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Efficient Generation of Terahertz Pulses from InN The terahertz region of the electromagnetic spectrum, lying between microwave frequencies (100 GHz) and photonic frequencies (30 THz, wavelength = 10 µm), is an area rich with possibilities. Terahertz signals can penetrate through many dry materials like plastics, paper, fabrics, dust, and cardboard, but unlike x-rays THz radiation is non-ionizing and harmless to tissue. Many molecules exhibit distinct rotational and vibrational spectral features in the THz range. Because of these unique properties, THz radiation has attracted attention for a range of applications including chemical identification, concealed weapon detection, and biological imaging. In spite of its potential, the THz regime remains one of the least explored portions of the electromagnetic spectrum, in part because of the difficulty in efficiently generating and detecting terahertz radiation. One promising and powerful technique for generating broadband terahertz radiation is to illuminate a target material with ultrafast optical pulses. The target may be either a non-linear crystal or semiconductor , or a photoconductive dipole antenna . Such systems generate electromagnetic bursts that contain frequencies extending into the terahertz regime. The electric field of the THz pulse is detected using an electrooptic technique in which the THz field modulates the polarization state of an optical probe pulse. These THz time-domain systems allow one to directly measure the electric field of a THz pulse, from which it is possible to determine the spectrum via Fourier transform . Traditionally, pulsed-THz systems utilize femtosecond mode-locked lasers with a wavelength near 800 nm. The large size and cost of these table-top mode-locked lasers make such systems impractical for field-deployable applications. There is a growing need for compact, robust terahertz systems, especially in military applications where reliability, size, weight, and portability are of paramount importance. The goal of this work is to demonstrate THz emission from indium-nitride (InN) films using short optical pulses at 1550 nm. There are significant advantages to using 1550 nm laser pulses instead of shorter wavelengths. Mode-locked fiber lasers operating at 1550 nm are much more compact and stable than free-space Ti-sapphire lasers, and the extremely low loss of optical fiber at 1550 nm could enable remote delivery of the THz-generating pulses. THz systems based 1550 nm technology could take advantage of the vast array of optoelectronic technologies originally developed for the telecommunication industry.
Fig. 1. Diagram of proposed system for generating TeraHertz radiation from InN. An optical parametric oscillator is used to generate tunable short pulses at wavelengths near 1550 nm. References
Note: This is not a comprehensive list of references such as you might find in a well-written journal article; it is provided only as a suggested starting point for visitors interested in learning more about this subject.
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