This PDF file contains the front matter associated with SPIE Proceedings Volume 6900, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

Electrically-pumped photonic-crystal distributed-feedback lasers with interband-cascade active regions operating in single spectral mode at 3.3 μm are demonstrated. At 78 K, a stripe of width 400 μm emits up to 67 mW of cw power into a single spectral mode with side-mode suppression ratio ≈ 27 dB. The full-width at half-maximum of the farfield divergence angle is ≈ 0.5°, which combined with the near-field profile yields an effective M² of 1.7-2.0.

The paper describes the heterostructures and device output parameters of Type-I quantum-well (QW) laser diodes with InGaAsSb active regions designed for room-temperature operation near 2.3 μm and 3.1 μm. For both designs decrease of the threshold current density and increase of the room-temperature output power have been achieved with increase of the QW depth for holes. For the 2.3 μm laser diodes, confinement of holes in the QW embedded into the AlGaAsSb waveguide was improved with increase of the hole energy level with compressive strain. Arrays of 1-mm-long 100-μmwide laser diode emitters with a fill-factor of 30 % have been fabricated. A quasi-CW (30 μs, 300 Hz) output power of 16.7 W from a 4-mm-wide array has been obtained with conductive cooling. For the laser diodes designed for roomtemperature operation above 3 μm, the hole confinement was improved by lowering the valence band energy in the waveguide. Two approached were implemented: one with increase of the Al composition, and another with utilization of quinternary InAlGaAsSb waveguide with increased As composition compared to the conventional AlGaAsSb waveguide. With the quinternary waveguide approach, a room-temperature CW output power in excess of 130 mW and a threshold current as low as 0.6 A have been obtained at λ = 3 μm from 2-mm-long 100-μm-wide emitters.

Free-space optical communications has recently been touted as a solution to the "last mile" bottleneck of high-speed data networks providing highly secure, short to long range, and high-bandwidth connections. However, commercial near infrared systems experience atmospheric scattering losses and scintillation effects which can adversely affect a link's operating budget. By moving the operating wavelength into the mid- or long-wavelength infrared enhanced link uptimes and increased operating range can be achieved due to less susceptibility to atmospheric affects. The combination of room-temperature, continuous-wave, high-power quantum cascade lasers and high operating temperature type-II superlattice photodetectors offers the benefits of mid- and long-wavelength infrared systems as well as practical operating conditions for next generation free-space communications systems.

This paper presents recent progress in the field of semiconductor lasers based on self-assembled quantum dots grown either on GaAs or InP substrates. Quantum dot (QD) based lasers are attracting a lot of interest owing to their remarkable optoelectronic properties that result from the three dimensional carrier confinement. They are indeed expected to exhibit much improved performance than that of quantum well devices. Extremely low threshold currents as well as high temperature stability have readily been demonstrated in the InAs/GaAs material system. The unique properties of quantum dot based active layers such as broad optical gain spectrum, high saturation output power, ultrafast gain dynamics and low loss are also very attractive for the realization of mode-locked lasers. Recent results in the field of directly modulated InAs/GaAs lasers emitting in the 1.3 μm window are discussed. We report in particular on temperature independent linewidth enhancement factor (or Henry factor α_H) up to 85°C. This is a key parameter which determines many laser dynamic properties. Optical feedback insensitive operation of specifically band-gap engineered devices, compatible with high bit rate isolator-less transmission is also reported at 1.55 μm. Monolithic mode locked lasers based on InAs/InP quantum dashes have been investigated for 1.55 μm applications. Subpicosecond pulse generation at very high repetition rates (> 100 GHz) is reported for self-pulsating one-section Fabry Perot devices. Specific applications based on these compact pulse generators including high bit rate clock recovery are discussed.

This paper presents the development of a nanofabricated mid-infrared optical source, thermally emitting linearly polarized light. The optical source in the current study is a heated series of one-dimensional metal-insulator-metal cavities with a closed end on a Au surface. This closed cavity exhibits the so-called organ pipe resonance resulting in specific frequencies being selectively emitted from the blackbody heat source. This characteristic results in the control of the thermal radiation, thereby emitting a narrow infrared spectrum at a specific wavelength of 2.5-5.35 micro-meters. The wavelength is specified by a theoretical model and 100nm wide, 1000nm deep dimensions of the cavity were accurately manufactured. The maximum emittance reaches 0.90, and the peak width Δλ/λ is as narrow as 0.13-0.23. As a demonstration, the Cyclohexane concentration in Benzene is determined with a simple optical system. This simple emitter is expected to play a key role in the infrared sensing technologies for analyzing our environment.

We demonstrate electrically pumped, room temperature, single mode operation of photonic crystal distributed feedback (PCDFB) quantum cascade lasers emitting at λ ~ 4.75 μm. Ridge waveguides of 50 μm and 100 μm width were fabricated with both PCDFB and Fabry-Perot feedback mechanisms. The Fabry-Perot device has a broad emitting spectrum and a broad far-field character. The PCDFB devices have primarily a single spectral mode and a diffraction limited far field characteristic with a full angular width at half-maximum of 4.8 degrees and 2.4 degrees for the 50 μm and 100 μm ridge widths, respectively.

Over the past several years, our group has endeavored to develop high power quantum cascade lasers for a variety of remote and high sensitivity infrared applications. The systematic optimization of laser performance has allowed for demonstration of high power, continuous-wave quantum cascade lasers operating above room temperature. In the past year alone, the efficiency and power of our short wavelength lasers (λ~4.8 μm) has doubled. In continuous wave at room temperature, we have now separately demonstrated ~10% wallplug efficiency and ~700 mW of output power. Up to now, we have been able to show that room temperature continuous wave operation with >100 mW output power in the 3.8< λ<11.5 μm wavelength range is possible.

Quantum information processing and quantum cryptography are expected to realize highly secure future information networks. How to control photon qubits will be one of the major issues for this direction, but the technologies related to generations and detections of individual photons are still being developed. Semiconductor quantum dots (QDs) have been expected to play major role for on-demand generations of single photons as well as entangled photon pairs. In this talk, photon generation processes from a single QD will be studied for the control of the quantum states of generated photons. Generation of entangled photon pairs from QDs based on biexciton-exciton cascade recombination processes is presently in a difficult situation due to exciton states energy splitting related to growth-related issues. An approach to open new paradigm will be discussed with preliminary results.

We demonstrate mid-infrared continuous-wave vertical-cavity surface-emitting lasers based on Bragg mirrors using IV-VI semiconductors and BaF₂. This material combination exhibits a high ratio between the refractive indices of up to 3.5, leading to a broad mirror stop band with a relative width of 75 %. Thus, mirror reflectivities higher than 99.7 % are gained for only three layer pairs. Optical excitation of microcavity laser structures with a PbSe active region results in stimulated emission at various cavity modes between 7.3 μm and 5.9 μm at temperatures between 54 K and 135 K. Laser emission is evidenced by a strong line width narrowing with respect to the line width of the cavity mode and a clear laser threshold at a pump power of 130 mW at 95 K. Furthermore, we study a similar microcavity but without an active region. The resonance of such an empty microcavity has a narrow line width of 5.2 nm corresponding to a very high finesse of 750, in good agreement to transfer matrix simulations and to the expected mirror reflectivities.

The Missile Defense Agency's Advanced Technology Office is developing advanced passive electro-optical and infrared sensors for future space-based seekers by exploring new infrared detector materials. A Type II strained layer superlattice, one of the materials under development, has shown great potential for space applications. Theoretical results indicate that strained layer superlattice has the promise to be superior to current infrared sensor materials, such as HgCdTe, quantum well infrared photodetectors, and Si:As. Strained layer superlattice-based infrared detector materials combine the advantages of HgCdTe and quantum well infrared photodetectors. The bandgap of strained layer superlattice can be tuned for strong broadband absorption throughout the short-, mid-, long-, and very long wavelength infrared bands. The electronic band structure can be engineered to suppress Auger recombination noise and reduce the tunneling current. The device structures can be easily stacked for multicolor focal plane arrays. The III-V semiconductor fabrication offers the potential of producing low-defect-density, large-format focal plane arrays with high uniformity and high operability. A current program goal is to extend wavelengths to longer than 14 μm for space applications. This paper discusses the advantages of strained layer superlattice materials and describes efforts to improve the material quality, device design, and device processing.

Integration of active photonic components on silicon and silicon on insulator (SOI) would be versatile for nanophotonics since CMOS compatible processes are available for fabricating passive devices on Si/SOI. Selective area growth of III-V semiconductors is also attractive for realising periodic structures for nanophotonics. Here we report on the recent results of high quality InP on Si and InP on SOI achieved by means of nanopatterning. MQW structures have been realised on InP/Si and InP/SOI. We would elaborate routes for monolithic integration of active and passive devices for nanophotonics.

We report the molecular beam epitaxial growth of InSb quantum dots (QD) inserted as sub-monolayers in an InAs matrix and grown using Sb₂ and As₂ fluxes. These InSb QD nanostructures exhibit intense mid-infrared photoluminescence up to room temperature. The nominal thickness of the sub-monolayer insertions can be controlled by the growth temperature (T_Gr = 450-320 °C) which gives rise to the variation of the emission wavelength within the 3.6-4.0 μm range at room temperature. Light emitting diodes where fabricated using ten InSb QD sheets and were found to exhibit bright electroluminescence with a single peak at 3.8 μm at room temperature. A comparative analysis of the optical properties of the structures grown using (Sb₂,As₂) and (Sb₄,As₄) is also presented.

Quantum cascade detectors (QCDs) have been introduced recently as a photovoltaic candidate to infrared detection. Since QCDs work with no applied bias, longer integration time and different read-out circuits can be used. Depending on the application, QCDs could be preferred to QWIPs. The systematic comparison between QCDs and QWIPs is difficult due to the large number of parameters in a thermal imager for a given application. Here we propose a first comparison between these two devices, starting with several examples, based on specific cases. In particular, it is shown that QCDs in the 8-12 µm band are an interesting alternative to QWIPs if higher operating temperature is required.

This paper discusses the potential attributes of (110)-grown InAs/GaSb superlattices for infrared detection applications. In comparison to (001)-grown structures, (110) SLs will be thinner, have higher mobilities, diffusion lengths, quantum efficiencies, and gains. Unless growth issues arise, they should also have higher minority carrier lifetimes, higher responsivities, lower noise, and higher detectivities. The first 8x8 envelope-function approximation calculation for a (110)-oriented structure shows the bands to be slightly anisotropic and the oscillator strengths to be polarization dependent. Layer widths for specific absorption thresholds were calculated.

Recent progress in growth techniques, structure design and processing has lifted the performances of Type-II InAs/GaSb superlattice photodetectors. A double heterostructure design, based on a low band gap (11 μm) active region and high band gap (5 μm) superlattice contacts, reduced the sensitivity of the superlattice to surface effects. The heterodiodes with an 11 µm cutoff, passivated with SiO₂, presented similar performances to unpassivated devices and a one order of magnitude increase of the resistivity of the sidewalls, even after flip-chip bonding and underfill. Thanks to this new design and to the inversion of the polarity of the devices, a high performance focal plane array with an 11 μm cutoff was demonstrated. The noise equivalent temperature difference was measured as 26 mK and 19 mK for operating temperatures of 81 K and 67 K. At an integration time of 0.08 ms, the FPA presented a quantum efficiency superior to 50%.

In order for solar and visible blind III-nitride based photodetectors to effectively compete with the detective performance of PMT there is a need to develop photodetectors that take advantage of low noise avalanche gain. Furthermore, in certain applications, it is desirable to obtain UV photon counting performance. In this paper, we review the characteristics of III-nitride visible-blind avalanche photodetectors (APDs), and present the state-of-the-art results on photon counting based on the Geiger mode operation of GaN APDs. The devices are fabricated on transparent AlN templates specifically for back-illumination in order to enhance hole-initiated multiplication. The spectral response and Geiger-mode photon counting performance are analyzed under low photon fluxes, with single photon detection capabilities being demonstrated in smaller devices. Other major technical issues associated with the realization of high-quality visible-blind APDs and Geiger mode APDs are also discussed in detail and solutions to the major problems are described where available. Finally, future prospects for improving upon the performance of these devices are outlined.

Interband and intersubband transitions in self-assembled InAs quantum dots embedded in an InGaAs graded well have been investigated for their use in visible to mid-infrared (0.4 - 20 μm) detection applications. The materials were grown by molecular beam epitaxy and characterized using atomic force microscopy and photoluminescence. Devices were fabricated from the multiple quantum dot structures in order to measure the normal incident photoresponse at 77 and 300 K. In addition, the dark current was measured in the temperature range of 77 - 300 K for the devices. A dual broadband photoresponse from the interband and intersubband transitions was measured to be 0.5 to 1.0 μm and 2.0 to 14.0 μm, respectively.

The performance of infrared focal plane arrays and quantum cascade lasers manufactured from InAs/GaSb type- II superlattices (SLs) depends on the mobility of carriers along the growth axis. In turn, the longitudinal mobility depends on the quality of SL interfaces. In-plane transport is a sensitive measure of interface quality and the degree of interface roughness scattering (IRS). In this paper, we demonstrate the IRS-limited transport regime in InAs/GaSb SL samples grown for this study. We find that the in-plane mobility μ as a function of InAs layer width L behaves as μ ∝ L⁵ , which closely follows the classic sixth power dependence expected from theory. Fits to the mobility data indicate that, for one monolayer surface roughness, the roughness correlation length is about 35 Å.

Nitride-semiconductor light-emitting-devices such as blue, green, and white LEDs, and 400nm-wavlength LDs have been commercially available since 1993. The active layers in all these devices consist of InGaN, which composition is designed for the wavelength of the emitted light. In this paper, the current status of MOVPE growth in GaN to InN, including InGaN is reviewed. The GaN growth mechanism of two-step growth on a sapphire substrate, polarity- controlled GaN growth, and the possibility in In-rich InGaN growth are described. The InN research as an ultimate material of InGaN is also introduced. The future perspective of InN in device application is also mentioned.

Although CMOS technology scaling has provided tremendous power and circuit density benefits for innumerable applications, focal plane array (FPA) readouts have largely been left behind due to dynamic range and signal-to-noise considerations. However, if an appropriate pixel front end can be constructed to interface with a mostly digital pixel, it is possible to develop sensor architectures for which performance scales favorably with advancing technology nodes. Although the front-end design must be optimized to interface with a particular detector, the dominant back end architecture provides considerable potential for design reuse. In this work, digitally dominated long wave infrared (LWIR) active pixel sensors with cutoff wavelengths between 9 and 14.5 μm are demonstrated. Two ROIC designs are discussed, each fabricated in a 90-nm digital CMOS process and implementing a 256 x 256 pixel array on a 30-μm pitch. In one of the implemented designs, the feasibility of implementing a 15-μm pixel pitch FPA with a 500 million electron effective well depth, less than 0.5% non-linearity in the target range and a measured NEdT of less than 50 mK at f/4 and 60 K is demonstrated. Simple on-FPA signal processing allows for a much reduced readout bandwidth requirement with these architectures. To demonstrate the potential for commonality that is offered by a digitally dominated architecture, this LWIR sensor design is compared and contrasted with other digital focal plane architectures. Opportunities and challenges for application of this approach to various detector technologies, optical wavelength ranges and systems are discussed.

Recent advances in the development of sensors based on infrared quantum cascade lasers for the detection of trace gas species is reported. Several selected examples of applications in environmental and industrial process monitoring as well as in medical diagnostics using quartz enhanced photoacoustic spectroscopy and laser absorption spectroscopy will be described.

Silicon photonic crystal (PhC) waveguide based resonator is designed by introducing a micro-cavity within the line defect so as to form the resonant band gap structure for PhC. Free-standing silicon beam comprising this nanophotonic resonator structure is investigated. The output resonant wavelength is sensitive to the shape of air holes and defect length of the micro-cavity. The resonant wavelength shift in the output spectrum is a function of force loading at the center of a suspended beam with PhC waveguide resonator. The sensing capability of this new nanomechanical sensor is derived as that vertical deformation is about 20nm at center and the smallest strain is 0.005% for defect length.

We investigated the use of a pulsed, distributed feedback (DFB) quantum cascade (QC) laser centered at 970 cm^-1 in combination with an astigmatic Herriot cell with 150 m path length for the detection of ammonia and ethylene. This spectrometer utilizes the intra-pulse method, where a linear frequency down-chirp, that is induced when a top-hat current pulse is applied to the laser, is used for sweeping across the absorption line. This provides a real time display of the spectral fingerprint of molecular gases, which can be a few wave numbers wide. A 200 ns long pulse was used for these measurements which resulted in a spectral window of ~1.74 cm^-1. A room temperature mercury-cadmium-telluride detector was used, resulting in a completely cryogen free spectrometer. We demonstrated detection limits of ~3 ppb for ammonia and ~5 ppb for ethylene with less than 10 s averaging time.

With the advance of nano-lithography and nano-fabrication, individual sizes of electronic, photonic, and mechanical components, as well as their integration densities, have progressed steadily towards the sub-100 nm regime. Therefore, being able to image such feature sizes becomes imperative. Many conventional high-resolution imaging tools such as SEM, STM, AFM, and NSOM either require operation under high vacuum or slow scanning across the sample. A far-field optical imaging instrument would thus be highly desirable. Optical imaging, however, is subject to the diffraction limit, which limits the size of the smallest resolvable feature to be ~ λ/2, where λ is the wavelength of the imaging light. Recently, negative-index materials and super lens have been proposed to overcome this limit and achieve high-resolution optical imaging [1-4]. In this paper, we propose a different approach to achieve sub-diffraction optical imaging with far-field microscopy. The technology builds on a high-spatial resolution quantum-dot (QD) photodetector with high sensitivity that we have demonstrated [5]. The photodetector consists of several nanocrystal QDs between a pair of electrodes with 50-nm width spaced ~ 25 nm apart. An optically effective area of 13515 nm² was determined by modeling the electric field distribution in-between and around the electrodes using FEMLab. High-sensitivity photodetection has been demonstrated by measuring the tunneling photocurrent through the QDs, with a detection limit of 62 pW of the input optical power. The proposed sub-diffraction optical imaging system consists of an array of such photodetectors. We performed theoretical simulations assuming a two slit source and then pixilated the far-field diffraction pattern to simulate the photodetector array. A Fourier transform of the detector signal is then performed to determine how much of the original aperture information remains. Using a wavelength of 500 nm and a screen distance of 10 cm, we found that, as expected, the quality of the resultant image generally degraded with larger pixilation size. With 50-nm one-dimensional spatial resolution at the detection plane, it appears that the original slit image with 100-nm width and 300-nm spacing can still be restored.

We discuss a scheme for a photon-counting detection system that overcomes the difficulties of photon-counting at extremely high rates. Our method uses an array of N detectors and a 1-by-N optical switch with a control circuit to direct input light to live detectors. Detector deadtime is significantly reduced by an active routing of single photons to the detector that has had the most time to recover from its last firing. In addition to deadtime reduction, our scheme reduces afterpulsing and background counts (such as dark counts). We present experimental results showing the advantageous performance of our system as compared to passive multi-detector detection systems. We conclude that intelligent active management of a group of N photon-counting detectors yields the highest photon counting rates, an important technological challenge for fast developing quantum metrology and quantum key distribution applications. Also, we report our experimental progress in developing an integrated device based on this scheme.

Recent developments in three-dimension imaging, quantum cryptography, and time-resolved spectroscopy have stimulated interest in single-photon counting avalanche photodiodes (APD) operating in the short wavelength infrared region. For visible and near infrared wavelengths, Silicon Geiger-mode APDs have demonstrated excellent photon detection efficiency (PDE) and low dark current rate (DCR)¹. Recently, MIT Lincoln Laboratories, Boeing Spectrolab, and Boeing SVS have demonstrated Geiger-mode (GM) APD focal plane arrays (FPA) operating at 1.06 μm. However for longer wavelength sensitivity around 1.55 μm, GM-APDs have to be cooled to 180~240 K to achieve a usable DCR. Power consumption, package weight and size and APD PDE all suffer with this cooling requirement. In this paper we report the development of an InP/InGaAs GM-APD structure with high PDE and low DCR at 273K. The photon collection efficiency was optimized with a single step-graded quaternary layer and a 3.5 μm InGaAs absorption layer, which provides a broadband coverage from 0.95 μm to 1.62 μm. The InP multiplication layer and the charge layer are carefully tailored to minimize the DCR and maximize the PDE. Despite having a low bandgap absorber layer InGaAs, these APDs demonstrated excellent dark current, optical responsivity, and superior DCR and PDE at 1.55 μm. The DCR and PDE were evaluated on 25 μm diameter APDs at 273 K. DCRs as low as 20 kHz have been measured at a 2 V overbias, while PDEs at 1.55 μm exceed 30% at 2 V overbias.

Time-correlated single-photon counting techniques using individual optimized detectors have been applied to time-of-flight ranging and depth imaging. This paper describes recent progress in photon-counting systems performing surface mapping of non-cooperative targets. This includes systems designed for short ranges of the order of 1-50 meters, and longer ranges of up to ten kilometers. The technique has also been applied to distributed surfaces. We describe the measurement approach, techniques used for scanning, as well as the signal analysis methodology and algorithm selection. The technique is fundamentally flexible: the trade-off between the integrated number of counts (or acquisition time) against range repeatability or depth resolution allows its application in a number of diverse fields. The inherent time gating of the technique, allied to the spatial filtering provided by small active area single-photon detectors, can lead to operation under high ambient light conditions even with low average optical power pulsed sources. We have demonstrated three-dimensional imaging of meter-dimensioned objects where reverse engineering methods using cooperative targets cannot be routinely employed: e.g. delicate objects, or objects with more than one reflective surface. Using more advanced signal processing algorithms, we have been able to improve the system performance significantly, as measured by the depth resolution at short and long ranges. Furthermore, the application of these methodologies has allowed us to characterize the positions and amplitudes of multiple returns. Hence, the approach can be used for characterization of distributed non-cooperative targets at kilometer ranges, even in environments where low-light level and and/or eye-safe operation is necessary. The technique has also been applied in conjunction with a rapid scanning approach, to acquire three-dimensional information of a target scene with frame times of approximately 1 second.

Single photon counting (SPC) and time correlated single photon counting (TCSPC) techniques have been developed in the past four decades relying on photomultiplier tubes (PMT), but interesting alternatives are nowadays provided by solid-state single photon detectors. In particular, silicon Single Photon Avalanche Diodes (SPAD) fabricated in planar technology join the typical advantages of microelectronic devices (small size, ruggedness, low operating voltage and low power dissipation, etc.) with remarkable basic performance, such as high photon detection efficiency over a broad spectral range up to 1 μm wavelength, low dark count rate and photon timing jitter of a few tens of picoseconds. In recent years detector modules employing planar SPAD devices with diameter up to 50 µm have become commercially available. SPADs with larger active areas would greatly simplify the design of optical coupling systems, thus making these devices more competitive in a broader range of applications. By exploiting an improved SPAD technology, we have fabricated planar devices with diameter of 200 μm having low dark count rate (1500 c/s typical @ -25 °C). A photon timing jitter of 35 ps FWHM is obtained at room temperature by using a special pulse pick-up network for processing the avalanche current. The state-of-the-art of large-area SPADs will be reviewed and prospects of further progress will be discussed pointing out the challenging issues that must be faced in the design and technology of SPAD devices and associated quenching and timing circuits.

InP-based single photon avalanche diodes (SPADs) have proven to be the most practical solution currently available for many applications requiring high-performance photon counting at near-infrared wavelengths between 1.0 and 1.6 µm. We describe recent progress in the design, characterization, and modeling of InP-based SPADs, particularly with respect to the dark count rate vs. photon detection efficiency metric of devices optimized for use at both 1.55 μm and 1.06 μm. In this context, we report for the first time dark count probabilities as low as 7 x 10-7 ns-1 for fiber-coupled 1.55 μm SPADs operated at 20% detection efficiency and 215 K. Additionally, because of the critical role played by afterpulsing in limiting photon counting rates, we describe recent characterization of the dependence of afterpulsing effects on SPAD operating conditions such as photon detection efficiency, repetition rate, and bias gate length.

In this paper, we report a new quenching circuit for single photon avalanche diodes, passive-quenching-with-activereset. This circuit uses a large resistor to passively quench the avalanche current and a transistor to actively reset the diode to the operating voltage after a specified hold-off time. Afterpulsing is reduced by minimizing the total charge flow during avalanche, which can be achieved by minimizing the stray capacitance in the circuit. The simulation of the circuit using PSpice showed that the circuit is operating correctly. The operation of the circuit was demonstrated using discrete components. A quenching time of ~2ns and a dynamic range of ~100dB have been achieved. The performance can be further improved by integrating the transistor with the photodiode to reduce the stray capacitance.