Abstract-Terahertz excitation spectra of InP single crystals

Ričardas Norkus, Andrius Arlauskas, Arūnas Krotkus

http://iopscience.iop.org/article/10.1088/1361-6641/aac5a4

Investigation of terahertz (THz) pulse generation from semi insulating and n type InP crystals surfaces is presented in this letter. In order to determine energy separation between the main and subsidiary conduction band valleys, THz pulse amplitude dependences on the photoexcitation wavelength (in a range of 410 – 950 nm) were measured. These dependences had a clear maximum at ~540 nm, which was used to determine the intervalley energy separation in the conduction band of InP as equal to 0.75 eV. Moreover, THz generation mechanisms in InP surface were investigated by additionally analysing the azimuthal angle dependences of the emitted THz signal amplitude and power. It has been shown that the main physical mechanism of the surface THz emission is spatial separation of photoexcited electrons and holes, which can also lead to a symmetry similar to the second-order optical non-linearity. Photocurrent surge in the surface electric field can also contribute to the THz emission from a semi-insulating crystal illuminated by optical pulses with wavelengths close to the absorption edge.

Abstract-Strong THz emission and its origin from catalyst-free InAs nanowire arrays

Andrius Arlauskas , Julian Treu , Kai Saller , Ieva Beleckaitė , G. Koblmueller , and Andrius Krotkus

Nano Lett., Just Accepted Manuscript

DOI: 10.1021/nl404737r

Publication Date (Web): February 6, 2014

Copyright © 2014 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/nl404737r?journalCode=nalefd

The unique features of nanowires (NW), such as high aspect ratio and extensive surface area, are expected to play a key role in the development of very efficient semiconductor surface emitters in the terahertz (THz) spectral range. Here, we report on optically excited THz emission from catalyst-free grown arrays of intrinsically n-type InAs NWs using THz time-domain spectroscopy. Depending on aspect ratio, the THz emission efficiency of the n-type InAs NWs is found to be up to ~3 times stronger than that of bulk p-type InAs, known as currently the most efficient semiconductor-based THz surface emitter. Characteristic differences from bulk p-type InAs are particularly revealed from excitation wavelength-dependent measurements, showing monotonously increasing THz pulse amplitude in the NW arrays with increasing photon energy. Further polarization-dependent and two-color pump-probe experiments elucidate the physical mechanism of the THz emission: In contrast to bulk p-type InAs, where the anisotropic photoconductivity in the surface electric field is the dominant cause for THz pulse generation, the origin of the intrinsic THz emission in the NWs is based on the photo-Dember effect. The strong THz emission from high aspect ratio NW arrays further suggests an improved out-coupling of the radiation, while further enhancements in efficiency using core-shell NW geometries are discussed.