Surface-Field Terahertz Emission Enhancement via 2D-Materials

Summary form only given. Narrow-bandgap III-V semiconductors are proving to be excellent terahertz (THz) emitters when excited by ultrashort optical pulses. The specific nonlinear process contributing to the emission is inherently complicated by the fact that many different mechanisms contributing to the nonlinearity exist [1]. Surface field surge currents, photo-Dember and third order surface field-seeded optical rectification (SOR) are the most commonly acknowledged source phenomena. SOR relies on the static electric field resulting from charges populating the material surface states, which is responsible for the band bending and formation of a depletion region at the interface [2]. SOR in Indium Arsenide has been singled out as the dominant generation mechanism at high excitation energies. THz generation is typically limited by the saturation of the relevant field-matter interaction mechanism [3], the damaging threshold of nonlinear crystals or the lack of suitable phase matching between THz and optical fields in real materials. We have recently demonstrated in [4] that for SOR, the quadratic dependence between the emitted THz energy and the pump energy is re-established at extreme excitations. Increasing the impinging optical pump further, however, becomes impractical as the optical damage threshold of the semiconductor is approached. It would, therefore, be greatly beneficial to instead enhance the surface field to enable a continued increase in the emitted THz field. One such technique involves biasing the surface using a thin, transparent electrode layer. Such as that which has been used to assess the surface potential of silicon surfaces [5]. However, this will add significant cost and complexity to the system, especially if high voltages are required. Alternatively, a passive modification to the surface field can be achieved by introducing additional material layers, thus developing additional interfacial fields. 2D-materials such as graphene represent a promising material to achieve this aim, being both large-area scalable and can be easily manufactured in large quantities [6]. Here we present our experimental characterisation of the THz field emitted from 2D-material coated semiconductor surfaces. We demonstrate that the addition of graphene flakes with varying surface coverage concentrations enables a progressively enhanced surface field (until material percolation kills the 2D nature of the flakes) which we probe by observing as much as a 26.6% increase in the peak THz field relative to the uncoated semiconductor, as shown in figure 1 (b). We believe that this work enables new applications for 2D materials in the THz range, specifically in the development of ultrathin THz sources based on parametric nonlinearities