Saroj R. Tripathi

Random error estimation in refractive index measured with the terahertz time domain spectroscopy

This study proposes a practical method to estimate the random error in real part of refractive index measured with terahertz time domain spectroscopy (THz-TDS) for the single measurement of sample by using the phase spectra of the reference terahertz electric field and their standard deviation. The validity of this method is based on the experimental result that the phase correlates with its standard deviation and the signal to noise ratio is almost equal in the phase spectra of reference and sample signal. The random error estimated from the proposed method fitted well to the statistically computed standard deviation.

Accurate Complex Refractive Index with Standard Deviation of ZnTe Measured by Terahertz Time Domain Spectroscopy

Accurate complex refractive index of ZnTe crystal in terahertz spectral region from 0.2 to 3.5 THz is obtained by measuring three samples with different thickness using transmission terahertz time-domain spectroscopy. Each sample is measured a number of times and the mean and standard deviation (SD) of the complex refractive index are calculated to quantify the random errors in the measurement. The ratios of SD to mean of the real and imaginary parts of the refractive index are approximately 1.0×10-4 and 1.2×10-2 at 1 THz respectively. The absorption coefficient of our ZnTe sample is found to be comparable with or smaller than the values reported in previous literatures.

Exploration of the origin of random error in spectrum intensity measured with THz-TDS

We explored an origin of the random error in intensity spectra measured with THz-TDS. The optical-delay scanning, variations in temperature and humidity, the intensity fluctuation of fs-laser, and the THz detectors of LT-GaAs PC antenna and ZnTe EO-crystal are not the predominant origin of the random error.

Study on random errors in THz signal and optical constants observed with THz time-domain spectroscopy

We have observed that random errors in THz intensity spectra which are much larger than the noise floor. The standard deviation in intensity and phase spectra measured with THz-TDS are almost proportional to THz intensity and phase respectively and common for reference and sample. We discuss on the characteristics related to the origin of the random errors.

Accurate optical constants of ZnTe measured by THz-TDS with their standard deviations

Accurate optical constants of the ZnTe are essential to optimize its performance as a terahertz emitter and detector. We measured the optical constants like complex refractive index of ZnTe crystal using transmission terahertz time domain spectroscopy (THz-TDS) from 0.2 to 3.5 THz at room temperature and their standard deviation is calculated to quantify the random errors. The uncertainty in sample thickness measurement, which is important to consider especially in transmission THz-TDS, is also incorporated to compute the resultant standard deviations in all optical constants. The percentage errors of real and imaginary parts of refractive index at 1 THz are 0.01 and 1.22 respectively.

Polarization-changeable THz time-domain spectroscopy system with a small incident-angle beam-splitter

When the polarization of terahertz (THz) light is measured with THz time-domain spectroscopy (TDS) by switching the polarization angle of THz light, it is preferable that intensities of pump and probe lights are almost constant. Pump and probe lights, which are produced through dividing a femto second (fs) laser pulses by a beam-splitter, must be correlated with orientations of THz emitter and detector as well as the THz polarization angle especially in a TDS adopting nonlinear crystal like ZnTe as an emitter and/or a detector. To achieve small change in the signal against change of the THz polarization, a THz-TDS system with a small incident-angle beam-splitter has been developed, which have smaller polarization dependency in pump and probe lights than ordinary THz-TDS systems adopting a 45-degree incident-angle beam-splitter. The change in THz signal observed with a 10-degree incident-angle THz-TDS was sufficiently small in both transmission and reflection measurements when polarization angle of THz light is switched by 90 degrees.

Random Error in Intensity Spectrum Measured with THz-TDS - No Relation to the Intensity Fluctuation of Fs-Laser

We explored sources of random errors in intensity spectra measured with THz-TDS. Influences of detection accuracies, optical-delay scanning, atmospheric absorption, and intensity fluctuations of fs-laser on the detected THz pulses are discussed based on the standard deviations of measurement data. Those influences on the THz pulses are considerably small and not a predominant cause of the random error.

Investigation of reflection-type cone condenser used for THz detectors

A low cost and easy to fabricate THz triangle cone condenser with a high condensing efficiency in comparison to Winston cone has been made. A simulation and experimental results on condensing efficiency of both triangle cone and Winston cone show good consistency at the wavelength of 632.8 nm. In the case of Triangle cone condenser at the terahertz spectral region, simulation result showed that the condensing efficiency becomes approximately 100% at normal incident. In contrast to this, the condensing efficiency of Winston cone decreases at normal incident owing to the loss due to reflection. In general, the triangle cone has a broader field of view (FOV) than Winston cone, it is improved by attaching a baffle to the condenser.

Efficient method for estimating random errors in optical constants measured with THz-TDS

We propose practical models for standard deviations in intensity and phase spectra of electric field measured with THz-TDS, which makes possible to estimate random errors in the optical constants from single measurement of a sample.

Background-limited operation of 4K-cryocooled THz photoconductive detector system with a wide frequency range of 0.8 to 4THz

We have demonstrated a terahertz (THz) detector system which employs three terahertz photoconductive semiconductor detectors to realize a response to a wide frequency range of 0.8 to 4THz and a mechanical 4K GM refrigerator instead of a liquid helium container to perform practical and convenient use. Optical and electrical performance of the THz detector system was evaluated at an operation temperature of 4K under 300K background radiation environment, which have proved the detector system can achieve background limited NEP (noise equivalent power) for THz detection even if vibration noise of mechanical cooler is present.