On-chip pulsed terahertz systems and their applications
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On-chip pulsed terahertz systems and their applications C. Wood, J. Cunningham, I. C. Hunter, P. Tosch, E. H. Linfield and A. G. Davies School of Electronics and Electrical Engineering University of Leeds Woodhouse Lane, Leeds, West Yorkshire, LS2 9JT, UK E-mail: [email protected]
Abstract The generation and detection of guided wave terahertz (THz) transients in microstrip transmission line systems is demonstrated at both room and cryogenic (~ 4 K) temperatures using thin film low-temperature-grown GaAs (LT-GaAs) switches, excited by a 100 fs, 80 MHz repetition rate pulsed Ti:Sapphire laser. The characterisation of passive filter elements formed in the microstrip line is reported, together with their response to the application of dielectric loads of varying thickness at room temperature. Introduction The generation and propagation of THz pulses on-chip, in a transmission line geometry, is a relatively unexplored technique for spectroscopic applications in comparison to complementary free-space technologies. Whilst free-space studies have seen development for an increasingly wide range of applications as diverse as analysis of bone structure [1] through to detection of drug polymorphs [2], on-chip applications have only recently begun to be explored. Recent research has demonstrated the operation of microstrip bandpass filter systems at THz frequencies, the electrical responses of which have been shown to be affected by the presence of thin overlaid dielectric films. Measurements on such systems have, for example, enabled distinction between the hybridization state of nanolitre volumes of overlaid DNA [3, 4], and are a potential alternative to present fluorescent labelling methods employed for the detection of DNA hybridisation states. Analysis of dielectric films using THz bandpass filters is somewhat restricted, however, since such filters limit the frequency bandwidth available for measurements. Peak shift analysis of a bandstop system, however, circumvents this issue, since only a selected range of frequencies is removed from the device bandwidth [5, 6], allowing the prospect of cascading several filters of different operating frequencies on a single device. Another benefit of the bandstop design is its comparatively small active area, which is important in 557
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the proposed sensing applications. Spectroscopic analysis using an on-chip system at cryogenic temperatures also offers advantages over free-space studies when considering measurements of certain condensed matter systems. Whilst for the biological applications mentioned above room temperature operation is necessary, a range of other studies which require cryogenic temperatures would benefit by containment of the THz transients in an on-chip geometry. This is owing to the increase in interaction length between the THz radiation and the sample under investigation in an on-chip study. A major hindrance to the detailed analysis of a broader range of condensed matter systems, however, is that the photoconductive properties of modern semi
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