At BESSY II it is demonstrated that far-infrared coherent synchrotron radiation (CSR) can be generated by a controlled, steady-state process at storage rings. As an indication for coherent emission, the radiated power grows with the square of the beam current. The spectrum was analyzed by an interferometer in the 1-mm to 0.3-mm wavelength range. The CSR was enhanced more than 3000 times above background; the incoherent radiation remained below the background level. Steady-state and bursting CSR were discriminated by time resolved analysis from micro seconds to seconds.
We present a model describing high power stable broadband coherent synchrotron radiation (CSR) in the terahertz frequency region in an electron storage ring. The model includes distortion of bunch shape from the synchrotron radiation (SR), which enhances higher frequency coherent emission, and limits to stable emission due to an instability excited by the SR wakefield. It gives a quantitative explanation of several features of the recent observations of CSR at the BESSY II storage ring. We also use this model to optimize the performance of a source for stable CSR emission.
PTB and DLR join their expertise and experience in optical radiometry and in THz techniques to perform what is to our knowledge the first traceable measurement of radiant power of a THz quantum cascade laser and the first absolute calibration of a THz radiation detector against a cryogenic radiometer (CR). A total standard uncertainty of 7.3% was achieved at a frequency of 2.5 THz corresponding to a wavelength of 120 µm. This uncertainty is dominated by the limited knowledge of the absorptance of the CR cavity. All other uncertainty contributions including those arising from diffraction are only 2%.
The principles of stimulated emission for the terahertz frequency range from shallow impurity centers in bulk semiconductors are discussed. Evidence of stimulated emission from group V donors embedded in silicon (P, Sb, Bi) as well as spontaneous emission from Li acceptor in monocrystalline ZnSe under excitation by CO 2 laser radiation at low temperatures is demonstrated. The temporal behavior and the emission spectra of the lasers are presented. IntroductionToday, semiconductor lasers bridge a wide range of the electromagnetic spectrum. Many everyday applications in information technology and communications are covered by well-known band-gap visible and infrared diode lasers.Far-infrared (or terahertz, THz) semiconductor lasers are required for applications in molecular and solid-state spectroscopy. However, there is a problem in creating population inversions of charged carriers in semiconductors for the THz spaced states because of the fast phonon-assisted and Auger relaxations that usually tend to thermalize carrier distributions. New proposals for THz semiconductor oscillators are based on the intra-band optical transitions: p-Ge intra-valence band bulk lasers [1], inter-subband quantum cascade [2, 3] and fountain lasers [4] as well as SiGe strained heterostructure lasers [5]. These lasers use the fast low-temperature relaxation processes to maintain the population inversion. In this report the results of an investigation of semiconductor oscillators based on the intra-center optical transitions of shallow donors and acceptors are presented. It is shown that both the acoustic phonon-assisted (AP) and optical phonon-assisted (OP) relaxations can be successfully used to reach population inversion between bound excited states of impurity centers and to obtain lasing under optical excitation.Different mechanisms of population inversion for THz light generation based on the phonon-assisted relaxation of non-equilibrium electrons captured on shallow impurity centers in silicon have been proposed recently [6]. One of them is based on the suppression of the AP relaxation for a particular excited state. This results in a relatively long lifetime (~10 −9 s) of the 2p 0 excited neutral donor (P, As, Sb) state, which can be used for the accumulation of charge carriers [7]. Another mechanism uses the resonant coupling of the 2s and 2p 0 excited states of the Bi donor to the ground state via inter-valley optical pho-
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