Thermoelectric-cooled terahertz quantum cascade lasers.

We demonstrate the first lasing emission of a thermo-electrically cooled terahertz quantum cascade laser (THz QCL). A high temperature three-well THz QCL emitting at 3.8 THz is mounted to a novel five-stage thermoelectric cooler reaching a temperature difference of ΔT = 124 K. The temperature and time-dependent laser performance is investigated and shows a peak pulse power of 4.4 mW and a peak average output power of 100 µW for steady-state operation.

Mid-infrared upconversion based hyperspectral imaging.

Mid-infrared hyperspectral imaging has in the past decade emerged as a promising tool for medical diagnostics. In this work, nonlinear frequency upconversion based hyperspectral imaging in the 6 to 8 µm spectral range is presented for the first time, using both broadband globar and narrowband quantum cascade laser illumination. AgGaS2 is used as the nonlinear medium for sum frequency generation using a 1064 nm mixing laser. Angular scanning of the nonlinear crystal provides broad spectral coverage at every spatial position in the image. This study demonstrates the retrieval of series of monochromatic images acquired by a silicon based CCD camera, using both broadband and narrowband illumination and a comparison is made between the two illumination sources for hyperspectral imaging.

External-cavity quantum cascade laser using intra-cavity out-coupling.

We describe an external-cavity quantum cascade laser with an intra-cavity out-coupling optical system, tunable from 8.4 µm to 10.8 µm. The new optical configuration allows higher output power compared to a conventional Littrow-type external cavity approach, while keeping the broad tunability of wavelength. We achieved 1 W output power at the maximum in pulse-mode operation, which is more than twice the optical power using the Littrow design with the same gain chip.

Broadband multilayer anti-reflection coating for mid-infrared range from 7 µm to 12 µm.

We describe a design concept for broadband anti-reflection coatings for the mid-infrared that allows thinner film thickness and characterize its properties in the mid-infrared range from 7 µm to 12 µm. Two demonstrator coatings comprising zinc sulfide, yttrium fluoride, and germanium, all of which are deposited by electron beam evaporation on indium phosphide substrates, are optimized for the 7-12 µm and the 10-12 µm ranges. They exhibit less than 1.0% reflection over the target wavelength ranges with notably thin coating thickness.

Alignment-stabilized interference filter-tuned external-cavity quantum cascade laser.

A passively alignment-stabilized external cavity quantum cascade laser (EC-QCL) employing a "cat's eye"-type retroreflector and an ultra-narrowband transmissive interference filter for wavelength selection is demonstrated and experimentally investigated. Compared with conventional grating-tuned ECQCLs, the setup is nearly two orders of magnitude more stable against misalignment of the components, and spectral fluctuation is reduced by one order of magnitude, allowing for a simultaneously lightweight and fail-safe construction, suitable for applications outdoors and in space. It also allows for a substantially greater level of miniaturization and cost reduction. These advantages fit in well with the general properties of modern QCLs in the promise to deliver useful and affordable mid-infrared-light sources for a variety of spectroscopic and imaging applications.

Mid-infrared optical properties of thin films of aluminum oxide, titanium dioxide, silicon dioxide, aluminum nitride, and silicon nitride.

The complex refractive index components, n and k, have been studied for thin films of several common dielectric materials with a low to medium refractive index as functions of wavelength and stoichiometry for mid-infrared (MIR) wavelengths within the range 1.54-14.29 µm (700-6500 cm(-1)). The materials silicon oxide, silicon nitride, aluminum oxide, aluminum nitride, and titanium oxide are prepared using room temperature reactive sputter deposition and are characterized using MIR variable angle spectroscopic ellipsometry. The investigation shows how sensitive the refractive index functions are to the O2 and N2 flow rates, and for which growth conditions the materials deposit homogeneously. It also allows conclusions to be drawn on the degree of amorphousness and roughness. To facilitate comparison of the materials deposited in this work with others, the index of refraction was also determined and provided for the near-IR and visible ranges of the spectrum. The results presented here should serve as a useful information base for designing optical coatings for the MIR part of the electromagnetic spectrum. The results are parameterized to allow them to be easily used for coating design.

Second harmonic generation in gallium phosphide photonic crystal nanocavities with ultralow continuous wave pump power.

We demonstrate second harmonic generation in photonic crystal nanocavities fabricated in the semiconductor gallium phosphide. We observe second harmonic radiation at 750 nm with input powers of only nanowatts coupled to the cavity and conversion effciency P(out)/P(2)(in,coupled)=430%/W. The large electronic band gap of GaP minimizes absorption loss, allowing effcient conversion. Our results are promising for integrated, low-power light sources and on-chip reduction of input power in other nonlinear processes.