Exploring dynamics in the far-infrared with terahertz spectroscopy.

Terahertz fields and applications

Widely tunable terahertz (THz) wave generation by optical parametric processes based on laser light scattering from the polariton mode of nonlinear crystals is reviewed. Using parametric oscillation of LiNbO3 or MgO-doped LiNbO3 crystal pumped by a nanosecond Q-switched Nd : YAG laser, we have realized widely tunable coherent THz-wave sources in the range between 0.7 and 3 THz, with a simple configuration. For the efficient coupling of the THz wave, a monolithic grating coupler or an Si-prism array coupler was used. We report the detailed characteristics of the oscillation and the radiation, including tunability, spatial and temporal coherency, uni-directivity, and efficiency. Further, Fourier transform limited THz-wave spectrum narrowing was achieved by introducing the injection seeding method. A linewidth of about 100 MHz (0.0033 cm-1) was assured by the absorption spectrum measurement of low-pressure water vapour. At the same time, the THz-wave output was increased hundreds of times higher than that of a conventional generator which has no injection seeder. In addition, a wider tunability was observed using a tunable diode laser as the injection seeder. This room-temperature-operated, tabletop system promises to be a new widely tunable THz-wave source that is suited to a variety of applications.

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Terahertz wave parametric source

An injection-seeded nanosecond terahertz (THz) wave parametric generator was demonstrated using nonlinear crystals that were pumped by a single frequency Nd:yttrium–aluminum–garnet laser. Spectrum narrowing to the Fourier transform limit (ν=1.58 THz, Δν 100 mW peak) approximately 300 times higher than that of a conventional THz-wave parametric generator, which has no injection seeder. This compact system operates at room temperature and promises to be a widely tunable THz-wave source that will compete with free-electron lasers and p-Ge lasers.

Transform-limited, narrow-linewidth, terahertz-wave parametric generator

Over the past decades, signicant attention has been paid to biomedical applications of terahertz (THz) technology. Nowadays, THz spectroscopy and imaging allow for solving numerous demanding problems in biological, medical, food, plant and pharmaceutical sciences. Among all biomedical applications, a label-free diagnosis of malignant and benign neoplasms represents one of the most attractive branches of THz technology. Despite the attractiveness, the methods of THz diagnosis are still far from medical practice. In this review, we consider the modern research results in THz diagnosis of malignant and benign neoplasms, along with the topical research and engineering problems, which restrain the THz technology translation to clinics. We start from analysis of the common models of THz-wave { tissue interactions and the eects of tissue exposure to THz waves. Then, we discuss existing modalities of THz spectroscopic and imaging systems, which either have been already applied in medical imaging, or hold strong potential. We summarize the earlier-reported and the original results of THz measurements of neoplasms with dierent nosology and localization. We pay attention to the origin of contrast between healthy and pathological tissues in the THz spectra and images and discuss the prospective of THz technology in non-invasive, minimally-invasive and intraoperative diagnosis, as well as in aiding histology. Finally, we review the challenging problems of THz diagnosis, as well as attempts to solve them, which should bring the THz technology much closer to the medical practice. This review allows one to objectively uncover benets and weaknesses of the THz technology in diagnosis of malignant and benign neoplasms.

https://iopscience.iop.org/article/10.1088/2040-8986/ab4dc3/pdf

The progress and perspectives of terahertz technology for diagnosis of neoplasms: a review

Coherent tunable terahertz waves were generated successfully using a terahertz-wave parametric oscillator (TPO) based on laser light scattering from the A/sub 1/-symmetry polariton mode of LiNbO/sub 3/. This method has several advantages, such as continuous and wide tunability (frequency: 0.9-3.1 THz), a relatively high peak power (more than a few milliwatts), and compactness of its system (tabletop size). In addition, the system simply requires a fixed-wavelength pump source and it is easy to tune. This paper deals with the general performance of this terahertz-wave source using the prism output-coupler method as well as the development and applications of the system. Its tunability, coherency, power, and polarization were measured, and this tunable source was used for terahertz spectroscopy to measure the absorption spectra of LiNbO/sub 3/ and water vapor. Also, the use of MgO-doped LiNbO/sub 3/ (MgO:LiNbO/sub 3/) in our terahertz regime, as well as its far-infrared properties, is described. We found that the MgO:LiNbO/sub 3/ TPO is almost five times more efficient than the undoped LiNbO/sub 3/ TPO, and we have proven that the enhancement mechanism originates from the enhanced scattering cross section of the lowest A/sub 1/-symmetry mode in a spontaneous Raman experiment.

Tunable terahertz-wave parametric oscillators using LiNbO/sub 3/ and MgO:LiNbO/sub 3/ crystals

Widely tunable coherent terahertz (THz) wave generation from has been successfully demonstrated based on the laser light scattering from the lowest A/sub 1/-symmetry polariton mode of LiNbO/sub 3/. This method exhibits multiple advantages like wide tunability, coherency and compactness of its system. In this paper, we describe the general performances of this optical parametric oscillator and its application, including the recent study using MgO doped LiNbO/sub 3/.

H. Ito

Papers

Tunable terahertz-wave parametric oscillators using LiNbO/sub 3/ and MgO:LiNbO/sub 3/ crystals

Tunable THz-wave generation from LiNbO/sub 3//MgO:LiNbO/sub 3/ optical parametric oscillators