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Terahertz spectroscopy detects and controls properties of matter with electromagnetic fields that are in the frequency range between a few hundred gigahertz and several terahertz (abbreviated as THz). In many-body systems, several of the relevant states have an energy difference that matches with the energy of a THz photon. Therefore, THz spectroscopy provides a particularly powerful method in resolving and controlling individual transitions between different many-body states. By doing this, one gains new insights about many-body quantum kinetics and how that can be utilized in developing new technologies that are optimized up to the elementary quantum level. Different electronic excitations within semiconductors are already widely used in lasers, electronic components, computers, to mention a few. At the same time, they constitute an interesting many-body system whose quantum properties can be modified, e.g., via a nanostructure design. Consequently, THz spectroscopy on semiconductors is relevant in revealing both new technological potentials of nanostructures as well as in exploring the fundamental properties of many-body systems in a controlled fashion. ==Background== There are a great variety of techniques to generate THz radiation and to detect THz fields. One can, e.g., use an antenna, a quantum-cascade laser, a free-electron laser, or optical rectification to produce well-defined THz sources. The resulting THz field can be characterized via its electric field ''E''THz(''t''). Present-day experiments can already output ''E''THz(''t'') that has a peak value in the range of MV/cm (megavolts per centimeter).〔Junginger, F.; Sell, A.; Schubert, O.; Mayer, B.; Brida, D.; Marangoni, M.; Cerullo, G.; Leitenstorfer, A. et al. (2010). "Single-cycle multiterahertz transients with peak fields above 10 MV/cm". ''Optics Letters'' 35 (15): 2645. doi:(10.1364/OL.35.002645 )〕 To estimate how strong such fields are, one can compute the level of energy change such fields induce to an electron over microscopic distance of one nanometer (nm), i.e., ''L'' = 1 nm. One simply multiplies the peak ''E''THz(''t'') with elementary charge ''e'' and ''L'' to obtain ''e'' ''E''THz(''t'') ''L'' = 100 meV. In other words, such fields have a major effect on electronic systems because the mere field strength of ''E''THz(''t'') can induce electronic transitions over microscopic scales. One possibility is to use such THz fields to study Bloch oscillations〔Feldmann, J.; Leo, K.; Shah, J.; Miller, D.; Cunningham, J.; Meier, T.; von Plessen, G.; Schulze, A.; Thomas, P.; Schmitt-Rink, S. (1992). "Optical investigation of Bloch oscillations in a semiconductor superlattice". ''Physical Review B'' 46 (11): 7252–7255. doi:(10.1103/PhysRevB.46.7252 )〕〔Ben Dahan, Maxime; Peik, Ekkehard; Reichel, Jakob; Castin, Yvan; Salomon, Christophe (1996). "Bloch Oscillations of Atoms in an Optical Potential". ''Physical Review Letters'' 76 (24): 4508–4511. doi:(10.1103/PhysRevLett.76.4508 )〕 where semiconductor electrons move through the Brillouin zone, just to return to where they started, giving rise to the Bloch oscillations. The THz sources can be also extremely short,〔Jepsen, P.U.; Cooke, D.G.; Koch, M. (2011). "Terahertz spectroscopy and imaging - Modern techniques and applications". ''Laser & Photonics Reviews'' 5 (1): 124–166. doi:(10.1002/lpor.201000011 )〕 down to single cycle of THz field's oscillation. For one THz, that means duration in the range of one picosecond (ps). Consequently, one can use THz fields to monitor and control ultrafast processes in semiconductors or to produce ultrafast switching in semiconductor components. Obviously, the combination of ultrafast duration and strong peak ''E''THz(''t'') provides vast new possibilities to systematic studies in semiconductors. Besides the strength and duration of ''E''THz(''t''), the THz field's photon energy plays a vital role in semiconductor investigations because it can be made resonant with several intriguing many-body transitions. For example, electrons in conduction band and holes, i.e., electronic vacancies, in valence band attract each other via the Coulomb interaction. Under suitable conditions, electrons and holes can be bound to excitons that are hydrogen-like states of matter. At the same time, the exciton binding energy is few to hundreds of meV that can be matched energetically with a THz photon. Therefore, the presence of excitons can be uniquely detected〔Timusk, T.; Navarro, H.; Lipari, N.O.; Altarelli, M. (1978). "Far-infrared absorption by excitons in silicon". ''Solid State Communications'' 25 (4): 217–219. doi:(10.1016/0038-1098(78)90216-8 )〕〔Kira, M.; Hoyer, W.; Stroucken, T.; Koch, S. (2001). "Exciton Formation in Semiconductors and the Influence of a Photonic Environment". ''Physical Review Letters'' 87 (17). doi:(10.1103/PhysRevLett.87.176401 )〕 based on the absorption spectrum of a weak THz field.〔Kaindl, R. A.; Carnahan, M. A.; Hägele, D.; Lövenich, R.; Chemla, D. S. (2003). "Ultrafast terahertz probes of transient conducting and insulating phases in an electron–hole gas". ''Nature'' 423 (6941): 734–738. doi:(10.1038/nature01676 )〕〔Kira, M.; Hoyer, W.; Koch, S.W. (2004). "Terahertz signatures of the exciton formation dynamics in non-resonantly excited semiconductors". ''Solid State Communications'' 129 (11): 733–736. doi:(10.1016/j.ssc.2003.12.015 )〕 Also simple states, such as plasma and correlated electron–hole plasma〔Kira, M.; Koch, S.W. (2006). "Many-body correlations and excitonic effects in semiconductor spectroscopy". ''Progress in Quantum Electronics'' 30 (5): 155–296. doi:(10.1016/j.pquantelec.2006.12.002 )〕 can be monitored or modified by THz fields. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Terahertz spectroscopy and technology」の詳細全文を読む スポンサード リンク
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