Scanning laser terahertz near-field imaging system

Abstract

We have proposed and developed a scanning laser terahertz (THz) near-field imaging system using a 1.56 μm femtosecond fiber laser for high spatial resolution and high-speed measurement. To obtain the two-dimensional (2D) THz images of samples, the laser pulses are scanned over a 2D THz emitter plate [DASC: 4'-dimenthylamino-N-methyl-4- stilbazolium p-chlorobenzenesulfonate] by a galvano meter. In this system, THz wave pulses locally generated at the laser irradiation spots transmit through the sample set on the emitter, and the amplitude of the transmitted THz wave pulse is detected by using a typical THz time-domain spectroscopy (THz-TDS) technique. Using this system, we have succeeded in obtaining THz transmission images of a triangle shaped metal sheet of millimeter-size and a human hair sample with a spatial resolution of sub-wavelength order up to ~27 μm (~λTHz/28) at an imaging speed of about 47 seconds/image for 512 x 512 pixels.

Invited Article: Terahertz microfluidic chips sensitivity-enhanced with a few arrays of meta-atoms

“…In contrast to the standard THz-TDS, we can utilize the near-field interactions between the THz waves and samples; therefore, high-sensitive spectroscopy of sub-THz wavelength scale samples can be realized. 26 The significant change of the sensitivity by using the proposed near-field approach is shown in detail in Multiple reflections are due to the THz wave traveling and reflection in GaAs. It can be seen that the waveforms near the main peak differ from each other.…”

“…[22][23][24][25] In the recently developed system, a nonlinear optical crystal (NLOC) is employed as a 2D THz emitter, and THz waves are locally generated in the process of optical rectification at the irradiation spots of femtosecond (fs) laser beams. 26 Since the fs laser beams are focused at the output surface of the NLOC, the beam diameter of the generated THz waves is approximately the same size as that of the focused beam spot which overcomes the diffraction limit of THz waves. 26 By setting samples in the vicinity of the THz generation spot on the 2D-THz emitter, THz-TDS and THz-imaging have been successfully achieved on the samples of sub-THz wavelength scale, such as dielectric thin-film, 27 a human hair strand, 28 metallic small apertures, 29 THz meta-atom, 30 and nanoparticles.…”

“…26 Since the fs laser beams are focused at the output surface of the NLOC, the beam diameter of the generated THz waves is approximately the same size as that of the focused beam spot which overcomes the diffraction limit of THz waves. 26 By setting samples in the vicinity of the THz generation spot on the 2D-THz emitter, THz-TDS and THz-imaging have been successfully achieved on the samples of sub-THz wavelength scale, such as dielectric thin-film, 27 a human hair strand, 28 metallic small apertures, 29 THz meta-atom, 30 and nanoparticles. [31][32][33] Therefore, it is interesting to see if this technique can be also applied to the measurements of trace-amount of liquid solutions.…”

Terahertz Application for Non-Destructive Inspection of Coated Al Electrical Conductive Wires

“…Examples of the application of THz waves in industrial fields include internal defect inspection and structural analysis of polymer materials [4] and applications of non-destructive imaging inspection to space-shuttle ceramic style bonded state [5], as well as tablet surface defect analysis [6]; in the medical field, the non-destructive inspection of cancer cells [7] has been reported. For imaging measurement, high spatial resolution measurements have been studied by using near-field techniques [8][9][10][11].…”

Observation of Change of Moisture Retention of Single Human Hairs using THz Waves

“…To apply the THz imaging technique to micro-scale samples, we have fabricated a scanning laser THz near-field imaging system as shown in Fig. 1 [2]. The system features near-field THz emission and far-field THz detection.…”