Group III-V wide band gap materials are widely used in developing solar blind, radiation-hard, high speed optoelectronic devices. A device detecting both ultraviolet ͑UV͒ and infrared ͑IR͒ simultaneously will be an important tool in fire fighting and for military and other applications. Here a heterojunction UV/IR dual-band detector, where the UV/IR detection is due to interband/intraband transitions in the Al 0.026 Ga 0.974 N barrier and GaN emitter, respectively, is reported. The UV threshold observed at 360 nm corresponds to the band gap of the Al 0.026 Ga 0.974 N barrier, and the IR response obtained in the range of 8-14 m is in good agreement with the free carrier absorption model.

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An all photonic terahertz communication link operating at 3.8 THz using a quantum cascade laser 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 2 m of room air. Carrier strength at the photodetector was 100 times greater than the noise level measured.It is anticipated that many new applications of the terahertz (THz) spectrum are possible if simple compact sources and photodetectors were readily available. Many groups around the world are in the process of developing semiconductor sources and photodetectors for the THz spectrum [1][2][3][4]. In this Letter, we report on a demonstration of a free space link we have constructed to show the feasibility of laser generated free space communications at THz frequencies. We show that the basic characteristics needed for a simple telecommunications link can now be performed easily using a quantum cascade laser as a source and a quantum well photodetector as a receiver. A previous demonstration of a sub-THz analogue transmission system was reported by Jastrow et al. at 300 GHz using an electronic system [5]. The work we report here operates at 3.8 THz and is an all photonic design for THz generation, radiation, collimation and detection. Fig. 1 is a schematic representation of the link. At the left a quantum cascade laser housed in a vacuum dewar provides 3.8 THz radiation which is collected and collimated by a parabolic mirror labelled M1. The laser transmitter was constructed from a multiple quantum well structure described earlier [6] with a 1 mm-long and 100 mm-wide surface plasmon waveguide formed on a semi-insulating GaAs substrate. A reflecting mirror was formed on the back facet by first coating the facet with an aluminium oxide insulator, to prevent short circuiting the electrodes, followed by evaporating a gold layer over the facet. The exit coupler of the laser is the cleaved surface. M1 is a 50 mm focal length off-axis parabolic reflector while M2 is 76 mm focal length. Both mirrors are 50 mm in diameter. The laser was mounted on an aluminium cold finger in a liquid nitrogen dewar with the laser facet approximately 2 mm from the low density polyethylene window. This permitted collecting a large fraction of the diverging beam with an off-axis parabolic mirror. A temperature sensor was mounted near the base of the QCL die and showed that the base temperature was maintained at 78 or 77 K for all of the work we report here. MOD LNA MP3 player 77k M1 M2 2 m path AM radio Fig. 1 Schematic of link showing quantum cascade laser at left and quantum well photodetector on right

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Results are presented for a dual-band detector that simultaneously detects UV radiation in the 250-360 nm and IR radiation in the 5-14 microm regions with near zero spectral cross talk. In this detector having separate UV- and IR-active regions with three contacts (one common contact for both regions) allows the separation of the UV and IR generated photocurrent components, identifying the relative strength of each component. This will be an important development in UV-IR dual-band applications such as fire-flame detection, solar astronomy, and military sensing, eliminating the difficulties of employing several individual detectors with separate electronics-cooling mechanisms.