Nanoscale object detection using a whispering gallery mode resonator

Detection of individual objects using a light source and a whispering gallery mode (WGM) resonator. Light from the whispering gallery mode (WGM) resonator is analyzed. The presence of an object is determined based on mode splitting associated with the light received by the photodetector. For example, the presence of the object may be determined based on the distance between two whispering gallery modes and/or the linewidths of the two modes in a transmission spectrum. Alternatively, the presence of the object may be determined based on a beat frequency that is determined based on a heterodyne beat signal produced by combining split laser modes in the received light from a WGM microcavity laser.

BACKGROUND

With recent progress in nanotechnology, nanoparticles of different materials and sizes have been synthesized and engineered as key components in various applications ranging from solar cell technology to the detection of biomolecules. Meanwhile, nanoparticles generated by vehicles and industry have become recognized as potential threats to health and environment. Microscopy and spectroscopy techniques have played central roles in single nanoparticle/molecule detection. However, their widespread use has been limited by bulky and expensive instrumentation, long processing time, and/or the need for labeling. Light scattering techniques, while suitable for label-free detection, are hindered by the extremely small scattering cross-sections of single nanoparticles.

SUMMARY

The presence of an object, such as a nanoparticle and/or a virion, is detected using a whispering gallery mode (WGM) resonator. An optical fiber is positioned adjacent the WGM resonator, such that light passing through the optical fiber is coupled into the WGM resonator, where it propagates in two opposite directions. The propagation of the light is affected by the presence of objects proximate to (e.g., adsorbed on) the WGM resonator. The light is decoupled from the WGM resonator to the optical fiber, which carries the light to a device that includes photodetector. At the device, the presence of an object is determined based on mode splitting associated with the light received by the photodetector. For example, a transmission spectrum may be created based on the received light, and the distance between two modes and/or the linewidths of the two modes may be used to determine the presence of an object and measure its polarizability. In another example, two modes in the received light are combined to create a heterodyne beat signal, and a beat frequency is determined based on the heterodyne beat signal. The presence of an object is determined based on the beat frequency.

DETAILED DESCRIPTION

Embodiments described herein facilitate detecting the presence and the polarizability, which is related to the size, the composition, and/or the refractive index, of one or more objects on the surface of a whispering gallery mode (WGM) resonator. Accordingly, such embodiments enable the creation of a portable, inexpensive, and high-resolution device capable of real-time and in-situ detection of particles surpassing current detection limits.

In exemplary embodiments, the presence of an object, such as a nanoscale object, is determined based on light received from WGM resonator. As used herein, the term “nanoscale object” refers to any synthetic or natural subwavelength (e.g., smaller than the wavelength of the light used to detect) object that scatters light. Nanoscale objects may also be referred to as nanoparticles and may include, for example metallic particles, non-metallic particles, plasmonic particles, non-plasmonic particles, viruses (e.g., virions), bacteria, and/or biomolecules.

A WGM resonator offers a highly confined microscale mode volume and an ultra-high quality factor (“Q”), enabling strong light-matter interactions that can be used for ultra-sensitive optical detection. Such detection may be enabled, at least in part, by the existence of two standing wave modes produced by the presence of an object on the WGM resonator and/or within an evanescent field of the WGM resonator. More specifically, object binding splits a WGM into two spectrally shifted resonance modes. The split modes share a single resonator and are therefore subject to the same noise, allowing for a self-referencing detection system relatively immune to noise.

Such embodiments facilitate compact and/or portable in-situ detection and sizing systems with single-object resolution which do not require labeling of objects or predetermined information regarding the presence of objects in the medium tested. For example, an entire detection system may be integrated into a single chip or die, facilitating cost-efficient manufacture and packaging. Furthermore, this technique enables extracting accurate object size information with a single-shot measurement in a micro-scale device.

Some embodiments are described herein in with reference to particular objects, such as virions. However, the methods described are generally applicable to nanoscale objects, regardless of material and/or internal structure. It is contemplated that the embodiments provided may be practiced with single or multiple nanoscale objects (e.g., nanoparticles, atoms, and/or virions).

In exemplary embodiments, a silica microtoroidal resonator includes two degenerate WGMs with the same resonant frequency and evanescent field distributions but opposite propagation directions. The two WGMs may be referred to as a clockwise mode and a counter-clockwise mode. Other types of WGM resonators that support such degenerate modes, such as a sphere, a disk, or a cylinder, may be used in addition to or in place of a microtoroidal resonator. It is contemplated that a resonator with a substantially circular structure may be used. In one embodiment, the resonator is approximately 10 micrometers (also known as microns, μm) to 1000 μm in size. Resonators of other dimensions are also contemplated.

A perturbation in the mode volume, such as surface roughness, material inhomogeneity, or a scatterer, causes the resonator to deviate from perfect azimuthal symmetry, lifting the degeneracy of the WGM modes to split the resonance into a doublet. When light received from a WGM resonator is represented in a transmission spectrum, such “mode splitting” appears as a distance (in hertz or megahertz, for example) between the two modes of the doublet. Mode splitting may be used to determine that an object is present and/or to determine one or more properties of the object. In some embodiments, object presence and/or at least one object property is determined based on the distance between the standing wave modes and the linewidths of the standing wave modes.

FIG. 1is a diagram illustrating a system100for detecting an object. System100includes a whispering gallery mode (WGM) resonator102and a detection device104. Detection device104includes a photodetector106configured to receive light emitted by or coupled out of WGM resonator102. Detection device104also includes a processor108that is coupled to photodetector106. Processor108is capable of executing instructions and may include one or more processing units (e.g., in a multi-core configuration).

In some embodiments, WGM resonator102is “passive” (e.g., not populated with a gain medium). In such embodiments, processor108is programmed to create a transmission spectrum based on light received from WGM resonator102and to determine the presence of an object based on the transmission spectrum, as described in more detail below with reference toFIGS. 4-11.

In some embodiments, WGM resonator102includes a gain medium and may be referred to as “active.” In such embodiments, photodetector106is configured to combine split laser modes that are included in the light received from WGM resonator102, optionally filtered by a wavelength-division multiplexer (WDM)109, to create a heterodyne beat signal. Processor108is programmed to determine a beat frequency based on the heterodyne beat signal and to detect the presence of an object based on the beat frequency, as described in more detail below with reference toFIGS. 12-21.

In some embodiments, detection device104includes a memory area110coupled to processor108. Memory area110is any device allowing information, such as executable instructions and/or other data, to be stored and retrieved. Memory area110may include one or more computer readable media.

Memory area110may be configured to store data, including encoded instructions that are executable by processor108to perform one or more of the operations described herein. Memory area110may also be configured to store object detection data, such as, but not limited to, transmission spectra, heterodyne beat signals, beat frequencies, object detection events, and/or object properties.

In one embodiment, memory area110is configured to store transmission spectra, and processor108is programmed to compare a current transmission spectrum to a previously stored transmission spectrum from memory area110. For example, processor108may be programmed to subtract the previously recorded transmission spectrum from the current transmission spectrum to create a difference and to determine a presence and/or a property of one or more objects based on the difference. For example, the appearance of a second mode where only one mode was previously present may indicate the presence of an object. Similarly, a change in the distance between the first mode and the second mode may indicate the presence of an additional object. Memory area110may be configured to store the current transmission spectrum, which may be subsequently used by processor108as a previously stored transmission spectrum.

System100may also include a light source112and an optical fiber114. In one embodiment, such as with a passive resonator, light source112is a laser, which may be optimized so that no thermal effect is present in the transmission spectrum created by processor108. Light source112may be tunable, such that light may be produced over a range of frequency.

Optical fiber114includes a first normal portion116, a second normal portion118, and a tapered portion120between first normal portion116and second normal portion118. Tapered portion120has a diameter smaller than the wavelength of light transmitted by light source112. An evanescent field surrounds at least a part of tapered portion120. With WGM resonator102positioned proximate to tapered portion120(e.g., within the evanescent field), at least some light carried by optical fiber114is transmitted to or coupled into WGM resonator102. Similarly, light is coupled out of or decoupled from WGM resonator102and coupled into tapered portion120.

In exemplary embodiments, WGM resonator102is configured to receive light from tapered portion120and to allow the light to propagate within WGM resonator102. For example, a photon may travel around an ultra-high-Q WGM resonator102over one million times. The repeated circulation of light in WGM resonator102may amplify the effect of standing wave modes, facilitating more accurate detection of objects, as described herein. Similarly, light is coupled out of, or decoupled from, WGM resonator102. Light is coupled out of WGM resonator102. Light may be emitted by WGM resonator102proximate to tapered portion120of optical fiber114and transmitted by second normal portion118to detection device104.

System100may include a plurality of WGM resonators102. As illustrated inFIG. 1, system100includes a first WGM resonator122and a second WGM resonator124. Optical fiber114is split, such that light source112provides light to both first WGM resonator122and second WGM resonator124. In an alternative embodiment, a light source112is provided for each WGM resonator102.

Detection device104includes one photodetector106for each WGM resonator102. As illustrated, detection device104includes a first photodetector126configured to receive light from first WGM resonator122and a second photodetector128configured to receive light from second WGM resonator124. Both first photodetector126and second photodetector128are coupled to processor108. Processor108may be programmed to create a transmission spectrum for each photodetector106and to determine a presence, a size, a refractive index, and/or a position of one or more objects based on each created transmission spectrum, as described above.

In some embodiments, system100includes one or more particle sources130. Particle source130is configured to acquire one or more nanoparticles and direct the nanoparticles to a nozzle132. Particle source130may be configured to filter or select particles based on one or more particle properties, including size, electrical mobility, shape, composition, and/or any other property of interest. In one embodiment, particle source130includes a differential mobility analyzer (DMA). Particle source may include one or more collections of nanoparticles (e.g., having known properties) and/or may draw samples from a medium to be tested, such as, but not limited to, ambient air, a fluid in a surrounding environment, and/or a fluid in a container. In addition, or alternatively, WGM resonator102may be directly exposed to the medium to be tested. In some embodiments, particle source130and nozzle132are omitted.

Nozzle132is positioned proximate to WGM resonator102. For example, nozzle132may be separated from WGM resonator102by approximately 150 μm. In one embodiment, nozzle132has a tip inner diameter of approximately 80 μm.

In one embodiment, nozzle132is configured to direct an object134received from particle source130toward WGM resonator102, such that object134is adsorbed on WGM resonator102. If multiple WGM resonators102are provided, system100may include a nozzle132for each WGM resonator102. Multiple nozzles132may be configured to receive nanoparticles from a single particle source130. Alternatively, system100may include multiple particle sources130, each of which is coupled to one or more nozzles132.

Detection device104may include an output device, such as a communication interface136and/or a presentation device138. Communication interface136may include, for example, at least one electrical conductor, serial data communication device, parallel data communication device and/or network adapter, whether wired or wireless. In one embodiment, communication interface136is configured to transmit a detection signal indicating the presence, the size, and/or the refractive index of one or more detected objects. The transmitted detection signal may be received by one or more remote devices, such as an operating console, a monitoring device, and/or any other computing device.

Presentation device138may include, but is not limited to, a display device and/or an audio output device. In one embodiment, presentation device138is configured to indicate the presence, the size, and/or the refractive index of one or more detected objects. For example, presentation device138may emit an audible noise when an object is detected and/or may display information about detected objects.

In some embodiments, multiple components of system100are integrated into a single hardware package. For example, light source112, optical fiber114, one or more WGM resonators102, and detection device104may be included on a single die or silicon chip.

In one embodiment, system100is implemented as a fly-by particle counting and sizing system. In such a configuration, nozzle132is configured to direct object134through an evanescent field of WGM resonator102, rather than directly at the surface of WGM resonator102. As object134passes through the evanescent field of WGM resonator102, the presence of object134results in mode splitting, which is detected and/or analyzed to detect the presence and/or a property of object134, as described herein. When the particle departs the evanescent field, the transmission spectrum reverts to its previous state.

FIG. 2is a diagram illustrating an example microtoroidal WGM resonator200for use with system100. Microtoroidal WGM resonator200is fabricated on or mounted to a surface205by a base210. In exemplary embodiments, surface205and base210are constructed of silicon, and microtoroidal WGM resonator200is constructed of silica. Microtoroidal WGM resonator200may be fabricated from a silica layer (e.g., approximately 2 millimeters in thickness) on a silicon wafer. For example, microtoroidal WGM resonator200may be formed from the silica layer by laser reflow, xenon difluoride (XeF2) etching, photolithography followed by hydrofluoric acid (HF) etching, and/or any suitable fabrication means. Such an embodiment facilitates integrating one or more WGM resonators102with other components of system100on a single silicon wafer. In some embodiments, microtoroidal WGM resonator200is doped with a gain medium and is referred to as an active resonator. In other embodiments, no gain medium is included, and microtoroidal WGM resonator200is referred to as a passive resonator.

In some embodiments, microtoroidal WGM resonator200has a major diameter215of approximately 30 μm to 200 μm. In one exemplary embodiment, microtoroidal WGM resonator200has a minor diameter220of approximately 5 μm to 30 μm and has a mode volume of approximately 200 μm3. Microtoroidal WGM resonator200is positioned proximate to tapered portion120of optical fiber114.

Microtoroidal WGM resonator200includes two degenerate WGMs with the same resonant frequency and the same evanescent field distribution but opposite propagation directions, as shown inFIG. 3.FIG. 2illustrates a distribution of a Wgm evanescent field225on the periphery of microtoroidal WGM resonator200. Nozzle132(shown inFIG. 1) may be configured to direct object134toward Wgm evanescent field225. For example, nozzle132may be configured to deposit object134in the evanescent field of Wgm evanescent field225near a center230of Wgm evanescent field225and/or the mode, where the object may have a pronounced effect on the transmission spectrum, as described in more detail below. In addition, surface205may be configured to exert an electrical field on object134to attract object134toward microtoroidal WGM resonator200.

Microtoroidal WGM resonator200includes a cavity235, which may be doped with a gain medium in an “active” application, such that an input light of shorter wavelength with power above a lasing threshold generates a laser light of longer wavelength. The structure of microtoroidal WGM resonator200surrounds cavity235, defining an outer surface240. In some embodiments, outer surface240includes (e.g., is coated with) a selective coating. The selective coating may be selected to bind one or more particular types of objects (e.g., specific compounds and/or virions) to outer surface240, while other types of objects may not easily bind to the object adhesive. Such embodiments facilitate detecting the presence of one or more objects of interest.

FIG. 3is a diagram illustrating light propagation in optical fiber114and microtoroidal WGM resonator200. Proximate to tapered portion120, light300transmitted by optical fiber114produces an evanescent field302about tapered portion120. Microtoroidal WGM resonator200is positioned within the evanescent field of tapered portion120and receives at least some light305from optical fiber114. Viewed from above, within microtoroidal WGM resonator200, a clockwise WGM310is associated with light propagating in a clockwise direction, and a counter-clockwise WGM315is associated with light propagating in a counter-clockwise direction.

Light is confined within microtoroidal WGM resonator200. For example, light may circulate through microtoroidal WGM resonator200up to approximately one million times before being completely dissipated. Light coupled out of microtoroidal WGM resonator200is received by tapered portion120and carried toward detection device104.

As a result of repeated interactions between the confined light and object134, which is deposited on the surface of microtoroidal WGM resonator200, the effect of object134on light320is amplified, producing very high quality output.

Embodiments provided herein are operable with passive and/or active WGM resonators200, as described in more detail below.

Passive WGM Resonator

Referring toFIG. 1, in some embodiments, WGM resonator102is a passive WGM resonator that includes no gain medium, and light source112is a tunable laser. In such embodiments, processor108is programmed to create a transmission spectrum based on the light received from WGM resonator102. The transmission spectrum indicates transmission of light by WGM resonator102over a frequency range. Processor108may be programmed to create the transmission spectrum based on input received from photodetector106over a sampling period. In exemplary embodiments, the sampling period is approximately 1 millisecond or less. If the light coupled out of WGM resonator102originates at a tunable laser, the sampling period may be defined based on a wavelength scanning speed of the tunable laser. For example, the sampling period may be substantially equal to the amount of time required for the tunable laser to scan a frequency range of interest (e.g., spanning about 500 to 1000 megahertz).

Processor108is also programmed to identify within the transmission spectrum a first mode and a second mode, each of which represents a portion of the transmission spectrum associated with decreased transmission. Processor108is further programmed to determine a presence, a size, a composition, a refractive index, and/or a position of an object based on the first mode and the second mode. For example, processor108may be programmed to determine the presence of an object and/or to measure the polarizability of an object based on the distance between the first mode and the second mode, the linewidth of the first mode, and/or the linewidth of the second mode.

Processor108may also be programmed to determine a presence of one or more additional objects based on the first mode and the second mode. For example, processor108may be programmed to determine, based on the distance between the first mode and the second mode, the linewidth of the first mode, and/or the linewidth of the second mode, that more than one object is adsorbed on and/or proximate to WGM resonator102.

FIG. 4is a chart400illustrating an example zero-particle transmission spectrum405and an example one-particle transmission spectrum410based on light coupled out of WGM resonator102. For comparison, zero-particle transmission spectrum405, produced in the absence of a nanoparticle, is overlaid on one-particle transmission spectrum410, produced in the presence of one nanoparticle.

Zero-particle transmission spectrum405indicates a single Lorentzian resonance or a single mode415. After a particle is deposited on WGM resonator102, standing wave modes (SWMs) are formed, as indicated by double Lorentzian resonances, depicted as a first mode420and a second mode425in one-particle transmission spectrum410. Successive depositions of particles may introduce variation in first mode420and second mode425, as described below with regard toFIG. 6.

Referring again toFIGS. 2-4, the WGMs within WGM resonator102are associated with a distribution of evanescent fields225, and a nanoparticle in evanescent field225acts as a scatterer. A portion325of the scattered light is lost to the environment, creating an additional damping channel, while the remaining light couples back into the resonator and induces coupling between clockwise WGM310and counter-clockwise WGM315. The degeneracy of clockwise WGM310and counter-clockwise WGM315is consequently lifted, creating SWMs that are split in frequency, as represented by first mode420and second mode425of one-particle transmission spectrum410. In some embodiments, in the absence of object134, clockwise WGM310and counter-clockwise WGM315share a single set of evanescent fields225.

FIG. 5is an example illustration of WGM evanescent fields relative to the position of object134deposited on microtoroidal WGM resonator200. In the presence of object134, clockwise WGM310and counter-clockwise WGM315are redistributed according to the position of object134, creating a symmetric mode (SM) with SM evanescent fields350and an asymmetric mode (ASM) with ASM evanescent fields355.

Referring again toFIGS. 4 and 5, the symmetric mode (SM) locates object134at an anti-node360, and the asymmetric mode (ASM) locates object134at a node365. Consequently, the SM experiences frequency shift and linewidth broadening, as indicated by first mode420of one-particle transmission spectrum410. First mode420corresponds to the SM, and second mode425corresponds to the ASM.

In one embodiment, a frequency shift is determined by calculating a distance (δ) between first mode420and second mode425. Specifically, the distance δ is determined between a nadir430of first mode420and a nadir435of second mode425. The linewidth (γ) of a mode may be calculated by determining the width of the mode at a half-amplitude level. For example, a linewidth (γ2) of second mode425is determined at a vertical position440, which is equidistant from nadir435and a baseline445. A linewidth (γ1) is similarly determined for first mode420. In one embodiment, a single nanoparticle is detectable if δ>(γ1+γ2)/2.

A coupling strength g is quantified by the doublet splitting g=πδ, where δ is the distance between first mode420and second mode425, as described above. The additional linewidth broadening may be expressed as ΓR=π|γ1−γ2|.

In some embodiments, the resonance wavelength prior to splitting, denoted as λ, is equal to the absolute wavelength at a nadir450of zero-particle transmission spectrum405or at nadir435of one-particle transmission spectrum410. The size of object134is expressed as a radius length R. If radius R<<λ, particle-WGM interaction may induce a dipole represented by particle polarizability α, as expressed by Equation 1 below.

In Equation 1, ∈pand ∈mdenote dielectric permittivities of the particle and the medium, respectively. The parameters g and ΓRmay be expressed by Equations 2 and 3 below.

In Equations 2 and 3, ωcis the angular resonant frequency, f(r) designates normalized mode distribution, Vcis the mode volume, and ν=c/√{square root over (∈m)} with c representing the speed of light.

Particle size may be derived from Equation 4 below.
α=−(3λ3/8π2)(ΓR/g)  (4)

Because the value of ΓR/gis independent of the particle position r on the resonator, this technique has advantages over schemes using resonance spectral shift, which is affected by particle positions. If ∈p>∈m, the symmetric mode experiences a red-shift. If ∈p<∈m, the symmetric mode experiences a blue-shift. In exemplary embodiments, ∈p>∈mis always satisfied, and a low-Q mode (an SM) therefore appears on the lower frequency side of the transmission spectrum.

More specifically, in one embodiment the radius of a particle is determined using Equation 5 below.

In Equation 5, npdenotes the refractive index of the particle. Given n=√{square root over (∈μ)}, in which μ, the relative permeability of the particle, is approximately equal to 1, np2is approximately equal to ∈p.

In exemplary embodiments, microtoroidal WGM resonator200has a quality value (Q) of approximately 4×108. A theoretical lower limit of measurable nanoparticle radius R may be estimated using 2g>ΓR+ωc/Q. For example, at λ=670 nm, a radius of approximately 9.2 nm may be determined for potassium chloride (KCl), and a radius of approximately 8.7 nm may be determined for polystyrene.

FIG. 6is a chart500illustrating example transmission spectra based on light coupled out of WGM resonator102in the presence of varying quantities of particles. A zero-particle transmission spectrum505depicts only a single mode, which represents two degenerate modes within WGM resonator102, as described above. A one-particle transmission spectrum510depicts a splitting of the degenerate modes into two distinct modes separated by a relatively small distance and having relatively small linewidths.

Consecutive particle depositions on WGM resonator102affect both the distance between modes and the linewidths of the modes, as shown by a two-particle transmission spectrum515, a three-particle transmission spectrum520, and a four-particle transmission spectrum525. The distance between the modes and the linewidths of the modes may be used to determine a quantity of nanoparticles. Although the progression from one-particle transmission spectrum510to four-particle transmission spectrum525indicates an increase in both distance and linewidths, the presence of an additional particle may instead result in a decrease in distance and/or linewidth(s), as described with regard toFIG. 7below.

FIG. 7is a chart600illustrating distances between modes (“splitting”) in example transmission spectra based on light coupled out of WGM resonator102in the presence of varying quantities of particles. More specifically, chart600illustrates splitting by both particle size and quantity of particles. Chart600includes a 150-nanometer (nm) line605, a 100-nm line610, and a 40-nm line615. 40-nm line615indicates relatively small changes in splitting based on quantity of particles. A high-resolution 40-nm line620depicts discrete splitting levels for varying quantities of 40-nm particles.

Referring to 150-nm line605, a continual increase in splitting is apparent between a one-particle splitting level625and a four-particle splitting level630. This trend is consistent with the transmission spectra illustrated inFIG. 6. However, a five-particle splitting level635indicates a decrease in splitting compared to four-particle splitting level630. In one embodiment, the splitting level depends on the location of a successively deposited particle with respect to the distribution of the SM and ASM, as shown inFIG. 5. Regardless of whether the addition of a particle increases or decreases splitting, detection of a particular level of splitting and/or a detection of a change in the level of splitting may be used to determine a quantity of and/or one or more properties of nanoparticles.

In some embodiments, mode splitting directly reveals particle polarizability, which depends at least in part on particle size and refractive index. Accordingly, a nanoparticle property (e.g., size, refractive index, or composition) may be determined based on mode splitting and a known value for one or more other properties. For example, nanoparticles with the same size but different composition may be discriminated. Embodiments providing such property determinations facilitate classifying biomolecules, for example.

FIG. 8is an illustration of example transmission spectra700based on light coupled out of WGM resonator102in the presence of nanoparticles of varying sizes. Depicted inFIG. 8are a 50-nm transmission spectrum705, a 75-nm transmission spectrum710, a 100-nm transmission spectrum715, and a 125-nm transmission spectrum720. Nanometer measurements correspond to particle size, expressed as a radius length. The refractive index npis constant at 1.48, and the normalized mode distribution f(r) is constant at 0.3.

50-nm transmission spectrum705includes split modes at a very small distance (i.e., approximately 20 Hertz (Hz)) from each other. As the particle size increases, the distance between the modes also increases. For example, 50-nm transmission spectrum705depicts a distance of approximately 20 MHz between the split modes, whereas 125-nm transmission spectrum720depicts a distance of approximately 320 MHz between the split modes. Because the level of mode splitting varies with radius R, a particle size may be determined based at least in part on the distance between the split modes.

FIG. 9is an illustration of example transmission spectra800based on light coupled out of microtoroidal WGM resonator200in the presence of object134at various positions relative to a WGM evanescent field225. Nanoparticle refractive index npis constant at 1.48, and normalized mode distribution f(r) is constant at 0.3.

Physical position charts805illustrate the position of object134relative to the surface of microtoroidal WGM resonator200. Field position charts810illustrate the position of object134relative to WGM evanescent field225, which is most pronounced near center230. Evanescent field center230corresponds to a right-most portion of physical position charts805and a peak of field position charts810. Transmission spectra800are generated based on light coupled out of microtoroidal WGM resonator200.

Physical position charts805, field position charts810, and transmission spectra800are provided for a first scenario820, a second scenario825, a third scenario830, and a fourth scenario835. In first scenario820, object134is positioned almost completely outside evanescent field225. In fourth scenario835, object134is positioned at evanescent field center230. In second scenario825and third scenario830, object134resides at intermediate positions within evanescent field225.

A first transmission spectrum840indicates that in first scenario820, in which object134is mostly removed from evanescent field225, mode splitting is not apparent. As indicated by a second transmission spectrum845, a third transmission spectrum850, and a fourth transmission spectrum855, mode splitting increases as object134approaches evanescent field center230. For example, second transmission spectrum845indicates a distance860of approximately 50 Hz between modes, whereas fourth transmission spectrum855indicates a distance865of approximately 465 Hz between modes. Because mode splitting varies with position, a position of object134may be determined based at least in part on the amount of mode splitting.

FIG. 10is an illustration of example transmission spectra900based on light coupled out of WGM resonator102in the presence of nanoparticles having varying refractive indices. Nanoparticle radius R is constant at125nm, and normalized mode distribution f(r) is constant at 0.3.

A first transmission spectrum905corresponds to a nanoparticle having a refractive index npof 1.1. A second transmission spectrum910corresponds to a nanoparticle having a refractive index npof 1.3. A third transmission spectrum915corresponds to a nanoparticle having a refractive index npof 1.5. A fourth transmission spectrum920corresponds to a nanoparticle having a refractive index npof 1.7. As indicated by transmission spectra905,910,915,920, mode splitting varies with refractive index. Specifically, in the example ofFIG. 10, mode splitting varies directly with refractive index. Because of the relationship between mode splitting and refractive index, a refractive index may be determined based at least in part on a distance between modes.

FIG. 11is an example flow chart of a method1000for detecting an object based on mode splitting in a whispering gallery mode (WGM) resonator, such as WGM resonator102. Method1000includes receiving1005light emitted by or coupled out of a WGM resonator. The light may be received via a photodetector. In one embodiment, light is received1005from an optical fiber configured to transmit the light emitted by or coupled out of the WGM resonator. For example, the optical fiber and WGM resonator may be arranged as shown inFIGS. 1-3.

A transmission spectrum is created1010based on the received light. A first mode and a second mode are identified1015within the transmission spectrum. The first mode and the second mode represent portions of the transmission spectrum associated with decreased transmission.

A presence of an object (e.g., a virion or nanoparticle) adsorbed on the WGM resonator and/or within an evanescent field of the WGM resonator is determined1020by a processor based on the first mode and the second mode. For example, the presence of the object may be determined1020based on a distance between the first mode and the second mode and, optionally, a linewidth of the first mode and/or a linewidth of the second mode.

In addition, or alternatively, one or more object properties, such as size, refractive index, and/or composition, may be determined1025based on the first mode and the second mode. For example, an object property may be determined based on a distance between the first mode and the second mode, a linewidth of the first mode, and/or a linewidth of the second mode.

In some embodiments, the WGM resonator is cleaned1030after detection of object presence and/or properties. Hydrophilic or water (solvent)-soluble particles, such as potassium chloride (KCl), may be removed by condensing water vapor on the surface of the WGM resonator and then by drying the surface with dry air or nitrogen. Hydrophobic or insoluble particles may be removed by steam laser cleaning; by high-speed steam and purified water droplet cleaning; by high-velocity aerosol cleaning with ultrapure water and/or a dilute aqueous solution; by applying solid argon, a nitrogen aerosol, or a CO2aerosol; or by dry laser cleaning.

After determining1020a presence of an object, determining1025a property of an object, and/or or cleaning1030the WGM resonator, method1000may be repeated. In some embodiments, a current transmission spectrum is created1010and compared1035to a previous transmission spectrum. For example, the previous transmission spectrum may be subtracted from the current transmission spectrum to create a difference. The first mode and the second mode may be identified1015based on the comparison (e.g., based on the difference). In addition, or alternatively, an object presence and/or an object property may be determined1020,1025based on the comparison.

Active WGM Resonator

Referring toFIG. 1, in some embodiments, an active WGM resonator102includes or defines a cavity235that includes (e.g., is populated or doped with) a gain medium. In exemplary embodiments, the gain medium includes ions of one or more rare earth metals, such as erbium (Er), neodymium (Nd), or ytterbium (Yb), and/or other types of light emitters including quantum dots.

In such embodiments, light source112includes a pump light with a wavelength that overlaps with the absorption band of the gain medium and is used to pump the gain medium. A tunable wavelength may not be required for light source112. The power of light from light source112is adjusted above a lasing threshold associated with the gain medium in WGM resonator to achieve a lasing effect. Normally, the lasing effect produces a laser mode (e.g., light within a relatively narrow linewidth) at a frequency that is different from the frequency of output from light source112. When an object is proximate to (e.g., adsorbed on) WGM resonator102, the laser mode is split into two modes.

Some residual light from pump light source112may exist in the light coupled out of WGM resonator102. Accordingly, in some embodiments, detection device104includes a wavelength-division multiplexer (WDM)109that is configured to receive the light coupled out of WGM resonator102and to separate the lasing light from the residual pump light, creating filtered light. The filtered light is passed to photo detector106. In other embodiments, WDM109is omitted.

Photodetector106is configured to receive the light emitted by WGM resonator102(optionally filtered by WDM109) and to combine the split laser modes that are included in the received light to create a heterodyne beat signal. Processor108is programmed to determine a beat frequency based on the heterodyne beat signal and to detect the presence of an object based on the beat frequency.

In exemplary embodiments, an active WGM resonator102and light source112(which may collectively be referred to as a “WGM microcavity laser”) produce two frequency-degenerate but counter-propagating traveling laser modes: clockwise and counter-clockwise modes. The laser modes are highly confined with evanescent tails probing the surrounding medium many times during circulating within the cavity. A particle that enters the evanescent field of the cavity mode couples these two degenerate laser modes to each other via intracavity Rayleigh backscattering, and leads to the splitting of the laser frequency. This reflects itself as a transition from a single frequency lasing spectrum to a two-frequency lasing spectrum with the spectral distance between the two laser modes determined by the polarizability α (e.g., size and shape of the particle and its refractive index contrast with the surrounding medium) of the particle and by the location of the particle in the mode volume. The polarizability of a spherical particle of radius R is given by Equation 1 above. Thus, a change in a of the particle may be translated into a change in the amount of frequency splitting. Similarly, a subsequent particle binding event may induce excess polarizability that will be observed as another change in the frequency splitting.

In exemplary embodiments, frequency splitting information is extracted by mixing the split modes at a photodetector of sufficient bandwidth to create a heterodyne beat note signal with a beat frequency corresponding to the frequency splitting. In such embodiments, single object adsorption events may be revealed in real time by monitoring the beat note signal and its frequency component.

Exemplary WGM microcavity lasers include toroidal cavities fabricated from Erbium (Er)-doped silica. Such resonators may have a diameter of 20-40 μm and may have quality factors (Q) of approximately 6×106. Further, an exemplary WGM microcavity laser includes a silica WGM resonator doped with Er ions at a concentration of approximately 5×1018ions/cm3. Such a concentration facilitates continuous-wave (CW) laser operation.

The resonator is continuously pumped by a CW laser diode with a wavelength of 1.46 μm, which lies within the Er absorption band. A lasing effect is produced within WGM resonator, producing from the input light a laser emission in the 1.55 μm band. This laser emission is monitored by a photodiode.

FIG. 12is an illustration of example lasing spectra1105and heterodyne beat signals1110based on light coupled out of an active WGM resonator when in the presence of varying quantities of nanoparticles. In the absence of a nanoparticle, a zero-particle lasing spectrum1115is produced, with a single laser mode1120. A corresponding zero-particle heterodyne beat signal1125represents constant laser intensity, or a beat frequency of zero.

When a first particle is present, the laser mode splits. A one-particle lasing spectrum1130includes a first laser mode1135and a second laser mode1140. A corresponding one-particle heterodyne beat signal1145fluctuates with a beat frequency that corresponds to the amount of frequency splitting (e.g., a distance1150between first laser mode1135and second laser mode1140).

Subsequent particle adsorption events further change the observed beat frequency. For example, a two-particle lasing spectrum1155indicates an increase in mode splitting, and a corresponding two-particle heterodyne beat signal1160represents a higher beat frequency than is shown in one-particle heterodyne beat signal1145. In exemplary embodiments, because the split laser modes reside in the same microcavity, environmental noise, such as a temperature fluctuation, affects both modes in the same way. Accordingly, although each split mode undergoes a spectral shift, as indicated by a heated two-particle lasing spectrum1165, the amount of frequency splitting and, therefore, the beat frequency, does not change. For example, a heated two-particle heterodyne beat signal1170is equal to two-particle heterodyne beat signal1160. Such embodiments enable detecting objects with an apparatus that is largely resistant to environmental noise.

FIG. 13is a chart1200illustrating example beat frequencies based on laser emitted from an active WGM resonator in the presence of varying quantities of Influenza A (InfA) virions. In chart1200, each discrete upward or downward change in the beat frequency corresponds to a single virion adsorption event, also referred to as a binding event. A first point1205represents the adsorption of a first particle, and a second point1210represents the adsorption of a second particle. A positive change1215in the beat frequency is shown between first point1205and second point1210. The heights and the signs (e.g., positive or negative) of the changes in the beat frequency are related to the polarizability of each arriving particle and to the location of each particle with respect to the previously adsorbed particles in the field distribution of the laser modes.

Aside from the incorporation of a gain medium, the structure of an active WGM resonator may be similar to the structure of a passive WGM resonator. Accordingly, in some embodiments, an active WGM resonator is represented by WGM resonator200shown inFIG. 3. As indicated by Wgm evanescent field225, the optical field on the surface of WGM resonator200is non-uniform, such that the light-matter interaction strength varies depending on the position of a particle on WGM resonator200. Consequently, a single particle adsorbed in different locations in the mode volume induces different amount of frequency splitting.

In exemplary embodiments, when individual polystyrene (PS) nanoparticles of the same size are adsorbed onto WGM resonator200at random locations, the resultant frequency splitting either increases or decreases with different step heights. For an ensemble of particles with the same polarizability adsorbed one by one onto the microcavity laser, the beat frequency steps are not constant. Instead, the beat frequency steps form a statistical distribution with a standard deviation that scales linearly with particle polarizability. Such results have been verified with a Monte Carlo simulation in which PS nanoparticles were continuously and randomly deposited in a microcavity mode volume. In the simulation, the PS particles had a radius R=50 nanometers (nm) and refractive index np=1.59. The zero-particle light emission from the resonator had a wavelength λ=1550 nm. The surrounding medium was air, with a refractive index ns=1.0. The resonator had a mode volume V=300 μm3. In such a simulation, each nanoparticle adsorption event leads to an upward or downward change in the frequency splitting. The step height of each change depends on the particle location in the mode volume. Nanoparticles with smaller size lead to a narrower distribution of step changes. Because the polarizability is proportional to R3, the size of the particles with a known refractive index can be estimated by proper calibration using particles of known size.

FIG. 14is a chart1300illustrating example beat frequencies based on the laser emitted from an active WGM resonator in the presence of varying quantities of gold (Au) nanoparticles with a radius of 15 nm.FIG. 15is a chart1350illustrating example beat frequencies based on the laser emitted from an active WGM resonator in the presence of varying quantities of Au nanoparticles with a radius of 25 nm. In chart1300and chart1350, particles are individually deposited at random locations on the surface of the microcavity laser. The measurements of beat frequency were performed using the same microcavity laser and the same laser mode to minimize cavity- and mode-related effects. As shown by chart1300, changes in beat frequency are apparent as Au particles are individually adsorbed to the active WGM resonator, even with a radius of only 15 nm.

FIG. 16is a histogram1400illustrating example changes in beat frequency versus the number of binding events for gold particles with radii of 15 nanometers and 25 nanometers. Histogram1400indicates binding events for a total of 816 Au nanoparticles, measured using the same active WGM resonator and the same laser mode. More specifically, 397 binding events are illustrated for R=15 nm, and 419 binding events are illustrated for R=25 nm. In exemplary embodiments, small nanoparticles do not cause significant change in the cold cavity-Q and the linewidth of the laser mode.

As indicated by histogram1400, the standard deviation for R=25 nm (e.g., as shown inFIG. 15) is larger than the standard deviation for R=15 nm (e.g., as shown inFIG. 14). Accordingly, the standard deviation of beat frequency changes may be used to extract the polarizability of particles and, therefore, the size of an unknown particle by using measurements associated with reference particles.

In exemplary embodiments, linewidth broadening of the laser modes due to the losses induced by nanoparticles of R<250 nm is significantly less than the induced frequency splitting between the laser modes. Therefore, such embodiments facilitate detecting a relatively large quantity of binding events using the same laser mode in a single microcavity laser without significantly degrading the lasing linewidth.

Embodiments described above detect particle binding events using a microcavity laser that produces a single laser mode. In some embodiments, a microcavity laser produces multiple laser modes.

FIG. 17is an illustration of an example lasing spectrum1500based on a laser emitted from a two-mode active WGM resonator. Lasing spectrum1500illustrates input light1505pumped at a wavelength λp=1443 nm. A first laser mode1510is illustrated at a first lasing wavelength λs1=1549 nm, and a second laser mode1515is illustrated at a second lasing wavelength λs2=1562 nm.

When a particle is adsorbed onto the two-mode active WGM resonator, first laser mode1510and/or second laser mode1515splits, as described above, and a heterodyne beat signal can be created from the split modes. When both first laser mode1510and second laser mode1515split, a total of four laser modes may be present in the light emitted by the active WGM resonator.FIG. 18is an illustration of an example heterodyne beat signal1550based on the laser emitted from a two-mode active WGM resonator in the presence of one or more particles.

In exemplary embodiments, a fast Fourier transform (FFT) is applied to a heterodyne beat signal, such as heterodyne beat signal1550, to determine one or more beat frequencies.FIG. 19is an illustration of a fast Fourier transform spectrum1575based on heterodyne beat signal1550. Referring toFIGS. 17 and 18, a first peak1580and a second peak1585in FFT spectrum1575correspond to frequency splitting in the first laser mode1510and the second laser mode1515, respectively.

FIG. 20is an illustration of an intensity graph1600based on an FFT spectrum, such as FFT spectrum1575(shown inFIG. 19), as gold particles with a radius R=50 nm are deposited onto the surface of an active WGM resonator. A sidebar1605indicates the magnitude of FFT spectrum1575in decibels (dB).

Referring toFIGS. 19 and 20, a first beat frequency group1610corresponds to first peak1580, and a second beat frequency group1615corresponds to second peak1585. As described above with reference toFIG. 13, changes in the beat frequency within a group over time indicate binding events. As shown in intensity graph1600, beat frequency changes for first beat frequency group1610and second beat frequency group1615differ for the same binding events (e.g., events occurring at the same time).

An expanded view of a portion of intensity graph1600is shown in a close-up graph1620. A first beat frequency point1625represents a binding event that is not indicated by first beat frequency group1610. More specifically, first beat frequency point1625is not vertically offset from preceding beat frequency points1630. Conversely, a second beat frequency point1635is vertically offset from preceding beat frequency points1640in second beat frequency group1615. Accordingly, second beat frequency group1615indicates the binding event.

Embodiments in which a microcavity laser produces multiple laser modes enable redundant detection of binding events. In such embodiments, a binding event that does not significantly affect a first laser mode may significantly affect a second laser mode, such that the binding event may be detected.

More generally, embodiments including an active WGM resonator facilitate eliminating the need for a narrow linewidth tunable laser source to detect induced spectral shift and/or mode splitting, thus enabling a reduction in the cost of the detection system. Moreover, the use of an active WGM resonator and a pump light source may increase detection speed, as no tuning delay is incurred, and noise, such as thermal effects and piezo-motion, may be reduced or eliminated.

The split laser modes in an active WGM resonator reside in the same microcavity and are affected in the same way by the noise sources which affect the microcavity homogenously (e.g., environmental noise, the pump laser source, etc.). Accordingly, an active WGM resonator provides a self-referencing system. For example, while an arriving nanoparticle leads to a change in the amount of frequency splitting, changes in the environmental or the cavity temperature does not change the amount of frequency splitting but rather shift both modes consistently. Thus, high detection sensitivity, real-time and in-situ measurements may be facilitated without the need for active stabilization or temperature control.

FIG. 21is an example flow chart of a method1700for detecting an object based on mode splitting that may be used with an active WGM resonator. Method1700includes receiving1705, by a photodetector, light emitted by a whispering gallery mode (WGM) resonator. The WGM resonator may be an active WGM resonator, doped with a gain medium. In exemplary embodiments, the received light includes a pair of split laser modes, with a first split laser mode representing light transmission in a first frequency range and a second split laser mode representing light transmission in a second frequency range.

In some embodiments, the light from the WGM resonator is filtered1702before it is received by the photodetector. In exemplary embodiments, the emitted light includes laser modes emitted by the WGM resonator based on light from a light source. The light is filtered1702by a wavelength-division multiplexer to reduce or remove residual light from the light source (e.g., light other than light corresponding to the laser modes), creating filtered laser light that is transmitted to the photodetector.

A heterodyne beat signal is created1710based on the received light. For example, the photodetector may create1710the heterodyne beat signal at least in part by combining the first split laser mode and the second split laser mode. In some embodiments, the WGM resonator produces a single initial laser mode in the absence of a particle and two split laser modes in the presence of one or more particles. In other embodiments, the WGM resonator produces two or more initial laser modes in the absence of a particle and a pair of split laser modes corresponding to each initial laser mode in the presence of one or more particles. For example, if the WGM resonator produces two initial laser modes and a total of four split laser modes, all four split laser modes may be combined to create1710the heterodyne beat signal.

One or more beat frequencies are determined1715by a processor based on the heterodyne beat signal. For example, the processor may apply a fast Fourier transform to the heterodyne beat signal to determine1715a beat frequency. In embodiments in which the WGM resonator produces multiple pairs of split laser modes, a plurality of beat frequencies may be determined1715, with each beat frequency corresponding to a pair of split laser modes.

The presence of an object proximate to the WGM resonator is determined1720by the processor based on the beat frequency. For example, when the beat frequency is greater than zero, the presence of at least one object may be determined1720. In some embodiments, method1700is performed repeatedly (e.g., continuously and/or periodically). Each iteration of method1700is associated with a time of execution, and the current beat frequency (e.g., determined1715at a current time) is compared to a previous beat frequency (e.g., determined1715at a previous time). The presence of an object is determined1720based on a comparison of the current beat frequency to the previous beat frequency. For example, if the current beat frequency does not equal (e.g., differs by more than 1%, 2%, or 5% from) the previous beat frequency, the presence of an additional object may be determined1720.

In addition, one or more attributes (e.g., size, refractive index, and/or composition) of the object may be determined1725. For example, the size of an object may be determined1725by monitoring the changes in both the amount of frequency splitting and linewidths of the split laser modes in the light received1705from the WGM resonator. This may be done by employing linewidth measurement techniques. In some embodiments, the WGM resonator is cleaned1730, similar to cleaning1030, as shown inFIG. 11, to remove objects from the WGM resonator.

Active WGM resonator embodiments facilitate an object (e.g., nanoparticle and/or virion) detection scheme using an on-chip WGM microcavity laser. Detection and counting of individual objects may be achieved by monitoring the changes in the heterodyne beat frequency of the split laser modes in the microcavity laser. Individual object depositions are resolved as discrete step changes in the frequency splitting of the laser mode. Histograms of the frequency splitting steps may be used to extract the size of objects. Although embodiments described herein involve the use of a microtoroidal cavity laser, the principles and detection scheme can be applied to any other WGM microcavity lasers and/or other WGM resonator systems. For example, the techniques described herein with regard to passive resonators and active resonators may be applied to an aqueous environment and/or any other environment for detecting single biomolecules and/or particles.

Embodiments described herein enable the detection of nanoscale objects using a microcavity laser. Such microcavity lasers produce a narrow laser linewidth and facilitate a self-referencing detection scheme. For example, given linewidths as narrow as 4 Hz have for lasing in Er-doped WGM microcavities, detection of frequency splittings as small as a few tens of Hz, which translates into a lower detection limit of R˜0.5 nm, may be possible with WGM microcavity lasers.

While the making and use of various embodiments of the invention are discussed in detail above, the embodiments of the invention provide many applicable inventive concepts that may be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.

The order of execution or performance of the operations in embodiments of the invention illustrated and described herein is not essential, unless otherwise specified. For example, it is contemplated that executing or performing a particular operation before, contemporaneously with, or after another operation is within the scope of aspects of the invention. Embodiments of the invention may include additional or fewer operations than those disclosed herein.

Exemplary Operating Environment

Collection and analysis of object detection data such as described herein is typically performed by a computer or computing device. A computer or computing device includes one or more processors or processing units, system memory, and some form of computer readable media. By way of example and not limitation, computer readable media comprise computer storage media and communication media. Computer storage media include volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Communication media typically embody computer readable instructions, data structures, program modules, or other data in a modulated data signal such as a carrier wave or other transport mechanism and include any information delivery media. Combinations of any of the above are also included within the scope of computer readable media.

Although described in connection with an exemplary computing system environment, embodiments of the invention are operational with numerous other general purpose or special purpose computing system environments or configurations. The computing system environment is not intended to suggest any limitation as to the scope of use or functionality of any aspect of the invention.

Embodiments of the invention may be described in the general context of computer-executable instructions, such as program modules, executed by one or more computers or other devices. The computer-executable instructions may be organized into one or more computer-executable components or modules. Generally, program modules include, but are not limited to, routines, programs, objects, components, and data structures that perform particular tasks or implement particular abstract data types. Aspects of the invention may be implemented with any number and organization of such components or modules. For example, aspects of the invention are not limited to the specific computer-executable instructions or the specific components or modules illustrated in the figures and described herein. Other embodiments of the invention may include different computer-executable instructions or components having more or less functionality than illustrated and described herein.