Opportunities for Wideband Wavelength Conversion in Foundry-Compatible Silicon Waveguides Covered With Graphene

Abstract

We numerically examine the opportunities for wideband wavelength conversion through four-wave mixing (FWM) in a foundry-compatible 220 nm-thick silicon-on-insulator (SOI) waveguide covered with the highly nonlinear two-dimensional material of graphene. As a case study, we consider a foundrycompatible SOI waveguide shaped as a double spiral and covered with two separate graphene sheets, which are covered in turn by two solid polymer electrolyte gates. When combining subwatt level pump powers with a short waveguide length of only a few hundreds of microns, 'perfectly phase-matched' conversion with significant efficiencies close to −20 dB can be obtained over a more than 40 T Hz-wide signal band adjacent to the pump frequency. Because of the tunability of the graphene properties using the electrolyte top gates, it is also possible to obtain 'quasi-phase matched' FWM conversion through a periodic sign reversal of the graphene third-order nonlinearity along the waveguide. Conversion efficiencies exceeding −30 dB can be achieved over a 3.4 T Hz-wide signal band that is situated as much as 58 T Hz away from the pump frequency. Finally, the tunability of the graphene also allows for switching the converter from the perfectly phase-matched to the quasi-phase-matched operation mode.

Negative Kerr Nonlinearity of Graphene as seen via Chirped-Pulse-Pumped Self-Phase Modulation

“…For a Fourier-transformed ampli-tudeÃ(z, ω − ω 0 ) centered at frequency ω 0 , the square of the rms spectral width can be written as [11,12], which in the time domain can be expressed as [11]. For the experiments reported in this Letter, the pulse propagation is mostly determined by Kerr-nonlinear effects rather than dispersion and free-carrier effects (which was also verified numerically with the inclusion of graphenegenerated free carriers [12,13]). Hence, we can adopt a simplified version of the nonlinear Schrödinger equation incorporating only SPM effects and linear loss: ∂ z A = −(α/2)A + iγ K |A| 2 A, with γ K the Kerr nonlinearity and α the linear loss coefficient.…”

“…with γ SOI,weighted and γ gr,weighted the weighted nonlinearity contributions from the underlying SOI waveguide and the graphene top layer, respectively. v p and I p are scaling factors defined in [13], ǫ 0 is the dielectric permittivity, ω p is the pump frequency, χ is the third-order Kerr-nonlinear conductivity of graphene. The latter can be converted to a susceptibility using χ = σ/(−iωǫ 0 d gr ) [19].…”

Electrically Tunable Optical Nonlinearities in Graphene-Covered SiN Waveguides Characterized by Four-Wave Mixing

“…3 in Ref. [13]). The majority of experiments have been performed in weakly doped graphene at high frequencies (near-IR, visible), where ω ≫ 2|E F |.…”

“…Quantitatively, the theory predicts about one order of magnitude larger response than was experimentally observed (this contrasts to a number of previous publications, where the measured nonlinear response was claimed to be larger than the theoretically calculated one, see discussions in Refs. [3,4,13]). This discrepancy should be subject to further investigation.…”

High-efficiency and broadband four-wave mixing in a silicon-graphene strip waveguide with a windowed silica top layer

“…In our previous study, we demonstrated a large Kerr coefficient of graphene (n 2 ∼10 −12 m 2 · W −1 ), which is 6 orders of magnitude larger than that of silicon [14]. Combining graphene with silicon photonic devices offers a possible solution towards high-efficiency and broadband FWM [15,16]. Nevertheless, the high absorption of the graphene sheet greatly limits the performance of the FWM in the silicon-graphene hybrid devices [17].…”