Magnetic-field-induced fourfold azimuthal angle dependence in THz power of InAs
by Elmer S. Estacio
Institute of Laser Engineering, Osaka University, Osaka, Japan
The terahertz (THz) frequency region, spanning the 100 GHz to 10 THz range, bridges the gap between electronics (millimeter and sub millimeter waves) and photonics (far infrared) as shown in Fig. 1. At present, interest in THz research has grown because of the wealth of promising applications in the fields of fingerprint spectroscopy of biomolecules, environment monitoring, semiconductor and medical imaging, and even law enforcement. The progress in THz technology, however, has been hampered by the lack of intense radiation sources and the sheer size and difficulty of operation of several proposed designs. Several THz sources include optically pumped THz lasers, frequency mixers, and semiconductors (surfaces and photoconductive emitters) An optically pumped THz laser consists of a grating-tuned carbon dioxide pump laser and a far-infrared gas cell mounted in a laser resonator. Frequency mixers, on the other hand, rely on a nonlinear optical process called difference frequency generation in a nonlinear crystal and utilize two lasers of slightly different wavelengths. Our research group works with semiconductor-based THz emitters. Femtosecond laser pulses, incident on semiconductor surfaces cause the generation of THz electromagnetic radiation. Moreover, these materials may be fabricated as photoconductive antennae to function as either emitters or detectors in the THz regime. In particular, our interest lies with InAs which is gaining grounds as a compact and easy-to-use THz emitter with frequencies spanning the 0.2 to 2 THz range.
Figure 1. The THz radiation spectral region bridges the gap between photonics (infrared) and electronics (microwaves). Due to its long wavelength and the fact that important “fingerprint” vibrational and rotational molecular motions are in this regime, a multitude of important applications await its advancement into a full-blown technology. (Image from http://www.zomega-terahertz.com)
The generation of intense terahertz (THz) radiation from InAs surfaces illuminated with ultrashort optical pulses has been the subject of immense work in the past decade. Consequently, numerous studies have been conducted to explain the prevalent THz radiation mechanism in these materials. The established THz radiation mechanisms are optical rectification, and the surge current. Optical rectification is a nonlinear optical process that generates THz radiation via a proportional 2nd order nonlinear polarization. In this case, the radiation is strongly dependent on the crystal orientation relative to the laser polarization. Thus, azimuthal angle-dependence of the radiation is observed by rotating the sample about its surface normal. The surge current THz emission is primarily due to Ampere’s Law which states that a moving charge generates an electric field. After optical excitation, electrons and holes are accelerated in opposite directions forming a surge current normal to the semiconductor surface. With a magnetic field applied parallel to the surface, the electron-hole dipole moment is rotated according to the Lorentz force and THz radiation collection is enhanced. Surge current radiation is generally thought to be independent of azimuthal rotation. This work presents results on the investigation of the THz radiation mechanism in (100) p-type InAs under 1 T magnetic field and reports a symmetry-folding of its azimuthal angle dependence.
Figure 2. The two-fold symmetry that was expected for the azimuthal angle dependence of the p-polarized THz radiation power was modified to a four-fold symmetry with an applied 1-T magnetic field. These results are attributed to an anisotropic inter-valley scattering of the photogenerated carriers.
In our experiment, the optical excitation a mode-locked, 100-fs Ti-Sapphire laser was set at incidence 45º with respect to the InAs surface and the THz radiation was collected in the reflection direction. A 1-T magnet field was applied parallel to the sample surface, normal to the plane of incidence. The THz radiation was then detected using an InSb hot electron bolometer. The semiconductor wafer was rotated about its surface normal and the azimuthal angle dependence of the THz emission was taken. In general, the optical rectification contribution will have an oscillatory, two-fold symmetric dependence and will be superposed to a DC offset due to the angularly independent surge current contribution. Although the dominant THz mechanism was determined to be the surge current as seen from the high DC offset in Fig. 2, we were also able to observe a contribution from the optical rectification effect. A two-fold azimuthal symmetry for the THz radiation with no applied field was observed as shown by the closed circle trace. The open circle trace shows the behavior of the “with magnetic field” case. Interestingly, a four-fold symmetric, azimuthal angle-dependent feature appeared to be superposed on the usual two-fold dependence. Initially, this modified symmetry was thought to originate from a nonlinear effect due to a magnetization-induced nonlinear optical susceptibility. However, calculations failed to yield the four-fold symmetry that was observed in the experiment. Tentatively, it is suggested that the anisotropy in the inter-valley scatterings of carriers in four equivalent directions in the crystallographic half-space imply the creation of an electric quadrupole moment. This relatively weak quadrupole response is thought to cause the four-fold symmetry in the with field case regardless of the actual THz radiation mechanism being surge current or a nonlinear optical effect. With a transversely-applied field, the quadrupole and dipole-related emission may be enhanced with the associated tilting of their electric moments from the sample surface normal according to the Lorentz force. Knowledge of such carrier scattering is important in the design considerations for future InAs-based terahertz emitters.
- E. Estacio, H. Sumikura, H. Murakami, M. Tani, N. Sarukura, M. Hangyo, C.Ponseca, R. Pobre, R Quiroga, and S. Ono, App. Phys. Lett., 90, 151915(2007)
- M. Yamashita, K. Kawase, C. Otani, T. Kiwa ,M. Tonouchi, Optics Express, 13, 115(2005)
- P. Gu and M. Tani, in Terahertz Optoelectronics, Topics in Applied Physics 97, edited by K. Sakai (Springer-Verlag, Germany, 2005)
Dr. Elmer Estacio got his BS, MS, and Ph.D in Physics at the National Institute of Physics, University of the Philippines under the supervision of Professor Arnel Salvador. He joined Professor Nobuhiko Sarukura’s group at the Institute for Molecular Science (IMS) in Okazaki as a postdoctoral fellow in 2005. After 1 year, the laboratory transferred to the Institute of Laser Engineering in Osaka University. Prior to his current stay in Japan, he was working on MBE-grown, GaAs-based optoelectronic devices, which was not entirely different from his new area of study, semiconductor-based terahertz research. He currently lives in Toyonaka City in Osaka with his wife and his son, Damien.