Kenichi Oguchi

High-speed terahertz time-domain polarimeter based on an electro-optic modulation technique

We present a method of developing a high-speed terahertz time-domain polarimeter based on an electro-optic sampling setup, which consists of a rapid polarization analysis of the probe optical beam within 1 ms with an angular resolution of ~100 mrad using an electro-optic modulator (EOM). A fast optical delay line enabled us to obtain the vector waveform of the terahertz pulse within 100 ms. The setup does not utilize a manual/mechanical rotational stage, but the use of the polarization modulation technique employing the EOM ensures a high-speed and high-accuracy measurement of the state of polarization of terahertz waves.

A Real-Time Terahertz Time-Domain Polarization Analyzer with 80-MHz Repetition-Rate Femtosecond Laser Pulses

We have developed a real-time terahertz time-domain polarization analyzer by using 80-MHz repetition-rate femtosecond laser pulses. Our technique is based on the spinning electro-optic sensor method, which we recently proposed and demonstrated by using a regenerative amplifier laser system; here we improve the detection scheme in order to be able to use it with a femtosecond laser oscillator with laser pulses of a much higher repetition rate. This improvement brings great advantages for realizing broadband, compact and stable real-time terahertz time-domain polarization measurement systems for scientific and industrial applications.

Polarization-sensitive electro-optic detection of terahertz wave using three different types of crystal symmetry: Toward broadband polarization spectroscopy

We investigated polarization-sensitive electro-optic (EO) detection of terahertz (THz) waves by using two uniaxial crystals: a c-cut gallium selenide and a c-cut lithium niobate crystals. We formulated a general frequency-domain description of EO detection by in-plane isotropic EO crystals, which holds regardless of the frequency. Based on this description, the polarization of THz waves can be derived by analyzing EO sampling signals measured with two orthogonal configurations of the in-plane isotropic EO crystals as well as typical (111) zinc-blende EO crystals. In addition, we experimentally demonstrated that the frequency-dependent polarization of THz waves can be reproducibly retrieved using three EO crystals with different crystal symmetries and with different phase matching conditions. Our description provides essential information for practical polarization sensing in the THz frequency range as well as in the mid-infrared range.

Polarization detection of terahertz radiation via the electro-optic effect using zinc blende crystal symmetry

We provide a frequency-domain picture of the electro-optic (EO) detection of elliptically polarized terahertz radiation utilizing the crystal angle dependence of the EO effect using zinc blende crystal symmetry. In contrast to EO detection of a linearly polarized terahertz pulse, the measured EO signal would not be a replica of the terahertz pulse, even though the phase-matching condition is completely fulfilled. However, the rotation of the EO crystal reduces the complexity, and the ellipticity and the angle of rotation of each frequency component of the elliptically polarized terahertz pulse can be determined irrespective of what kind of zinc blende crystals are used for the EO detection. We experimentally show that the ellipticity and angle of rotation are reproducibly obtained by using three different crystals with different thicknesses and compositions. Direct detection of the elliptically polarized terahertz radiation enables us to precisely determine the optical axis and retardation of a quartz crystal using the Poincare sphere representation of the state of polarization.

Retrieving the undistorted terahertz time-domain electric-field vector from the electro-optic effect

We formulate and experimentally demonstrate a method to retrieve the undistorted temporal profile of an elliptically polarized terahertz pulse from the electric-field (E-field) vector waveform detected by electro-optic (EO) sampling. This method takes into account the distortion effects due to pulse width of the probe laser, absorption by the EO crystal, and frequency-dependent phase mismatch between the probe and terahertz pulses inside the EO crystal. Even with this distortion, the detected elliptically polarized terahertz vector waveform can be simply interpreted as that at the center of the EO crystal for detection. This interpretation enables us to retrieve the undistorted terahertz E-field vector waveform in front of the EO crystal by analyzing the detected EO signals in the frequency domain and then reconstructing those in the time domain. In order to verify our method, we experimentally demonstrate that the same elliptically polarized terahertz waveform can be retrieved even when we use different EO crystals for detection. Although the terahertz E-field vector waveforms measured by different EO crystals are significantly different due to the distortions, the retrieved waveforms show a good agreement without need for additional corrections.