Development and evaluation of high-sensitivity terahertz camera

Abstract

Summary form only given. Terahertz (THz) imaging technology attracts a lot of attentions for security and biochemical applications because many biochemical molecules have characteristic absorption spectra in the THz frequency region. THz camera, which can take real-time THz images is one of the most important detection devices for the development of THz imaging technology. NEC has developed a THz camera called IRV-T0830 consisting of uncooled microbolometer THz focal plane arrays (THz-FPAs) [1]. Although the operation of real-time imaging system with this camera was demonstrated recently [2], improvement of performance is still required. In this study, we have improved structure of the THz-FPAs to increase the sensitivity in the THz frequency region, and evaluated frequency-dependent sensitivity of the improved THz camera systematically.IRV-T0830 detects THz waves by sensing resistance change of the microbolometers. The resistance change is originated from the temperature increase by the absorption of incident THz waves. In addition, reflection layer is formed at the bottom of air gap located under the microbolometer layer in order to increase the THz absorption efficiency. Optical cavity is formed between reflection layer and THz absorption layer and it was found that the thickness of air gap is closely related to sensitivity of the THz camera [1]. Here, we have developed a new THz camera which air-gap thickness is near to wave length of THz frequency region to increase the sensitivity. THz beam used in sensitivity-measurement experiment was generated by tilted-pulse-front excitation of lithium niobate (LN) crystal. Spectrum of generated THz beam was obtained by electro-optic (EO) sampling method. The spectrum extended to 2 THz. We have measured frequency dependence of sensitivity of THz camera by putting THz band-pass filters in front of THz camera, and measured incident polarization direction angle dependence by rotating THz camera. It was found that sensitivities of both the new THz camera and IRV-T0830 depend on polarization of incident THz beam. Therefore, we estimated frequency dependence of sensitivities by averaging count numbers of each THz camera throughout all polarization angles. Frequency dependence of sensitivity for each THz camera is shown in Fig.1. The sensitivity of the new THz camera is three or four times larger than IRV-T0830 in frequency range of 1-2 THz. THz beam-spot images obtained by two cameras are shown in Fig.2. A band-pass filter which has transmission peak at 1.0 THz and full width at half maximum of 0.24 THz was used, and the power of incident THz beam penetrating the band-pass filter is about 100 μW. The image obtained by the new THz camera is much clearer than IRV-T0830. These results indicate that the structural modification of FPA successfully improved the performance of the THz camera, which should become a critical tool for the development of THz imaging technology.