Emitter Optimization
The realization of many attractive applications for coherent THz-spectroscopy fails due to the low efficiency of existing sources. Conventional amplification mechanisms based on stimulated emission (like lasers) cannot be transferred to optically excited THz emitters. The necessary population inversion for this purpose - if possible at all - is very difficult to obtain. The THz emission of an optically excited semiconductor emitter is based on coherent acceleration of charge carriers. Dephasing caused by scattering processes destroys the coherence of this translation and limits the coherent emission period. Therefore, the emitted coherent energy meets the total available energy only at a fraction. One possibility to improve the emitter efficiency thus results in an increase of emission rate and therefore in amplification of the coherently emitted energy. As part of this goal, the Institute of Semiconductor Electronics follows two different approaches: (i) the coherent superposition with a THz- background field, which is based on a special THz-resonator (Fig. 1) (Inversionless amplification [1]) and (ii) the optimization of dielectric ambient conditions (Fig. 2) (Increase of photonic density of states [2]). An increase of efficiency on the order of one magnitude has been demonstrated with both techniques.
Fig. 1: (a) Standard-THz setup for generation and detection of THz radiation, (b) THz-resonator setup. The emitter simultaneously undertakes the task of the outcoupler.
Fig. 2: Standard surface emitter (left) and modified emitter with sapphire covering. Back side metallization (gold) and copper heat sink (right).
Publications:
[1] C. Janke, P. Haring Bolivar, A. Bartels, H. Künzel, and H. Kurz, "Inversionless amplification of coherent THz radiation", Phys. Rev. B 67, 155206 (2003).
[2] M. Zedler, C. Janke, P. Haring Bolivar, H. Künzel, and H. Kurz, "Improved coherent THz emission by modification of the dielectric environment", Appl. Phys. Lett. 83 (20), 4196 (2003).
