We demonstrate the use of a tunable external cavity quantum cascade laser system for measurement of broad absorption
features in the mid-infrared spectral region. The thermoelectrically cooled external cavity laser was tuned over a 65 cm-1range centered at 8.7 microns using stepper motor control. Pulsing the laser at a high duty cycle provided 2-4.5 mW
average output power over the tuning range, and enabled phase-sensitive detection of amplitude-modulated signals. We
used the laser system to measure the absorption spectra of Freon-125 using a Herriott cell. In addition, the absorption
spectrum of water in the laboratory air was measured. The measurements showed excellent agreement with reference
spectra, in both wavelength and amplitude. The measured scan resolution of 0.14 cm-1 is suitable for measurement of the
absorption features of complex molecules as well as simple molecules with atmospherically broadened lines. We
discuss the limits to the scan resolution due to effects of spectral chirp and mode-hops during pulsed operation.
In this paper, we compare the open-path FTIR to the differential optical absorption spectroscopy (DOAS) approach in
the Mid-Infrared region for the continuous retrieval of trace gases. After consideration of FTIR capabilities and results,
we explore the potential for a compact quantum cascade laser (QCL) based DOAS system to continuously monitor
ambient concentration levels of Ozone and Ammonia. Based on absorption spectra obtained from the HITRAN2000
database and processed within the GENSPECT environment, we find the optimal window for simultaneous retrieval of
Ozone and Ammonia to be between 1045 (cm-1) and 1047 (cm-1). We further show that for a QCL-based transmitter
with 0.1cm-1 spectral resolution and 10mW power and using nitrogen cooled MCT detector (D* ~ 1010W-1m Hz1/2) in the receiver, it is possible to detect total path ambient concentrations of ozone and ammonia to within 10% accuracy
using suitable targets of opportunity such as an building. Details of the optimal frequency sweeping methodology,
optical path length, shot averaging, and SNR considerations are considered for comparison of the QCL-based standoff
DOAS performance to more conventional open-path FTIR sensors.
In conventional lidar, calibration of the backscatter signal is performed by comparing the returns to aerosol clean layers
where the backscatter signal can be calculated directly. Unfortunately, in the IR, this approach is not practical since the
aerosol contribution to the signal is always a significant fraction even for high altitudes. Two approaches which have
been suggested include the calibration from high altitude cirrus clouds and the calibration from low altitude
stratocumulous (water phase) clouds. In this paper, we perform an inter-comparison of these methods over long time
periods both as a means to assess the accuracy of the calibration methods as well as determining the stability of the lidar
calibration coefficient. To improve upon the standard calibration using the cirrus cloud approach, a forward iterative
integration scheme is employed from below cloud base to determine the backscatter at 532nm. It is found that the cirrus
cloud approach in reasonable agreement with the low altitude water cloud method. Furthermore, a small 15%
discrepancy between the methods is explained based on reasonable multiple scattering corrections which must be
introduced into the low altitude water cloud technique.
A planar waveguide design is presented that integrates As2S3 chalcogenide glass with Ti-diffused lithium-niobate (LN)
waveguides to increase functionality. As2S3 is a higher index material that is introduced to create small bend radii
structures that are crucial for making high density optical circuits. As2S3 waveguide patterns are aligned on top of the
straight LN waveguide. Power launched into LN is coupled to the higher index As2S3 cladding structure and propagates
through a tight bend before coupling back to the LN waveguide. Experimental results are given for s-bends of 2.48cm
minimum radius of curvature.
We demonstrate doping-tunable mid-infrared extraordinary transmission through periodic sub-wavelength openings in
thin metal films. This effect, known as extraordinary optical transmission, is thought to result from the excitation of
Surface Plasmon Polaritons at the metal/dielectric interface. The metal aperture arrays studied were fabricated upon
GaAs substrates. Because the dielectric constant of a semiconductor changes with carrier concentration, identical
metallic grating show different spectral characteristics as a function of GaAs epilayer doping. Thus, the resonant
transmission peaks of our grating structures can be shifted by varying the doping of the n-GaAs epilayer upon which
they are fabricated. We demonstrate peak shifts of 37 cm-1, or approximately 0.33 μm, as we move from undoped GaAs
layer to highly doped n-GaAs layers. Additionally, we study the effect of doping layer thickness on the resonant
transmission peak position, which allows for an estimate of the surface plasmon strength as a function of distance from
the metal/dielectric interface. Furthermore, we present calculated results for our samples and compare them to our
experimental results and propose an explanation for the slight discrepancy between theoretical and experimental results.
The devices presented could eventually lead to voltage-tunable mid-IR mirrors, filters, or modulators.
Split grating-gate field effect transistors (FETs) detectors made from high mobility quantum well two-dimensional
electron gas material have been shown to exhibit greatly improved tunable resonant photoresponse compared to single
grating-gate detectors due to the formation of a 'diode-like' element by the split-gate structure. These detectors are
relatively large for FETs (1mm x 1mm area or larger) to match typical focused THz beam spot sizes. In the case where
the focused THz spot size is smaller than the detector area, we have found evidence, through positional scanning of the
detector element, that only a small portion of the detector is active. To further investigate this situation, detectors with
the same channel width (1mm), but various channel lengths, were fabricated and tested. The results indicate that indeed,
only a small portion of the split grating gated FET is active. This finding opens up the possibility for further
enhancement of detector sensitivity by increasing the active area.