A THz detector with both high sensitivity and fast time response has been required for industrial applications such as nondestructive testing (NDT), security, and spectroscopy. Through a collaboration with the Technical University of Denmark (DTU), we have recently developed a THz-sensitive point detector and imager based on metasurface and photomultiplier tube (PMT) and image intensifier (I.I.) technologies, respectively. A fast time response is one of the unique characteristics of these devices: the PMT-based point detector provides a nanosecond response time while the I.I.- based imager is capable of frame rates up to 1000 fps. These devices have a double split-ring resonator (DSRR) at the photocathode for THz-electron conversion (metasurface). In this paper, we discuss the two devices and report on the development and results for increasing their sensitivity for ultrafast, broadband THz pulses by sharpening the fieldenhancing antenna tips. This leads to a smaller tip diameter, which increases the electric field confinement and thus intensity at the tip, making the field emission more likely to occur at lower field strengths as a result. Both devices thus offer a sensitive and simple method to detect THz frequencies easily, with the I.I. offering a handheld, 9V batterypowered device.
We developed a stable, compact, terahertz attenuated total reflected (ATR) spectrometer using an integrated prism, which unifies a terahertz emitter, a terahertz detector, and an ATR prism. Because the prism confines terahertz wave propagation within it and shortens the terahertz path length, our spectrometer has the following two advantages: 1) because water vapor does not disturb the terahertz wave propagation, a high-quality terahertz spectrum is obtained even without a nitrogen purge, and 2) the shorter propagation length provides distinct stability and compact features. To verify our spectrometer's stability, we successively measured water absorption for 9 days and found the relative error to be ±3%. We also provide three distinctive examples by adapting our spectrometer. The first is the quantitative measurement of a solute. We determined the detection limit of H2SO4 to be 0.21% using the calibration curve of the refractive index. The second example is monitoring fermentation. Because terahertz absorption is a useful, new indicator against the potential of hydrogen (pH) as a standard indicator, we confirmed that the absorption reduction agreed with the pH during the fermentation of milk to yogurt by our prism for 1 day. The third is the observation of a crystal transition. The transition process of theophylline anhydrate to hydrate was measured on the prism because of the sensitivity of terahertz to the crystal and our spectrometer's stability. Therefore, our apparatus has potential application to different types of new quantitative measurements and real-time monitoring.
We demonstrate terahertz imaging using a terahertz nonlinear quantum cascade laser source (THz NL-QCL). THz NL-QCLs are ultrabroad terahertz source which can be operated at room temperature. The maximum operating temperature of conventional THz-QCLs has been limited to 210.5 K so far. Therefore, THz NL-QCL sources are the only electrically pumped monolithic terahertz semiconductor sources operable at room temperature. Currently, various room temperature compact THz sources have been reported. However, the operation frequencies of these sources were basically below 1 THz. Although several devices demonstrate THz emission above 1THz, output powers are still quite low; thus, it is very difficult to apply to practical THz applications. THz NL-QCL sources are able to operate above 1 THz, and the average THz output powers have exceeded 10 μW (duty cycle >5%) at room temperature, which can potentially be applied to THz applications. Also, in edge-emitting metal-metal THz QCLs, ring-like fringe patterns in their far-field beams are frequently observed due to far-field interference of coherent radiation in deep sub -wavelength apertures. Otherwise, the beam profile of THz NLQCL is Gaussian-like far-field pattern. The beam quality of nonlinear quantum cascade laser is better than that of conventional terahertz quantum cascade laser. Therefore, THz NL-QCL sources are suitable for terahertz imaging. We demonstrated terahertz imaging with the THz NL-QCL sources.
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