A terahertz wave is an electromagnetic wave having a frequency of approximately 0.01 THz to 100 THz corresponding to an intermediate range between light waves and radio waves, and has an intermediate property between light waves and radio waves. As an application of such a terahertz wave, a technology for acquiring information on a measuring object by measuring a temporal waveform of an electric field amplitude of a terahertz wave which is transmitted through or is reflected by the measuring object has been studied.
A technology for measuring information on a measuring object by use of a terahertz wave is generally as follows. That is, pulsed light output from a light source (for example, a femtosecond laser light source) is split into two components to be pulsed excitation light and pulsed detection light by a branching part. The pulsed excitation light among those is input to a terahertz wave generating element (for example, a nonlinear optical crystal or a photoconductive antenna element), and a pulsed terahertz wave is generated from the terahertz wave generating element thereby. This generated terahertz wave is transmitted through or reflected by a measuring object part as a measurement light, to acquire information on the measuring object (for example, an absorption coefficient, a refractive index), and thereafter, the terahertz wave is made incident on a terahertz wave detecting element (for example, an electro-optic crystal or a photoconductive antenna element) in substantially the same timing as that of the pulsed detection light.
In the terahertz wave detecting element to which the terahertz wave and the pulsed detection light are input, a correlation between both light components is detected. For example, in the case in which an electro-optic crystal is used as the terahertz wave detecting element, the terahertz wave and the pulsed detection light are coupled by a coupling part, to be made incident on the electro-optic crystal, and birefringence is induced in accordance with propagation of the terahertz wave in the electro-optic crystal, and a polarization state of the pulsed detection light is changed by the birefringence. A change in the polarization state of the pulsed detection light in the electro-optic crystal is detected, and as a result, an electric field amplitude of the terahertz wave is detected, which enables to acquire information on the measuring object.
Terahertz wave generating apparatus of various configurations for generating terahertz waves have been known. One of those terahertz wave generating apparatus is configured to make pulsed excitation light incident into a nonlinear optical crystal to generate a terahertz wave from the nonlinear optical crystal. In such a conventional terahertz wave generating apparatus, a type of nonlinear optical crystal suitable for terahertz wave generation is determined according to a central wavelength of the pulsed excitation light. For example, in the case where pulsed excitation light with a central wavelength of 800 nm is used, a ZnTe crystal is broadly used as a nonlinear optical crystal on the basis of a phase matching condition for the pulsed excitation light and the terahertz wave in the nonlinear optical crystal. When pulsed excitation light with a central wavelength of 800 nm is incident into a ZnTe crystal, a pulsed terahertz wave is generated coaxially with the pulsed excitation light.
In recent years, it has become apparent that, even in a combination of pulsed excitation light of a central wavelength and a nonlinear optical crystal in which a phase matching condition is not usually satisfied, the pulsed excitation light whose pulse front is tilted is made incident into the nonlinear optical crystal, to enable to satisfy the phase matching condition (refer to Non Patent Documents 1 to 3).