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We report standoff detection of explosives using quantum cascade laser (QCL) and photoacoustic technique. In our experiment, a QCL with emission wavelength near 7.35 μm was used and operated at pulsed mode. The output light was focused on Trinitrotoluene (TNT) sample in its powder form. Photoacoustic signals were generated and detected by an ultra-sensitive low-noise microphone with one inch diameter. A detection distance up to 8 inches was obtained using the microphone alone. With the increasing detection distance the measured photoacoustic signal not only decayed in amplitude but also delayed in phase, which clearly verified the source location. To further increase the detection distance, a parabolic sound reflector was used for effective sound collection. With the help of the sound reflector, standoff photoacoustic detection of TNT with distance of 8 feet was demonstrated.

We temporally resolve ultrafast modulation of quantum cascade lasers (QCLs) using a near-infrared pump mid-infrared probe technique. We compare interband and intersubband transition mechanism assisted modulation of QCLs by using different pump wavelengths.

Neuronal optical excitation can provide non-contacting tools to explore brain circuitry and a durable stimulation interface for cardiac pacing and visual as well as auditory sensory neuronal stimulation. To obtain accurate absorption spectra, we scan the transmission of neurons in cell culture medium, and normalize it by subtracting out the absorption spectrum of the medium alone. The resulting spectra show that the main neuronal absorption peaks are in the 3000- 6000nm band, although there is a smaller peak near 1450nm. By coupling the output of a 3μm interband cascade laser (ICL) into a mid-IR fluorozirconate fiber, we can effectively deliver more than 1J/cm2 photon intensity to the excitation site for neuronal stimulation.

We report the operation of quantum cascade lasers (QCLs) as a mid-infrared photodetector as well as a tunable resonant amplifying detector. When biased at low voltage or without bias, QCL works as a photoconductive or photovoltaic detector. When biased near threshold, QCL operates as a tunable resonant amplifying detector. The detected signal reaches its maximum when the QCL bias is close to its threshold. The maximum signal power is over 28 dB higher compared with the case without bias. These interesting mid-IR detection and amplification characteristics of QCL gain material can help with mid-IR optical filtering and photonics integration.

We report in this work the near-Infrared (IR) induced negative photoconductance (NPC) on mid-IR Quantum Cascade Lasers (QCLs) and its relationship with the quenching of QCL. The NPC behaviour depends on the coupled near-IR intensity, wavelength and the QCL bias voltage amplitude. If the QCL is under pulse operations, it depends on the pulse width as well. From our interpretation, the NPC is related to electron-hole pair generation and hole-trapping, which produces an internal field counteracting to the external field. The created band-bending effect will reduce the current, increase the resistance, and reduce the mid-IR light output, which has been reported earlier as the "optical quenching effect." We observed that only a near-IR laser with good quenching ability can modulate the mid-IR laser. Since the modulation speed exceeds more than 100 MHz, this excludes the possibility of a thermal origin of these results. The near-IR induced optical quenching effect has the potential to provide all optical modulation of mid-IR QCLs.

The temperature effect on the characteristics of strain-balanced mid-infrared quantum cascade lasers is theoretically investigated and compared to experimental results. Our theoretical model simulates the temperature dependence of modal gain, current threshold density, and optical output power of a QCL at pulsed, continuous wave (CW), and quasi-CW operation conditions. Simulation results show that QCLs operated at quasi-CW driving condition expresses much lower laser core temperature than CW mode while generates comparable or even higher average power than CW mode. The model can be used to predict the optimum driving condition in order to generate maximum average output power.

In this paper, quasi-continuous-wave (quasi-CW) operation of quantum cascade lasers (QCLs) is studied. A group of strain-balanced InGaAs/InAlAs QCLs emitting around λ~4.8μm were used in the experiment. QCLs were tested at different driving conditions (i.e. pulsed mode, CW mode and quasi-CW mode) at various temperatures. Experimental measurements show that thermal effect plays an important role in deteriorating QCL performance. This is especially true at high temperatures. At low temperature (~100K), L-I curves with different pulse widths exhibit no significant difference. While at a higher temperature (~200K), we observed that the longer the pulse width, the lower the roll-over power and the worse the laser performance. At 100K, the roll-over power increases as duty cycle increases until it reaches CW operation, indicating that the thermal generation difference for quasi-CW mode and CW mode is negligible. At 200K, however, a maximum roll-over power is about 50mW at duty cycle of 65% corresponding to a CW roll-over power of 40mW. It reveals that at high temperature, quasi-CW operation generates less heat and more average output power. Therefore, quasi-CW operation is obviously a favorable way to achieve high performance operations at high temperature.

We report in this paper the study of achieving low current threshold quantum cascade lasers (QCLs) by using highreflection (HR) coating to short-cavity QCLs. Experimental results show that the QCL current threshold (Ith) and injecting electrical power are significantly decreased with HR coatings applied to one side or both sides of a QCL. The QCL structures used in the experiment are strain balanced InGaAs/InAlAs supperlattices based on InP substrate with the emission wavelength at λ~4.8μm. QCL devices with 28 periods and 60 periods are studied. With HR coatings on both laser facets, a 28 periods QCL device cleaved with 1mm long cavity expresses a low current threshold of 70mA at cryogenic temperature and a 0.5mm long cavity device shows a current threshold of 65mA at 50K. A 0.5mm long 60 periods QCL device with both sides HR coated is found to have a current threshold as low as 30mA at 20K and still below 50mA at liquid Nitrogen cooled temperature.

Leakage current in quantum-cascade-lasers with oxide-blocked-ridge waveguide and buried-heterostructure (with/without n-InP top-cover) are compared using electrical derivative measurements. Oxide-blocking structure provides the least leakage current although the top-covered BH QCLs show the toughest durability.

Testing QCL devices usually requires cryogenic systems, mid-IR windows, optics and detectors. We report here the demonstration of an electrical derivative method that can quickly measure QCL lasing thresholds without using mid-IR optics or detectors.

Standoff detections of explosives using quantum cascade lasers (QCLs) and the photoacoustic (PA) technique were studied. In our experiment, a mid-infrared QCL with emission wavelength near 7.35 μm was used as a laser source. Direct standoff PA detection of trinitrotoluene (TNT) was achieved using an ultrasensitive microphone. The QCL output light was focused on explosive samples in powder form. PA signals were generated and detected directly by an ultrasensitive low-noise microphone with 1 in. diameter. A detection distance up to 8 in. was obtained using the microphone alone. With increasing detection distance, the measured PA signal not only decayed in amplitude but also presented phase delays, which clearly verified the source location. To further increase the detection distance, a parabolic sound reflector was used for effective sound collection. With the help of the sound reflector, standoff PA detection of TNT with distance of 8 ft was demonstrated.

Standoff chemical detection with a distance of more than 41 feet using photoacoustic effect and quantum cascade laser (QCL) operated at relatively low power, less than 40 mW, is demonstrated for the first time. The option of using QCL provides the advantages of easy tuning and modulation besides the benefit of compact size, light weight and low power consumption. The standoff detection signal can be calibrated as a function of different parameters such as laser pulse energy, gas vapor concentration and detection distance. The results yield good agreements with theoretical model. Techniques to obtain even longer detection distance and achieve outdoor operations are in the process of implementation and their projection is discussed.

The electrical derivative characteristics of quantum cascade lasers (QCLs) are investigated to test the QCL threshold, leakage current, and possibly explore carrier transport. QCL thresholds can be identified by searching for the slope peak of the first derivative of the I-V curves and can be further confirmed with its alignment to the peak of the second derivative of the I-V curves. Leakage current in QCLs with oxide-blocked ridge waveguides and buried heterostructure (BH) waveguides are studied and compared. The oxide-blocking structures provide the lowest leakage current although the capped-mesa-BH (CMBH) QCLs provide the toughest durability under highly stressful operations. The leakage current of CMBH QCLs are also compared at different temperatures.

Heat dissipation issues are critical for quantum-cascade-laser (QCL) high-power operations. Simulation shows multi-emitter array can better dissipate heat than broad area laser does. QCL arrays with 5 and 16 elements are fabricated and results are presented.