Cavity-enhanced optical Hall effect in two-dimensional free charge carrier gases detected at terahertz frequencies

Abstract

The effect of a tunable, externally coupled Fabry-Pérot cavity to resonantly enhance the optical Hall effect signatures at terahertz frequencies produced by a traditional Drude-like two-dimensional electron gas is shown and discussed in this communication. As a result, the detection of optical Hall effect signatures at conveniently obtainable magnetic fields, for example by neodymium permanent magnets, is demonstrated. An AlInN/GaN-based high electron mobility transistor structure grown on a sapphire substrate is used for the experiment. The optical Hall effect signatures and their dispersions, which are governed by the frequency and the reflectance minima and maxima of the externally coupled Fabry-Pérot cavity, are presented and discussed. Tuning the externally coupled Fabry-Pérot cavity strongly modifies the optical Hall effect signatures, which provides a new degree of freedom for optical Hall effect experiments in addition to frequency, angle of incidence and magnetic field direction and strength.The optical Hall effect (OHE) in semiconductor layer structures is the occurrence of magneto-optic birefringence detected in response to incident electromagnetic radiation, caused by movement of free charge carriers under the magnetic field-induced influence of the Lorentz force [1]. In general, this birefringence leads to polarization mode coupling which is conveniently detected by generalized ellipsometry at oblique angle of incidence and at terahertz (THz) frequencies, for example. THz-OHE has recently been demonstrated as non-contact and therefore valuable tool for the investigation of free charge carrier properties in semiconductor heterostructures [2][3][4][5][6][7][8][9]. Previous instrumental approaches, discussed more detailed in Refs. 2 and 9, rely on high magnetic fields provided either by conventional, water-cooled or superconducting, liquid He-cooled electromagnets resulting in comparably large and costly experimental setups. In general, OHE configurations capable of detecting signals at low and conveniently obtainable magnetic fields are desirable. The use of small magnetic fields for THz magnetooptic measurements was demonstrated recently by Ino et al. for bulk-like InAs [10]. Due to low effective mass and high charge carrier concentration, the low field still yielded large enough signals for detection. The signalto-noise separations of the OHE signatures depend on many factors, the most important are low effective mass, high mobility and high carrier density, but also crucial is the thickness of the physical layer that contains the charge carriers. The OHE signatures scale, in first approximation, linearly with the magnetic field amplitude. Hence, the first approach to detect OHE signatures in samples with very thin layers, or low-mobile, heavy-mass and low-density charge carriers where the OHE signatures are weak is to increase the magnetic field amplitude.In this communication, we demonstrate and exploit the enhancement of the OHE signal obtained from samples IG. 1. Schematic drawing of the beam pa...

Properties of two‐dimensional electron gas in AlGaN/GaN HEMT structures determined by cavity‐enhanced THz optical Hall effect

“…Details on the THz OHE methodology can be found in Ref. [8]. Ellipsometric data was acquired and analyzed using the Stokes vector formalism which describes the sample optical response via a 4x4 Mueller matrix [9].…”

Advanced Terahertz Frequency-Domain Ellipsometry Instrumentation for In Situ and Ex Situ Applications

“…Furthermore, the gas cell is equipped with a high-grade neodymium permanent magnet with a surface field of B = 0.55 T Tesla for in-situ optical Hall effect measurements (see Sec.V-D). Figure 3 c) depicts a newly designed air-gap-manipulator sample stage for cavity-enhanced and cavity-enhanced optical Hall effect measurements [42]. The air-gap manipulator sample stage allows for opening an air gap between a highly reflective (metal) surface and the backside of the sample.…”

“…The OHE allows for detection of three free charge carrier parameters, charge density, charge mobility and effective mass [54][55][56][57][58]. The magnitude of the OHE birefringence can be further tuned by adding a cavity with varying thickness on the backside of a (transparent) sample [42,59,60].…”

Study of novel electronic materials by mid-infrared and terahertz optical Hall effect

“…It has been shown that the use of a backside cavity with a highly reflective backside surface can enhance the OHE in HEMT structures and EG, as a result of the formation of Fabri-Pérot modes, within the sample-cavity system [37][38][39]. An optical scheme of the cavity-enhanced (CE)-OHE measurement for a sample containing a transparent substrate and a conductive layer on top is shown in Figure 1.3.…”