Waveguide-integrated mid-infrared photodetection using graphene on a scalable chalcogenide glass platform

Abstract

The development of compact and fieldable mid-infrared (mid-IR) spectroscopy devices represents a critical challenge for distributed sensing with applications from gas leak detection to environmental monitoring. Recent work has focused on mid-IR photonic integrated circuit (PIC) sensing platforms and waveguide-integrated mid-IR light sources and detectors based on semiconductors such as PbTe, black phosphorus and tellurene. However, material bandgaps and reliance on SiO₂ substrates limit operation to wavelengths λ ≲ 4 μm. Here we overcome these challenges with a chalcogenide glass-on-CaF₂ PIC architecture incorporating split-gate photothermoelectric graphene photodetectors. Our design extends operation to λ = 5.2 μm with a Johnson noise-limited noise-equivalent power of 1.1 nW/Hz^1/2, no fall-off in photoresponse up to f = 1 MHz, and a predicted 3-dB bandwidth of f_3dB > 1 GHz. This mid-IR PIC platform readily extends to longer wavelengths and opens the door to applications from distributed gas sensing and portable dual comb spectroscopy to weather-resilient free space optical communications.

Fig. 1. Device geometry.

a Illustration of the device cross-section perpendicular to the waveguide axis. The optical mode supported by the GSSe waveguide evanscently couples to and is absorbed by the graphene channel, which is gated by two graphene back-gates to induce a pn-junction. b Optical image of the device depicting source, drain and gate contact pads. c Depiction of the optical guided mode at λ = 5.2 μm.

Fig. 2. Gate voltage maps.

a Measured zero-bias photovoltage produced by the device as a function of the two gate voltages. b Total device resistance as a function of the two gate voltages. c Lock-in signal reflecting power measured by an InAsSb photodetector at the focal point of our output facet collection lens, used to monitor transmission of the device as a function of the gate voltages. The star, triangle, and cross symbols on each gate voltage map represent the optimum operating points for maximum voltage responsivity, maximum current responsivity, and minimum NEP, respectively. The power-normalized transmittance is plotted in Supplementary Fig. 3b. d, e, f Plots of line sections indicated with dashed lines in panels a, b, and c, respectively.

Fig. 3. Experiment/model comparison.

a, b Contour plots of the a measured and b modeled responsivity maps of our device, evaluated with τ_DC = 3.5 fs, τ_IR = 40 fs, σ_n = 2 × 10¹² cm⁻², τ_eph = 50 ps, and α_e = 2.5 mm⁻¹. c Electron temperature increase ΔT_el and absorbed optical power per area $\stackrel{°}{Q}$ profiles in the graphene channel per guided optical power at gate voltages of {−2.35 V,  0.35 V}, chosen to maximize the modeled photoresponse, and other parameters as above.

Fig. 4. Bandwidth and noise properties.

a Comparison of the frequency response of our photodetector with that of the laser current modulation itself. The consistency between the two indicates that the photodetector frequency response exceeds 1 MHz. Inset: Simulated GHz-range photodetector frequency response, with and without considering the impact of the electron-phonon cooling time τ_eph. b Measured noise spectral density versus resistance and corresponding Johnson noise spectral density of Device B, without illumination, for the 49 pairs of gate voltages {V_g1, V_g2} where each V_gn is varied from −6 V to 6 V in steps of 2 V. Measurement was performed at T = 293 K.