The development of technology in the THz frequency band has seen rapid progress recently. Considered as an extension of the microwave and millimeter wave bands, the THz frequency offers greater communications bandwidth than is available at microwave frequencies. The development of sources and detectors for this frequency range has been driven by other applications such as spectroscopy, imaging, and impulse ranging. Only recently modulators and filters have been added to enable the development of communications applications. APL's contributions to date in THz research have been primarily in the areas of spectroscopy and imaging. This article gives an overview of THz technology for communications and sensing applications, with some discussion of the sources, detectors, and modulators needed for a practical THz communications system.

Terahertz waves for communications and sensing

A terahertz wave detector is for detecting a terahertz wave which is emitted from a terahertz wave generator and which is transmitted through a sample. The timing, at which a probe light is irradiated on an optical switching device in the terahertz wave detector, is vibratingly varied by driving a movable reflector in a variable optical delay device at a predetermined vibration frequency. The resultant detection signal generated thereby and changing periodically and vibratingly is subjected to frequency analysis by a spectrum analyzer in a spectroscopic processor. The detection signal has the same temporal waveform as that of the terahertz wave and subjected to scale conversion. Therefore, the frequency analysis of the detection signal enables frequency measurement of a terahertz wave in real time. As a result, real-time spectroscopy is possible, and the device configuration is simplified.

Terahertz wave spectrometer

The authors demonstrate the operation of an electro-optic terahertz transceiver based on the reciprocal behaviour of optical rectification and an electro-optic effect in a nonlinear optical crystal. With optical excitation and sampling of an ultrafast laser, the electro-optic terahertz transceiver alternately transmits pulsed electromagnetic radiation (optical rectification) and receives the return signal (electro-optic effect) from the same crystal.

Electro-optic terahertz transceiver

Various frequencies are spaced along the frequently used electromagnetic spectrum, including microwaves, infrared, visible light, and X rays. Terahertz radiation between microwave and infrared frequencies lies. In the electromagnetic spectrum, radiation at 1 THz has a period of 1 ps, a wavelength of 300 µm, a wave number of 33 cm−1, photon energy of 4.1 meV, and an equivalent temperature of 47.6 K. In the same way that visible light can create a photograph, radio waves can transmit sound, and X rays can see shapes within the human body, terahertz waves (Trays) can create pictures and transmit information.



Until recently, however, the very large terahertz portion of the spectrum has not been particularly useful because there were neither suitable emitters to send out controlled terahertz signals nor efficient sensors to collect them and record information. Recent developments in terahertz time-domain spectroscopy and related terahertz technologies now lead one to view the world in a new way. As a result of developing research, terahertz radiation now has widespread potential applications in medicine, microelectronics, agriculture, forensic science, and many other fields.



Three properties of THz wave radiation triggered research to develop this frequency band for applications and are discussed.



Coherent terahertz time-domain spectroscopy that has an ultrawide bandwidth provides a new method for characterizing the electronic, vibronic, and compositional properties of solid, liquid, and gas phase materials, as well as flames and flows. In theory, many biological and chemical compounds have distinct signature responses to terahertz waves due to their unique molecular vibrations and rotational energy levels, this implies that their chemical compositions might be examined using a terahertz beam.



A T-ray imaging modality would produce images that have “component contrast” enabling analysis of the water content and composition of tissues in biological samples. Such capability presents tremendous potential to identify early changes in composition and function as a precursor to specific medical investigations and treatment. Moreover, in conventional optical transillumination techniques that use near-infrared pulses, large amounts of scattering can spatially smear out the objects to be imaged. T-ray imaging techniques, due to their longer wavelengths, can provide significantly enhanced contrast as a result of low scattering (Rayleigh scattering).


Keywords:

terahertz wave;
generation of beams;
free space electro-optic detectors;
electro-optic measurement;
chirped probe pulse;
terahertz imaging

Terahertz Electric Field Imaging

We present the use of an electro-optic THz transceiver, which emits and receives THz signal by using one ZnTe crystal with femtosecond optical pulses, for THz sensing ranging and imaging applications. We demonstrate the operation of an electro-optic THz transceiver based on the reciprocal behavior of optical rectification and electro-optic effect in the nonlinear optical crystal. With optical excitation and sampling of an ultrafast laser, the electro-optic THz transceiver alternately transmits pulsed electromagnetic radiation (optical rectification) and receives the return signal (electro-optic effect) from the same crystal.

Electro-optic THz transceiver

The use of terahertz (THz) beams for imaging applications was first demonstrated by Hu and Nuss. We report on an alternative method for real-time terahertz (THz) imaging applications. By illuminating an electro-optic crystal with a THz beam and an optical readout beam, then detecting the optical beam with a linear diode array or a CCD camera, we achieved time-resolved ID or 2D images of pulsed far-infrared radiation. The temporal and spatial resolution of this imaging system are mainly limited by the laser pulse duration and the diffraction limit of the THz beam, respectively.

Q. Wu

Papers

Electro-Optic Imaging of THz Beams