1.

INTRODUCTION

In 2D layered materials, atomic layers with strong in-plane covalent bonds stack together via weak van der Waals forces. The 2D material family that has recently attracted renewed attention is group-IV monochalcogenides GeS, GeSe, SnS and SnSe. They are non-toxic and composed of earth-abundant elements. With high optical absorption in the visible and near-infrared range, combined with predicted ultrahigh carrier mobility within the layers, they are promising for applications in solar energy conversion and optoelectronics. Recent theoretical predictions also suggest that group-IV monochalcogenide monolayers combine robust room temperature ferroelectricity and ferroelasticity with giant spontaneous electric polarization.^1-5 In-plane electric polarization results from an elastic distortion of the lattice, as the top and the bottom atoms shift in the armchair direction, as shown in Fig. 1(a) ^{2, 6}. Of group-IV monochalcogenides, GeS and GeSe are predicted to exhibit the highest polarization, ~5 x 10^–10 C/m and ~3 x 10^–10 C/m at zero temperature, respectively.². In-plane polarization breaks inversion symmetry of the monolayer, and can enable second order nonlinear optical effects such as optical rectification, a non-resonant effect, and a shift current, a resonant effect that is expected to dominate under above the band gap photoexcitation conditions.^{4, 6, 7}. A shift current is an instantaneous spatial shift of electron density distribution along S-Ge or Se-Ge bond (Fig. 1 (a)). ^{6, 8}, and it has been proposed as a mechanism behind the bulk photovoltaic effect (BPVE), a bias-free photocurrent generation that has been proposed as a mechanism for efficient third generation photovoltaics (Butler, Frost et al. 2015, Tan, Zheng et al. 2016). Here, we have used terahertz (THz) emission spectroscopy to explore generation of THz radiation in multi-layer GeS and GeSe crystals in response to the above band gap excitation with ultrashort laser pulses. We attribute the observed THz generation to the surface shift current in these 2D materials.

Figure 1.

(a) A spontaneous electric polarization vector along the armchair direction in the surface layer of GeS or GeSe^4,7 THz waveforms taken at different sample orientations, indicating that rotating sample by 180° reverses polarity of the emitted pulse in (c) GeS (15 μJ/cm² excitation fluence) and (d) GeSe (190 μJ/cm² excitation fluence).

2.

EXPERIMENT

3.

RESULTS AND DISCUSSION

Excitation with 400 nm (3.1 eV) pulses ensures direct interband excitation in both GeS and GeSe. We find that both systems emit nearly single-cycle electromagnetic field transients (Fig. 1 (c) and (d)). Normal incidence geometry, above band gap, linearly polarized excitation in the absence of external electrical bias suggest that the observed emission is indeed a result of a shift current.^10,19 Data shown in Figures 1(c) and 1(d) have been taken with the pump polarization unchanged and parallel to the THz detection. The figures shows that rotating the sample by 180° reverses the polarity of emission for both GeS and GeSe while the temporal shape of the waveform show only minimal change. This suggests the emission polarity is dictated by the symmetry breaking due to the intrinsic ferroelectric polarization, and supports the surface shift current as a mechanism of THz generation. Unlike the monolayers, bulk group-IV monochalcogenides do possess inversion symmetry, ruling out bulk shift current response. However, inversion symmetry is broken at the surface, indicating that the observed THz emission is due to a surface rather than bulk shift current. Shift current in response to photoexcitation suggests applications of these layered materials in BOVE photovoltaics and THz sources. Surface selectivity of THz emission in GeS and GeSe may also lead to new applications of these 2D materials in chemical sensing.