Low noise optical pre-amplifier for ultra-low-light detectors and FPAs

An optical pre-amplifier is described. The optical pre-amplifier has an optical amplifier region that has a semiconductor active region having a direct electronic band gap with a conduction band edge. The semiconductor active region is embedded within a photonic crystal having an electromagnetic band gap having photon energies overlapping the energy of the conduction band edge of the electronic band gap such that spontaneous emission of photons in the semiconductor active region is suppressed.

BACKGROUND

The present invention relates to a low noise optical pre-amplifier for low light detection, and to optical systems employing such a low noise optical pre-amplifier.

Low light level imaging is important for applications ranging from photography to night vision. One example application is in helmet mounted displays, such that may be deployed on a helmet for a pilot operating an aircraft. Other applications include astronomical imaging. Low light levels may occur, for example, for night time imaging under overcast conditions.

Light detection systems include imaging devices that may employ focal plane array (FPA) detectors. Typically, an FPA will have very many pixels, each pixel appropriate for separately detecting a plurality of impinging photons. Light detectors and FPAs, however, eventually are unable to operate in ultra-low light conditions. At long range, laser illuminator and imaging systems, typically operating at 1064 nm wavelength, also fail.

SUMMARY OF THE INVENTION

According to one embodiment, there is provided an optical pre-amplifier, comprising: an optical amplifier region comprising a semiconductor active region having a direct electronic band gap with a conduction band edge, the semiconductor active region embedded within a photonic crystal having an electromagnetic band gap having photon energies overlapping the energy of the conduction band edge of the electronic band gap such that spontaneous emission of photons in the semiconductor active region is suppressed.

According to one aspect of the embodiment, the optical amplifier region comprises a p-n junction.

According to another aspect of the embodiment, the photonic crystal comprises a photonic band gap material having a three-dimensional, two-dimensional or one-dimensional structure.

According to another aspect of the embodiment, the photonic crystal comprises a plurality of Bragg gratings.

According to another aspect of the embodiment, the semiconductor active region comprises one or more quantum well (QW) structures.

According to another aspect of the embodiment, the QWs are confined spatially into QW wires or quantum dots.

According to another aspect of the embodiment, the QWs are confined spatially in an arrangement of one or more of nanotubes, bucky balls made of carbon, graphene, germanene, or boron-nitride.

According to another aspect of the embodiment, the optical pre-amplifier further comprises one or more electrodes arranged to provide current injection into the conduction band edge of the semiconductor active region.

According to another aspect of the embodiment, the photonic crystal comprises a dielectric or semiconductor material.

According to another aspect of the embodiment, the semiconductor active region comprises at least one of group IV semiconductors, group II-VI semiconductors, or group III-V semiconductors.

According to another aspect of the embodiment, the semiconductor active region comprises a group III-V semiconductor.

According to another aspect of the embodiment, the optical pre-amplifier is configured to amplify a photon impinging on the semiconductor active region.

According to another aspect of the embodiment, the photonic crystal has a predetermined pattern of holes arranged within the semiconductor active region.

According to another embodiment, there is provided an optical detector system comprising: an optical pre-amplifier, comprising an optical amplifier region comprising a semiconductor active region having a direct electronic band gap, the semiconductor active region embedded within a photonic crystal having an electromagnetic band gap having photon energies overlapping the energy of the conduction band edge of the electronic band gap such that spontaneous emission of photons in the semiconductor active region is suppressed; and an optical detector arranged to receive and detect amplified electromagnetic radiation from the optical pre-amplifier.

According to one aspect of the embodiment, the optical pre-amplifier comprises an array of optical pre-amplifiers, and the optical detector comprises an array of optical detectors, each of the optical pre-amplifiers may correspond to a respective one of the optical detectors.

According to another aspect of the embodiment, the optical detector system is a focal plane array (FPA) detector device.

According to another aspect of the embodiment, the optical detector system comprises a dielectric spacer separating the optical pre-amplifier and the optical detector.

According to another embodiment, there is provided an optical system comprising: an optical detector system comprising: an optical pre-amplifier, comprising an optical amplifier region comprising a semiconductor active region having a direct electronic band gap, the semiconductor active region embedded within a photonic crystal having an electromagnetic band gap having photon energies overlapping the energy of the conduction band edge of the electronic band gap such that spontaneous emission of photons in the semiconductor active region is suppressed; and an optical detector arranged to receive and detect amplified electromagnetic radiation from the optical pre-amplifier; and imaging optics arranged to image electromagnetic radiation onto the optical detector system.

According to one aspect of the embodiment, the optical system is one of a camera, telescope, or microscope.

According to another aspect of the embodiment, the optical system is a head worn imaging system.

DETAILED DESCRIPTION

In low light level imaging conditions, only a few photons per pixel may arrive each frame-time. In this case, the imaging quality in optical imaging system may be very grainy or noisy, which can render images taken at night unusable.

The present inventors have realized that a substantially improved optical imaging system is possible by amplifying those few photon arrivals per frame time per pixel by a low (near-zero) noise pre-amplifier. In particular, such a low noise pre-amplifier may be provided by disposing a semiconductor amplifier within a photonic crystal, which has a photonic band gap. By appropriately aligning the energy of the conduction band edge of electronic band gap of the semiconductor amplifier with the photon energy of forbidden wavelength within the photonic band gap, the spontaneous emissions of photons in the semiconductor amplifier may be strongly suppressed. Thus the noise due to spontaneous emissions of photons in the semiconductor amplifier within the amplification process may be strongly suppressed.

Inhibited Spontaneous Emission and Photonic Crystals

Spontaneous emission in an atom occurs when an excited state undergoes a transition to a state with a lower energy (ground state) and a photon is emitted. The rate of spontaneous emission depends partly on the environment of a photon source. This means that by placing the photon source in a special environment, the rate of spontaneous emission can be modified. The spontaneous emission rates of atoms may be enhanced when the atoms are matched in a resonant cavity due to the Purcell Effect. The enhancement is given by the Purcell factor [E. M. Purcell, “Spontaneous emission probabilities at radio frequencies,” Phys. Rev. 69, 681 (1946)]:

FP=34⁢π2⁢(λcn)3⁢(QV),
where (λc/n) is the wavelength within the material, and Q and V are the quality factor and mode volume of the cavity, respectively.

The use of a photonic crystal may suppress the spontaneous emission in a semiconductor, which may spontaneously emit a photon from an electronic transition from the conduction band to the valence band. Photonic crystals are periodic dielectric structures that have an electromagnetic band gap that forbids propagation of a certain frequency range of electromagnetic radiation. In particular, spontaneous emission can be suppressed by appropriate alignment of the photonic crystal band gap with respect to an electronic conduction band edge of a semiconductor material [E. Yablonovitch, ‘Inhibited Spontaneous Emission in Solid-State Physics and Electronics’, Phys. Rev. Letts, 58, 2059 (1987)].

If a photonic crystal with a periodic dielectric structure has an electromagnetic band-gap that overlaps the electronic conduction band edge of the semiconductor material embedded in the photonic crystal, then spontaneous emission of photons in the semiconductor material can be suppressed. The photonic crystal may have a three-dimensional structure. Alternatively, the photonic crystal may have a two-dimensional or one-dimensional structure, such as a surrounding Bragg grating, or a surrounding thin layer of appropriately chosen refractive index. The photonic band gap structure in the photonic crystal is provided by the photonic crystal structure, which typically may have a periodic array of dielectric index values in some geometric manner, such as in glass, polymer or semiconductor.

Optical Amplification in Semiconductor Material

Optical amplification in the semiconductor material of the semiconductor active region of the pre-amplifier may be provided by electrically injecting (pumping) the semiconductor band edge of the semiconductor material suitable for amplifying an incoming photon. Appropriate semiconductor materials for optically amplifying photons impinging on the pre-amplifier may be selected from semiconductor materials appropriate for laser light amplification (although lasing does not occur in the semiconductor material of the pre-amplifier). For example, the semiconductor material of the semiconductor active region may be a Group III-V material. Alternatively, the semiconductor material may be a Group IV or Group II-VI material.

The semiconductor active region may be in the form of a p-n junction, or layers in the multiple quantum well (MQW). The semiconductor active region may be in the form of quantum wires or quantum dots.

Examples are provided below of appropriate semiconductor materials for the active material of a p-n junction for different colors/wavelengths of light:

Blue and Green: GaN;

Examples of stacks for III-V systems for an MQW structure are provided below for different colors/wavelengths of light:

Optical Detector with Optical Pre-Amplifier

FIG. 1is a schematic illustrating an optical detector system10with an optical pre-amplifier20and an optical detector70according to an embodiment.FIG. 2is a top view ofFIG. 1, and shows a triangular hexagonal array pattern of holes52with a hole spacing a51. The optical pre-amplifier20comprises an optical amplifier region24which has a semiconductor active region26. An appropriate voltage is applied to the optical pre-amplifier20through electrode30and electrode31to inject current and pump the semiconductor conduction band edge of the semiconductor active region26. The optical pre-amplifier amplifies the incoming electromagnetic radiation40impinging on the semiconductor active region26into outgoing radiation44, which is directed to the optical detector70.

The optical pre-amplifier20further comprises a photonic crystal50, which is embedded within the semiconductor active region26. InFIG. 1, the photonic crystal50has an array of holes52, and a defect in the center where there is a lack of a hole. The semiconductor active region26of the optical amplifier region24is embedded within the photonic crystal50, and inFIG. 1the semiconductor active region26is a part of the photonic crystal50.

The semiconductor active region26is formed of a semiconductor material which has an electronic band edge having a predetermined energy. The photonic crystal50has a hole size and spacing, and a dielectric constant to provide an electromagnetic band gap having a range of photon energies. Photons having energies falling with the electromagnetic band gap are forbidden to propagate within the photonic crystal50. The photonic crystal50has a hole size and spacing, and a dielectric constant to provide an electromagnetic band gap such that the electromagnetic band gap has photon energies that overlap the energy of the conduction band edge of the direct electronic band gap of the semiconductor active region26.

The outgoing radiation44, which has been optically amplified, therefore comprises many photons. These photons may be used directly in a light amplification system for direct viewing by a user's eye, or they may be detected by an optical detector70. The optical detector70may be any appropriate optical detector, such as an avalanche photodiode, CMOS photodetector, or photomultiplier, for example, where the amplified radiation is detected and converted into photo-current. Dielectric spacers80separate the optical detector from the optical pre-amplifier20. An electrical connection may be made to circular ring bias contact31using any of a set of standard semiconductor connection methods.

FIG. 3illustrates the condition where the electromagnetic band gap has photon energies that overlap the energy of the conduction band edge of the electronic band gap, where the electronic band gap is a direct band gap. As can be seen inFIG. 3, the band energies vary as a function of the wave vector k. The electromagnetic band gap320inFIG. 3has photon energies within the range of E0to E1. The semiconductor has an electronic band structure with a valence band edge energy Ev, which is the highest energy in the valence band, and a conduction band edge energy Ec, which is a lowest energy in the conduction band. The valence band edge energy Ev is taken as the ground energy of zero. Thus, the electronic band gap energy is Ec. As can be seen inFIG. 3, the electromagnetic band gap320has photon energies that overlap the conduction band edge energy Ec. In this case, spontaneous emission of a photon based on an electronic transition from the conduction band edge to the valence band edge is prohibited.

AlGaAs Optical Preamplifier

As an example, parameters for photon energies of the electromagnetic band of a photonic crystal for an AlGaAs optical preamplifier are described. The electromagnetic band structure for AlGaAs is provided inFIG. 4, where band energies are shown as a function of wave vector.FIG. 4shows a photonic band structure of a regular triangular array hole pattern in AlGaAs. The two-dimensional photonic crystal of AlGaAs should have an electromagnetic band gap whose longest wavelength is at 860 nm, but the photonic crystal parameters may be manipulated to change both the electromagnetic band width and the longest wavelength.

The photonic crystal may be embedded in the AlGaAs material by forming a predetermined pattern of holes in the AlGaAs material. For an array of holes in the AlGaAs, the electromagnetic center energy and band width will depend upon the index of refraction of the AlGaAs, and the spacing and size of the holes. This predetermined pattern of holes can be triangular, rectangular, or quasiperiodic, or randomly spaced. In this example, the center energy and electromagnetic bandwidth may be calculated for a triangular lattice of holes with a diameter s, and a spacing between hole centers of a. AlGaAs has a core index of refraction of 3.42 at a photon wavelength of 860 nm. The effective index of refraction, neffwill change due to the presence of holes in the AlGaAs. The spacing a of the triangular hole lattice is ˜180 nm-300 nm, the hole diameter s is ˜110 nm-160 nm, and the hole heights are 750 nm or more. The bandwidth of the electromagnetic gap is ⅓ the center frequency, which provides an appropriate electromagnetic band gap for use with AlGaAs as the material for active region of the photon amplifier.

For an effective index of refraction, neff, a ratio of s/a=0.35 between the hole diameter and the spacing is enough for an omni-directional TE (transverse electromagnetic) electromagnetic band gap to appear. This corresponds to a fill factor of no more than 15%. A ratio s/a of 0.75, corresponding to a fill factor of 60%, would be required to simultaneously obtain complete TE and TM (transverse magnetic) gaps.

FIG. 5shows calculated gain-bandwidth curves for an AlGaAs p-n junction amplifier for injection currents of 2.5×1018electrons/cm3, 5.0×1018electrons/cm3, and 7.5×1018electrons/cm3, as a function of photon wavelength over a photon wavelength range of 750 nm to 880 nm. For optimum performance, the photonic crystal should have an electromagnetic band gap that includes the range of 750 nm to 870 nm to ensure suppression of spontaneous emission over the appropriate photon amplification range of the AlGaAs detector.

While an AlGaAs detector is described above for a photon amplification range of about 760 nm to 870 nm, semiconductor materials with other photon amplification ranges are appropriate. For example, InGaAsP, with an index of refraction of 3.35, provides a photon amplification range within a band from ˜1.05 microns to ˜1.55 microns with a photon amplifier gain between 20× and 60×. As another example, AlGaInP, with an index of refraction of 3.56, provides a photon amplification range centered at 617 nm with a bandwidth about 617 nm of ˜40 nm to ˜60 nm. In any case, the crystal should have an electromagnetic band gap that includes the appropriate photon amplification range to ensure suppression of spontaneous emission over the appropriate photon amplification range of the detector.

MQW Detector System

FIG. 6is a schematic illustrating an optical detector system610with an optical pre-amplifier620and an optical detector670according to an embodiment, where the optical amplifier region624of the optical pre-amplifier620has a MQW structure. The optical amplifier region624has a semiconductor active region626, comprising layers within the MQW where electronic and photonic confinement occur. The optical pre-amplifier620may also include current injecting electrodes630, which function to inject current and pump the semiconductor band edge of the semiconductor active regions626within the MQW to allow for amplification of incoming electromagnetic radiation640impinging on the semiconductor active region626into outgoing radiation644, which is directed to the optical detector670.

The optical pre-amplifier620further comprises a photonic crystal650, which is arranged within the semiconductor active region626. InFIG. 6, the photonic crystal650has an array of holes652, and a defect in the center where there is a lack of a hole. The semiconductor active region626is embedded within the photonic crystal650, and inFIG. 6the semiconductor active region626is a part of the photonic crystal650.

The semiconductor active region626is formed of a semiconductor material that has an electronic band gap having a desired energy. Typical materials for a MQW structure are semiconductor heterojunctions formed of one or more of: InAs, InGaAs, GaAs and AlGaAs. The photonic crystal650has a hole size and spacing, and a dielectric constant to provide an electromagnetic band gap having photon energies. Photons having energies falling with the electromagnetic band gap are forbidden to propagate within the photonic crystal650. The photonic crystal650has a predetermined pattern of hole sizes and spacings, and a dielectric constant to provide an electromagnetic band gap such that the electromagnetic band gap has photon energies that overlap the energy of the conduction band edge of the electronic band gap such that spontaneous emission of photons in the semiconductor active region626is suppressed.

The outgoing radiation644, which has been optically amplified is detected by an optical detector670. The optical detector670may be any appropriate optical detector, such as an avalanche photodiode, or photomultiplier, for example where the amplified radiation is detected and converted into photo-current. Dielectric spacers680separate the optical detector from the optical pre-amplifier620. An electrical contact631provides a return path for the pumping current.

The MQW structure ofFIG. 6provides good gain qualities. One of the advantages of a narrow active MQW region for gain is the expectation of low gain threshold current densities. The carrier (e) density n in a quantum-well structure is given by:
n=(Jτ)/(eL),
where J is the gain-threshold current, τ is the carrier lifetime, e is the carrier charge, and L is the QW width. There is a minimum value of J, the gain-threshold current, needed to overcome any losses in the amplifier. This has a dependency on the carrier density n as shown inFIG. 7, which illustrates carrier density curves between 1.5×1018electrons/cm−3and 2.5×1018electrons/cm−3. At the onset of gain, n is usually ˜1-3×1018cm−3, depending on the emission wavelength. If the QW width L is made smaller, J can be made correspondingly smaller to maintain n. J is usually in the region 100-300 A/cm2for the threshold of positive amplification.

The MQW structure may be confined within a photonic band gap structure, where the MQW is within one-dimensional, two-dimensional, or three-dimensional photonic crystal microcavities. Such microcavities may comprise nanotubes, rolled graphene, germanene, boron nitride sheets, bucky balls of C60, QW-wires or quantum dots, for example.

The optical pre-amplifier using MQW structure, as well as further confined quantum-wire and quantum dot structures, and MQWs incorporating super-lattices, is not limited to photon detection in the visible and near infra-red (NIR), but may be applicable to the Mid-wave infra-red (MWIR) (˜3-5 microns), Long-wave infra-red (LWIR) (˜8-14 microns) and far infra-red (˜25-100 microns). Such structures may be placed inside a photonic crystal structure, such as in a manner as described above, and used for low noise optical pre-amplification.

FIG. 8is a schematic illustrating an optical detector system800according to an embodiment of the invention. The optical detector system800includes an array of optical pre-amplifiers820, and an array of optical detectors870. The array of optical detectors870may be an FPA. The optical pre-amplifiers820may each be a pre-amplifier20as shown inFIG. 1or a pre-amplifier620as shown inFIG. 6, for example. The optical detectors870may each be an optical detector70as shown inFIG. 1or an optical detector670as shown inFIG. 6, for example. Each of the optical pre-amplifiers820corresponds to a respective one of the optical detectors870, and provides amplified electromagnetic radiation to a respective optical detector.

FIG. 9illustrates a schematic of an optical system900including an optical detector system910and imaging optics920. The optical detector system910may be one of the optical detector systems10,610or800ofFIGS. 1, 6 and 8, respectively, for example. The imaging optics920is arranged to image electromagnetic radiation onto the optical detector system910. The imaging optics920may include, for example, one or more lenses or mirrors, for example. The optical system900may be one of a camera, telescope, microscope, or head mounted display, for example.

The embodiments of the invention have been described in detail with particular reference to preferred embodiments thereof, but it will be understood by those skilled in the art that variations and modifications can be effected within the spirit and scope of the invention.