Masatsugu Yamashita

Terahertz imaging system based on a backward-wave oscillator

We present an imaging system designed for use in the terahertz range. As the radiation source a backward-wave oscillator was chosen for its special features such as high output power, good wave-front quality, good stability, and wavelength tunability from 520 to 710 GHz. Detection is achieved with a pyroelectric sensor operated at room temperature. The alignment procedure for the optical elements is described, and several methods to reduce the etalon effect that are inherent in monochromatic sources are discussed. The terahertz spot size in the sample plane is 550 microm (nearly the diffraction limit), and the signal-to-noise ratio is 10,000:1; other characteristics were also measured and are presented in detail. A number of preliminary applications are also shown that cover various areas: nondestructive real-time testing for plastic tubes and packaging seals; biological terahertz imaging of fresh, frozen, or freeze-dried samples; paraffin-embedded specimens of cancer tissue; and measurement of the absorption coefficient of water by use of a wedge-shaped cell.

Terahertz imaging diagnostics of cancer tissues with a chemometrics technique

The terahertz (THz) spectroscopic images of the paraffin embedded cancer tissues have been measured by a THz time domain spectrometer. Though the potential of the THz spectrum to the cancer diagnostics have been reported, it is sometimes difficult to distinguish normal and cancer parts due to the variety of the tissues. Thus, the systematic analysis should be introduced to clarify their difference. In this paper, the chemometrics technique has been applied to the analysis of the THz spectroscopic images of plural tumor samples. The capability of the method was discussed.

Laser terahertz-emission microscope for inspecting electrical faults in integrated circuits

A laser terahertz-emission microscope (LTEM) system is proposed and developed for inspecting electrical faults in integrated circuits (IC). We test a commercial operational amplifier while the system is operating. Two-dimensional terahertz-emission images of the IC chip are clearly observed while the chip is scanned with a femtosecond laser. When one of the interconnection lines is cut, the damaged chip has a LTEM image different from that of normal chips. The results indicate that the LTEM system is a potential tool for IC inspection.

Imaging of large-scale integrated circuits using laser terahertz emission microscopy

We present the redesign and improved performance of the laser terahertz emission microscope (LTEM), which is a potential tool for locating electrical failures in integrated circuits. The LTEM produces an image of the THz waves emitted when the circuit is irradiated by a femtosecond laser; the amplitude of the THz emission is proportional to the local electric field. By redesigning the optical setup and improving the spatial resolution of the system to below 3 microm, we could extend its application to examining of large-scale integration circuits. As example we show the THz emission pattern of the electric field in an 8-bit microprocessor chip under bias voltage.

High-resolution time-of-flight terahertz tomography using a femtosecond fiber laser

High-resolution tomographic imaging is demonstrated using a reflection-type terahertz time-domain spectroscopy. To realize a practical system for general use, a robust all-fiber laser was used as the pump light source. Broadband terahertz waves were generated with the combination of optical pulses compressed to 17 fs using optical fibers and a DAST crystal. Using deconvolution signal processing, the wideband spectrum of the generated terahertz waves provided high-axial resolution leading to successful imaging of a multilayered structure containing a 2-microm-thin GaAs layer. To our knowledge, this is the first demonstration of terahertz tomographic imaging of such a thin layer.

Terahertz radiation imaging of supercurrent distribution in vortex-penetrated YBa2Cu3O7−δ thin film strips

We have developed a supercurrent distribution imaging system for high Tc superconductive thin films and demonstrated the visualization of the supercurrent distribution in the vortex-penetrated YBa2Cu3O7−δ thin film strips. The terahertz (THz) radiation and detection system with a scanning femtosecond laser was employed to visualize the distribution. The imaging system utilizes the principle that the femtosecond optical pulses excite THz radiation into the free space by optical supercurrent modulation, and the radiation amplitude is proportional to the local supercurrent density at the optically excited area. Prior to the observation of the supercurrent distribution, we studied optical excitation effects on the vortices trapped in the strips, calibration of the current density from the THz radiation amplitude, temperature dependence of the THz radiation properties, etc. The laser power dependence of the THz radiation in the remanent state revealed that the excitation with powers larger than the relatively ...

Effect of solvent on carrier transport in poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) studied by terahertz and infrared-ultraviolet spectroscopy

The effect of solvent such as ethylene glycol (EG) on the carrier transport in poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) was studied by the combination of terahertz (THz) time-domain transmission spectroscopy (0.1–3 THz) and broadband reflectance spectroscopy (4–800 THz). While the broadband reflectance showed a plasma edge in the near-infrared, the frequency dependence of the THz conductivity of PEDOT/PSS indicated weakly localized carrier behavior. The simultaneous fitting of both spectra by the localization-modified Drude model allowed us to conclude that the significant increase in conductivity with the addition of EG is due to improved carrier mobility in the PEDOT/PSS.

Observation of supercurrent distribution in YBa2Cu3O7−δ thin films using THz radiation excited with femtosecond laser pulses

We have demonstrated that the supercurrent distribution in current-biased YBa2Cu3O7−δ thin films can be obtained by measuring the radiation power of THz electromagnetic pulses excited with femtosecond laser pulses. As the radiation power is proportional to the square of the bias current density at the laser spot position, the two-dimensional current distribution can be obtained from the intensity distribution of THz radiation by scanning the laser spot. The characteristic supercurrent distribution is analyzed by using the critical-state model.

Terahertz wideband spectroscopic imaging based on two-dimensional electro-optic sampling technique

We extended the bandwidth of a terahertz (THz) spectroscopic imaging system based on two-dimensional electro-optic sampling. By using an optical-rectification emitter, formed by a ZnTe crystal, a high-frequency component of more than 2.0THz was detected. A calibration-free procedure with polarity inversion of the THz beam ensures us precise spectroscopy: the polarity was inverted by rotating the emitter crystal. The spatial patterns of the chemicals were extracted from the THz multispectral images in the extended spectral region.

Laser terahertz emission microscope for inspecting electrical failures in integrated circuits

The inspection and fault analysis of semiconductor devices has become a critical issue with increasing demands for quality and reliability in circuits as stated in L. A. Krauss et al. (2001), K. Nikawa (2002) and K. Nikawa et al. (2003). Recently, we have developed a laser-terahertz (THz) emission microscope (LTEM) that can be applied for the noncontact and nondestructive inspection of the electrical faults in circuits presented in K. Nikawa et al. (2003). The LTEM can image the amplitude profile of the THz wave emitted by scanning the sample with femtosecond (fs) laser pulses. The amplitude of the THz emission generated by the transient photocurrent is proportional to the local electric field at the laser-irradiated area according to T. Kowa et al. (2003). Therefore, the LTEM image of the semiconductor device while it operates reflects the electric field distribution in the chip. By comparing the LTEM image of a damaged chip with that of a normal one, we can localize the electrical faults. In this work, we report experimental results on a biased 8-bit microprocessor, as well as unbiased MOSFETs embedded in a test element group (TEG).