body <NUM>. The plurality of holes <NUM> may define an array of holes. The plurality of holes <NUM> may reduce a refractive index mismatch between the dielectric body <NUM> and an ambient environment thereof (e.g., air) when the example vapor cell <NUM> receives electromagnetic radiation. The plurality of holes <NUM> may also reduce a scattering cross-section of the example vapor cell <NUM> when receiving the electromagnetic radiation as well as increasing a uniformity of the electromagnetic radiation in the cavity <NUM>. In some implementations, the example vapor cell <NUM> is configured to detect a target radiation, such as an electromagnetic radiation having a frequency ranging from <NUM> to <NUM> THz. In such implementations, the plurality of holes <NUM> may have a largest dimension no greater than a wavelength of the target radiation, and the target radiation may have a wavelength of at least <NUM>.

In some implementations, the plurality of holes <NUM> encircles a perimeter defined by the opening <NUM> of the cavity <NUM>. In these implementations, the opening <NUM> may define an inner perimeter and one or more sides <NUM> of the dielectric body <NUM> may define an outer perimeter. <FIG> depicts the plurality of holes <NUM> as having the same shape. However, in some variations, a portion (or all) of the holes <NUM> may have different shapes. For example, the plurality of holes <NUM> may include two or more subsets of holes, each having a different shape. In some implementations, the plurality of holes <NUM> includes a pattern of holes repeating around a perimeter. For example, if the plurality of holes <NUM> includes two or more subsets of holes, the two or more subsets of holes may be arranged relative to each other along the perimeter to define a pattern.

In some implementations, the plurality of holes <NUM> extends completely through the dielectric body <NUM>. For example, the surface <NUM> of the dielectric body <NUM> may be a first surface, and the dielectric body <NUM> includes a second surface <NUM> opposite the first surface <NUM>. The plurality of holes <NUM> may then extend from the first surface <NUM> to the second surface <NUM>. However, in other implementations, a portion (or all) of the holes <NUM> extends only partially through the dielectric body <NUM>. Such extension may start at the first surface <NUM> or the second surface <NUM> of the dielectric body <NUM>. Although <FIG> depicts a constant cross-section for each of the plurality of holes <NUM>, in some variations, one or more holes may vary in cross-section along an extension into the dielectric body <NUM>. Moreover, the extension into the dielectric body <NUM> need not be perpendicular to the surface <NUM> or be straight. In some instances, the extension is angled relative to the surface <NUM>. In some instances, the extension follows a curved pathway into the dielectric body <NUM>.

The example vapor cell <NUM> includes a vapor (not shown) in the cavity <NUM> of the dielectric body <NUM>. The vapor may include constituents such as a gas of alkali-metal atoms, a noble gas, a gas of diatomic halogen molecules, or a gas of organic molecules. For example, the vapor may include a gas of alkali-metal atoms (e.g., K, Rb, Cs, etc.), a noble gas (e.g., He, Ne, Ar, Kr, etc.), or both. In another example, the vapor may include a gas of diatomic halogen molecules (e.g., F<NUM>, Cl<NUM>, Br<NUM>, etc.), a noble gas, or both. In yet another example, the vapor may include a gas of organic molecules (e.g., acetylene), a noble gas, or both. Other combinations for the vapor are possible, including other constituents.

The example vapor cell <NUM> may also include a source of the vapor in the cavity <NUM> of the dielectric body <NUM>. The source of the vapor may generate the vapor in response to an energetic stimulus, such as heat, exposure to ultraviolet radiation, and so forth. For example, the vapor may correspond to a gas of alkali-metal atoms and the source of the vapor may correspond to an alkali-metal mass sufficiently cooled to be in a solid or liquid phase when disposed into the cavity <NUM>. In some implementations, the source of the vapor resides in the cavity of the dielectric body, and the source of the vapor includes a liquid or solid source of the alkali-metal atoms configured to generate a gas of the alkali-metal atoms when heated.

The example vapor cell <NUM> additionally includes the optical window <NUM>. As shown in <FIG>, the optical window <NUM> covers the opening <NUM> of the cavity <NUM> and has a surface <NUM> bonded to the surface <NUM> of the dielectric body <NUM>. This bond forms a seal around the opening <NUM>. The surface <NUM> of the optical window <NUM> is configured to mate to the surface of the dielectric body <NUM> and may be planar surface. However, other types of surfaces are possible (e.g., curved). Examples of the bond between the two surfaces <NUM>, <NUM> includes an anodic bond, a contact bond, and a fired glass-frit bond.

For example, a contact bond may form the seal around the opening <NUM>. The seal may include metal-oxygen bonds formed by reacting a first plurality of hydroxyl ligands on the surface <NUM> of the dielectric body <NUM> with a second plurality of hydroxyl ligands on the surface <NUM> of the optical window <NUM>. If one or both of the dielectric body <NUM> (or an adhesion layer thereon) and the optical window <NUM> include silicon oxide, the metal-oxide bonds may include siloxane bonds (i.e., Si-O-Si). However, other types of metal-oxygen bonds are possible, including hybrid oxo-metal bonds. For example, if the dielectric body <NUM> and the optical window are both formed of sapphire (e.g., Al<NUM>O<NUM>), the metal-oxygen bonds may include oxo-aluminum bonds (e.g., Al-O-Al). If the dielectric body <NUM> is formed of a glass that includes silicon oxide and the optical window <NUM> is formed of sapphire, the metal-oxygen bonds may include silicon-oxo-aluminum bonds (e.g., Si-O-Al, Al-O-Si, etc.).

The optical window <NUM> may be formed of a material highly transparent to electromagnetic radiation (e.g., laser light) used to probe the vapor sealed within the cavity <NUM> of the dielectric body <NUM>. For example, the material of the optical window <NUM> may be transparent to infrared wavelengths of electromagnetic radiation (e.g., <NUM> - <NUM>), visible wavelengths of electromagnetic radiation (e.g., <NUM> - <NUM>), or ultraviolet wavelengths of electromagnetic radiation (e.g., <NUM> - <NUM>). Moreover, the material of the optical window <NUM> may be an insulating material having a high resistivity, e.g., ρ > <NUM><NUM> Ω·cm, and may also correspond to a single crystal, a polycrystalline ceramic, or an amorphous glass. For example, the material of the optical window <NUM> may include silicon oxide (e.g., SiOz, SiOx, etc.), such as found within quartz, vitreous silica, or a borosilicate glass. In another example, the material of the optical window <NUM> may include aluminum oxide (e.g., Al<NUM>O<NUM>, AlxOy, etc.), such as found in sapphire or an aluminosilicate glass. In some instances, the material of the optical window <NUM> is an oxide material such as magnesium oxide (e.g., MgO), aluminum oxide (e.g., Al<NUM>O<NUM>), silicon dioxide (e.g., SiO<NUM>), titanium dioxide (e.g., TiOz), zirconium dioxide, (e.g., ZrO<NUM>), yttrium oxide (e.g., Y<NUM>O<NUM>), lanthanum oxide (e.g., La<NUM>O<NUM>), and so forth. The oxide material may be non-stoichiometric (e.g., SiOx), and may also be a combination of one or more binary oxides (e.g., Y:ZrO<NUM>, LaAlO<NUM>, etc.). In other instances, the material of the dielectric body <NUM> is a non-oxide material such as diamond (C), calcium fluoride (CaF), and so forth.

In many implementations, the surface <NUM> of the dielectric body <NUM> and the surface <NUM> of the optical window <NUM> may have a surface roughness Ra, no greater than a threshold surface roughness. The threshold surface roughness may ensure that, during contact bonding, pathways are not formed that leak through the seal. Such pathways, if present, might allow contaminates to enter the cavity <NUM> and vapor to exit the vapor cell <NUM>. In some variations, the threshold surface roughness is less than <NUM>. In some variations, the threshold surface roughness is less than <NUM>. In some variations, the threshold surface roughness is less than <NUM>. In some variations, the threshold surface roughness is less than <NUM>.

Although <FIG> depict the example vapor cell <NUM> as having a single optical window, two or more optical windows are possible for the example vapor cell <NUM>. Moreover, in some variations, the cavity <NUM> may extend entirely through dielectric body <NUM>. <FIG> presents an exploded view, in perspective, of an example vapor cell <NUM> having two optical windows. The example vapor cell <NUM> may be analogous in many features to the example vapor cell <NUM> shown by <FIG>. <FIG> presents the example vapor cell <NUM> of <FIG>, but in which both optical windows are bonded to a dielectric body <NUM> of the example vapor cell <NUM>. The example vapor cell <NUM> includes a dielectric body <NUM> and a cavity <NUM> in the dielectric body <NUM>. The cavity <NUM> extends completely through the dielectric body <NUM>. A first surface <NUM> of the dielectric body <NUM> defines a first opening <NUM> to the cavity <NUM>, and a second surface <NUM> of the dielectric body <NUM> defines a second opening <NUM> to the cavity <NUM>. The second surface <NUM> may be opposite the first surface <NUM>, and in some instances, one or both of the first and second surfaces <NUM>, <NUM> are planar surfaces. A vapor or a source of the vapor resides in the cavity <NUM> of the dielectric body <NUM>.

The example vapor cell <NUM> also includes a first optical window <NUM> covering the first opening <NUM> of the cavity <NUM>. The first optical window <NUM> has a surface <NUM> bonded to the first surface <NUM> of the dielectric body <NUM> to form a first seal around the first opening <NUM>. The example vapor cell <NUM> additionally includes a second optical window <NUM> covering the second opening <NUM> of the cavity <NUM>. The second optical window <NUM> has a surface <NUM> bonded to the second surface <NUM> of the dielectric body <NUM> to form a second seal around the second opening <NUM>. In some instances, one or both of the two surfaces <NUM>, <NUM> are planar surfaces. Such planar configuration may allow one or both of the two surfaces <NUM>, <NUM> to mate to, respectively, the first and second surfaces <NUM>, <NUM> of the dielectric body <NUM>. For example, the second surface <NUM> of the dielectric body <NUM> and the surface <NUM> of the second optical window <NUM> may be planar surfaces.

The dielectric body <NUM> and the optical windows <NUM>, <NUM> may share features in common with, respectively, the dielectric body <NUM> and the optical window <NUM> described in relation to the example vapor cell <NUM> of <FIG>. For example, the dielectric body <NUM> may be formed of silicon (Si), aluminum oxide (e.g., Al<NUM>O<NUM>), or a glass that includes silicon oxide (e.g., SiOz, SiOx, etc.). In another example, one or both of first and second optical windows <NUM>, <NUM> may be formed of a material transparent to electromagnetic radiation (e.g., laser light) used to probe the vapor sealed within the cavity <NUM> of the dielectric body <NUM>. Other features and combinations are possible. Similarly, the vapor and the source of the vapor may share features in common with, respectively, the vapor and the source of the vapor described in relation to the example vapor cell <NUM> of <FIG>. For example, the vapor may include a gas of alkali-metal atoms, a noble gas, a gas of diatomic halogen molecules, a gas of organic molecules, or some combination thereof. In another example, the source of the vapor may reside in the cavity <NUM> of the dielectric body <NUM>, and the source of the vapor may include a liquid or a solid source of alkali-metal atoms configured to generate a gas of the alkali-metal atoms when heated. Other features and combinations are possible.

Similar to the example vapor cell <NUM> of <FIG>, the dielectric body <NUM> of the example vapor cell <NUM> includes a plurality of holes <NUM> between the cavity <NUM> and a side <NUM> of the dielectric body <NUM>. The plurality of holes <NUM> may define an array of holes, and may extend into the dielectric body <NUM> from one or both of two opposing surfaces of the dielectric body <NUM>. In some variations, the plurality of holes <NUM> extend through dielectric body <NUM>, while in other variations, the plurality of holes <NUM> extend only partially into the dielectric body <NUM>. The plurality of holes <NUM> may reduce a refractive index mismatch between the dielectric body <NUM> and an ambient environment thereof (e.g., air) when the example vapor cell <NUM> receives electromagnetic radiation. The plurality of holes <NUM> may also reduce a scattering cross-section of the example vapor cell <NUM> when receiving the electromagnetic radiation as well as increasing a uniformity of the electromagnetic radiation in the cavity <NUM>. In some implementations, the example vapor cell <NUM> is configured to detect a target radiation, such as an electromagnetic radiation having a frequency ranging from <NUM> to <NUM> THz. In such implementations, the plurality of holes <NUM> may have a largest dimension no greater than a wavelength of the target radiation, and the target radiation may have a wavelength of at least <NUM>.

In some implementations, the plurality of holes <NUM> encircles a perimeter defined by the first and second openings <NUM>, <NUM> of the cavity <NUM>. In these implementations, the first and second openings <NUM>, <NUM> may define respective first and second inner perimeters and one or more sides <NUM> of the dielectric body <NUM> may define an outer perimeter. <FIG> depicts the plurality of holes <NUM> as having the same shape. However, in some variations, a portion (or all) of the holes <NUM> may have different shapes. For example, the plurality of holes <NUM> may include two or more subsets of holes, each having a different shape. In some implementations, the plurality of holes <NUM> includes a pattern of holes repeating around a perimeter. For example, if the plurality of holes <NUM> includes two or more subsets of holes, the two or more subsets of holes may be arranged relative to each other along the perimeter to define a pattern.

In some implementations, the plurality of holes <NUM> extends completely through the dielectric body <NUM>. For example, the plurality of holes <NUM> may extend from the first surface <NUM> to the second surface <NUM>. However, in other implementations, a portion (or all) of the holes <NUM> extends only partially through the dielectric substrate <NUM>. Such extension may start at the first surface <NUM> or the second surface <NUM> of the dielectric body <NUM>. For example, the plurality of holes <NUM> may be a first plurality of holes extending from the first surface <NUM> into the dielectric body <NUM>. The first plurality of holes may be between the first opening <NUM> of the cavity <NUM> and the side <NUM> of the dielectric body <NUM>. The dielectric body <NUM> may then include a second plurality of holes extending from the second surface <NUM> into the dielectric body <NUM>. The second plurality of holes may be between the second opening <NUM> of the cavity <NUM> and the side of the dielectric body <NUM>. In some instances, the second plurality of holes encircles a second perimeter defined by the second opening <NUM> of the cavity <NUM>. The second plurality of holes may include a pattern of holes repeating around the second perimeter.

Although <FIG> depicts a constant cross-section for each of the plurality of holes <NUM>, in some variations, one or more holes may vary in cross-section along an extension into the dielectric body <NUM>. Moreover, the extension into the dielectric body <NUM> need not be perpendicular to the first and second surfaces <NUM>, <NUM> or be straight. In some instances, the extension is angled relative to the first and second surfaces <NUM>, <NUM>. In some instances, the extension follows a curved pathway into the dielectric body <NUM>.

In implementations where the dielectric body <NUM> is formed of a non-oxide material, an adhesion layer may be disposed on the dielectric body <NUM> to define the first surface <NUM> of the dielectric body <NUM>. The adhesion layer may be capable of bonding to the non-oxide material of the dielectric body <NUM> while also being capable of forming a contact bond with the surface <NUM> of the first optical window <NUM>. For example, the dielectric body <NUM> may be formed of silicon and the example vapor cell <NUM> may include an adhesion layer that includes silicon oxide (e.g., SiO<NUM>, SiOx, etc.) on the dielectric body <NUM>. This adhesion layer defines the first surface <NUM> of the dielectric body <NUM> and is capable of forming a contact bond that includes siloxane bonds. In some implementations, the second seal comprises metal-oxygen bonds formed by reacting a third plurality of hydroxyl ligands on the second surface <NUM> of the dielectric body <NUM> with a fourth plurality of hydroxyl ligands on the surface <NUM> of the second optical window <NUM>. In these implementations, example vapor cell <NUM> may include an adhesion layer disposed on the dielectric body <NUM> to define the second surface <NUM> of the dielectric body <NUM> if the dielectric body is formed of a non-oxide material.

In some implementations, such as shown in <FIG>, the first and second surfaces <NUM>, <NUM> of the dielectric body <NUM> are planar surfaces opposite each other, and the surface <NUM> of the first optical window <NUM> and the surface <NUM> of the second optical window <NUM> are planar surfaces. In some implementations, the second surface <NUM> of the dielectric body <NUM> and the surface <NUM> of the second optical window <NUM> have a surface roughness, Ra, no greater than a threshold surface roughness. In some variations, the threshold surface roughness is less than <NUM>. In some variations, the threshold surface roughness is less than <NUM>. In some variations, the threshold surface roughness is less than <NUM>. In some variations, the threshold surface roughness is less than <NUM>. In further implementations, the threshold surface roughness is a second threshold surface roughness, and the first surface <NUM> of the dielectric body <NUM> and surface <NUM> of the first optical window <NUM> have a surface roughness, Ra, no greater than a first threshold surface roughness. The first threshold surface roughness need not be the same as the second threshold surface roughness.

In some implementations, the second seal includes an anodic bond between the second surface <NUM> of the dielectric body <NUM> and the surface <NUM> of the second optical window <NUM>. For example, the dielectric body <NUM> may be formed of silicon and the second optical window <NUM> may include silicon oxide (e.g., SiOz, SiOx, etc.). The second seal may then include an anodic bond between the second surface <NUM> of the dielectric body <NUM> and the surface <NUM> of the second optical window <NUM>. In another example, the dielectric body <NUM> may be formed of a glass comprising silicon oxide (e.g., SiO<NUM>, SiOx, etc.) and the second optical window <NUM> may include silicon oxide (e.g., SiO<NUM>, SiOx, etc.). In this example, the vapor cell <NUM> may include a layer of silicon disposed between the second surface <NUM> of the dielectric body <NUM> and the surface <NUM> of the second optical window <NUM>. The second seal includes an anodic bond between the layer of silicon and one or both of the second surface <NUM> of the dielectric body <NUM> and the surface <NUM> of the second optical window <NUM>.

In some implementations, the dielectric body <NUM> is formed of a glass comprising silicon oxide (e.g., SiO<NUM>, SiOx, etc.) and the second optical window <NUM> includes silicon oxide (e.g., SiO<NUM>, SiOx, etc.). In such cases, the example vapor cell <NUM> includes a fired layer of glass frit bonding the second surface <NUM> of the dielectric body <NUM> to the surface <NUM> of the second optical window <NUM>. The fired layer of glass frit defines the second seal.

The example vapor cells <NUM>, <NUM> described in relation to <FIG> may correspond to vapor cells for Rydberg atom-based electric field sensing. Such vapor cells have a low scattering cross-section and uniform electromagnetic field in the region of the vapor cell (e.g., the cavities <NUM>, <NUM>) where the electromagnetic field is measured. As such, the measured electromagnetic field may be immune to manufacturing variations in one or more aspects of the vapor cell (e.g., a width of a side wall). The vapor cells use metamaterial walls that are defined by holes, voids, and so forth to create a more index-matched vapor cell that is structurally sound. Two examples are presented where a pure alkali gas is filled into the vapor cell - one vapor cell is made entirely of glass while the other is made of silicon and glass.

The vapor cells may be constructed from at least one optical window (e.g., <NUM>-<NUM> optical windows) and a dielectric body that serves as a frame. The frame include a single cavity or array of cavities machined in the dielectric body to make a chip that can be subsequently cut ( e.g., with a dicing saw, a laser, etc.) into individual vapor cells at a later date. As described above in relation to the example vapor cells <NUM>, <NUM>, the frame (or dielectric body) can be formed of various materials such as glass or silicon. Such materials can be machined with a laser, and a dielectric body formed of silicon, may also be machined using deep reactive ion etching (DRIE).

The at least one optical window can be made out of thin glass so that light of a desired wavelength can pass into the cavity. Such passage allows the light to interact with a vapor (e.g., gaseous atoms or molecules) contained in the vapor cell. The at least one optical window can be antireflection coated or coated to transmit one or more specific colors of light. The at least one optical window may have a thermal expansion coefficient that matches (or closely matches) that of the frame. For example, if the frame is formed of silicon, the at least one optical window may be formed of a borosilicate glass (e.g., a MEMpax wafer from Schott). If the frame is not closed on one of its surfaces (e.g., the surface <NUM> of dielectric body <NUM>, etc.), then the frame may be bonded to an optical window or a plate that will serve as an entry or exit optic for the vapor cell.

Bonding can be accomplished with a technique such as anodic or glass frit bonding, which is done at high voltages and/or temperatures since the frame is open and can outgas during the process. Such bonds are capable of forming leak-tight seals for high vacuum operation. Once this bond is complete, the frame can be contact bonded to the remaining window or plate in an atmosphere of the vapor or gas that will fill the vapor cell. The frame is machined with small sub wavelength holes to better index match the frame's structure to its environment (e.g., air, free space, etc.). This machining can be done via DRIE etching (e.g., for silicon) or via laser machining with a pulsed laser to avoid melting the sample (e.g., for glass or silicon).

<FIG> shows a possible wall pattern that may be manufactured into a frame or dielectric body using a plurality of holes or voids. The wall pattern has the structural integrity to support the necessary vacuum pressure within the cavity. However, other patterns are possible. In many implementations, the structures defined by the plurality of holes are voids having sub-wavelength dimensions for a target electromagnetic radiation that is to be measured by the vapor cell. Moreover, the structures maintain the mechanical integrity of the vapor cell with regards to a vacuum-tight seal and handling during manufacturing and deployment. The vapor cell may be optically coupled over free space or through a waveguide such as an optical fiber. In some implementations, the plurality of holes or voids may be larger than a wavelength of the target electromagnetic radiation if scattering of the incident radiation and interference are used to either enhance or eliminate the electromagnetic field. Multiple vapor cells may be connected together to form arrays since their scattering cross-sections are small relative to their geometric cross-sections. As such, the vapor cell sensors interfere minimally with each other, if at all.

In some implementations, the vapor cell consists of two optical windows affixed to a frame (e.g., the vapor cell <NUM> of <FIG>). The optical windows or the frame may have additional coatings on their respective surfaces to tailor the vapor cell for optical transmission, optical reflection, and adhesion of the optical windows to the frame. The vapor cell may be constructed by first laser machining (or by some other type of microstructuring such as etching) a cavity and metamaterial walls of the vapor cell. The surfaces are then prepared so that the optical windows can be bonded to the frame. A first optical window is affixed to the frame using a method such as glass frit bonding or anodic bonding. This bonding operation can be done at high temperatures and/or voltages. After completion of the bonding operation, the remaining exposed surfaces are prepared for contact bonding, e.g., the first surface <NUM> of the dielectric body <NUM> and the surface <NUM> of the first optical window <NUM>. Contact bonding is then carried out in an atmosphere of the vapor or gas to be filled into the vapor cell. The contact bonding operation is done at low temperatures (e.g., less than <NUM>) and zero voltage to prevent outgassing of undesirable gasses into the vapor cell's cavity.

Gasses produced in the contact bonding operation, such as water vapor, can be reacted to form products that are solid at room temperature. For example, if the vapor in the cavity is a gas of cesium atoms, water vapor produced during the contact bonding operation will react with a portion of the cesium atoms to the form solids, such as CszO (Tmelt ≅ <NUM>), CsOH (Tmelt ≅ <NUM>), or CsH (Tmelt ≅ <NUM>). The vapor cell can be coated with epoxy and parylene if there is a desire to protect the vapor cell further. The vapor cell can also be coupled to a waveguide or be used in a free-space application. In some variations, the vapor cell is annealed to strengthen the bond (e.g., at a temperature less than <NUM>).

Multiple vapor cells can be connected together or arranged in an array to make multiple simultaneous measurements in a region of space. For example, multiple vapor cells can be arranged in a planar array so that an electromagnetic field can be characterized in a plane of the array. Three-dimensional arrays are also possible. Such capabilities may be allowed by the dielectric nature of the vapor cells (e.g., their dielectric bodies) since the vapor cells minimally interact with each other due to low scattering cross-sections. Light to probe the vapor in the vapor cells can be transported through optical waveguides, such as fiber, in parallel or series, but have to be readout independently (the signal light has to be split off at each vapor cell to give a measurement that reflects the absorption or dispersive signal associated with the individual vapor cell). In essence, this is a multipixel array, but the transparent nature of vapor cells makes <NUM>-dimensional imaging possible. Thick cells can be manufactured by this method by stacking unit cells together or to make unique shapes, such as taking anodically bonded glass + frames (laser cut together), stacking them and anodically bonding several together one at a time, and then capping the structure with a contact bond.

Now referring to <FIG>, a schematic diagram is presented of an example method of manufacturing chips that include one or more vapor cells. Each vapor cell includes a plurality of holes defining a pattern. The pattern reduces the index of refraction or impedance matching of a vapor cell relative to an incident wave of target radiation to be measured. <FIG> depicts the chips as having three or six vapor cells. However, other numbers of vapor cells are possible. The method of manufacturing may start after a simulation phase is used to design a metamaterial wall for the one or more vapor cells. In particular, the simulation may allow those skilled in the art to design a pattern for the plurality of holes. The method of manufacturing includes removing material from a chip to define cavities for each vapor cell and a respective plurality of holes. An anodic bond may then be formed between an optical window and the chip, followed by a contact bond between another optical window and the chip. The chip may then be disposed between the two optical windows in a sandwiched configuration.

Now referring to <FIG>, a comparison is presented of scattering cross-sections for three example vapor cells, one having a solid wall and two having respective metamaterial walls. Modeled values of the scattering cross sections (RCS) are presented in a lower left graph for frequencies of electromagnetic radiation ranging from <NUM> to <NUM>. A lower right graph presents the frequencies of electromagnetic radiation from the lower left graph, but within a narrower range from <NUM> to <NUM>. The two example vapor cells with metamaterial walls each have a different pattern of holes disposed around a square cavity. The example vapor cells all have frames (or dielectric bodies) with square cross-sections. An edge length of the square cross-sections is <NUM>, and a height of the example vapor cells is <NUM>. The scattering cross-sections are notably reduced for the two example vapor cells with metamaterial walls relative to the single example vapor cell with the solid wall. In particular, the RCS values are lower for the two example vapor cells with metamaterial walls over a range from about <NUM> to <NUM> (see lower right graph) and a range from about <NUM> to <NUM> (see lower left graph).

Now referring to <FIG>, simulated graphs of electric field distribution are presented for four example square vapor cells, each of which has a different pattern of holes to define a respective metamaterial wall. The simulated graphs were generated by modeling an <NUM> plane wave of unit amplitude incident on the example vapor cells. Each vapor cell was sampled across a central circular region in its respective cavity. The central circular region corresponds to the region where the laser beams may be used to initialize and measure a response of atoms or molecules (e.g., the sealed vapor) to electromagnetic radiation passing through a vapor cell. The distribution of measured electric fields is a much narrower when the vapor cell is better index-matched to free space. The patterns of holes illustrated in <FIG> illustrate that virtually any two-dimensional shape can be machined in the frame to define the metamaterial wall, especially if laser cutting is used.

Now referring to <FIG>, a simulated contour plot is presented of an electric field distribution in two example vapor cells having metamaterial walls. A geometric cross-section of each example vapor cell is disposed adjacent and to the left of its corresponding simulated contour plot. The circular region in the center of each example vapor cell is the region probed by one or more laser beams to characterize electromagnetic radiation incident on a respective vapor cell. The electric field distributions are represented by greyscale intensities that correspond to an electric field strength in V/m. The electric field distributions are uniform and occur within a range from <NUM> V/m to <NUM> V/m for the example vapor cell associated with the upper portion of <FIG> and a range from <NUM> V/m to <NUM> V/m for the example vapor cell associated with the lower portion of <FIG>.

Now referring to <FIG>, a graph is presented showing a standard deviation of a measured electric field in a vapor cell and the structural stability of the vapor cell as a function of wall thickness. The vapor cell is a square vapor cell with no holes present in a frame (or wall). To achieve a standard deviation of the measured electric field of less than about <NUM>%, the wall thickness is about <NUM>. The factor of safety (FoS) for a vapor cell with <NUM>-µm walls is <NUM>. In contrast, the equivalent metrology vapor cell has a factor of safety of about <NUM> for an equivalent field standard deviation. <FIG> shows that the vapor cell (or metrology vapor cell) has both structural stability and a high uniform electric field in comparison to a conventional vapor cell.

In some implementations, a method of manufacturing a vapor cell includes obtaining a dielectric body. The dielectric body includes a surface that defines an opening to a cavity in the dielectric body, and a plurality of holes between the cavity and a side of the dielectric body. The method also includes obtaining an optical window having a surface. The surface of the dielectric body and the surface of the optical window may be planar surfaces. A vapor or a source of the vapor is disposed into the cavity. The method additionally includes bonding the surface of the optical window to the surface of the dielectric body to form a seal around the opening to the cavity. In some implementations, bonding the surface includes covering the opening of the cavity with the optical window to enclose the vapor or the source of the vapor in the cavity.

In some implementations, disposing the vapor of the source of the vapor includes exposing the cavity to a vacuum environment that includes a gas of alkali-metal atoms. However, other types of vacuum environments are possible (e.g., those that include a gas of diatomic halogen molecules). In some implementations, obtaining the dielectric body includes removing material from the dielectric body to form the cavity, the plurality of holes, or both. Removing material may include machining material from the surface of the dielectric body with a laser. Removing material may also include etching material from the surface of the dielectric body. Such etching may involve one or both of a dry or wet etching process. Other types of subtractive processes are possible for the operation of removing material (e.g., ablation, grinding, polishing, etc.).

In some implementations, the plurality of holes encircles a perimeter defined by the opening of the cavity. The plurality of holes may include a pattern of holes repeating around the perimeter. In some implementations, the surface of the dielectric body is a first surface and the dielectric body includes a second surface opposite the first surface. In these implementations, the plurality of holes extends from the first surface to the second surface. In some implementations, the vapor cell, when manufactured, is configured to detect a target radiation (e.g., an electromagnetic radiation having a frequency ranging from <NUM> to <NUM> THz). Each of the plurality of holes has a largest dimension no greater than a wavelength of the target radiation. In some instances, the target radiation has a wavelength of at least <NUM>.

In some variations, the dielectric body may be formed of silicon. In such variations, the method may include forming an adhesion layer on the dielectric body that defines the surface of the dielectric body. The adhesion layer may include silicon oxide (e.g., SiOz, SiOx, etc.). In some variations, the dielectric body is formed of a glass that includes silicon oxide (e.g., SiO<NUM>, SiOx, etc.). In some variations, the optical window includes silicon oxide (e.g., SiOz, SiOx, etc.).

In some implementations, the method includes altering the surface of the dielectric body and the surface of the optical window to include respectively, a first plurality of hydroxyl ligands and a second plurality of hydroxyl ligands. In these implementations, bonding the surfaces includes contacting the altered surface of the dielectric body to the altered surface of the optical window to form the seal around the opening of the cavity. The seal includes metal-oxygen bonds formed by reacting the first plurality of hydroxyl ligands with the second plurality of hydroxyl ligands during contact of the altered surfaces. In some instances, altering the surfaces includes activating one or both of the surfaces of the dielectric body and the optical window by exposing the respective surfaces to a plasma. Altering the surfaces may also include washing one or both of the activated surfaces of the dielectric body and the optical window in a basic aqueous solution.

The method may also be used to manufacture vapor cells having at least two optical windows. In some implementations, the surface is a first surface, the opening is a first opening, the optical window is a first optical window, and the seal is a first seal. Moreover, the dielectric body includes a second surface that defines a second opening to the cavity of the dielectric body. The first and second surfaces of the dielectric body may be opposite each other. In these implementations, the method includes obtaining a second optical window having a surface, and bonding the surface of the second optical window to the second surface of the dielectric body to form a second seal around the second opening of the cavity. The second surface of the dielectric body and the surface of the second optical window may be planar surfaces.

In some variations, the plurality of holes is a first plurality of holes extending from the first surface into the dielectric body, and the dielectric body includes a second plurality of holes extending from the second surface into the dielectric body. The first plurality of holes is between the first opening of the cavity and the side of the dielectric body, and the second plurality of holes is between the second opening of the cavity and the side of the dielectric body. The second plurality of holes may encircle a second perimeter defined by the second opening of the cavity. The second plurality of holes may include a pattern of holes repeating around the second perimeter.

In some implementations, the dielectric body is formed of silicon and the second optical window includes silicon oxide (e.g., SiO<NUM>, SiOx, etc.). In these implementations, bonding the surface of the second optical window includes anodically bonding the surface of the second optical window to the second surface of the dielectric body to form the second seal. In other implementations, the dielectric body is formed of a glass that includes silicon oxide (e.g., SiO<NUM>, SiOx, etc.) and the second optical window includes silicon oxide (e.g., SiOz, SiOx, etc.). The method then includes depositing a layer of silicon on the second surface of the dielectric body. Moreover, bonding the surface of the second optical window includes anodically bonding the layer of silicon to the surface of the second optical window to form the second seal.

In some implementations, the dielectric body is formed of a glass that includes silicon oxide (e.g., SiOz, SiOx, etc.) and the second optical window includes silicon oxide (e.g., SiO<NUM>, SiOx, etc.). In such implementations, bonding the surface of the second optical window includes applying a glass frit to one or both of the second surface of the dielectric body and the surface of the second optical window, and contacting the second surface of the dielectric body to the surface of the second optical window. At least one of the glass frit, the dielectric body, or the second optical window is then heated to a firing temperature to form the second seal.

In some implementations, the method includes altering the second surface of the dielectric body and the surface of the second optical window to include, respectively, a third plurality of hydroxyl ligands and a fourth plurality of hydroxyl ligands. In these implementations, bonding the surfaces includes contacting the altered second surface of the dielectric body to the altered surface of the second optical window to form the second seal around the second opening of the cavity. The second seal includes metal-oxygen bonds formed by reacting the third plurality of hydroxyl ligands with the fourth plurality of hydroxyl ligands during contact of the altered surfaces.

The methods of manufacturing vapor cells may also be described by the following examples. However, examples are for purposes of illustration only. It will be apparent to those skilled in the art that many modifications, both to materials and methods, may be practiced without departing from the scope of the appended claims.

A p-type silicon wafer was obtained with a double-sided polish and an <<NUM>> orientation. The silicon wafer had a diameter of <NUM>-inches and was <NUM> thick with a surface roughness, Ra, no greater than <NUM> on each side. Electrical properties of the silicon wafer included a resistance that ranged from <NUM>Ω-cm to <NUM>Ω-cm. A glass wafer formed of borosilicate glass was also obtained from Schott. The glass wafer was a MEMpax wafer having a diameter of <NUM> inches and a thickness of <NUM>. The surface roughness was less than <NUM>.

The silicon and glass wafers were inspected in preparation for anodic and contact bonding. In particular, the wafers were visually inspected for chips, micro-cracks, and scratches. The wafers were also verified to have a surface roughness less than <NUM>. A <NUM>-nm layer of SiO<NUM> was grown on both sides of the silicon wafer using a wet growth process in an oxidation furnace. The temperature of the oxidation furnace was set to about <NUM> and the processing time of the silicon wafer was about <NUM>. A thickness uniformity of the silicon wafer (with the SiO<NUM> layers) was verified to be within <NUM> ± <NUM> over its <NUM>-inch diameter area. The surface roughness was also verified to be less than <NUM>.

Multiple silicon chips were cut from the silicon wafer using either a Protolaser U3 micro-laser tool, a Protolaser R micro-laser tool, or a DISCO DAD <NUM> dicing saw. Each silicon chip had dimensions of <NUM> x <NUM>. Nine holes were subsequently machined through each of the silicon chips using the Protolaser U3 micro-laser tool or the Protolaser R micro-laser tool. The holes were each circular with a <NUM>-mm diameter or square with a <NUM>-mm edge length. In some cases, combinations of circles and holes were machined in a silicon chip. A plurality of holes was also machined around each of the nine holes to create metamaterial walls in the silicon chips. The silicon chips were inspected visually with 5x and 10x magnification loupes for cracks or chips that might have occurred during cutting. Silicon chips with zero or minimal surface defects were selected for subsequent vapor-cell fabrication.

The selected silicon chips were then cleaned with methanol and isopropanol using cotton swabs and optical tissue paper. Next, the silicon chips were submerged in a buffered oxide etch (BOE) solution having a <NUM>:<NUM> volume ratio and an etch rate of <NUM>/min at room temperature. The buffered oxide etch solution contained hydrofluoric acid buffered with ammonium fluoride. The silicon chips were submerged for at least <NUM> minutes to remove the <NUM>-nm layer of SiOz from the surface of each side of the silicon chips. After being removed from the buffered oxide etch, the silicon chips were visually inspected. If embedded material from the cutting process was found on a silicon chip, the silicon chip was discarded. If regions of SiOz remained on a silicon chip, the silicon chip was re-submerged in the buffered oxide etch solution, removed, and then re-inspected. Silicon chips with both sides free of the <NUM>-nm layer of SiO<NUM> were selected for final cleaning and a <NUM>-nm SiO<NUM> layer was sputtered onto one surface.

The selected silicon chips were then cleaned with acetone and isopropanol using cotton swabs and optical tissue paper. An ultrasonic cleaner was optionally used to assist the cleaning process by agitating baths of acetone or isopropanol in which the selected silicon chips were submerged. A <NUM>-nm layer of SiO<NUM> was then grown on one side of the silicon chips. The temperature of the oxidation furnace was set to a minimum of <NUM> to obtain a surface roughness no greater than <NUM> for the <NUM>-nm layer of SiOz. A thickness uniformity of the <NUM>-nm SiO<NUM> layer was verified to be within <NUM> ± <NUM> over an area of a silicon chip. Silicon chips failing the uniformity criterion were discarded.

Silicon chips with the <NUM>-nm SiO<NUM> layer were then cleaned with methanol and isopropanol using cotton swabs and optical tissue paper to eliminate loose residues on their surfaces (e.g., such as due to handling). The silicon chips were subsequently deep-cleaned with acetone and isopropanol using cotton swabs and optical tissue paper. A low magnification loupe (e.g., 10x) was used during the deep cleaning process for a first visual inspection followed by a high magnification microscope (e.g., 50x - 200x) for a second visual inspection. Silicon chips passing the second vision inspection were placed in a bath of acetone for ultrasonic cleaning at <NUM> (e.g., in a Branson Ultrasonic Cleaner CPX-<NUM>-117R). For example, the silicon chips could be placed in a glass beaker of acetone and cleaned ultrasonically for <NUM> minutes at room temperature. After ultrasonic cleaning, the silicon chips were dried with particulate-free compressed air and stored in an air-tight container until needed for bonding.

Separately, a dicing saw was used to cut the glass wafers into suitable sizes for bonding to the (stored) silicon chips. Two glass chips were prepared for each silicon chip. If a glass chip was intended for an anodic bond, the glass chip was cut to have the same dimensions as the silicon chip. However, if a glass chip was intended for a contact bond, the glass chip was cut to have longer dimensions than the silicon chip. For example, glass chips for anodic bonding had dimensions of <NUM> x <NUM> and glass chips for contact bonding had dimensions of <NUM> x <NUM>. After cutting, each glass chip was inspected to ensure that its optical clarity was not degraded (e.g., hazing), or that scratches or cracks were not present. Glass chips found to be acceptable were then cleaned with acetone using cotton swabs and optical tissue paper. If necessary, the glass chips were placed in a glass beaker of acetone and ultrasonically cleaned form <NUM> minutes at room temperature. After ultrasonic cleaning, the glass chips were dried with particulate-free compressed air and then stored in an air-tight container until needed for bonding.

One silicon chip and one glass chip were then placed into an assembly for anodic bonding. For the silicon chip, the planar surface opposite the planar surface defined by the <NUM>-nm layer of SiO<NUM> participated in the anodic bonding process. In the assembly, planar surfaces of silicon and glass chips were contacted to define an interface, and the interface was visually inspected to confirm that optical fringes were present. The silicon chip was then heated to a temperature of about <NUM>. After this temperature was reached, 600V was applied across the silicon and glass chips for about <NUM> minutes, which drove the formation of an anodic bond. The interface was inspected again to confirm the disappearance of the optical fringes, which indicated the anodic bond was complete. Next, the anodic bond was inspected for defects (e.g., bubbles, micro-cracks, unbonded areas, etc.). If <NUM>% or more of an area around the holes was free of defects, the anodic bond was then further inspected for open channels (e.g., from a hole to the environment, a hole to another hole, etc.). If an open channel was discovered, the anodically-bonded chips were discarded as the anodic bond was not deemed leak-tight.

Bonded silicon and glass chips with leak-tight anodic bonds were cleaned in acetone and methanol. During this cleaning process, the unbonded surface of the silicon chip was cleaned with acetone and methanol using cotton swabs and optical tissue paper to eliminate any residues (e.g., residues from a graphite plate of the assembly used to form the anodic bond). The unbonded surface of the silicon chip was then visually inspected to ensure defects (e.g., scratches, pitting, etc.) were not present that might compromise a soon-to-be formed contact bond. The anodically-bonded chips were then individually cleaned. In particular, the anodically-bonded chips were placed individually (i.e., with no other chips) in a glass beaker of acetone and cleaned ultrasonically for <NUM> minutes at room temperature. After ultrasonic cleaning, the anodically-bonded chips were dried with particulate-free compressed air. A low magnification loupe (e.g., 10x) was used for a first visual inspection of the anodically-bonded chips, followed by a high magnification microscope (e.g., 50x - 200x) for a second visual inspection. The first and second visual inspections were used to ensure no visual residues or deposits remained on the anodically-bonded chips.

The anodically-bonded chips - along with glass chips - were then taken into a clean room environment (e.g., Class <NUM> or better) for contact bonding. Single instances of the anodically-bonded chips were paired with single instances of the glass chips to define a pair of chips for contact bonding. For each pair, a planar surface defined by the <NUM>-nm layer of SiO<NUM> on the silicon chip and a planar surface of the glass chip were wiped with optical paper and acetone to clean any macroscopic deposits or contaminants from them. Each pair was then submerged in an acetone bath (e.g., acetone in a beaker) and cleaned via ultrasonic cleaning for <NUM> minutes. Each pair of chips was subsequently removed from the acetone bath, rinsed with isopropanol (e.g., submerged in an isopropanol bath), and blown dry with dry nitrogen gas.

A pair of chips was placed in a YES-CV200RFS plasma cleaner and cleaned for <NUM> seconds using a nitrogen plasma. (In some instances, multiple pairs of chips were place in the plasma cleaner. ) In particular, the planar surface defined by the <NUM>-nm layer of SiOz on the silicon chip and the planar surface of the glass chip were activated by plasma cleaning. The RF-power of the plasma cleaner was set at about <NUM> W, and the pressure inside was maintained at about <NUM> mTorr. Nitrogen gas introduced into the plasma cleaner at a volume flow rate of about <NUM> sccm. After activation by plasma cleaning, the pair of chips was removed from the YES-CV200RFS plasma cleaner and rinsed in de-ionized water for <NUM> minutes. The rinsing process served to hydroxylate the activated surfaces. In some variations, the rinsing process was conducted with a basic aqueous solution (e.g., an aqueous solution of ammonium hydroxide). Care was taken not to touch the two hydroxylated and activated surfaces together.

The pair of chips was then transferred into a vacuum chamber and mounted into a fixture having a "press finger". The fixture held the glass chip adjacent the silicon chip of the anodically-bonded chip to define a gap. The activated and hydroxylated surface of the glass chip faced the activated and hydroxylated SiOz surface of the silicon chip. The vacuum chamber was then sealed and pumped down to a reduced pressure (e.g., less than <NUM>-<NUM> Torr) to remove volatile species (e.g., water vapor) that might react with a vapor of cesium atoms used to fill the cavities of the anodically-bonded chip. The fixture was then chilled to by a thermoelectric cooler, which in turn, chilled at least the anodically-bonded chip to a temperature between -<NUM> and <NUM>.

After the temperatures of the pair of chips stabilized, the vapor of cesium atoms was introduced into the vacuum chamber by opening a valve connecting a source of cesium vapor to the vacuum chamber. The source of the cesium vapor was an oven containing a mass of cesium heated to a processing temperature. A target pressure of cesium vapor in the vacuum chamber could be controlled by altering an opening of the valve, altering the processing temperature induced by the oven, or both. Once the pressure in the vacuum chamber stabilized to the target pressure of cesium vapor, the pair of chips was exposed to the vapor of cesium atoms for a length of time.

The pressure of cesium vapor in the vacuum chamber influences the length of time needed to fill the anodically-bonded chip. One or both of the pressure of cesium vapor in the vacuum chamber and the period of time can be varied to control an amount of cesium vapor that condenses in the cavities of the anodically-bonded chip. Once the length of time had elapsed, the value to the source of cesium vapor was closed. The vacuum chamber was subsequently pumped down to the reduced pressure (e.g., less than <NUM>-<NUM> Torr) and the power to the thermoelectric cooler turned off.

Once the pair of chips reached ambient temperature, the fixture was actuated to contact the activated and hydroxylated surface of the glass chip to the activated and hydroxylated SiOz surface of the silicon chip. The "press finger" was used to hold the contacted surfaces together for <NUM> minutes, which drove the formation of a contact bond. In some variations, the "press finger" was used to apply a target pressure (e.g., about <NUM> MPa) during the <NUM>-minute duration.

A thick glass wafer was obtained from Howard Glass Co. with a thickness of <NUM> and a diameter of <NUM> inches. The thick glass wafer had a surface roughness, Ra, no greater than <NUM> on each side. Electrical properties of the silicon wafer included a resistance that ranged from <NUM>Ω-cm to <NUM>Ω-cm. A thin glass wafer formed of borosilicate glass was also obtained from Schott. The thin glass wafer was a MEMpax wafer having a diameter of <NUM> inches and a thickness of <NUM>. The surface roughness was less than <NUM>. The thick and thin glass wafers were inspected in preparation for anodic and contact bonding. In particular, the glass wafers were visually inspected for chips, micro-cracks, and scratches. The wafers were also verified to have a surface roughness less than <NUM>.

Next, multiple thick glass chips were cut from the thick glass wafer using either a Protolaser R micro-laser tool or a DISCO DAD <NUM> dicing saw. Each thick glass chip had dimensions of <NUM> x <NUM>. Nine holes were subsequently machined through each of the thick glass chips with a Protolaser R micro-laser tool. The holes were each circular with a <NUM>-mm diameter or square with a <NUM>-mm edge length. A plurality of holes was also machined around each of the nine holes to create metamaterial walls in the thick glass chips. In some cases, combinations of circles and holes were machined in a thick glass chip. The thick glass chips were inspected visually with 5x and 10x magnification loupes for cracks or chips that might have occurred during cutting. Thick glass chips with zero or minimal surface defects were selected for subsequent vapor-cell fabrication.

The selected thick glass chips were then cleaned with acetone and isopropanol using cotton swabs and optical tissue paper. An ultrasonic cleaner was optionally used to assist the cleaning process by agitating baths of acetone or isopropanol in which the selected thick glass chips were submerged. A less than <NUM> layer of Si (e.g., a <NUM> layer of Si) was then grown on one side of the thick glass chips using plasma-enhanced chemical vapor deposition (PECVD). A thickness uniformity of the Si layer was verified to be within ± <NUM> over an area of a thick glass chip. Thick glass chips failing the uniformity criterion were discarded.

The thick glass chips were then cleaned with methanol and isopropanol using cotton swabs and optical tissue paper to eliminate loose residues on their surfaces (e.g., such as due to handling). The thick glass chips were subsequently deep-cleaned with acetone and isopropanol using cotton swabs and optical tissue paper. A low magnification loupe (e.g., 10x) was used during the deep cleaning process for a first visual inspection followed by a high magnification microscope (e.g., 50x - 200x) for a second visual inspection. Thick glass chips passing the second vision inspection were placed in a bath of acetone for ultrasonic cleaning at <NUM> (e.g., in a Branson Ultrasonic Cleaner CPX-<NUM>-117R). For example, the thick glass chips could be placed in a glass beaker of acetone and cleaned ultrasonically for <NUM> minutes at room temperature. After ultrasonic cleaning, the thick glass chips were dried with particulate-free compressed air and stored in an air-tight container until needed for bonding.

Separately, a dicing saw was used to cut the thin glass wafers into suitable sizes for bonding to the (stored) thick glass chips. Two thin glass chips were prepared for each thick glass chip. If a thin glass chip was intended for an anodic bond, the thin glass chip was cut to have the same dimensions as the thick glass chip. However, if a thin glass chip was intended for a contact bond, the thin glass chip was cut to have longer dimensions than the thick glass chip. For example, thin glass chips for anodic bonding had dimensions of <NUM> x <NUM> and thin glass chips for contact bonding had dimensions of <NUM> x <NUM>. After cutting, each thin glass chip was inspected to ensure that its optical clarity was not degraded (e.g., hazing), or that scratches or cracks were not present. Then glass chips found to be acceptable were then cleaned with acetone using cotton swabs and optical tissue paper. If necessary, the thin glass chips were placed in a glass beaker of acetone and ultrasonically cleaned form <NUM> minutes at room temperature. After ultrasonic cleaning, the glass chips were dried with particulate-free compressed air and then stored in an air-tight container until needed for bonding.

One thick glass chip (with a layer of Si up to <NUM> thick) and one thin glass chip were then placed into an assembly for anodic bonding. For the thick glass chip, the planar surface defined by the up to <NUM> layer of Si participated in the anodic bonding process. In the assembly, planar surfaces of the thick and thin glass chips were contacted to define an interface, and the interface was visually inspected to confirm that optical fringes were present. The thick glass chip was then heated to a temperature of about <NUM>. After this temperature was reached, 600V was applied across the thick and thin glass chips for about <NUM> minutes, which drove the formation of an anodic bond. The interface was inspected again to confirm the disappearance of the optical fringes, which indicated the anodic bond was complete. Next, the anodic bond was inspected for defects (e.g., bubbles, micro-cracks, unbonded areas, etc.). If <NUM>% or more of an area around the holes was free of defects, the anodic bond was then further inspected for open channels (e.g., from a hole to the environment, a hole to another hole, etc.). If an open channel was discovered, the anodically-bonded chips were discarded as the anodic bond was not deemed leak-tight.

Bonded thick and thin glass chips with leak-tight anodic bonds were cleaned in acetone and methanol. During this cleaning process, the unbonded surface of the thick glass chip was cleaned with acetone and methanol using cotton swabs and optical tissue paper to eliminate any residues (e.g., residues from a graphite plate of the assembly used to form the anodic bond). The unbonded surface of the thick glass chip was then visually inspected to ensure defects (e.g., scratches, pitting, etc.) were not present that might compromise a soon-to-be formed contact bond. The anodically-bonded chips were then individually cleaned. In particular, the anodically-bonded chips were placed individually (i.e., with no other chips) in a glass beaker of acetone and cleaned ultrasonically for <NUM> minutes at room temperature. After ultrasonic cleaning, the anodically-bonded chips were dried with particulate-free compressed air. A low magnification loupe (e.g., 10x) was used for a first visual inspection of the anodically-bonded chips, followed by a high magnification microscope (e.g., 50x - 200x) for a second visual inspection. The first and second visual inspections were used to ensure no visual residues or deposits remained on the anodically-bonded chips.

The anodically-bonded chips - along with unbonded thin glass chips - were then taken into a clean room environment (e.g., Class <NUM> or better) for contact bonding. Single instances of anodically-bonded chips were paired with single instances of thin glass chips to define a pair for contact bonding. For each pair, an unbonded planar surface of the thick glass chip (i.e., without the layer of Si up to <NUM>) and a planar surface of the thin glass chip were wiped with optical paper and acetone to clean any macroscopic deposits or contaminants from them. Each pair was then submerged in an acetone bath (e.g., acetone in a beaker) and cleaned via ultrasonic cleaning for <NUM> minutes. Each pair of chips was subsequently removed from the acetone bath, rinsed with isopropanol (e.g., submerged in an isopropanol bath), and blown dry with dry nitrogen gas.

A pair of chips was placed in a YES-CV200RFS plasma cleaner and cleaned for <NUM> seconds using a nitrogen plasma. (In some instances, multiple pairs of chips were place in the plasma cleaner. ) In particular, the unbonded planar surface of the thick glass chip and the planar surface of the glass chip were activated by plasma cleaning. The RF-power of the plasma cleaner was set at about <NUM> W, and the pressure inside was maintained at about <NUM> mTorr. Nitrogen gas introduced into the plasma cleaner at a volume flow rate of about <NUM> sccm. After activation by plasma cleaning, the pair of chips was removed from the YES-CV200RFS plasma cleaner and rinsed in de-ionized water for <NUM> minutes. The rinsing process served to hydroxylate the activated surfaces. In some variations, the rinsing process was conducted with a basic aqueous solution (e.g., an aqueous solution of ammonium hydroxide). Care was taken not to touch the two hydroxylated and activated surfaces together.

The pair of chips was then transferred into a vacuum chamber and mounted into a fixture having a "press finger". The fixture held the thin glass chip adjacent the thick glass chip of the anodically-bonded chip to define a gap. The activated and hydroxylated surface of the thin glass chip faced the activated and hydroxylated unbonded surface of the thick glass chip. The vacuum chamber was then sealed and pumped down to a reduced pressure (e.g., less than <NUM>-<NUM> Torr) to remove volatile species (e.g., water vapor) that might react with a vapor of cesium atoms used to fill the cavities of the anodically-bonded chip. The fixture was then chilled to by a thermoelectric cooler, which in turn, chilled at least the anodically-bonded chip to a temperature between -<NUM> and <NUM>.

The pressure of cesium vapor in the vacuum chamber influences the length of time needed to fill the anodically-bonded chip. One or both of the pressure of cesium vapor in the vacuum chamber and the length of time can be varied to control an amount of cesium vapor that condenses in the cavities of the anodically-bonded chip. Once the length of time had elapsed, the value to the source of cesium vapor was closed. The vacuum chamber was subsequently pumped down to the reduced pressure (e.g., less than <NUM>-<NUM> Torr) and the power to the thermoelectric cooler turned off.

Once the pair of chips reached ambient temperature, the fixture was actuated to contact the activated and hydroxylated surface of the glass chip to the activated and hydroxylated unbonded surface of the thick glass chip. The "press finger" was used to hold the contacted surfaces together for <NUM> minutes, which drove the formation of a contact bond. In some variations, the "press finger" was used to apply a target pressure (e.g., about <NUM> MPa) during the <NUM>-minute duration.

In some aspects of what is described, a method of manufacturing a vapor cell may additionally be described by the following examples:.

In some aspects of what is described, a vapor cell may be also be described by the following examples:.

While this specification contains many details, these should not be understood as limitations on the scope of what may be claimed, but rather as descriptions of features specific to particular examples. Certain features that are described in this specification or shown in the drawings in the context of separate implementations can also be combined. Conversely, various features that are described or shown in the context of a single implementation can also be implemented in multiple embodiments separately or in any suitable sub-combination.

Claim 1:
A method of manufacturing a vapor cell (<NUM>, <NUM>), the method comprising:
obtaining a dielectric body (<NUM>, <NUM>) comprising:
a surface that defines an opening to a cavity (<NUM>, <NUM>) in the dielectric body (<NUM>, <NUM>), and
a plurality of holes (<NUM>, <NUM>) between the cavity (<NUM>, <NUM>) and a side of the dielectric body (<NUM>, <NUM>), the plurality of holes is separated from the cavity (<NUM>, <NUM>) by a solid portion of the dielectric body (<NUM>, <NUM>) and configured to define a metamaterial wall of the dielectric body (<NUM>, <NUM>);
obtaining an optical window (<NUM>, <NUM>) that comprises a surface;
disposing a vapor or a source of the vapor into the cavity (<NUM>, <NUM>); and
bonding the surface of the optical window (<NUM>, <NUM>) to the surface of the dielectric body (<NUM>, <NUM>) to form a seal around the opening to the cavity (<NUM>, <NUM>);
wherein the vapor cell (<NUM>, <NUM>), when manufactured, is configured to detect a target electromagnetic radiation; and
wherein each of the plurality of holes (<NUM>, <NUM>) that define the metamaterial wall has a largest dimension no greater than a wavelength of the target electromagnetic radiation.