Single-mode surface-emitting distributed feedback terahertz quantum-cascade lasers operating around 2.9 THz are developed in metal-metal waveguides. A combination of techniques including precise control of phase of reflection at the facets, and use of metal on the sidewalls to eliminate higher-order lateral modes allow robust single-mode operation over a range of approximately 0.35 THz. Single-lobed far-field radiation pattern is obtained using a pi phase-shift in center of the second-order Bragg grating. A grating device operating at 2.93 THz lased up to 149 K in pulsed mode and a temperature tuning of 19.7 GHz was observed from 5 K to 147 K. The same device lased up to 78 K in continuous-wave (cw) mode emitting more than 6 mW of cw power at 5 K. In general, maximum temperature of pulsed operation for grating devices was within a few Kelvin of that of multi-mode Fabry-Perot ridge lasers.

The authors demonstrate the use of a terahertz quantum cascade laser ͑QCL͒ for real-time imaging in transmission mode at a standoff distance of 25 meters. Lasing frequency was selected for optimum transmission within an atmospheric window at ϳ4.9 THz. Coarse frequency selection was made by design of the QCL gain medium. Finer selection ͑to within 0.1 THz͒ was made by judicious choice of laser cavity length to adjust facet losses and therefore lasing threshold bias, in order to overlap the peak frequency of the Stark-shifted gain spectrum with the atmospheric window. Images are shown using an uncooled 320ϫ 240 microbolometer camera.

This paper describes a real-time terahertz (THz) imaging system, using the combination of a palm-size THz camera with a compact quantum cascade laser (QCL). The THz camera contains a 320x240 microbolometer focal plane array which has nearly flat spectral response over a frequency range of ca. 1.5 to 100 THz, and operates at 30 Hz frame rate.The QCL is installed in compact cryogen-free cooler. A variety of QCLs are prepared which can cover frequency range from ca. 1.5 to 5 THz. THz images of biochemical samples will be presented, using the combined imaging system. Performance of the imaging system, such as signal-to-noise ratio of transmission-type THz microscope, is predicted.

This paper describes a real-time transmission-type Terahertz (THz) microscope, with palm-size THz camera and compact quantum cascade laser (QCL). The THz camera contains 320x240 microbolometer focal plane array which operates at 30 Hz frame rate and has lock-in imaging function as well as integration functions such as frame integration and spatial filter. These functions are found very powerful in improving signal-to-noise ratio. QCL is installed in compact Stirling cycle cooler. A variety of QCLs covers frequency range from 1.5 to 5 THz and provides time-average power of 0.5~2 mW. The illumination area for sample is changed by adjusting one lens in the illumination optics. Performances of the THz microscope, such as signal-to-noise ratio and so on, were measured and are found consistent with the calculations. THz images taken with the THz microscope are finally presented.