Terahertz Resonances in the Dielectric Response Due to Second Order Phonons in a GaSe Crystal
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Terahertz Resonances in the Dielectric Response Due to Second Order Phonons in a GaSe Crystal Baolong L. Yu1, Hakan Altan1, Fanang Zeng1, Vladimir Kartazayev1, Robert Alfano1, and Krishna C. Mandal2 1 IUSL-Physics, City College of New York, Marshak Bldg, Rm J201, 160 Convent Avenue, New York, NY, 10031 2 EIC Laboratories INC., 111 Downey Street, Norwood, MA, 02062
ABSTRACT The dielectric function and momentum relaxation time of carriers for a single-crystal GaSe were investigated using terahertz time-domain spectroscopy over the frequency range from 0.4 to 2.4 THz. The key parameters determined from THz data using the Drude model are: the plasma frequency ωp =6.1 ± 0.5 THz, the average momentum relaxation time = 51 ± 6 fs, and the mobility µ = 89 cm2 / Vs for electrons. The THz absorption spectrum showed resonance structures attributed to the difference frequency combinations associated with acoustical and optical phonons. INTRODUCTION Gallium selenide (GaSe) is a promising semiconductor crystal for nonlinear optics and THz generation with an extremely large bandwidth up to 41 THz. 1, 2 GaSe is a negative uniaxial ___
layered semiconductor with hexagonal structure of 62 m point group with a direct band gap at E gd = 2.202 eV and an indirect gap E gi = 1.995 eV at T= 300 K. The longitudinal and transverse optical phonons are located at 254 and 213 cm-1, respectively. 3, 4 Recently, GaSe was used in the generation of 0.18 to 5.27 THz electromagnetic waves using a difference-frequency technique 4 and also used in phase-matched optical rectification with a Ti: sapphire laser from 0.4 to 41 THz. 1 GaSe is becoming a material of choice for a THz emitter and sensor. Timedomain spectroscopy (THz-TDS) can achieve a high signal-to-noise ratio (~104) using coherent phase detection for characterizing a variety of materials, including molecular vapors, liquids, semiconductors, superconductors, biomedical molecules and tissues, nanostructures materials, and artificial metallic structures. 5-7 Recently, THz-TDS has also been used to investigate the lattice absorption associated with the fundamental and second order phonon resonances in semiconductors.8 Here we report on the dielectric response measurements in the far-infrared absorption of z-cut crystals of doped GaSe using THz-TDS in the frequency range 0.4 - 2.4 THz. Absorption peaks at 0.80 THz (26.7 cm-1), 1.24 THz (41.3 cm-1), 1.57 THz (52.3 cm-1) and 1.84 THz (61.3 cm-1) were observed which are attributed to the difference frequency combinations of acoustical and optical phonons (see Table 1). The key parameters extracted from data analysis using the Drude model for the dielectric response are the plasma frequency ωp, the average momentum relaxation time , the mobility and the frequencies of the combinations of acoustical and optical phonons in the THz region.
THEORY (OR EXPERIMENT) THz pulses were generated using 70 fs optical pulses by optical rectification in a ZnTe crystal. The details of the experimental system are described in references 7 and 9. The e
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