This Small Business Innovation Research (SBIR) Phase II research project is aimed at developing devices for the detection of terahertz (THz) signals and a spectrometer based on these devices. To date, the lack of suitable electronic devices have made the THz region of the electromagnetic spectrum inaccessible except by use of large and costly scientific instruments. The aim is to develop a simple, low-cost, low-power receiver which will make this important region accessible. The three critical components of the THz heterodyne receiver are an antenna, microbolometer/mixer and quantum cascade laser which functions as a local oscillator. With these novel components it is possible to develop a portable, field-deployable THz spectrometer capable of monitoring a wide variety of gases in its vicinity. The high-sensitivity spectrometer will allow rapid identification of chemicals and remote sensing of gases for environmental, global warming, and homeland security applications.
The broader impacts of this research are that the THz receiver, which has high sensitivity and high spectral resolution not achievable with existing devices, can be used in a much wider variety of imaging and screening devices. THz screening of personnel is non-invasive and harmless. Explosives and biological agents can be detected and identified even if concealed in clothing and suitcases because the THz radiation is transmitted through clothing and luggage. The proposed receiver also has a potential of providing THz imaging of biological materials and broad-band transmitting of digital signals.
The development of the tunable receiver for the terahertz band consisted of combining several components into a front end similar to a conventional heterodyne receiver except for the extremely high frequency response in the Thz region. The receiver consists (see figure) of an antenna, and integrated local oscillator and mixer. The output is processed by readily available electronics operating in MHz and GHz amplifier, demodulator and signal processor. The mixer consists of a hot electron bolometer, based on a novel 2-D electronic gas structure with ultrafast response. We were able to demonstrate that the hot electron bolometer technology can be extended from films of superconductors to low-dimensional semiconductor structures operating at higher temperatures. The local oscillator, which provides a reference frequency in the THz region, consists of a quantum cascade laser. Both a ring laser configuration and dual midIR were tested. The application of the receiver is for the identification of chemical vapors and biological agents by means of their unique spectra. The THz detection technique has significant potential for the remote monitoring of public and industrial facilities for toxic industrial chemicals, chemical agents, and explosives. Specifically heterodyne sensing enables information about both amplitude and phase of the radiation to be recovered. In THz spectroscopy, the heterodyne receiver enables the sampling of a very large number of spectrum channels simultaneously, in contrast to direct detection where one channel at a time only is analyzed. In this way, heterodyne sensing drastically decreases the data acquisition time and significantly improves the resolution. The receiver has these desirable features: [1] high sensitivity (low noise), [2] broad spectral coverage, [3] low local oscillator power, [4] low cost using available fabrication technologies, [5] good impedance matching to antenna and IF amplifier output and [6] Feasibility for imaging applications.
