1. INTRODUCTION

Photonic quantum technologies provide next-generation tools for the implementation of quantum information processing schemes using classical nanophotonic circuitry [1–8]. Operating waveguide-based systems with single-photon input grants access to the established fabrication and design techniques used for integrated optics and enables circuit complexity, which is out of reach for table-top, free-space implementations. Current research efforts in optical quantum information processing pursue the full integration of active quantum optical components—single-photon sources [9–12] and single-photon detectors [7,8,13–16]—with reconfigurable photonic circuitry on a single chip. This development holds potential to resolve current limitations in terms of stability, robustness, and component scalability. While a joint integration of all relevant components on a chip is yet to be achieved, the emergence of high-performance waveguide-integrated single-photon detectors [13,17–20] opens up additional avenues beyond applications in quantum optics. Here we demonstrate how the thriving fields of optical sensing and imaging benefit from nanophotonic integration through the use of such detectors.

State-of-the-art microscopy techniques such as fluorescence resonance energy transfer (FRET) or fluorescence lifetime imaging (FLIM) are indispensable tools in the modern life sciences, allowing molecular networks and intracellular activities to be monitored with great spatial and temporal precision [21–27]. Data acquisition often requires the detection of weak optical signals down to the single-photon level and necessitates highly sensitive detectors with broad spectral ranges [28,29]. Standard experimental and commercial implementations comprise single-photon detectors, wavelength-selective optics for spectroscopic analysis, and fast time-correlated single-photon counting (TCSPC) electronics for correlative investigation. For unperturbed, accurate measurements over extended time periods, these applications rely on the high efficiency, low noise level, and precise timing resolution of the single-photon detectors, as well as the robust, immovable installation of all optical elements. Current state-of-the-art single-photon detectors suffer from performance-degrading effects such as after-pulsing or low efficiency and large timing jitter, particularly in the near-infrared (NIR) spectral range [30,31]. In addition, the bulk optical components typically used in classical setups are prone to misalignment. Most experimental installations are therefore subject to stability constraints and offer only limited scalability. Photonic integration constitutes a promising resolution to these issues. By combining nanophotonic integrated circuits with superconducting single-photon detectors, we implement a scalable single-photon spectrometer, which provides single-photon resolution on multiple wavelength channels and is inherently stable.

2. METHODS

A. Integrated Single-Photon Spectrometer Concept and Design

Our spectrometer concept is based on the use of high-performance waveguide-integrated superconducting nanowire single-photon detectors (SNSPDs) in conjunction with wavelength-discriminating integrated photonic circuitry. SNSPDs provide significant advantages over current state-of-the-art single-photon detectors: they offer superb detection efficiency and impeccable timing characteristics over a wide spectral domain, encompassing visible and infrared wavelengths [13,17,32,33]. In addition, their inherently integrated design enables seamless integration of multiple SNSPDs with advanced on-chip photonic circuitry while maintaining the detectors’ efficiency and temporal precision. The co-integration of multiple SNSPDs with wavelength-separating photonic circuitry therefore offers a convenient approach to on-chip single-photon spectroscopy with high timing accuracy and fast data acquisition rates. Beside pure spectroscopic analysis at the single-photon level, the concept enables advanced techniques such as FLIM and FRET (or its exotic siblings multicolor and N-way FRET) [34,35] in a single device.

We realize fully integrated single-photon spectrometers (SPSs) by co-integrating eight SNSPDs with an arrayed waveguide grating (AWG) on a silicon-nitride (${\mathrm{Si}}_3{\mathrm{N}}_4$) on insulator substrate (Fig. 1). The chosen platform enables broadband optical operation, filtering, and multiplexing combined with highly efficient on-chip single-photon detection. We choose a spectral bandwidth of 24 nm at 1550 nm wavelength and 60 nm at 740 nm wavelength with a spectral resolution of, respectively, 2.2 nm at 1550 nm wavelength and 6.4 nm at 740 nm wavelength. The resolution is determined by the number of available RF channels in our cryogenic setup. The fabrication is based on a top-down approach, which allows reliable and reproducible integration of all active and passive photonic components, as described in the Methods section and Supplement 1.

 

Fig. 1. (a) Illustration of the single-photon spectroscopy setup including a confocal scanning system and the integrated SPS chip, which comprises an input grating coupler for light insertion and a reference port for output coupling; an AWG for spectral separation of the broadband input spectrum into eight output waveguides; and SNSPDs at the end of each output waveguide for single-photon detection. Inset: optical micrograph of a diamond nanocluster with embedded SiV color centers. (b) Scanning electron micrograph of the AWG circuit section. (c) False-color SEM micrograph of one of the eight SSPDs. (d) Optical micrograph of one of the fabricated devices with nanophotonic elements and an electrode array in false color.

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Light is coupled into the on-chip circuitry through a focusing grating coupler. The couplers and the photonic structure are opportunely designed for TE-like mode propagation. Although our grating couplers suffer from significant insertion loss (IL) around $-10\mathrm{dB}$, they provide a convenient way to characterize several devices in a cryogenic environment. The AWG spatially separates the broadband optical input signal and routes individual spectral components toward the SNSPD array for photon detection. Because the AWG distributes different wavelengths into different waveguides, the design is directly compatible with waveguide-integrated SNSPDs. The count rates measured by the individual detectors allow the reconstruction of the optical input spectrum. The utilization of an AWG leaves the temporal photon distribution unaffected, which allows us to exploit the SNSPDs’ high timing accuracy and thus expand the conventional spectroscopic concept by a temporal dimension.

B. Device Fabrication

The two sets of hybrid superconducting nanophotonic chips are fabricated using multistep electron-beam (e-beam) lithography with subsequent dry etching and thin-film deposition steps. Semiconductor multilayer structures consisting of silicon nitride (${\mathrm{Si}}_3{\mathrm{N}}_4$) and silicon dioxide (${\mathrm{SiO}}_2$) thin films on top of a silicon carrier wafer are used as starting materials. Different layer thickness configurations are used for the chips for 1550 nm and 738 nm optical wavelengths: the 1550 nm design employs 450 nm thick ${\mathrm{Si}}_3{\mathrm{N}}_4$ and 2.6 μm thick ${\mathrm{SiO}}_2$ layers, whereas the 738 nm design is based on 200 nm thick ${\mathrm{Si}}_3{\mathrm{N}}_4$ and 2.0 μm thick ${\mathrm{SiO}}_2$ layers.

The template wafers are diced into $15\mathrm{mm}\times 15\mathrm{mm}$. In a first step, a nominal 4 nm thick niobium nitride (NbN) layer is deposited by magnetron sputtering in an argon–nitrogen atmosphere [36]. Subsequently, gold contact pads and alignment markers are realized by e-beam lithography using positive tone polymethyl methacrylate 8.0 e-beam resist in the e-beam exposure system (50 kV JEOL 5500). The resist is developed in a $1:3$ methyl-isobutyl-ketone isopropanol solution, and 5 nm chromium and 120 nm gold layers are vapor-deposited by a custom-made e-beam physical vapor deposition (PVD) machine. The lift-off is performed in acetone with mild sonication. The NbN nanowire detector elements are structured in a second e-beam lithography step using hydrogen silsesquioxane (HSQ) as negative tone resist. To facilitate HSQ adhesion, a 5 nm thick ${\mathrm{SiO}}_2$ layer is vapor-deposited via PVD prior to resist application. Development of the exposed sample in a 6.25% tetramethylammonium hydroxide bath to produce a solid etch mask is followed by a carefully timed reactive-ion etching step in a tetrafluoromethane (${\mathrm{CF}}_4$) atmosphere to transfer the nanowire pattern into the NbN layer. Finally, the photonic circuit structures are fabricated using ma-N 2403 as negative tone resist in a final e-beam lithography step. The written sample is developed in a MF-319 solution and the structures are etched using a trifluoromethane (${\mathrm{CHF}}_3$) chemistry. Residual resist after etching is removed in an $N$-methyl-2-poyrrolidone bath.

On each fabricated sample, several spectrometer devices, including eight SNSPDs on the output waveguides as well as several single-detector reference devices, are realized. The latter allow additional detector characterization possibilities on the same chip and help to confirm the data obtained at the spectrometer devices.

C. Optical SPS Characterization at Room Temperature

The performance of the AWG spectrometers is characterized by inserting and extracting light into and out of the on-chip circuitry through focusing grating couplers and an array of fibers properly positioned above the couplers. A computer-controlled 4D piezo translation–rotation stage is used to facilitate alignment (see Fig. S1). The setup allowed us to investigate multiple photonic circuits on the same chip after nanofabrication. Light from a fiber-coupled laser (tunable NIR laser New Focus TLB-6600 for 1550 nm) or a white light continuum source (Leukos SM-30-UV to cover the wavelength range around 738 nm) is routed toward the input coupler. A fiber switch (Dicon GP700) is used for output channel selection. A power meter for the near-IR wavelength range (HP 8163A) and a modular spectrometer for the visible wavelength range (Ocean Optics JAZ) measure the transmitted powers in the 1500 nm and 738 nm spectral regions, respectively.

D. Cryogenic Measurement Setup

A liquid helium flow cryostat is used to cool the photonic chip containing both the AWGs and the detectors down to a stable base temperature of 1.6 K and ${10}^{-5}\mathrm{mbar}$ pressure inside the sample chamber. The photonic chip is mounted on a 4D rotation–translation stage (Attocube Systems) inside the cryostat’s sample chamber underneath an array of optical fibers and a multi-RF contact probe (Cascade Microtech). Optical connection is established by positioning the grating couplers of the photonic circuits directly below the fibers and the detectors are connected by bringing the on-chip contact pads in physical contact with the RF probe. The cryostat provides eight RF output lines for measuring each of the eight output channels of the AWG device. We note that the number of available RF lines therefore intrinsically limits the overall optical bandwidth of our spectrometer device. This bandwidth could be enhanced by employing larger numbers of RF lines. Each superconducting single-photon detector (SNSPD) is connected to a low-noise current source and read-out circuitry. The on-chip detector is current-biased by applying a constant voltage (Keithley 2400) over a 1 MΩ resistor via a high-frequency bias-tee (Mini-Circuits ZFBT-6GW+). The high-frequency components of the bias current are filtered using a low-pass filter (BLP-5, DC 5MHz). The voltage pulse generated upon detection of a photon is amplified by low-noise RF amplifiers (ZFL-1000LN+, $+50\mathrm{dB}$ gain), connected to a SSPD over the RF channel of a bias-tee. The arriving voltage pulses are registered with a time-correlated single-photon counter (Picoharp 300, Picoquant), a fast sampling oscilloscope (Infinium 6 GHz, Agilent), or a pulse counter (Agilent 53132A).

3. RESULTS

A. Spectrometer Characterization

Two circuit variants are fabricated: one implementation is designed for operation in the NIR at telecom wavelengths around 1550 nm, while the other targets the red visible regime around 738 nm. These wavelength regions thus cover the telecommunication C-band used for optical fiber communication and quantum communication, and the visible wavelength region where imaging applications in the life sciences are carried out. Both hybrid nanophotonic chips are realized through a multistep e-beam lithography process with subsequent dry etching and thin-film deposition steps, as described in detail in the Methods section and Supplement 1. We characterize the integrated SPS in the telecommunications range and demonstrate the device’s imaging and sensing capabilities by spectrally and temporally analyzing the fluorescence light emitted from silicon vacancy (SiV) color centers at 738 nm in diamond. In combination with a custom confocal scan head [Fig. 1(a)], we determine spatially resolved lifetime maps of the diamond nanocrystal sample.

First, we assess the optical transmission through the devices at room temperature using a custom multiport measurement system (see Methods section and Supplement 1). For this purpose, each output channel of the AWG is furnished with an additional grating coupler for light extraction [see Fig. 1(d)]. The transmission spectrum measured at the NIR device is depicted in Fig. 2(a). It shows eight separate transmission channels with a low cross-talk of $-18\mathrm{dB}$. The wavelength channels are designed to fall within the telecommunication C-band and feature a channel spacing of 2.2 nm. We define the NIR device’s overall on-chip detection efficiency as $\eta =1/8\sum _{i=1}^8{\eta }^{(i)}$, with contributions ${\eta }^{(i)}$ of the detectors $i\in \{1,\dots ,8\}$. Individual efficiencies ${\eta }^{(i)}$ are extracted by comparing a well-calibrated photon flux of $\mathrm{\varphi }={10}^6{\mathrm{s}}^{-1}$ inside the input waveguide to the measured detector count rates $R_c^{(i)}$ after correcting for the detectors’ dark count rates $R_{\mathrm{dc}}^{(i)}$, i.e., ${\eta }^{(i)}=[R_c^{(i)}-R_{\mathrm{dc}}^{(i)}]/\mathrm{\varphi }$. The detectors’ bias current $I_b$ is slowly raised from 50% to 90% of their respective critical currents $I_c$ and the input wavelength is swept across the device’s free spectral range from 1532.5 nm to 1557.5 nm. Figure 2(b) shows the resulting efficiency plot, which clearly exhibits the eight transmission windows in which the individual detectors are able to detect single photons. Exemplary pulse shapes are included in Supplement 1 along with a detailed characterization of the detector performance. The efficiency data agree well with the optical transmission data obtained at room temperature [Fig. 2(a)]. With the bias current set to 90% of the critical current [Fig. 2(c)], on-chip SPS device efficiency of $\eta =(19\pm 6)\%$ is obtained, including both IL and detection efficiency. For off-chip applications, considering the high IL of the input grating coupler for this particular device ($\mathrm{IL}=9.7\mathrm{dB}$), the estimated system efficiency for the SPS is 1.97%. While grating couplers offer a convenient prototyping solution for on-chip applications, for off-chip sources, more efficient coupling schemes, for example, butt coupling, could be employed to improve the system efficiency. The channel cross-talk level of the device was $-17.69\mathrm{dB}$. The dark count rate measured with a metal shielding cap on the fiber input terminal to prevent light insertion into the device was $<10\mathrm{Hz}$ throughout the investigated biasing range. Characterization of the temporal resolution in our detector system using a pulsed laser and an oscilloscope operated in histogram mode revealed a full width at half-maximum (FWHM) a timing jitter of $\tau =(47.5\pm 4.0)\mathrm{ps}$, in the single-photon detection sensitivity regime. The performance parameters determined enable multichannel single-photon operation in the important telecommunication C-band, which may benefit future applications in wavelength-division multiplexed (WDM) quantum communication.

 

Fig. 2. (a) Measured optical transmission in the telecommunication C-band showing eight separate transmission channels. (b) Corresponding single-photon on-chip detection efficiency as a function of wavelength and bias current. (c) On-chip device efficiency as a function of wavelength biased at 90% of the SNSPDs’ critical current.

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B. SPS Efficiency Characterization

A tunable, fiber-coupled NIR laser (New Focus TLB-6600 for the 1550 nm range/New Focus TLB-6700 Velocity for the 738 nm range) and a pair of calibrated attenuators (HP 8156A, 0–60 dB each) are used to generate a constant photon flux $\mathrm{\varphi }$ entering the AWG. By measuring the optical power transmitted through the circuit’s reference arm, $P_{\text{ref}}$, using an optical power meter (HP 8163A), we precisely monitor the photon flux $\mathrm{\varphi }$ inside the waveguide leading to the SPS. The count rate $R_c$ is measured with a specific flux of $\mathrm{\varphi }={10}^6{\mathrm{s}}^{-1}$ incident on the SPS and the dark count rate with disconnected fibers and metal shielding caps installed to prevent stray light insertion. The detection efficiency is obtained as the ratio of the dark count-corrected count rate and the incident photon flux, i.e., $\eta =(R_c-R_{\mathrm{dc}})/\mathrm{\varphi }$.

C. SPS Timing Jitter Characterization

The devices’ timing jitter is determined using a low-jitter picosecond laser (PriTel FFL 40-M for NIR/ALS PiLas PiL044X for visible; timing jitter $<2\mathrm{ps}$) and a fast digital sampling oscilloscope (Agilent Infinium 54855A DSO) operated in histogram mode. Half of the laser light is heavily attenuated and routed toward the on-chip device, while the other half is routed toward a fast reference photodetector (New Focus 1611, timing jitter $<1\mathrm{ps}$), which is installed outside the cryostat at room temperature to generate a stable trigger signal. The light guided to the detector is attenuated sufficiently to work in the single-photon detection range for a broad bias current range. The electrical outputs obtained from the SNSPD and reference detector are utilized as start and stop signals for the oscilloscope, respectively. The timing jitter values of the devices are measured as below 50 ps in both wavelength regimes. The timing jitter is device-limited and independent of the wavelength range.

D. Fluorescence Imaging and Lifetime Mapping of SiV Color Centers in Diamond

Beside single-photon characterization in the NIR wavelength range, the broadband detection capability of the SNSPDs enables the implementation of advanced devices for visible-light analysis. As an application example, we investigate the low-intensity fluorescence signal of SiV color centers in diamond nanocrystals using our visible-range SPS. Diamond nanocrystals with incorporated SiV defect states are emerging as promising candidates for labeling in biological tissue and also hold promise for single-photon sources. The hosting nanodiamonds can be easily functionalized and are biocompatible. In addition, the SiV emission in the deep red wavelength range complements genetic fluorophores, in particular, the green-fluorescent protein (GFP) and the red-fluorescent protein (RFP). It exhibits high brightness and virtually no photobleaching [37,38]. Moreover, our SPS circuit directly interfaces with the application of SiVs in single-photon generation [39,40]; it allows suppressing the optical excitation wavelength on chip and provides further capabilities for spectral analysis in a scalable fashion.

The investigated nanodiamond cluster specimen is created by drop-casting a colloidal solution of nanocrystals onto a microscope slide. Clusters of nanocrystals form upon solvent evaporation. The scanning confocal microscope depicted in Fig. 1(a) is used for sample alignment, its excitation, and for collection of the fluorescence light. The SiVs’ zero phonon emission line (ZPL) is found to be centered at 738 nm and optical excitation is possible from 300 nm to 580 nm (see Supplement 1).

We first analyze the emitted fluorescence spectrum using the eight-channel SPS upon continuous-wave (cw) excitation at 532 nm. The SPS’s channel spacing is designed such that the bandwidth of the eight-channel AWG covers the expected optical bandwidth of the SiV emission. Because the emission is collected at room temperature in our confocal setup, the necessary channel width is quite wide. The fluorescence signal is collected with a high-numerical-aperture microscope objective (Zeiss EC Plan-Neofluar $100\times /1.3\mathrm{NA}$) and coupled into an optical fiber after filtering out the excitation light using dichroic mirrors. The fiber is shielded from stray light with a metal coating and is used to route the light into our cryo-measurement platform for coupling into the photonic circuitry. The relative magnitudes of the measured count rates agree well with the reference spectrum obtained using a conventional spectrometer (Fig. 3), thus accurately reproducing the ZPL emission peak at 738 nm. By exploiting the scanning capabilities of the apparatus, we are able to include spatial information and thus spectrally image the nanodiamond cluster. The cluster shape is well reproduced on each wavelength channel [see Fig. 4(a)].

 

Fig. 3. Emission spectrum of SiV color centers embedded in a diamond nanocluster recorded by an eight-channel SPS (blue bars) optimized for operation at 738 nm. Overlaid in yellow is the emission spectrum obtained with a conventional spectrometer. The inset shows a three-dimensional molecular model of the SiV color center.

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Fig. 4. (a) Spatial map of the count rate obtained in each of the eight wavelength channels around 738 nm. (b) Single fluorescence decay trace of SiV centers obtained by TCSPC including a single-exponential fit (solid red line). (c) Mapped out fluorescence decay times of the SiV centers found within the cluster area. Inset: confocal intensity scan of the same cluster.

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Taking advantage of the SNSPDs high timing resolution and fast response time, in a complementary experiment we replace the exciting 532 nm cw laser with a passively mode-locked laser, which produces pulses of 32 ps duration (FWHM) at 440 nm wavelength (ALS PiLas PiL044X). In addition to the spectral image reported above, pulsed excitation allows correlative imaging in a start–stop measurement: upon emission of a pulse, the laser triggers a start event in our TCSPC electronics (Picoquant Picoharp 300) and the stop signal is provided by the registration of the fluorescence photons by the SPS. The collection of multiple start–stop-time delay data points allows extraction of the fluorescence decay time of the specimens [Fig. 4(b)]. Such data are available on all channels, thus providing temporal as well as spectral information simultaneously. In combination with the scanning confocal microscope setup, a lifetime map of the diamond cluster is obtained alongside the spectral information [Fig. 4(c)]. Because the lifetime did not vary significantly over the area of the cluster, we have chosen a coarser spatial sampling resolution to reduce the measurement time, which is mostly determined by the setting time of the piezostage.

4. DISCUSSION

Ultimately, the adoption of a quantum photonic approach to sensing and imaging holds potential to herald a new level of experimental fidelity. By integrating multiple SNSPDs with wavelength-discriminating on-chip circuitry, spectrally resolved single-photon detection with high timing precision can be realized in a single nanophotonic device. The two circuit devices presented here serve as prototypes of a rich class of hybrid nanophotonic superconducting systems. While the demonstrated resolution of the AWGs in the visible wavelength regime is adapted to the spectrum of the SiV emission, optimized designs with narrow channel spacing could be employed to provide higher spectral resolution.

AWGs with tens of output waveguides provide room for large SNSPD arrays similar to CCD arrays in conventional spectrometers. Such devices could enable live monitoring of numerous fluorophores over a wide spectral range. In particular, highly precise infrared and thermal imaging become possible owing to the enormous spectral range of the SNSPDs.

The presented SPSs operated in the telecommunication wavelength regime at 1550 nm hold promise for serving as receiver elements in WDM optical networks with low signal strengths and, in particular, for quantum optical implementations, which require reliable detection of individual photons on tailored wavelengths. Owing to the high temporal accuracy and good on-chip detection efficiency of the SNSPDs, even more advanced protocols involving quantum cryptography and quantum key distribution on multiple wavelength channels and thus with high bandwidth are brought within reach.