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Further, the development of a suitable waveguide is necessary Sirtori et al. It aims to confine light of the long wavelength within terahertz range to an epilayer compatible with molecular beam epitaxy technology and with low absorption losses onto the laser mode Tredicucci et al. Recent novel design concepts have led to CW operation and high output powers Rochat et al. In the recent years, terahertz QCLs have made rapid progress. More recent progress of QCLs in terahertz range is presented in Sect. One of considerable advantages of QCLs is the ability to tune the terahertz source to avoid regions of high water absorption.
A relevant experiment has been conducted Lee et al.
The output pulse is output from the end of the cavity dispersion, and then is tuned by a pair of matching extracavity prisms. Two concave focusing mirrors, a pair of Kerr lenses, allow passive mode locking, where M1 is a high reflector and M2 is an output coupler. This pair of Kerr lenses forms the optical cavity. When light travels between the two lenses, the energy is accumulated in the interval of the gain medium, which causes a population inversion. The TFP indicates a thin-film polarizing beam splitter.
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If the laser is Q-switched, a brief burst of light 2. Another important application of terahertz QCLs is in biomedical imaging; this is well illustrated by the work of Darmo et al. Emission of relatively strong CW terahertz radiation from cuprate superconductors is carried out by Ozyuzer et al. Josephson junctions are stacked within the layered high-temperature superconductors.
Currently, it is one of the most important laser sources for generating terahertz radiation Reid and Fedosejevs Ti:sapphire-based lasers are frequently applied for the generation of pulsed terahertz radiation. The principle of a Ti:sapphire-based laser is simply presented as follows.
It consists of a rod of Ti:sapphire titanium-sapphire , which can be pumped by a CW laser source. This light is focused into the Ti:sapphire rod collinearly with the laser axis through the back of a lightly silvered mirror. Dispersion arises from the variation of the refractive index of the crystal material across the gain bandwidth of the laser. The cavity dispersion output from the crystal material is Q-switched and self-mode-locked with the use of a pair of Kerr lenses Brabec et al. The schematic diagram regarding the Ti:sapphire pulsed laser oscillator is illustrated in Fig.
The output from the laser oscillator, in Fig. This alignment of the prism pair creates a longer path for red wavelengths propagating through the prism material, as compared with the blue, which yields a negative dispersion effect. It is illustrated in Fig. If the prism separation, l p defined tip to tip , is sufficiently large, the positive dispersion of the material can be balanced.
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The schematic shows a short injectoraccelerator combination compared to a stand FEL. After Grosse angle. Figure 2. The compressed output pulse is finally achieved from chirped cavity dispersion by the application of prism pairs. The photograph of a Ti:sapphire-based laser is shown in Appendix D in Fig. These features are essential for biological and medical studies Grosse and for the exploration of vibrational and configurational molecular transitions Kato et al.
Unlike femtosecond lasers, that is, Ti:sapphire-based lasers, where quantummechanical principles are employed, the FEL Krishnagopal et al. Two major components of the device are: electron accelerator and undulator. Generally, an undulator, such as in a linac or a synchrotron, is used to produce magnetic fields, which then drive an electron beam from an accelerator to achieve coherent terahertz pulsed radiation.
Different from the setup of conventional FELs, the terahertz FELs accelerate electrons directly after they leave the photocathode Volkov et al. The realization of a compact table-top full terahertz FEL system is now possible Grosse Terahertz radiation is generated when electrons propagate the serial connection of coupled quantum wells. These structures can be grown by molecular beam epitaxy with choice of the layer thickness, which determines the electron wave functions of the subbands.
The energy spacing of the lasing subbands determines the radiation frequency, allowing in principle to produce light at arbitrarily long wavelengths. The first THz QC laser with a photon energy less than the semiconductor optical phonon energy was demonstrated at 4. A chirped superlattice SL technique was adopted to design the QCL active region, with advantage of very large current-carrying capabilities and optical powers Tredicucci et al.
The bound-to-continuum QCL takes place between an isolated upper state and a miniband with a small well adjacent to the injection barrier Faist et al. The design combines the advantages of the three QW and superlattice active regions. It makes it possible to achieve selective injection into the upper state and fast extraction from the lower state.
Terahertz Imaging for Biomedical Applications
As a result, these designs display improved temperature and power performance compared with the CSL designs Williams ; Alton The laser was made at the University of Cambridge, by Jesse Alton, in collaboration with his colleagues Alton One period of the conduction band profile for each active region is displayed along with the calculated moduli squared of the most relevant wave functions. The upper, and lower state of the laser transition and the injector miniband ground state are labeled 2, 1, and g, respectively.
The transition energy, given by the energy difference between 2 and 1, is 12 meV.
At alignment, the wave functions of the upper and lower state are broadly overlapped. The upper state wave function is concentrated mainly in the two quantum wells adjacent to the injection barrier, reducing the overlap with the lower energy states in the injector miniband, thereby enhancing the non-radiative upper state lifetime.
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Electrons are injected into the upper state from state g through the injection barrier. The active region consists of 85 identical repeat periods. After Alton Both active regions are embedded between identical cladding layers: top contact layer is an nm-thick GaAs, and bottom contact layer is a nmthick GaAs. The two contact layers are designed regarding the surface plasmon waveguide. Illustration of the measured spectrum depending on the applied magnetic field.
The line on the right indicates the emission frequency of 2.https://icfersohan.tk
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After Scalari et al. Though the impact of this interlaced photon—phonon cascade laser has been limited, they are particularly notable for achieving very long wavelength operation Williams The other major active region type is the resonant-phonon RP scheme. The key design is to use resonant LO-phonon scattering to selectively depopulate the lower radiative level, while maintaining a long upper level lifetime. Owing to the lack of a miniband, the RP designs tend to have smaller oscillator strength Williams et al. At present, a QCL emitting at as low as 1.
An electrically switchable, two-color QCL emitting at 1. A magnetic field is applied perpendicularly to the layers to increase the gain enabling laser action. The structure is based on a large single quantum well, and multi-wavelength operation is obtained by selectively injecting carriers via resonant tunneling into one of the excited states of this large quantum well Sirtori et al. Bulk EO rectification difference frequency mixing and ultrafast charge transport techniques have been 18 2 Terahertz Sources and Detectors exploited.
The resulting nonlinear mixing of the Fourier components, produces a broadband extending up to THz frequencies.
Pattern Recognition and Tomographic Reconstruction
Considering a time-dependent polarization induced in the THz frequency range, it is proportional to the intensity of the incident pump pulse Davies et al. Since the resulting pulses have a bandwidth of a few THz, the difference frequencies fall in the THz range Schmuttenmaer In a material with normal dispersion, the refractive index n increases with the frequency. An alternative approach to obtain the phase matching condition is by exploiting the k-vectors of the extraordinary and the ordinary axes Kubler et al. Further information on the nonlinear phase matching for terahertz generation can be found in the literature Berger and Sirtori ; Sun et al.
Zinc telluride ZnnTe is regarded as the most popular EO material due to its physical durability and excellent phase-matching properties Rice et al. The organic crystal DAST has a very 2. The ultrafast pump pulse induces a transient polarization, p t , which in turn emits a T-ray bandwidth pulse. After Davies et al.
Saturation of OR due to second harmonic generation of the pump beam at high optical fluences has been studied in ZnTe Sun et al.
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