Terahertz radiation generation

Several different types of wide-band terahertz-radiation generation element are available these include a photoconductive antenna element, a nonlinear optical effective element (NOE), and a surface outgoing radiational-type semiconductor device. In this subsection, brief descriptions of the most commonly used PCAs are given. 

In theory, the full wavenumber region of the terahertz radiation generated by the PCA is inversely proportional to the pulse width of the exciting femtosecond pulse, so that it would be possible for this THz-TDS instrument to cover an extended wavenumber region... 

Sinyukov, A. M. Hayden, L. M., Generation and detection of terahertz radiation with multilayered electro optic polymer films, Opt. Lett. 2002, 27, 55 57... 

The Ti-sapphire oscillator is extremely useful as a stand-alone source of femtosecond pulses in the near-IR region of the spectrum. Some ultrafast experiments, especially of the pump-probe variety (see below), can be conducted with pulses obtained directly from the oscillator or after pulse selection at a lower repetition rate. Far-IR (terahertz) radiation is usually generated using a semiconductor (usually GaAs) substrate and focused Ti-sapphire oscillator pulses [7]. If somewhat higher-energy pulses are required for an experiment, the Ti-sapphire oscillator can be cavity dumped by an intracavity acousto-optical device known as a Bragg cell. 

The image of the generated terahertz radiation is then focused onto a sample through an optical system consisting of a Si hyper-spherical lens, a plane mirror, and an ellipsoid mirror. Although a sample is shown set at the position for a transmission measurement in Figure 19.2, other methods of spectral measurement are also possible, at least in principle. 

The terahertz radiation that has passed through the sample is detected with an optical system having essentially the same structure as the optical system for generating the terahertz radiation and focusing it onto the sample, as depicted in Figure 19.2. The terahertz radiation that has passed through the sample is focused onto the terahertz-radiation detection... 

Generation and Detection of the Terahertz Radiation Using a Photoconductive Antenna Element... 

However, it is not possible to determine directly Ej (t) and 4> t) of the terahertz radiation, because the terahertz radiation is emitted as an extremely short pulse. For this reason, as illustrated schematically in Figure 19.4, time delays are instigated between the laser pulses for generating the terahertz radiation and those for detecting the terahertz radiation. The shape of the terahertz radiation on the time axis is determined by scanning the delay time of the laser pulses. 

These relationships mean that, in order to expand the wavenumber region of the terahertz radiation, it is necessary to make the width of the laser pulse for generating the terahertz radiation narrower and the response of the photogenerated carrier faster in order to have a shorter lifetime. At present, LT-GaAs is widely used for the PCA as a material appropriate for the above-mentioned requirement. 

Terahertz radiation can be generated with the aid of certain polymer systems that are irradiated with 800 nm pulses (duration of less than 200 fs) emitted from a Ti sapphire laser system. A prerequisite for the generation of THz radiation is the... 

Nahata, A., Auston, D.H., Wu, C.J., and Yardley, J.T. (1995) Generation of terahertz radiation from a poled polymer. Appl. Phys. 

A. (2008) Generation of broadband radially polarized terahertz radiation directly from a cylindrical metal wire. Opt. Express, 16,... 

The main barrier to the use of free induction decay for FT spectroscopy in the infrared region is the lack of a convenient powerful source of broadband coherent radiation. In addition, the decay times for the coherently excited polarization in the system tend to be very short at higher frequencies. Coherent terahertz spectrometers operating in the far infrared region are now practical because of the success of ultrafast laser technology in generating broadband terahertz pulses. 

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