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Other Optical Effects
Published in Mary Anne White, Physical Properties of Materials, 2018
If light of frequency ν is incident on an NLO material, light of frequency ν and 2ν (and to a lesser extent 3ν, 4ν, etc.) will result. The production of light of frequency 2ν in this manner is known as frequency doubling by second-harmonic generation (SHG). This property is used in practice in NLO devices to achieve frequencies twice that of the incident light, for example using Ba2NaNb5O15, KH2PO4, LiNbO3 or HIO3. (The associated DC component of the electric field is referred to as optical rectification.) SHG is considered to be a three-wave mixing process since two photons with frequency ν are required to combine to give a single photon with frequency 2ν (energy is conserved). By analogy, third-order harmonics, at frequency 3ν from the γE3 term of Equation 5.9, require four-wave mixing.
O
Published in Philip A. Laplante, Comprehensive Dictionary of Electrical Engineering, 2018
optical proximity correction (OPC) a method of selectively changing the shapes of patterns on the mask in order to more exactly obtain the desired printed patterns on the wafer. optical proximity effect proximity effect that occurs during optical lithography. optical pumping excitation of an atom or molecule resulting from absorption of optical frequency electromagnetic radiation; the electromagnetically assisted accumulation of population into or out of one or more states of a quantum mechanical system. In practice, this generally involves selective absorption of the electromagnetic field to populate an excited state, followed by a less selective decay into more than one ground state. For example, a system having ground state spin sublevels can be optically pumped by circularly polarized light into a single ground state spin sublevel. In a multilevel system, more selective transfer of population from one state to another can be achieved by adiabatic passage. optical rectification the second-order nonlinear optical process in which a material develops a static electric field in response to and proportional to the square of the strength of an applied optical field. optical repeater optoelectric device that receives a signal and amplifies it and retransmits it. In digital systems, the signal is regenerated. optical representation of binary numbers the representation of binary numbers 0 and 1 using light. Since an optical detector is sensitive to light intensity, it is a very logical choice to represent
THz Photonics
Published in Chi H. Lee, Microwave Photonics, 2017
Albert Redo-Sanchez, X.-C. Zhang
The main factors that affect the effectiveness of optical rectification are the pulse duration of the laser, the phase matching condition, and the absorption of the EO crystal. The pulse duration determines the bandwidth of the THz pulse. Shorter pulses are expected to extend the bandwidth of the THz wave generated. Actually, with the development of sub 10 fs lasers, bandwidth of 100 THz could be generated. With such a broadband pulse, it is impossible to select EO materials that fulfill the condition of group velocity matching for all frequency components. To reduce the effects of different group velocity, the EO crystal must be thin in order to obtain broad bandwidth emission. On the other hand, if the EO crystal is very thin, secondary pulses or echoes generated by multiple reflections within the EO crystal will mix with the main pulse and make signal analysis difficult. The time between echoes should be much larger than the pulse duration itself. Finally, the THz wave absorption in the EO crystal also affects the performance. For instance, the thicker the crystal is the higher the absorption, which results in the lower THz output. Generally, materials with a large second-order nonlinear coefficient are good candidates for THz source [15]. Currently, ZnTe is the material of choice due to its high nonlinear coefficient, high damage threshold, and best phase match condition with a Ti:sapphire laser [16].
Terahertz generation and detection of 1550-nm-excited LT-GaAs photoconductive antennas
Published in Journal of Modern Optics, 2021
Zhi-Chen Bai, Xin Liu, Jing Ding, Hai-Lin Cui, Bo Su, Cun-Lin Zhang
Among typical THz spectroscopy, terahertz time-domain spectroscopy (THz-TDS) is a very effective coherent detection technology [5]. THz wave generation and detection is a crucial technology. Presently, THz waves are mainly generated via optical rectification and photoconductive antennas. Optical rectification generates a low-frequency polarization field through the interaction between a pulsed laser and a non-linear medium to radiate THz. Optical rectification-generated THz waves are related to pulsed lasers and non-linear media [6]. Under the action of an ultrafast laser pulse and a bias electric field, photoconductive materials produce carriers, which accelerate and radiate THz waves. These THz waves are related to the photoconductive material, antenna structure, and ultrafast laser pulse width [7]. Photoconductive antenna-generated THz waves are usually stronger than optical rectification-generated ones because the THz energy generated via optical rectification only comes from the incident laser. In contrast, photoconductive antenna-generated THz waves are also related to the incident laser but can be adjusted by setting a bias voltage. With the development of semiconductor technology, photoconductive antennas [8,9] have attracted increased attention. Currently, gallium arsenide (GaAs), indium gallium arsenide (InGaAs), and indium aluminium arsenide (InAlAs) [10,11] are the main materials used in THz wave generation and detection research.