External cavity quantum-cascade lasers (QCLs) emitting in the long-wave infrared (LWIR) around 10 μm and in the mid-wave infrared (MWIR) around 5.2 μm have been realized. The coupling facet of the QC chips was anti-reflection coated and the optical feedback was provided by a diffraction grating arranged in a Littrow configuration. The active regions of the gain elements were based on bound-to-continuum designs having broad gain curves (Approx = to 300 cm^-1 full width at half maximum). The LWIR laser was operated in pulsed mode on a thermoelectric (TE) cooler. It could be tuned over a frequency range of 150 cm^-1 from 9.1 to 10.55 μm with a peak power ⩾ 30 mW. The MWIR laser was operated in continuous-wave (CW) mode on a TE cooler. It could be tuned over more than 170 cm^-1 from 4.95 to 5.4 μm and was single-mode over more than 140 cm^-1. Its output power was in excess of 5 mW over 130 cm^-1. This broad tunability (10 times more than that of distributed-feedback QCLs) enables new applications of QCLs in high-resolution infrared spectroscopy.

In this paper we present single mode quantum cascade lasers (QCLs) based on the GaAs and the InP material systems. We show results for first- and second-order distributed feedback (DFB) QC lasers with surface gratings. The InP based lasers are grown by metalorganic vapor phase epitaxy (MOVPE) and show single mode continuous wave emission up to 200 K. In pulsed operation we achieved single mode surface emission peak output powers exceeding 1 Watt at room temperature. The presented GaAs/AlGaAs laser features an air/AlGaAs waveguide, combined with a second-order distributed feedback grating. That laser shows 3 Watts of single mode output power via the surface at 78 K.

Terahertz imaging is finding a wide range of interest in biology, communications, environment, medicine, security, and space applications. Here results are presented on heterojunction based terahertz detectors covering the range from 1-60 THz. One set of detectors is based on free carrier absorption, followed by internal photoemission aver a workfunction at the interface. These detectors can be produced using any III/V materials and the threshold frequency can be tailored by adjusting the material composition. Examples of GaAs/AlGaAs based detectors with thresholds ranging from 2.3 to 4.2 THz and GaN/AlGaN detectors with threshold of 1 THz are presented. Also briefly mentioned are the quantum dot detectors for the terahertz range.

Many applications are expected in the terahertz spectral region and terahertz technology is viewed as one of the most important ones in the coming decade. We report on the design and simulated performance of quantum-well photodetectors for the terahertz (1 - 10 THz) or the very-far-infrared region. We also report on our experimental demonstration of GaAs/AlGaAs photodetectors with background limited infrared performance (BLIP). The device dark current characteristics were optimized by employing thick barriers to reduce inter-well tunneling. BLIP operations were observed for all samples (three in total) designed for different wavelengths. BLIP temperatures of 17, 13, and 12 K were achieved for peak detection frequencies at 9.7, 5.4, and 3.2 THz, respectively. Furthermore, we discuss areas of improvement to make these detectors a viable technology.

We report the realization of microdisk and microring quantum-cascade lasers (QCLs) emitting in the terahertz (THz) region between 3.0 THz and 3.8 THz. The GaAs/Al_0.15Ga_0.85As heterostructure is based on longitudinal-optical phonon scattering for depopulation of the lower radiative state. A double metal waveguide is used to confine the whispering gallery modes in the gain medium. The threshold current density is 900 A/cm² at 5 K. Lasing takes place in pulsed-mode operation up to a heat-sink temperature of 140 K. Finite-Difference Time-Domaine (FDTD) simulations were performed in a strong field limit to obtain the field distribution within a microdisk THz QCL resonator.

Progress is described in experiments to generate coherent terahertz acoustic phonons in silicon doping superlattices by the resonant absorption of nanosecond-pulsed far-infrared laser radiation. Future experiments are proposed that would use the superlattice as a transducer in a terahertz cryogenic acoustic reflection microscope with sub-nanometer resolution.

Quantum cascade lasers coupled directly to unclad silver halide fibers were used to assemble mid-infrared fiber-optics evanescent-wave sensors suitable to measure the chemical composition of simple liquid droplets. Quantum cascade lasers can be designed to emit across a wide range of mid-infrared wavelengths by tailoring the quantum-well structure, and the wavelength can be fine tuned by a thermoelectric cooler. Here, laser wavelengths were chosen which offer the largest absorption contrast between two constituents of a droplet. The laser was coupled to an unclad silver halide fiber, which penetrates through the droplet resting on a hydrophobic surface. For the same liquid composition and droplet size, the transmitted intensity is weaker for a droplet on a 1H,1H,2H,2H-perfluoro-octyltrichlorosilane coated glass slide than for one on a hexadecanethiol (HDT) coated Au-covered glass slide because of the high reflectivity of the HDT/Au surface at mid-infrared wavelengths. The absorption coefficients of water, glycerol, α-tocophenol acetate, and squalane were measured by varying the immersion length of the fiber; i.e. the droplet size. A pseudo-Beer-Lambert law fits well with the experimental data. We tested both aqueous liquid mixtures (acetone/water and ethanol/water) and oil-base solutions (n-dodecane/squalane and α-tocophenol acetate/squalane); α-tocophenol acetate and squalane are common ingredients of cosmetics, either as active ingredients or for chemical stabilization. Using a 300μm diameter silver halide fiber with a 25mm immersion length, the detection limits are 1 vol.% for α-tocophenol in squalane and 2 vol.% for acetone in water for laser wavenumbers of 1208 cm^-1 and 1363 cm^-1, respectively. This work was previously been reported in J. Z. Chen et al. Optics Express 13, 5953 (2005).

Quantum cascade lasers (QCLs) are becoming well known as convenient and stable semiconductor laser sources operating in the mid- to long-wave infrared, and are able to be fabricated to operate virtually anywhere in the 3.5 to 25 micron region. This makes them an ideal choice for infrared chemical sensing, a topic of great interest at present, spanning at least three critical areas: national security, environmental monitoring and protection, and the early diagnosis of disease through breath analysis. There are many different laser-based spectroscopic chemical sensor architectures in use today, from simple direct detection through to more complex and highly sensitive systems. Many current sensor needs can be met by combining QCLs and appropriate sensor architectures, those needs ranging from UAV-mounted surveillance systems, through to larger ultra-sensitive systems for airport security. In this paper we provide an overview of various laser-based spectroscopic sensing techniques, pointing out advantages and disadvantages of each. As part of this process, we include our own results and observations for techniques under development at PNNL. We also present the latest performance of our ultra-quiet QCL control electronics now being commercialized, and explore how using optimized supporting electronics enables increased sensor performance and decreased sensor footprint for given applications.

Trace gas sensing and analysis by Tunable Diode Laser Absorption Spectroscopy (TDLAS) has become a robust and reliable technology accepted for industrial process monitoring and control, quality assurance, environmental sensing, plant safety, and infrastructure security. Sensors incorporating well-packaged wavelength-stabilized near-infrared (1.2 to 2.0 μm) laser sources sense over a dozen toxic or industrially-important gases. A large emerging application for TDLAS is standoff sensing of gas leaks, e.g. from natural gas pipelines. The Remote Methane Leak Detector (RMLD), a handheld standoff TDLAS leak survey tool that we developed, is replacing traditional leak detection tools that must be physically immersed within a leak to detect it. Employing a 10 mW 1.6 micron DFB laser, the RMLD illuminates a non-cooperative topographic surface, up to 30 m distant, and analyzes returned scattered light to deduce the presence of excess methane. The eye-safe, battery-powered, 6-pound handheld RMLD enhances walking pipeline survey rates by more than 30%. When combined with a spinning or rastering mirror, the RMLD serves as a platform for mobile leak mapping systems. Also, to enable high-altitude surveying and provide aerial disaster response, we are extending the standoff range to 3000 m by adding an EDFA to the laser transmitter.

Mid infrared Quantum Cascade (QCL) and Interband Cascade Lasers (ICL) coupled with cavity-enhanced techniques, have proven to be sensitive optical diagnostic tools for both atmospheric sensing as well as breath analysis. In this work, a TE-cooled, pulsed QCL and a cw ICL are coupled to high finesse cavities, for trace gas measurements of nitric oxide, carbon dioxide, carbon monoxide and ethane. QCL's operating at 5.26 μm and 4.6 μm were used to record ICOS spectra for NO, CO₂, and CO. ICOS spectra of C₂H₆ were recorded at 3.35 μm using an ICL. Ringdown decay times on the order to 2-3 μs are routinely obtained for a 50 cm cavity resulting in effective pathlengths on the order of 1000 meters. The sample cell is compact with a volume of only 60ml. Details of the QCL and ICL cavity enhanced spectrometers are presented along with the detection results for trace gas species. Here we report a detection limit of 0.7 ppbv in 4 s for NO in simulated breath samples as well as human breath samples. A preliminary detection limit of 250 pptv in 4 s for CO is obtained and 35 ppb in 0.4 s for C₂H₆.

We present a new approach to remote infrared temperature measurements over mid to long standoff ranges in varying atmospheric conditions. The sensor is intended for remote triage applications where direct access to victims may be hazardous or otherwise impossible. Our system employs a small reflector telescope followed by a series of interference filters which form spatially identical but spectrally separated images on two miniature uncooled microbolometer focal plane arrays. On a pixel by pixel basis, we algebraically combine and normalize the two images. By carefully selecting the spectral passbands of the two images, the mathematical process yields a result that is substantially free of errors caused by humidity, rain, light fog, and atmospheric carbon dioxide. The package measures 6 x 6 x 18-inches and weighs 6 lbs. We have also added an on-axis miniature visible imager to the system and present the user with a fused visible/infrared image with user-defined transparency levels. The visible/infrared combination provides good spatial resolution at large distances and ease of pointing along with the accurate temperature measurement across the field of view.

We investigated and demonstrated bio-medical imaging using a THz quantum cascade laser. With the THz quantum cascade laser (QCL) at 3.8 THz, we obtained large dynamic range and high spatial resolution in the transmission imaging technique. The various tissues images, such as lung, liver, and brain sections from the laboratory mouse were obtained and studied. The most important factor for this imaging scheme is to obtain high contrast with different absorption characteristics in tissues. We explored distinct images from the fat, muscles and tendon from the freshly cut tissues and investigated absorption coefficient and compared with FTIR measurement. We also demonstrated the image of distinct region of tumors progressed and normal tissues using this technique. The comparison of frequency dependent medical imaging with utilizing different wavelength of QCLs has been addressed.

Current optical technologies utilize changes in optical properties of tissue to distinguish diseased from normal tissue. This poses an important challenge to enhance this subtle intrinsic contrast with the use of novel nanoparticle based contrast agents. Gold nanoshells are a novel type of spherical concentric nanoparticle that possesses high optical efficiencies well into the near infrared. Gold nanoshells are typically made of a dielectric silica core and a thin metallic gold outer layer and a wide range of sizes are easily fabricated using current chemistries. Gold nanoshells can scatter and/or absorb light with optical cross-sections often several times larger than the geometric cross-section. To elucidate the effectiveness of gold nanoshells as a contrast agent for reflectance, it is important to understand how different optical properties of nanoshells affect reflectance, and ultimately provide insight into how reflectance is affected by gold nanoshells embedded in biological tissue. A fiber-probe based spectrometer was used to measure diffuse reflectance of gold nanoshells suspensions from 500nm to 900nm. We further characterize diffuse reflectance of gold nanoshell suspensions using Monte Carlo based computational tools. Our results show that gold nanoshells are capable of producing large changes in diffuse reflectance, and computer modeling results agreed well with the experimental observations. From the study, we also show that it may be feasible to use Monte Carlo based modeling to simulate biological medium embedded with gold nanoshells.

The modeling of through-wall sensing using ultra-wideband (UWB) signals is considered. The combined method of ray tracing and diffraction (CMRTD) is employed to analyze the interaction between the UWB signal and the target. The result is obtained in frequency domain, and then transformed into time domain by use of inverse Fourier transform (IFT). Scattering from a two-dimensional (2D) perfectly conducting circular cylinder is calculated and the result is shown in agreement with that obtained from the eigenfunction expansion method. Furthermore, the attenuating effects of walls are considered based on the geometrical optics. Numerical results of scattering from a 2D perfectly conducting circular cylinder behind a homogeneous, single-layered wall are given in both graphical and tabular formats.