Tunable, Grating‐Gated, Graphene‐On‐Polyimide Terahertz Modulators

Abstract

An electrically switchable graphene terahertz (THz) modulator with a tunable‐by‐design optical bandwidth is presented and it is exploited to compensate the cavity dispersion of a quantum cascade laser (QCL). Electrostatic gating is achieved by a metal grating used as a gate electrode, with an HfO2/AlOx gate dielectric on top. This is patterned on a polyimide layer, which acts as a quarter wave resonance cavity, coupled with an Au reflector underneath. The authors achieve 90% modulation depth of the intensity, combined with a 20 kHz electrical bandwidth in the 1.9–2.7 THz range. The modulator is then integrated with a multimode THz QCL. By adjusting the modulator operational bandwidth, the authors demonstrate that the graphene modulator can partially compensate the QCL cavity dispersion, resulting in an integrated laser behaving as a stable frequency comb over 35% of the operational range, with 98 equidistant optical modes and a spectral coverage ~1.2 THz. This paves the way for applications in the terahertz, such as tunable transformation‐optics devices, active photonic components, adaptive and quantum optics, and metrological tools for spectroscopy at THz frequencies.

Electrically tuneable nonequilibrium optical response of graphene

“…This exhibits a typical ambipolar behaviour, as seen by the V -shaped gate dependence of the source-drain current I ds . The channel resistance peaks at V CNP =0.84V, corresponding to the charge neutrality point (CNP), where the density of states in SLG reaches its minimum [22,23,84]. V CNP depends on E F , on the gate-metal work function [22], and on the choice of contact materials [85].…”

“…At the CNP, there is a residual p-doping E F ∼-190meV, in agreement with the Raman estimation (∼ -230±80meV). The finite electrical conductivity and doping at the CNP [22,23] have been attributed to the formation of electron-hole puddles [87], at the micrometer scale, that can be caused by charged impurities [88] located either in the dielectric, or at the SLG/dielectric interface [88].…”

“…In undoped graphene, the first ones dominate in the THz [14] and microwaves [15] ranges, and the second ones [16] in the near-infrared (NIR) [17] and visible (VIS) [16] ranges. Electrical control of E F , by exploiting the band-filling effect [18], allows one to vary the density of electronic states available for both intraband [17] and interband transitions [3,18], thus affecting the linear absorption of SLG over a broad range from THz [19][20][21][22][23] to NIR [24][25][26][27] and VIS [28]. This has led to the development of SLG-based electro-optic modulators [3,24,25,27,[29][30][31][32][33], which can reach higher modulation speed (up to 200GHz [34]) than LiNbO 3 [35] and Si [36] devices, due to the superior mobility of SLG charge carriers, and have demonstrated high modulation depths both in amplitude (up to∼ 60%) [3,19,21,24,26] and phase (up to∼ 65 • ) [3,31] .…”

III–VI and IV–VI van der Waals Semiconductors InSe, GaSe and GeSe: A Suitable Platform for Efficient Electrochemical Water Splitting, Photocatalysis and Chemical Sensing

“…Following the advent of graphene, [1][2][3][4][5] a large number of 2D materials were explored by the scientific community. [6][7][8][9][10] Especially, van der Waals semiconductors offer complementary characteristics to graphene, which, being a semimetal, does not possess a band gap.…”

Terahertz Quantum Cascade Lasers as Enabling Quantum Technology

“…c) Intermode beatnote linewidths plotted as a function of QCL driving current when the QCL is coupled with a SLG modulators having a grating period p = 10 μm; d) FTIR emission spectrum measured at 15 K, under vacuum with ≈0.075cm −1 resolution, whilst driving the QCL in CW with 780 mA under the experimental configuration of panel (c). Adapted with permission [72]. Copyright 2021, Wiley-VCH.…”