Research into terahertz technology is now receiving increasing attention around the world, and devices exploiting this waveband are set to become increasingly important in a very diverse range of applications. Here, an overview of the status of the technology, its uses and its future prospects are presented.

Cutting-edge terahertz technology

A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

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Principles Of Fluorescence Spectroscopy

Over the past three decades a new spectroscopic technique with unique possibilities has emerged. Based on coherent and time-resolved detection of the electric field of ultrashort radiation bursts in the far-infrared, this technique has become known as terahertz time-domain spectroscopy (THz-TDS). In this review article the authors describe the technique in its various implementations for static and time-resolved spectroscopy, and illustrate the performance of the technique with recent examples from solid-state physics and physical chemistry as well as aqueous chemistry. Examples from other fields of research, where THz spectroscopic techniques have proven to be useful research tools, and the potential for industrial applications of THz spectroscopic and imaging techniques are discussed.

Terahertz spectroscopy and imaging – Modern techniques and applications

The diffraction tomography theorem is adapted to one-dimensional length measurement. The resulting spectral interferometry technique is described and the first length measurements using this technique on a model eye and on a human eye in vivo are presented.

Measurement of intraocular distances by backscattering spectral interferometry

We propose a paintmeter for noncontact and remote monitoring of the thickness and drying progress of a paint film based on the time-of-flight measurement of the echo signal of a terahertz (THz) electromagnetic pulse. The proposed method is effectively applied to two-dimensional mapping of the painting thickness distribution for single-layer and multilayer paint films. Furthermore, adequate parameters for the drying progress are extracted from the THz pulse-echo signal and effectively applied to monitor the wet-to-dry transformation. The THz paintmeter can be a powerful tool for quality control of the paint film on the in-process monitoring of car body painting.

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Terahertz paintmeter for noncontact monitoring of thickness and drying progress in paint film

We propose a terahertz time-domain spectrometer based on asynchronous optical sampling. The spectrometer features ultrahigh spectral resolution equivalent to a laser mode-locked frequency and rapid data acquisition. The proposed method requires no mechanical translation stages for time-delay scanning, and hence, overcomes the inherent tradeoff between frequency resolution and data acquisition time. Time evolution of the picosecond terahertz pulse with a temporal window of 12.1 ns is measured directly on an oscilloscope using a time-scale magnification of 764 815. A frequency resolution of 82.6 MHz is achieved at a measurement time of 10 s. The effectiveness of the proposed method is confirmed by comparing it with conventional stage-scanning terahertz time-domain spectroscopy.

https://femto.me.tokushima-u.ac.jp/gyoseki/PDF2005/P05.pdf

Asynchronous optical sampling terahertz time-domain spectroscopy for ultrahigh spectral resolution and rapid data acquisition

We report a terahertz spectroscopy technique based on a stable terahertz frequency comb from a photoconductive terahertz emitter driven by a stabilized femtosecond laser. To this end, a photocurrent frequency comb is induced in a photoconductive terahertz detector by instantaneous photogating with another detuned femtosecond laser and is applied to read out the terahertz frequency comb. The detailed structure of the terahertz frequency comb was clearly observed with frequency accuracy of 2.5×10−7 and resolution of 81.8MHz using multifrequency-heterodyning photoconductive detection, which in turn is caused by the slightly mismatched frequency spacing between terahertz and photocurrent frequency combs.

/pdf/terahertz-frequency-comb-by-multifrequency-heterodyning-54zo5ff5ec.pdf

Terahertz frequency comb by multifrequency-heterodyning photoconductive detection for high-accuracy, high-resolution terahertz spectroscopy

Dual-comb spectroscopy holds the promise as real-time, high-resolution spectroscopy tools. However, in its conventional schemes, the stringent requirement on the coherence between two lasers requires sophisticated control systems. By replacing control electronics with an all-optical dual-comb lasing scheme, a simplified dual-comb spectroscopy scheme is demonstrated using one dual-wavelength, passively mode-locked fiber laser. Pulses with a intracavity-dispersion-determined repetition-frequency difference are shown to have good mutual coherence and stability. Capability to resolve the comb teeth and a picometer-wide optical spectral resolution are demonstrated using a simple data acquisition system. Energy-efficient, free-running fiber lasers with a small comb-tooth-spacing could enable low-cost dual-comb systems.

Picometer-resolution dual-comb spectroscopy with a free-running fiber laser

We have proposed an optical probe that can be used to characterize the orientation of collagen fibers in human dermis. A specific probing ability for collagen results from the use of second- harmonic-generation (SHG) light induced by collagen molecules in the tissue. Based on the concept of collagen SHG light, a reflection- type polarization measurement system (named SHG polarimetry) with a probe light spot of 15 mm in diameter has been constructed, and the human reticular dermis has been measured using this system. Result- ant data exhibit that the reticular dermis possesses approximately uniaxial orientation of the collagen fibers. Furthermore, we demon- strated a nondestructive measurement of the collagen orientation in the papillary dermis across an epidermis layer. For distribution mea- surement of the collagen fiber orientation in the reticular dermis, we have extended the SHG polarimetry to one- (1-D) and two- dimensional (2-D) measurement. By the macroscopic 2-D SHG polar- imetry, we have observed that the orientation angle and organization degree of collagen fibers vary widely depending on the discrete prob- ing positions in the reticular dermis. Furthermore, microscopic 1-D SHG polarimetry indicated a swell of the orientation angle and a large variance of the organization degree in the collagen fibers in the mi- croscopic region. These results imply that the reticular dermis posses a tangled structure of collagen fibers, which is highly consistent with the result of the anatomical examination of the skin. The proposed method will be a powerful tool for monitoring the microscopic distri- bution of the collagen fiber orientation in the human dermis. © 2004

/pdf/characterization-of-collagen-orientation-in-human-dermis-by-ryyzr7npqy.pdf

Takeshi Yasui

Papers

Terahertz paintmeter for noncontact monitoring of thickness and drying progress in paint film

Asynchronous optical sampling terahertz time-domain spectroscopy for ultrahigh spectral resolution and rapid data acquisition

Terahertz frequency comb by multifrequency-heterodyning photoconductive detection for high-accuracy, high-resolution terahertz spectroscopy

Picometer-resolution dual-comb spectroscopy with a free-running fiber laser

Characterization of collagen orientation in human dermis by two-dimensional second-harmonic-generation polarimetry.