Tielrooij, KJ., Principi, A., Reig, D.S. et al.
Milliwatt terahertz harmonic generation from topological insulator metamaterials. Light Sci Appl 11, 315 (2022).
An international research team has found a way of generating terahertz radiation by frequency conversion much more efficient than with previous technologies. In this study, a specially constructed quantum materials system acts as a high-efficiency frequency booster.
Abstract
Achieving efficient, high-power harmonic generation in the terahertz spectral domain has technological applications, for example, in sixth generation (6G) communication networks. Massless Dirac fermions possess extremely large terahertz nonlinear susceptibilities and harmonic conversion efficiencies. However, the observed maximum generated harmonic power is limited, because of saturation effects at increasing incident powers, as shown recently for graphene.
Here, we demonstrate room-temperature terahertz harmonic generation in a Bi2Se3 topological insulator and topological-insulator-grating metamaterial structures with surface-selective terahertz field enhancement. We obtain a third-harmonic power approaching the milliwatt range for an incident power of 75 mW—an improvement by two orders of magnitude compared to a benchmarked graphene sample. We establish a framework in which this exceptional performance is the result of thermodynamic harmonic generation by the massless topological surface states, benefiting from ultrafast dissipation of electronic heat via surface-bulk Coulomb interactions. These results are an important step towards on-chip terahertz (opto)electronic applications.
Introduction
One of the most canonical manifestations of the nonlinear interaction of electromagnetic waves with matter is harmonic generation, where the energy of generated photons is a multiple of the energy of incident photons. Until a few years ago, the most efficient way of generating harmonics in the terahertz (THz) regime was using two-dimensional quantum well systems.
During the past few years several other material systems, including SrTiO3 and superconductors have shown strong THz nonlinearities. In particular, it has been demonstrated that materials with massless Dirac fermions— charge carriers with a linear energy-momentum dispersion relation—can lead to efficient THz harmonic generation. This has been shown with the prototypical massless Dirac-fermion material graphene, with topological insulators (TIs), and with Dirac semi-metals.
Currently, the THz nonlinear photonics “toolbox” includes the possibility to electrically tune harmonic generation, and the ability to employ ultrathin metamaterials that generate an increased harmonic power by orders of magnitude, as a result of field enhancement in a metallic grating located nearby the nonlinear Dirac-fermion system. This has resulted in a record-high third-order susceptibility χ(3) above 10–8 m2 V–2 in the THz regime for a grating-graphene metamaterial, which is more than three orders of magnitude larger than the highest value obtained with quantum wells.
In order to fully exploit these ultrahigh THz nonlinearities towards practical technologies, there are fundamental and technological obstacles that need to be overcome. Important fundamental questions pertain to the mechanism that leads to THz harmonic generation in massless Dirac-fermion systems, and to whether THz harmonic generation in TIs originates mainly from the surface states or from bulk carriers without linear dispersion. For technological applications, it is necessary to significantly increase the amount of generated harmonic power.
a, b Schematic experimental layout of third-harmonic generation studies in graphene (a) and Bi2Se3 (b) metamaterials. c The THz third-harmonic power is enhanced by the metallic gratings for both graphene (black squares and lines) and Bi2Se3 (blue circles and lines). However, the power saturates strongly for graphene, and therefore eventually Bi2Se3 outcompetes graphene by around two orders of magnitude. The largest pump intensity corresponds to an incident peak field strength of 300 kV cm–1. d Transmitted spectral amplitude (normalized) after the Bi2Se3 metamaterial sample (blue line), with a 3f signal that is ~4% of the fundamental signal at f. Both the sample transmission (red line) and electrooptic sampling (EOS) sensitivity of the 2 mm thick ZnTe detection crystal (black line) are lower at 3f than at f. Taking into account the lower EOS sensitivity, the field-conversion efficiency is 8%, corresponding to 0.5 mW of generated harmonic power