Broadband on-chip single-photon spectrometer

Abstract

Single-photon counters are single-pixel binary devices that click upon the absorption of a photon but obscure its spectral information, whereas resolving the color of detected photons has been in critical demand for frontier astronomical observation, spectroscopic imaging and wavelength division multiplexed quantum communications. Current implementations of single-photon spectrometers either consist of bulky wavelength-scanning components or have limited detection channels, preventing parallel detection of broadband single photons with high spectral resolutions. Here, we present the first broadband chip-scale single-photon spectrometer covering both visible and infrared wavebands spanning from 600 nm to 2000 nm. The spectrometer integrates an on-chip dispersive echelle grating with a single-element propagating superconducting nanowire detector of ultraslow-velocity for mapping the dispersed photons with high spatial resolutions. The demonstrated on-chip single-photon spectrometer features small device footprint, high robustness with no moving parts and meanwhile offers more than 200 equivalent wavelength detection channels with further scalability.

Fig. 1

Device architecture and operation principle. a–d Three-dimensional sketch of the device. The on-chip focusing echelle grating operates as wavelength-discriminating microphotonic component while the superconducting nanowire functions simultaneously as a single-photon detector and a slow microwave delay line to continuously map the dispersed photons. The nanowire is capped with AlO_x as high-k dielectric material and Al as top metal ground to form a slow microwave transmission line. e Schematic illustration of the Rowland mounting. The input waveguide and the superconducting nanowire are mounted on the Rowland circle, which internally tangents with the focusing grating line. The focusing grating is projected from a flat grating with the radius of the curvature equal to the diameter of the Rowland circle. f Schematic representation of the signal transduction pathway and the device operation principle

Fig. 2

Broadband device images. a Overview optical micrograph image of the device. The input waveguide is first split into two waveguides, one to feed photons to the spectrometer, the other used for power calibration and monitoring the return power during the fiber-to-chip alignment. At the ends of the impedance tapers, the microstrip lines are converted to coplanar waveguides (CPWs) to match the modes of ground-signal-ground (GSG) RF probes. The yellow color represents Al and AlO_x beneath, while the signal pads in orange color are made of gold (Au). Scale bar, 250 µm. b Close-up SEM image of the impedance taper. Both ends of the nanowire detector are tapered from 60 nm to microns width to preserve the fast-rising edges of photon-excited microwave pulses. Scale bar, 10 µm. c Close-up SEM image of the meander nanowire detector. The nanowire is patterned from 8 nm-thick NbN film. The width, pitch and depth of the nanowire is 60 nm, 700 nm and 3 µm, respectively. Scale bar, 1. d Angular SEM view of a section of the echelle grating. The pitch of the grating teeth is 0.8 µm and the blaze angle is 20 degree. Scale bar, 1 µm. e Expanded SEM view of the waveguide splitter. Scale bar, 10 µm. The SEM images are taken prior to the deposition of AlO_x and Al for Device B (see Supplementary Note 1)

Fig. 3

Broadband device results. a Electric field distribution of the device from 2.5-dimensional FDTD simulation results at different wavelengths. Scale bar, 100 µm. b Normalized histogram of photon counts versus time difference Δt measured for different wavelength photons from 600 nm to 1970 nm. The main peaks are from the TE modes while the minor peaks marked by the dashed circles are from TM modes. The histogram is recorded with the nanowire detector biased at 80% of its switching current I_SW. c Comparison between experimentally measured Δt and the diffraction angle extracted from the simulation results. d Normalized photon counting rates (PCR) measured as a function of the bias current relative to the switching current of the device I_bias/I_SW at wavelengths from 750 to 1970 nm. The complete saturation trend of the curves indicates a near-unity internal quantum efficiency of the nanowire detector over the whole spectrum. The histogram results are taken from Device C with double-nanowire detector, while the efficiency curves are measured from Device B with single-nanowire structure for better comparison between different wavelengths (see Supplementary Note 1)

Fig. 4

Telecom-band device results. a Normalized histogram of photon counts versus time difference Δt measured for photons with different wavelength from 1480 to 1640 nm. b Normalized histogram measured for a mixture of two coherent light sources with their wavelengths separated by 2.5 nm. The red dashed line represents double-Gaussian fitting for the measured data, which is the sum of two Gaussian distribution displayed in blue dashed lines. c Normalized histogram measured for a single-color source with slightly varied wavelength step by 0.1 nm. d Normalized histogram measured for a 1560 nm pulsed source as a function of the arrival time difference between the averaged detector signal (t₁ + t₂)/2 and laser synchronization signal t₀. All the histogram results are measured with the nanowire detector biased at 80% of I_SW. All the results are taken from Device A (see Supplementary Note 1)