We report on our development of both detectors and lasers in the terahertz (THz) region. For detection, we
focus on the approach based on the extension of the celebrated quantum well infrared photodetectors
(QWIPs); whereas the quantum cascade lasers (QCLs) provide the source. We show our preliminary
demonstration of free space communication using our detectors and lasers. An all photonic THz
communication link operating at 3.8 THz using a QCL and quantum well photodetector has been
demonstrated. The link consists of a quantum cascade laser transmitter and a quantum well photodetector
receiver. The link was used to transmit audio through two meters of room air. Carrier strength at the
photodetector was 100 times above the noise level measured. THz free space communication may be of
interest in satellite based systems.
The development of quantum well infrared photodetector (QWIP) technologies for thermal imaging is well known. The high frequency and high speed capability is less known. Here we report on our recent advances of reaching over 100 GHz in heterodyne detection both at cryogenic and room temperatures. These advances may lead to new applications. One of such examples is free space optical communication.
Quantum well infrared photo-detectors (QWIP) have found numerous application
as sensitive fast photo-detectors. Applications for fast detectors include laser diagnostics, telecommunications and Compton scattering measurements. The high speed potential of QWIPs is enabled by a short carrier lifetime in the order of 5 pS. This short lifetime permit's design of a 40 GHz bandwidth detector. In this paper we report on the development of a QWIP with an integrated electrical co-planar waveguide. The QWIP is a 100 well structure that is almost completely absorbing over the wavelength range of 9 to 11 microns. Free space radiation at 105 GHz has been observed from these QWIPs, but devices made to exploit this speed have not yet been developed. It is necessary to integrate an electrical co-planar waveguide with the QWIP mesa. In this work we report on a QWIP fabricated with a gold air bridge to connect the top of the mesa to the center line of a gold co-planar waveguide. The co-planar waveguide is tapered to
allow direct connection to 2.4 mm and smaller electrical cable. Initial tests indicate that the device has a 40 GHz or greater bandwidth.
Excimer laser ablation is an excellent means of depositing high temperature superconducting (HTS) thin films because of high quality electrical properties of the resulting films and also because of the relative simplicity of the deposition system. The principal disadvantage of laser ablation is the contamination of the deposited film by particles, leading to a rough film surface. The influence of substrate temperature on surface morphology in YBa2Cu3O7 thin films deposited by excimer laser on r-plane sapphire with a CeO2 buffer layer has been investigated. The substrate temperature was measured using gold-nickel thermocouples on the substrate surface during deposition. The optimal deposition temperatures were found to be lower in comparison with films deposited on LaAlO3 because damage occurs to the buffer layer. Special attention was paid to the occurrence of droplets and outgrowths in the films. The ablation process produces droplets that range from 0.1 micrometers to a maximum of 1 micrometers in size. In the course of film growth these droplets undergo metamorphosis and form the basis of outgrowths.
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