This PDF file contains the front matter associated with SPIE Proceedings Volume 7222, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Over the past two decades mid-infrared laser spectroscopy has been increasingly utilized during airborne atmospheric studies to improve our understanding of atmospheric processes and transformations. Enhancing such understanding requires a suite of ever more sensitive, selective, versatile, and fast instruments that can measure trace atmospheric constituents at and below mixing ratios of 100-parts-per-trillion-by-volume. Instruments that can carry out such measurements are very challenging, as airborne platforms vibrate, experience accelerations, and undergo large swings in cabin temperature and pressure. These challenges notwithstanding, scientists and engineers at the National Center for Atmospheric Research (NCAR) have long been employing mid-infrared absorption spectroscopy to make atmospheric measurements of important trace gases like formaldehyde (CH₂O) on a variety of airborne platforms. The present paper discusses a new airborne spectrometer based upon a difference frequency generation (DFG) mid-IR laser source that was first deployed in 2006. Many of the fundamental components and concepts of this spectrometer closely follow those incorporated in our liquid-nitrogen cooled tunable lead-salt diode laser system, successfully employed for airborne CH₂O measurements over the past 10 years. However, a number of significant modifications were incorporated in the new DFG spectrometer and these will be briefly discussed here along with system performance. The DFG spectrometer was recently deployed during the 2008 Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign, and specific examples of its performance from this study will be discussed, as will prospects for the detection of other trace gases.

Quantum Cascade Lasers (QCLs) have been very successful at long wavelengths, >4μm, and there is now considerable effort to develop QCLs for short wavelength (2-3μm) applications. To optimise both interband and QC lasers it is important to understand the role of radiative and non-radiative processes and their variation with wavelength and temperature. We use high hydrostatic pressure to manipulate the band structure of lasers to identify the dominant efficiency limiting processes. We describe how hydrostatic pressure may also be used to vary the separation between the Γ, Χ and L bands, allowing one to investigate the role of inter-valley carrier scattering on the properties of QCLs. We will describe an example of how pressure can be used to investigate the properties of 2.9-3.3μm InAs/AlSb QCLs. We find that while the threshold current of the 3.3μm devices shows little pressure variation even at room temperature, for the 2.9μm devices the threshold current increases by ~20% over 4kbar at 190K consistent with carrier scattering into the L-minima. Based on our high pressure studies, we conclude that the maximum operating temperature of InAs/AlSb QCLs decreases with decreasing wavelength due to increased carrier scattering into the L-minima of InAs.

Mid infrared (MIR) absorption spectroscopy between 3 and 20 μm, known as Infrared Laser Absorption Spectroscopy (IRLAS) and based on tuneable semiconductor lasers, namely lead salt diode lasers, often called tuneable diode lasers (TDL), and quantum cascade lasers (QCL) has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas and for trace gas analysis. The increasing interest in molecular processing plasmas has lead to further applications of IRLAS. IRLAS provides a means of determining the absolute concentrations and temperatures of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetics. Since plasmas with molecular feed gases are used in many applications such as thin film deposition and semiconductor processing this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of QCLs offers an attractive new option for the monitoring and control of industrial plasma processes as well as for highly time-resolved studies on the kinetics of plasma processes and for trace gas analysis. The aim of the present contribution is threefold: (i) to report on selected studies of the spectroscopic properties and kinetic behaviour of the methyl radical, (ii) to review recent achievements in our understanding of molecular phenomena in plasmas and the influence of surfaces, and (iii) to describe the current status of advanced instrumentation for quantum cascade laser absorption spectroscopy (QCLAS).

We show that the internal quantum efficiency and the wall-plug efficiency of quantum cascade lasers (QCL) are correlated with the hot-electron cooling associated with photon emission. The experimental procedure for the assessment of these key device parameters is based on micro-probe photoluminescence (PL) that allows high resolution measurements of the electronic and local lattice temperatures in operating QCLs. By using a terahertz QCL as a prototype we demonstrate that the electronic distributions are Fermi-Dirac functions characterized by temperatures significantly larger than the lattice one. The lattice temperature is in turn well above the one of the heat sink bath. Combining the above observation with time-resolved PL experiments we assessed the characteristic time constants controlling the heating and cooling processes of terahertz QCLs that are limited by the presence of a high density of interfaces that causes phonon interference effects. The correlation between the above constants, the thermal diffusivities and the diffusion lengths have been extracted from the comparison with the outcome of a transient heat diffusion model.

The combination of infrared laser spectroscopy with a molecular jet expansion provides a powerful technique to investigate medium sized organic molecules and clusters. The coupling of quantum cascade lasers (QCLs) with two slit jet infrared spectrometers, namely an off-axis cavity enhanced absorption (CEA) spectrometer and a rapid scan spectrometer with an astigmatic multi-pass cell assembly, are described. Two types of QCLs, specifically a continuous wave (cw) liquid nitrogen cooled distributed-feedback QCL at 5.7 μm, and a cw room temperature mode-hop-free external cavity QCL centered at 6.1 μm, were employed as the light sources. A pair of 1 inch highly reflective cavity ring-down mirrors (R = 99.98% at 5.2 μm) separated by 55 cm or a pair of 1.5 inch astigmatic mirrors separated by 20 cm, served as the optical cavities. To automate and to synchronize the timing of the CEA or rapid scan experiments with a pulsed slit jet molecular expansion, two LabVIEW computer programs were developed. For the CEA experiments, one of the cavity mirrors was mounted on a piezoelectric actuator with 1 inch clear aperture to maximize the effective mirror size. The effects of mirror size and laser sweep rate were evaluated. A minimum detection sensitivity of 1.8×10^-8 cm^-1 was achieved. Jet-cooled molecules were generated using a homemade pulsed slit jet nozzle assembly. A jet-cooled infrared spectrum of methyl lactate was recorded to demonstrate the performance of the CEA spectrometer. Preliminary results obtained with the room temperature QCL coupled to the rapid scan spectrometer are also presented.

We describe a monolithic frequency comb generator. Using four-wave mixing enhanced by the high-Q optical modes of a microresonator, a continuous wave laser is converted into an optical frequency comb, whose equidistant mode spacing is verified to a precision better than 1 part in 10¹⁷. Balance of geometric and material dispersion allows to flatten dispersion and to create combs spanning 500 nm in the 1550 nm band. Moreover, scaling of this approach to microwave repetition rates is described.

Single-mode laser diodes on GaSb substrates were developed using InGaAsSb/AlGaAsSb triple quantum well active regions grown by molecular beam epitaxy. The devices were fabricated using lateral Cr gratings, with a grating pitch designed to coincide with a strong absorption feature of HF gas, deposited adjacent to a dry-etched narrow ridge waveguide. High sidemode suppression was achieved, and in 20°C continuous-wave operation, devices with a 400μm-long cavity provided 4.5mW total output power at the 2396nm target wavelength. Anti-reflection and high-reflection facet coatings exhibited no deleterious effects on the laser tunability or mode quality, thus allowing the preferential extraction of output power from a single laser facet.

Stand-off and extractive explosive detection methods for short distances are investigated using mid-infrared laser spectroscopy. A quantum cascade laser (QCL) system for TATP-detection by open path absorption spectroscopy in the gas phase was developed. In laboratory measurements a detection limit of 5 ppm*m was achieved. For explosives with lower vapor pressure an extractive hollow fiber based measurement system was investigated. By thermal desorption gaseous TATP or TNT is introduced into a heated fiber. The small sample volume and a fast gas exchange rate enable fast detection. TNT and TATP detection levels below 100 ng are feasible even in samples with a realistic contaminant background.

We recently reported the first Doppler-limited absolute frequency measurement of CO₂ transitions around 4.4 μm wavelength, by linking a DFB Quantum Cascade Laser (QCL) to an Optical Frequency Comb Synthesizer (OFCS). We further achieved sub-Doppler recording of these transitions, improving of about three orders of magnitude the measurement precision. We are exploring techniques able to significantly reduce the QCL jitter, in order to get metrological-grade QCLs for very demanding experiments in the frequency-domain. The latest experimental results in our group will be reported.

Spectroscopic concentration measurements of oxygen at high pressures are limited by the effect of pressure broadening and line mixing. These effects strongly depend on the gas mixture in which the oxygen concentration has to be determined as the pressure broadening coefficients of different gases vary over a large range. Line broadening coefficients of the oxygen a-band for a large number of different gases are well known in the literature, but up to now there is, to our best knowledge, no experimental data available which describes the line broadening of oxygen in hydrogen. In respect to a possible application for online-monitoring of oxygen in hydrogen electrolysis we have measured the pressure broadening coefficient of the oxygen P9P9 line in hydrogen and compared with the theoretical model. To confirm the result, also measurements of the well known broadening coefficient of oxygen in helium were accomplished. Measurements were obtained using laser absorption spectroscopy with vertical-cavity surface-emitting lasers in a Herriott-Cell with 15 m path length adapted for vacuum pressures.

A novel active region concept for GaSb-based optically pumped mid-infrared vertical external cavity surface emitting lasers (VECSELs, also referred to as optically pumped semiconductor disk lasers - OPSDLs) is presented. The concept is based on Ga_xIn_1-xAs_ySb_1-y type-I quantum wells (QWs) embedded between AlAs_0.08Sb_0.92 barrier layers designed for optical in-well pumping where the pump absorption at pump wavelengths between 1 μm and 2 μm takes place exclusively in the active QWs. This concept provides several advantages such as a high modal gain, the suppression of thermal leakage currents, and an improved thermal conductivity of the active region compared to a conventional GaInAsSb/AlGaAsSb active region design. Using the novel design approach an in-well pumped VECSEL emitting at 2.24 μm has been realized, yielding at a heat sink temperature of 20°C in continuous-wave operation a power slope efficiency of more than 32% and an absorption of the 1.96 μm pump light of more than 50% without pump recycling, These data constitute a significant improvement in device performance compared to previously reported data on in-well pumped GaSb-based VECSELs.

Excitation of the whispering gallery modes (WGM) of a CaF₂ ball resonator is demonstrated at 4.5 micron with a pulsed Quantum Cascade laser. A prism coupling scheme for mid-infrared is described. Future applications of WGM resonators as hyphenated inline chromatography sensors are discussed.

Over the past few years Daylight Solutions has introduced a family of tunable laser products tailored for scientific and OEM spectroscopic applications, ranging from gas sensing of complex matrices to condensed phase detection. The goal of this development is to create laser-based sources and instruments whose performance can rival and surpass that from conventional FTIRs. Specifically, these sources have the ability to obtain and analyze broad-band spectra in the midinfrared region with high spectral acquisition speed, perform with moderate to high spectral resolution and operate at room temperature, all while occupying minimal size and weight. This paper will report recent progress towards this goal. The performance of the novel laser capable of tuning 25% of the wavelength range centered at 8.8 um will be presented. A technology demonstration prototype employing a broadly tunable external cavity quantum cascade laser integrated into a portable, turn-key sensor will be described.

The advent of continuous wave quantum cascade lasers operating at near room temperature has greatly expanded the capability of spectroscopic detection of atmospheric trace gases using infrared absorption at wavelengths from 4 to 12 μm. The high optical power, narrow line width, and high degree of single mode purity result in minimal fractional absorptions of 5x10^-6 Hz^-1/2 detectable in direct absorption with path lengths up to 210 meters. The Allan plot minima correspond to a fractional absorbance of 1x10^-6 or a minimum absorption per unit path length 5x10^-11 cm^-1 in 50s. This allows trace gas mixing ratio detection limits in the low part-per-trillion (1 ppt = 10^-12) range for many trace gases of atmospheric interest. We present ambient measurements of NO₂ with detection precision of 10 ppt Hz^-1/2. The detection precision for the methane isotopologue ¹³CH₄ is 25 ppt Hz^-1/2 which allows direct measurements of ambient ratios of ¹³CH₄/¹²CH₄ with a precision of 0.5‰ in 100 s without pre-concentration. Projections are given for detection limits for other gases including COS, HONO and HCHO as CWRT lasers become available at appropriate wavelengths.

Novel materials, notably quantum-dot (QD) semiconductor structures offer the unique possibility of combining exploitable spectral broadening of both gain and absorption with ultrafast carrier dynamic properties. Thanks to these characteristics QD-based devices have enhanced the properties of ultrashort pulse lasers and opened up new possibilities in ultrafast science and technology. In this paper we will review recent results, which demonstrate that quantum-dot structures can be designed to provide compact and efficient ultrashort pulse laser sources with extremely high and low repetition rates.

This paper reviews the recent progress towards III-nitride intersubband devices based on either quantum wells or quantum dots. We first discuss the specific features of electron confinement in ultrathin GaN/AlN layers Recent achievements on fast electro-optical modulator devices are described. We then discuss a new concept of III-nitride quantum well detectors based on the quantum cascade scheme, which opens prospects for very fast devices. We finally review the progress towards light-emitting devices and saturable absorbers based on GaN/AlN quantum dots.

We report on the realization of a optoacoustic sensor based on mid-infrared quantum-cascade lasers (QCLs) for the detection of nitric oxide (NO) and formaldehyde (CH₂O). A resonant photoacoustic cell equipped with 4 electret microphones was excited in its first longitudinal mode by the modulated laser light. A detection limit for of 300 parts in 10⁹ (ppbv) for NO and 150 ppbv for CH₂O is found, using distributed feedback QCLs operating in pulsed mode at 5.34 μm and 5.6 μm, respectively.

Photonic crystal structures have been used to improve modal properties of broad area semiconductor lasers in recent years. It has been demonstrated that the width of single-mode semiconductor laser can be increased by at least two orders of magnitude using the transverse Bragg reflection of two dimensional periodic nanostructures. In this talk, we will describe a photonic crystal structure to obtain the single mode operation of large-area, edge-emitting semiconductor lasers. Pulsed and CW operation of electrically pumped, single-mode photonic crystal broad area lasers (100μm wide and 550μm long) with single-lobe, diffraction-limited far-fields are experimentally demonstrated at room temperature. A wavelength tuning sensitivity 80 times smaller than a conventional DFB laser is also achieved for the photonic crystal Bragg laser.

Nitric oxide detection with Faraday rotation spectroscopy offers excellent sensitivity and high specificity together with outstanding long term system performance. Development of a transportable, cryogen-free, prototype field instrument based on mode-hop-free external cavity quantum cascade laser targeting the optimum NO Q(3/2) transition at 1875.8 cm^-1 is reported . The system shows a minimum detection limit of 5.4 ppb with a 1sec. lock-in time constant. Continuous, unattended NO monitoring with >1 hour white noise limited averaging times is reported.

We propose a new method for using a MEMS device for measuring the power and spot size of a laser beam. The device consists of a doubly-clamped single crystal silicon micro-beam. A laser beam incident on the microstructure exerts an optical pressure on it and consequently the micro-beam gets strained. Analysis and simulations show that the laser power and spot size can be determined by measuring the strains at two different positions on the microstructure.

We demonstrate optimization of continuous wave (cw) operation of 4.6 μm quantum cascade lasers (QCLs). A 19.7 μm by 5 mm, double channel processed device exhibits 33% cw WPE at 80 K. Room temperature cw WPE as high as 12.5% is obtained from a 10.6 μm by 4.8 mm device, epilayer-down bonded on a diamond submount. With the semi-insulating regrowth in a buried ridge geometry, 15% WPE is obtained with 2.8 W total output power in cw mode at room temperature. This accomplishment is achieved by systematically decreasing the parasitic voltage drop, reducing the waveguide loss and improving the thermal management.

In this paper, we will review some of the recent progress that we have made on developing single pixel detectors and focal plane arrays based on dots-in-a-well (DWELL) heterostructure and Type II strained layer superlattice (SLS). The DWELL detector consists of an active region composed of InAs quantum dots embedded in InGaAs/GaAs quantum wells. By varying the thickness of the InGaAs well, the DWELL heterostructure allows for the manipulation of the operating wavelength and the nature of the transitions (bound-to-bound, bound-to-quasibound and bound-to-continuum) of the detector. Based on these principles, DWELL samples were grown using molecular beam epitaxy and fabricated into 320 x 256 focal plane arrays (FPAs) with Indium bumps using standard lithography at the University of New Mexico. The FPA evaluated was hybridized to an Indigo 9705 readout integrated circuit (ROIC). From this evaluation, we have reported the first two-color, co-located quantum dot based imaging system that can be used to take multicolor images using a single FPA. We have also been investigating the use of miniband transitions in Type II SLS to develop infrared detectors using PIN and nBn based designs.

Type II strained-layer superlattices promise superior performance as detectors in the LWIR and VLWIR wavelength regimes due to their potential for suppressing Auger recombination (and thus dark currents). These predictions for lower Auger rates have been verified experimentally, however the Shockley-Read-Hall recombination times remain shorter than for competing materials. Recent advances in understanding the interaction of defect/donor states with interfaces in strained materials indicates that in these short-period superlattices the behavior of defects and donors can be very different from in the bulk. The current understanding of these issues will be reviewed.

Quantum dot structures with tailored geometries were developed for different high power laser applications by molecular beam epitaxy based self-assembly techniques. 920 nm quantum dot laser material with new record values of 0.08 nm/K in temperature dependent wavelength shift could be obtained, which is a factor of 4 lower than for quantum well lasers. Tapered distributed Bragg reflector laser devices were processed, which exhibit single mode output powers of more than 1 W in cw with an M² value of 2. For display applications based on frequency doubling, 1060 nm quantum dot laser material is developed with and without tunnel injection quantum well active zones. With this new type of laser material an output power of 4.5 W could be obtained on 100 μm broad area lasers. An overview is given on this recent work performed within the frame of the EU project "WWW_BRIGHTER_EU".

We present theoretical and experimental results for a novel photon detector based on a strong opto-electro-mechanical effect. In this new architecture, photo-generated carriers are compresses under a suspended nano-injector to produce significant electrostatic pressure. The pressure results in a reduced gap between the nano-injector and the semiconductor, which in turn increases the tunnel based electron injection dramatically. Our experimental results show very good sensitivity at 1.55 μm at room temperature. We also show that Casimir force has a considerable effect on the device performance, due to the small gap between nano-injector and semiconductor.

In Germany, InAs/GaSb superlattice detector technology for the mid-wavelength infrared spectral range has been intensively developed in recent years. Mid-IR InAs/GaSb superlattice photodiodes achieve a very high quantum efficiency. The world's first high-performance infrared imagers based on InAs/GaSb superlattices were realized offering high spatial and excellent thermal resolution at short integration times. Additionally, the technology for dual-color superlattice detectors featuring simultaneous, pixel-registered detection of two separate spectral regimes in the mid-IR has been developed. Due to the ability to detect signatures of hot carbon dioxide, dual-color superlattice detectors are ideally suited for missile alerting sensors. The capability for small volume production of InAs/GaSb superlattice detectors has been established.

Research into avalanche photodiodes (APDs) is motivated by the need for high sensitivity ultraviolet (UV) detectors in numerous civilian and military applications. By designing photodetectors to utilize low-noise impact ionization based gain, GaN APDs operating in Geiger mode can deliver gains exceeding 1×10⁷. Thus with careful design, it becomes possible to count photons at the single photon level. In this paper we review the current state of the art in III-Nitride visible-blind APDs, and present our latest results regarding linear and Geiger mode III-Nitride based APDs. This includes novel device designs such as separate absorption and multiplication APDs (SAM-APDs). We also discuss control of the material quality and the critical issue of p-type doping - demonstrating a novel delta-doping technique for improved material quality and enhanced electric field confinement. The spectral response and Geiger-mode photon counting performance of these devices are then analyzed under low photon fluxes, with single photon detection capabilities being demonstrated. Other major technical issues associated with the realization of high-quality visible-blind Geiger mode APDs are also discussed in detail and future prospects for improving upon the performance of these devices are outlined.

We report theoretical and experimental investigations of infrared absorption characteristics for PbSe colloidal quantum dots in defect-free photonic crystal (PC) cavities, via Fano resonances. Angle and polarization independent transmission and absorption are feasible for surface normal incident beams with dispersion engineered modal design. Experimental demonstration was done on patterned single crystalline silicon nanomembranes (SiNMs) transferred on glass and on flexible PET substrates, with PbSe QDs backfilled into the air holes of the patterned SiNMs. These findings enable the design of spectrally selective photodetectors at near infrared regime with the desired angle and polarization properties.

Recent advances in the design and fabrication of Type-II InAs/GaSb superlattices allowed the realization of high performance long wavelength infrared focal plane arrays. The introduction of an Mstructure barrier between the n-type contact and the π active region reduced the tunneling component of the dark current. The M-structure design improved the noise performance and the dynamic range of FPAs at low temperatures. At 81K, the NEDT of the focal plane array was 23 mK. The noise of the camera was dominated by the noise component due to the read out integrated circuit. At 8 μm, the median quantum efficiency of the detectors was 71%, mainly limited by the reflections on the backside of the array.

Semiconductor nanostructures, such as quantum wells and quantum dots, are well known, and some have been incorporated in applications. Here we propose a new general approach to make use of polar optical phonons in quantum wells for terahertz (THz) devices. As the first example, we show the coupling of phonon and intersubband transition leading to Fano resonance in photocurrent spectra. We investigate the phenomenon experimentally in specially designed GaAs/AlGaAs quantum well infrared photodetectors. Finally, we discuss the future research and potentials.

The performance and operating temperature of infrared (IR) detectors is largely limited by thermal generation and noise processes in the active region of the device. Particularly, excess background charge carriers enhance Auger recombination and dark currents, and depress the detector figures of merit. Therefore, reducing background carriers in the undoped region of pin diodes is an important issue for developing high-operating temperature IR detectors. In this paper, we discuss how, through low-temperature Hall measurements, we optimized several growth and design parameters to lower residual carrier densities in various mid-IR InAs/GaSb superlattice (SL) designs. Among the growth/processing parameters investigated in the 21 Å InAs/24 Å GaSb SLs, neither growth temperature nor in-situ post-growth annealing significantly affected the overall carrier type and density. All of the mid-IR SL samples investigated were residually p-type. The lowest carrier density (1.8x10¹¹ cm^-2) was achieved in SLs grown at 400 °C and the density was not reduced any further by a post-growth anneal. The growth parameter that most affected the carrier density was interface composition control. With a minor variation in interface shutter sequence, the carrier density dramatically increased from ~2x10¹¹ to 5x10¹² cm^-2, and the corresponding mobility dropped from 6600 to 26 cm²/Vs, indicating a degradation of interface quality. However, the carrier density was further reduced to 1x10¹¹ cm^-2 by increasing the GaSb layer width. More importantly, a dramatic improvement on the photoluminescence intensity was achieved with wider GaSb SLs. The disadvantage is that as GaSb layer width increases from 24 to 48 Å, the photoluminescence peak position shifts from 4.1 to 3.4 μm, for a fixed InAs width of 21 Å, indicating a photodiode based on these wider designs would fall short of fully covering the 3 to 5 μm mid-IR spectral region.

A challenge for type-II InAs/GaSb superlattice (T2SL) photodetectors is to achieve high fill factor, high aspect ratio etching for third generation focal plane arrays (FPAs). Initially, we compare the morphological and electrical results of single element T2SL photodiodes after BCl₃/Ar inductively coupled plasma (ICP) and electron cyclotron resonance (ECR) dry etching. Using a Si₃N₄ hard mask, ICP-etched structures exemplify greater sidewall verticality and smoothness, which are essential toward the realization of high fill factor FPAs. ICP-etched single element devices with SiO₂ passivation that are 9.3μm in cutoff wavelength achieved vertical sidewalls of 7.7μm in depth with a resistance area product at zero bias of greater than 1,000 Ωcm² and maximum differential resistance in excess of 10,000 Ωcm² at 77K. By only modifying the etching technique in the fabrication steps, the ICP-etched photodiodes showed an order of magnitude decrease in their dark current densities in comparison to the ECR-etched devices. Finally, high aspect ratio etching is demonstrated on mutli-element arrays with 3μm-wide trenches that are 11μm deep.

This is an update of previous review by Ulmer in 2005 of UV detectors and UV astrophysics. The developments have been slow to come, both observationally and in terms of instrument development. Since it is redundant to devote significant space to repeating the previous review, we concentrate on updated science drivers based on recent NASA project studies such as the Large UV Optical Space Telescope (LUVO or LST) as well as several new areas of instrument advancement. We discuss recent advances in GaN-based avalanche photodiodes, complementary metal-oxide-semiconductor (CMOS) and low temperature devices (LTDs, operating near 50 mK) called Microwave Kinetic Inductance Detectors (MKIDs).

We present a novel charge self-consistent eight-band k•p envelope function method for the calculation of the electronic structure of type-II broken-gap heterostructures. Standard multiband k•p approaches fail to yield the correct occupation of electronic states in broken-gap heterostructures, because the strong hybridization of conduction band and valence band states is incompatible with the separate occupation of electron and hole states that is common to envelope function approaches. In our method, we occupy all included subbands with electrons according to the Fermi statistics and subsequently subtract a positive background ionic charge that guarantees charge neutrality. With this procedure, we have calculated local charge densities and subband dispersions of periodically n and p doped GaAs layers as well as effective band gaps of intrinsic InAs/GaSb superlattices.

A pulsed metalorganic chemical vapor deposition (MOCVD) technique, specifically designed for high quality AlN/GaN superlattices (SLs) is introduced. Optical quality and precise controllability over layer thicknesses are investigated. Indium is shown to improve interface and surface quality. An AlN/GaN SL designed for intersubband transition at a telecommunication wavelength of ~1.5 μm, is grown, and processed for intersubband (ISB) absorption measurements. Room temperature measurements show intersubband absorption centered at 1.49 μm. Minimal (n-type) silicon doping of the well is shown to be crucial for good ISB absorption characteristics. The potential to extend this technology into the far infrared and even the terahertz (THz) region is also discussed.

The use of nanostructures in semiconductor technology leads to the observation of new phenomena in device physics. Further quantum and non-quantum effects arise from the reduction of device dimension to a nanometric scale. In nanopillars, quantum confinement regime is only revealed when the lateral dimensions are lower than 50 nm. For larger mesoscopic systems, quantum effects are not observable but surface states play a key role and make the properties of nanostructured devices depart from those found in conventional devices. In this work, we present the fabrication of GaN nanostructured metal-semiconductor-metal (MSM) and p-i-n photodiodes (PIN PDs) by e-beam lithography, as well as the investigation of their photoelectrical properties at room temperature. The nanopillar height and diameter are about 520 nm and 200 nm, respectively. MSMs present dark currents densities of 0.4 A/cm² at ±100 V. A strong increase of the optical response with bias is observed, resulting in responsivities higher than 1 A/W. The relationship between this gain mechanism and surface states is discussed. PIN PDs yield peak responsivities (R_peak) of 35 mA/W at -4 V and show an abnormal increase of the response (R_peak>100 A/W) under forward biases.

The Type-II InAs/GaSb superlattice photon detector is an attractive alternative to HgCdTe photodiodes and QWIPS. The use of p⁺ - π - M - N⁺ heterodiode allows for greater flexibility in enhancing the device performance. The utilization of the Empirical Tight Binding method gives the band structure of the InAs/GaSb superlattice and the new M- structure (InAs/GaSb/AlSb/GaSb) superlattice allowing for the band alignment between the binary superlattice and the M- superlattice to be determined and see how it affects the optical performance. Then by modifying the doping level of the M- superlattice an optimal level can be determined to achieve high detectivity, by simultaneously improving both photo-response and reducing dark current for devices with cutoffs greater than 14.5 μm.

A monolithically integrated wire grid polarizer and quantum dot infrared photodetector is reported. The photodetector array is integrated with a single large area aluminum wire grid polarizer that was fabricated using standard photolithographic procedures. The polarizer has period of 4μm and a 50% duty cycle. The device has an extinction ratio of 3dB for normal incidence long wavelength infrared radiation.

We show how metallic waveguides offer the opportunity of implementing interesting functionalities for semiconductor lasers within a simple technological approach. In the THz, we show that the active region thickness of quantum cascade lasers can be reduced by a factor of 2 without effects on the threshold current density and maximum operating temperature of the laser. Pulsed and continuous-wave operation - with a low threshold J_th= 71 A/cm² - are obtained for a 5.86-μm-thick THz QC laser. The emission is peaked at λ≈115 μm and the waveguide resonator is based on a metal-metal geometry. In the mid-infrared, we demonstrate surface-plasmon distributed-feedback quantum cascade lasers with a first-order grating realised by the sole patterning the top metallic contact. The devices have a single mode emission with a side-mode suppression ratio greater than 20dB. The emission wavelength at 78K is centred at λ = 7.3 μm and has tuning rate as a function of the temperature of ≈0.4 nm/K.

The catalyst-assisted growth of semiconductor nanowires heterostructures offers a very flexible way to design and fabricate single photon emitters. The nanowires can be positioned by organizing the catalyst prior to growth. Single quantum dots can be formed in the core of single nanowires which can then be easily isolated and addressed to generate single photons. Diameter and height of the dots can be controlled and their emission wavelength can be tuned at the optical telecommunication wavelengths by the material composition. The final morphology of a wire can be shaped by the radial/axial growth ratio, offering the possibility to form single mode optical waveguides with a tapered end for efficient photon collection.

People working with optics and lasers usually try to avoid dust on their equipment as much as possible. Dust particles scatter light randomly in all directions and this is often detrimental to the performance of optical devices and lasers. In this articles we will see that it is possible to turn this situation upside down and actually make use of multiple light scattering to study interesting physical phenomena. In particular, we will discuss optical Lévy flights and super diffusion, and various interference effects like weak and strong localization of light waves.

We propose a novel concept of projection-type electron beam lithography to pattern nanometer scale periodic features with better throughput than a conventional-type of electron beam lithography system. Here, the nanometer scale periodic patterns are obtained from the phase contrast high resolution transmission electron microscopy images of the crystalline samples which are tenth of nanometer scales. We, thus, named this method as atomic image projection electron beam lithography (AIPEL). To realize this novel concept, we have modified the objective lens of conventional 200kV field emission transmission electron microscopy and also inserted patterning lens between the objective lens and the lithographic stage to vary the patterning magnification continuously from 50 times to 300 times. By using this AIPEL system, we successfully demonstrate nanopatterns with various sizes and shapes using the various high resolution lattice images obtained from single crystalline Si and polycrystalline β-Si₃N₄. We can vary the shapes of nanometer scale patterns by changing mask materials itself or the zone axis of observation in one mask material, and can vary the size of patterns by changing the magnification of patterning. Finally, we will discuss how one can improve the quality of image obtained from mask material.

A non-contact and low-cost nanomaterial based fiber-optic sensor is developed for measuring large values of electric currents. The magnetic field, generated by the electric current, changes the refractive index of a liquid in which nanomaterial particles are suspended. The change of refractive index is converted to a change in the intensity of light transmitted in an evanescent field based fiber optic sensor. The change in the intensity is proportional to the magnitude of the electric current and thus the current can be measured by measuring the resultant change in the intensity of light.

While InGaAs absorption material has been used for various applications up to 1.6μm wavelength, specific designs for low level detection have become of main interest using high responsivity and low dark current detectors. By adding an avalanche multiplication layer to form an avalanche photodiode (APD) using the Separated Absorption and Multiplication (SAM) structure, one can take advantage of the very low noise properties of multiplication process in large bandgap Al(Ga)(In)As material to improve receiver sensitivity by >10dB. Under high power level injection, specific PIN structures have been developed to improve space charge effects as needed for power applications such as microwave analog photonic links. Specific designs to achieve simultaneously broad bandwidth, high responsivity, very high power saturation and high linearity will be discussed.

This paper will review the development of single photon counting sensors at Boeing Spectrolab. Future development over the next five years will be discussed in the context of sensor requirements that have been established and will be established. Greater sensitivity through lower false event rates, higher bandwidths, lower after-pulsing rates, higher operating temperatures, and better uniformity are figures-of-merit that will be discussed in this presentation. We will present performance of large format InP/InGaAs Geiger mode avalanche photodiode arrays operating at 1.06 μm and 1.55 μm.

InGaAs/InP devices suitable as Single-Photon Avalanche Diodes (SPADs) for photon counting and photon timing applications in the near-infrared provide good detection efficiency and low time jitter, together with fairly low darkcount rate at moderately low temperatures. However, their performance is still severely limited by the afterpulsing effect, caused by carriers trapped into deep levels during the avalanche current flow and later released. We present preliminary experimental characterization of recently-developed InGaAs/InP detectors that can promisingly be operated slightly cooled. We investigate the primary dark-count rate, taking into account both thermal generation in the InGaAs absorption layer and trap-assisted tunnelling in the InP multiplication layer. We report on improvements obtainable by selecting the proper operating conditions and electronic circuit solutions. The fundamental role played by the front-end circuits in minimizing the effects of afterpulsing is assessed and demonstrated. We report the performance of a 25 μm-diameter InGaAs/InP SPAD at 1550 nm wavelength, with dark count rate of 400 cps (count per seconds) at 175 K and just 2000 cps at 225 K, with afterpulsing showing off only below T_OFF=10μs. The photon timing resolution is 100 ps (FWHM, Full Width at Half Maximum) at 7 V of excess bias.

We present a compact detection head based on a 32x1 array of Single-Photon Avalanche Diodes (SPAD) and associated electronics for time tagging single photons (photon counting) with 3μs time-resolution, with high photon detection efficiency (45% at 450nm) and sharp photon-timing resolution (55 ps). The array is composed by 32 "smart" pixels, working in photon-counting mode, with fully parallel and synchronous acquisition. The array is driven by an FPGA able to acquire data from the sensor and to upload them to a remote PC via an USB 2.0 link, for real-time continuous acquisition up to 312.5 kframe/s. The module is bus-powered for convenient use with laptops, and provides also direct timing outputs from two pixels for time-resolved measurements (photon timing).

Single photon sources are of extreme interest for future quantum communications networks. Several realizations of such sources where proposed but none of them corresponds to the needs of a quantum network, in terms of emission wavelength, repetition rate or quantum state purity. Using self organized InAs/InP quantum dots, it is possible to tune the emission wavelength up to 1.55 μm. Lifetime measurements confirm the high optical quality of these dots opening the possibility to engineer sources operate above 77K. With this material combination it is also possible to localized the growth of a single quantum dot, that can be to deterministically coupled to a photonic crystal cavity.

A semiconductor ridge microcavity is designed to generate counterpropagating twin photons by parametric fluorescence. This device is suitable as a narrow bandwidth source of twin photons at 1.55 μm working at room temperature. A sensible efficiency improvement due to the presence of the vertical cavity is demonstrated. The degree of frequency correlation can be controlled through the pump field spatial and spectral profiles.

The operation of InP-based single photon avalanche diodes (SPADs) in Geiger mode provides great utility for the detection of single photons at near-infrared wavelengths between 1.0 and 1.6 um. However, SPADs have performance limitations with respect to photon counting rate and the absence of photon number resolution that, at the most fundamental level, can be traced back to the positive feedback inherent in the impact ionization-driven avalanche process. In this paper, we describe the inclusion of negative feedback with best-in-class InP-based single photon avalanche diode (SPAD) structures to form negative feedback avalanche diodes (NFADs) in which many of the present limitations of SPAD operation can be overcome. The use of thin film resistors as monolithic passive negative feedback elements ensures rapid self-quenching with very low parasitic effects and wafer-level integration for creating multi-element NFAD arrays. To our knowledge, this is the first demonstration of this approach with InP-based avalanche diode structures. We present NFAD device properties, including pulse response, quenching dynamics, and photon counting performance parameters such as photon detection efficiency.

We report the design, fabrication, and test of a new InGaAs avalanche photodiode (APD) for short-wavelength infrared (SWIR) sensing applications at 950-1650 nm. The APD is grown by molecular beam epitaxy (MBE) on InP substrates from lattice-matched InGaAs and InAlAs alloys. Avalanche multiplication inside the APD occurs in a series of asymmetric gain stages whose layer ordering acts to enhance the rate of electron-initiated impact-ionization and suppress the rate of hole-initiated ionization when operated at low gain. Measurements have verified much lower excess multiplication noise and much higher avalanche gain than is characteristic of APDs fabricated from the same semiconductor alloys in bulk. At room temperature, multiplication-enhanced APDs (MAPDs) of this design were found to have excess noise characterized by an effective ionization coefficient ratio of k=0.02 to a gain of M=100. The impulse response duration of a 75-μm-diameter APD was measured to be less than 1 ns when operated at a gain of M=50, with a rise time of 225 ps and a fall time of 550 ps. High rate single photon counting at 1064 nm was demonstrated with multiple 10-stage APDs operated below their breakdown voltage, using a commercial 2-GHz transimpedance amplifier (TIA) chip. Single photon detection efficiencies as high as 70% were measured for signal photon rates of 50 MHz.