Spectrum-free integrated photonic remote molecular identification and sensing

Absorption spectroscopy is widely used in sensing and astronomy to understand molecular compositions on microscopic to cosmological scales. However, typical dispersive spectroscopic techniques require multichannel detection, fundamentally limiting the ability to detect extremely weak signals when compared to direct photometric methods. We report the realization of direct spectral molecular detection using a silicon nanophotonic waveguide resonator, obviating dispersive spectral acquisition. We use a thermally tunable silicon ring resonator with a transmission spectrum matched and cross-correlated to the quasi-periodic vibronic absorption lines of hydrogen cyanide. We show that the correlation peak amplitude is proportional to the number of overlapping ring resonances and gas lines, and that molecular specificity is obtained from the phase of the correlation signal in a single detection channel. Our results demonstrate on-chip correlation spectroscopy that is less restricted by the signal-to-noise penalty of other spectroscopic approaches, enabling the detection of faint spectral signatures.


INTRODUCTION
Absorption spectroscopy is an important tool for the determination of molecular composition where direct interaction with a target is not practical or feasible. Typical applications range from the measurement of trace gases in the atmosphere 1,2 , gas emissions 3 , hyperspectral ground 4 and satellite-based 5 remote sensing platforms, to deep-sky and solar system astronomical spectroscopy 6,7 . Many other high sensitivity molecular spectroscopy techniques have been pursued using active techniques such as dual comb spectroscopy 8-11 but these require laser sources, complex electronics and optics, and cannot probe distant targets. In particular, the compositional analysis of astronomical objects and distant atmospheric targets both must rely on simple absorption spectroscopy with a natural background light source. From such spectra, many molecular species can be detected and identified by their unique infrared absorption spectrum that results from their vibrational and rotational mode distributions.
Most absorption spectroscopy platforms are built around some form of grating spectrometer that disperses the incoming light across a detector array to capture the spectrum, which is subsequently analyzed to extract the molecular absorption features of interest. In astronomy, modern large telescopes must be matched to correspondingly large grating spectrometers 12 . Replacing full grating spectrometers with a compact device that can detect and quantify the presence of a specific target molecule may significantly reduce the size and complexity of such systems in applications where a complete spectrum is not required. Integrated photonic systems allow for the processing of light on the plane of a centimeter sized chip. Silicon photonics is among the most developed of these technologies, with many different types of integrated optical devices having been developed for telecommunications and sensing. Examples include integrated filters, modulators, wavelength (de)multiplexers (i.e. spectrometers), optical switches, phase shifters 13 , and label-free biosensors 14, 15 . Planar waveguide spectrometers can be implemented as echelle gratings 16 , arrayed waveguide gratings 17, 18 , Fourier transform spectrometers 19,19-21 , and photonic crystal superprisms 22 on integrated platforms. Waveguide ring resonators have been used as local gas sensors 23,24 but these devices rely on detecting the interaction of trace gases with the evanescent field extending outside the single mode waveguides and cannot be used for remote detection.
In the case of remote absorption spectroscopy, the detection of absorption features in the spectrum of the input light is used to infer the presence and type of molecular species lying between a broadband light source and the detector. As molecular species exhibit unique spectral fingerprints in the infrared, this uniqueness can be used to identify the molecule through a correlation of the broadband spectrum with a matching spectral filter, without the need to disperse and acquire a full spectrum. This principle has been demonstrated using bulk optic Fabry-Perot (FP) interferometers to produce a periodic transmission filter that can be correlated with a vibronic gas spectrum over a finite spectral region 25-28 . The infrared absorption spectrum of many gases exhibit quasi-periodic vibronic absorption features generated by their coupled vibrational and rotational degrees of freedom. While vibronic spectra are not perfectly periodic, the periodicity is sufficient over a finite spectral range to simultaneously overlap numerous transmission lines of a FP interferometer cavity with a suitably chosen cavity length. While the previously mentioned work used FP interferometers 25,26,28 waveguide ring resonators can be used for the same purpose since they are analogous to on-chip FP cavities. Silicon waveguide ring resonators are preferable as they are significantly more compact, mechanically stable, lower cost, and can be spectrally tuned at a much higher frequency with low power consumption.
In this paper, we design and characterize an integrated photonic HCN gas sensor in the near-infrared (NIR) using waveguide ring resonators on a silicon-on-insulator (SOI) platform. By actively modulating the ring resonator transmission comb spectrum while coupling light from the target through the ring, we generate a correlation signal to detect and identify the target gas based on the presence of vibronic absorption lines in the incoming broadband light signal. This allows for on-chip detection and identification of remote molecular species without spectrum acquisition using only a single detection channel. Gas specific integrated correlation filters can replace dispersive spectrometers in many targeted remote sensing applications where spectral information is secondary to instrument cost, size and detectivity.

Ring Resonator Design
We target the most abundant isotope of hydrogen cyanide (H 12 C 14 N) gas as a proof of concept due to its importance in astronomical spectroscopy and strong absorption cross-section in the telecommunications C-band, as shown in Figure 1a. To achieve the desired FSR of the ring resonator, the ring cavity length and the targeted vibronic spectral lines must be chosen carefully to simultaneously match as many ring and gas lines as possible. The optimization of this matching has two primary benefits: a larger possible correlation signal, and molecule specificity.
The HCN vibronic Stokes band centred at  = 1540 nm was chosen as the target spectral signature since these lines have a lower relative line spacing change with wavelength than the anti-Stokes band at shorter wavelengths, and is therefore more suitable for correlating with the more periodic ring resonator transmission spectrum. Note that both the ring spectrum comb and the vibronic absorption line spacings are not truly periodic and change at different rates with wavelength. The spacing between the strongest HCN absorption lines varies from 0.74 nm to 0.82 nm for the HCN stokes band shown in Figure 1b. Since the absorption lines at 1540 nm are the strongest in the HCN Stokes band, we have chosen to target an FSR of approximately 0.77 nm for greater detection sensitivity.
Ring resonator transmission spectra as a function of temperature and wavelength are correlated with the HCN absorption spectrum using Equation (7). The spectral products at ( , ) are then integrated using Equation (8) to produce the correlation signal, as shown in Figure 2a With HCN absorption lines present, correlation signal dips are observed every 11°C, or approximately 0.77 nm in spectral shift of the ring (free spectral range of the ring). In Figure 2b, we show the correlation signal as a function of temperature for various HCN absorption depths with a bandpass of 6 nm. A dip in the correlation signal becomes stronger as the deeper absorption lines from HCN increasingly block more light at the drop port of the ring, showing that the sensor can distinguish relative HCN concentration in the beam column. With HCN producing a 100% absorption depth feature, the correlation signal changes by ~8% for the chosen ring resonator parameters a, r1, r2=0.9. The magnitude of the correlation signal change is also a function of the gas temperature and pressure. Intrinsic or Doppler broadened absorption lines lead to improved overlap between multiple resonances and generally leads to stronger correlation signals. The correlation also benefits from strongly coupling in the ring, where broader resonances also lead to higher spectral overlap and increased absolute correlation signal change at the cost of relative signal change. We also investigated the effect of the bandpass filter width, which allows for more gas lines to contribute to the signal. The correlation signal is plotted as a function of temperature for different bandpass widths in Figure 2c, showing greater signal amplitude with a larger bandpass. Increased specificity can be confirmed by observing the change in the correlation signal amplitude with increasing bandpass. The signal is also broadened due to imperfect overlap of the gas lines with the ring resonances. As the gas lines are asymmetrically distributed, a slightly asymmetrical shape to the correlation signal is visible at the large bandpass filter widths.
We also investigate the molecular specificity of our sensor since multiple gases can have absorption features in the bandpass. In the case of acetylene (C2H2) present with HCN, we now have two additional non-periodic strong absorption lines which complicate the spectrum, as shown in

Remote Gas Detection with Tunable Laser
The racetrack ring resonators shown via microscopy in Figure 3a  To emulate operation using background broadband illumination, we integrate, as in Equation (8)

Remote Gas Detection with Weak Broadband Background
We now demonstrate sensing using a weak broadband background by the changing light source to a superluminescent LED. The spectral power density detected at the photodiode is around 30 pW/nm, or on the order of >200,000 weaker than the tunable laser. This source better represents a dim target typically encountered in astronomical observations. Different bandpass widths and centre wavelengths were selected using the tunable bandwidth filter to accommodate 1 to 8 gas lines. The normalized transmission spectra through the filter and the gas cell for varying bandpass widths are measured using a tunable laser sweep and are shown in Figure 4e. The microheater current is increased from 11 mA to 25 mA as before, allowing the resonances to simultaneously align with the gas lines. The power at the photodiode, which inherently integrates the output spectrum, is measured as a function of microheater current with a fixed stage temperature. The difference between the correlation signal with and without the gas cell in the light path is shown in Figure 4f. When the HCN gas cell is connected, a clear dip by about 8% in the correlation signal is seen centred at ~17-18 mA of microheater current, similar to what is seen by integrating the tunable laser spectra. The signal-to-noise ratio is significantly lower when using this source due to the weak spectral power density of the SLED source, as well as from additional insertion losses from the tunable

Discussion
We describe an integrated photonic remote gas sensor based on spectrum-free detection and identification of absorption features of a gas contained within a broadband background light. By engineering a silicon waveguide ring resonator with appropriate length and group index, a correlation filter can be matched to gases with quasi-periodic absorption features over a bandpass of a few nanometres. We show that the overlap of the ring resonator drop port with the absorption lines of HCN produces a unique modulation pattern that identifies HCN based on the phase of the modulated correlation signal. We additionally demonstrate HCN detection with a weak broadband source. While demonstrated with HCN, many other gases with similar periodic absorption features such as CO2 and CO can be detected similarly with different ring lengths. We also show by simulation that the sensor can operate in the presence of C2H2 and should distinguish other gases with non-overlapping absorption features in the same bandpass.
In this work, ring resonators have been used as a convenient device to generate a quasi-periodic comb filter. However, it also possible to create much more complex waveguide filters based on gratings to precisely match more complex gas spectra over wider spectral ranges, for example using layer peeling methods for Bragg gratings on the silicon nitride platform 29 . The precise matching of filter transmission to gas spectra should further increase the specificity and signal-to-noise ratio of the correlation method. This correlation technique reduces the detection requirement from a 1D photodetector array of typical dispersive spectrometer to a single channel, which introduces cost and sensitivity advantages while maintaining molecular specificity of full spectrum acquisitions. In extremely low signal applications, avalanche photodiodes or photomultiplier tubes can be used.
The multiplexed advantage of a single channel for gas sensing can also pave the way towards a form of gas mapping where arrays of ring resonators can process light from individual pixels. Possible applications range from low-cost remote monitoring of gas emissions to enabling astronomical observations of extremely weak absorption features such as exoplanet atmospheres during transits. Future work includes investigating devices which operates in the mid-infrared where device sensitivity can be significantly higher due to stronger absorption features.

Methods: Simulation
Here we outline the simulation of the transmission through a ring resonator filter from its physical and optical characteristics. The resonance wavelengths of a ring resonator occur under the condition where ne is the effective index of the waveguide mode, m is the longitudinal mode number, and L is the round-trip length of the ring. The FSR of a ring resonator is described by where is the group index of the waveguide mode. We used a single-mode silicon strip waveguide with a height of 220 nm and width of 450 nm.
We calculated the mode properties as a function of wavelength and temperature based on an empirical model 30 using a finite difference eigenmode solver (Lumerical Mode Solutions). The effective index of the calculated TE mode is ne ~ 2.37 and a group index of ng ~ 4.3 at a wavelength of  = 1539 nm and temperature of 300 K.
The transmission through an add-drop ring resonator is expressed by for the through port, and for the drop port 31 .
The parameters and are the coupling coefficients into and out of the ring, respectively, a is the self-coupling coefficient of the ring. The temperature dependent net phase shift of the light in the ring is described by where ( ) is the temperature dependent effective index. We can use this temperature dependence to dynamically correlate the ring resonator output spectrum to the absorption lines of HCN. The spectral shift is described by where is the thermo-optic coefficient of the mode is the resonance wavelength and is the change in temperature. The resulting amplitude and relative phase of the integrated intensity can be used to identify and quantify the gas in the incident beam path. Silicon has a large thermo-optic coefficient of 1.8×10 −4 K -1 , enabling tuning of the ring resonances with a small change in temperature.
The output spectrum of the ring resonator with absorption due to gas presence is where ( ) is the gas absorption spectrum and ( , ) is the ring resonator drop port transmission spectrum. The correlation signal ( , ) is measured using a photodetector with a signal that can be expressed by the integral A suitable ideal ring length can be calculated from Equation (2) by solving for ring length given a target FSR of 0.77 nm that matches the line spacing of the target molecule. From this simple calculation using a group index of 4.34 at =1535 nm and =295 K, we obtained a ring length of ~709 m. We verified the performance of this ring length ( ) by calculating the maximum correlation signal amplitude for ring lengths ranging from 400 m to 1200 m, as shown in Figure 2a. The self-coupling and loss coefficients (r1, r2, and a) were set at 0.9.
We generated the HCN absorption spectrum A() using line wavelengths and relative intensities from the high-resolution transmission molecular absorption database (HITRAN) 32 and modeled them as overlapping approximated Voigt functions. The final absorption spectrum was created by subtracting the Voigt functions from a flat, normalized background.

Ring resonator Fabrication
Silicon ring resonator devices were fabricated on the silicon-on-insulator (SOI) platform using E-beam lithography and reactive ion etching through Applied Nanotools. The ring resonator consists of a silicon waveguide formed into a racetrack loop. The coupling and chip configuration is shown in Figure 3a, with optical and scanning electron micrographs shown in Figures 3b and 3c 33 . A deep trench etch is made through the entire chip to provide a smooth waveguide facet to couple input and output light. Devices are designed for TE polarization to increase the waveguide thermo-optic coefficient which is proportional to the mode overlap ratio between silicon and SiO2. Coupling between the input waveguide and the racetrack ring resonator is accomplished using directional couplers with a closest coupling gap of 200 nm. A tapered waveguide is used in the drop output waveguide to reduce back-reflections from counter-propagating modes. The directional couplers were optimized with 5.4 µm and 6 µm parallel sections for the drop and through coupling sections, respectively. The asymmetric coupling length is designed to accommodate for some loss in the ring to better approach the critical coupling condition 31 . The racetrack ring bend radii are fixed at 20 µm to ensure negligible bend losses. The propagation loss of the TE mode in the waveguides was approximately 1.3 dB/cm. A range of ring lengths were fabricated. A ring length a 737 μm led to the best overlap with the HCN lines. This length differs slightly from the optimal length of 709 m found in the previous section, likely due to differences between the fabricated and the designed waveguide geometry and material constants.

Experimental Setup
The chips are placed on a temperature-controlled copper stage and maintained at 20 °C. The temperature of the stage is held constant while varying the current through the microheaters to modulate the drop port output spectrum. Tapered polarization maintaining fibers are used for light input and output coupling. The contact pads of the microheater were contacted with needle probes connected to a current source. The transmission spectra of the ring and gas cell are acquired using a tunable laser source and photodiode. The light polarization is controlled using a polarization controller that was adjusted to couple only TE polarized light to the chip. An H 12 C 14 N gas cell at a pressure of 10 Torr with an effective path length of 80 cm was used to introduce absorption line features during a wavelength scan. The gas cell produces very sharp absorption line features from 1515 nm to 1560 nm with maximum absorption depths in excess of 11 dB and linewidths of approximately 1pm full-width at half minimum (FWHM).
The optical set-up was also configured with a broadband light source, a superluminescent diode system centred at 1555 nm. In this case, the input light is passed through a tunable filter which supports tuning of the center wavelength and spectral width of the bandpass. In this configuration, the photodiode measures the total light power exiting the drop port. This configuration uses an incoherent source, and enables real-time hardware correlation, therefore better representing the actual operating characteristics of the sensor.
Disclosures. The authors declare no conflicts of interest.

Data availability
The authors declare that the data regarding this work are available from the corresponding author upon reasonable request.