Nonlinear silicon waveguides generating broadband, spectrally engineered frequency combs spanning 2.0-8.5 um

TL;DR

This paper demonstrates fully air clad suspended-silicon waveguides capable of generating broadband mid-infrared frequency combs spanning 2.0-8.5 um, enabling advanced spectroscopy and chemical analysis with high resolution and stability.

Contribution

The work introduces novel suspended-silicon waveguides with engineered dispersion for broadband mid-IR frequency comb generation, including efficient coupling and high coherence.

Findings

Spectra span 2.1 octaves in the mid-infrared range.

Achieved high signal-to-noise ratio with 112,200 comb lines.

Enabled broadband dual-comb spectroscopy with high resolution.

Abstract

Nanophotonic waveguides with sub-wavelength mode confinement and engineered dispersion profiles are an excellent platform for application-tailored nonlinear optical interactions at low pulse energies. Here, we present fully air clad suspended-silicon waveguides for infrared frequency comb generation with optical bandwidth limited only by the silicon transparency. The achieved spectra are lithographically tailored to span 2.1 octaves in the mid-infrared (2.0-8.5 um or 1170--5000 cm-1) when pumped at 3.10 um with 100 pJ pulses. Novel fork-shaped couplers provide efficient input coupling with only 1.5 dB loss. The coherence, brightness, and the stability of the generated light are highlighted in a dual frequency comb setup in which individual comb-lines are resolved with 30 dB extinction ratio and 100 MHz spacing in the wavelength range of 4.8-8.5 um (2100-1170 cm-1). These sources are used for broadband gas- and liquid-phase dual-comb spectroscopy with 100 MHz comb-line resolution. We achieve a peak spectral signal-to-noise ratio of 10 Hz^0.5 across a simultaneous bandwidth containing 112,200 comb-lines. These results provide a pathway to further integration with the developing high repetition rate frequency comb lasers for compact sensors with applications in chip-based chemical analysis and spectroscopy.

Full text

I Introduction

Nanophotonic waveguides offer sub-wavelength mode confinement with effective modal area of $\sim$1 µm2. This enhances the nonlinear optical interactions significantly and enables efficient wavelength conversion at low pulse energies. Subsequently, this results in the reduction of size, complexity, and power consumption of nonlinear optical systems. Moreover, the recent development of compact mode-locked lasers Shoji et al. (2016) with repetition rates, $f$rep, on the order of 1–10 GHz has increased the demand for integrated on-chip nonlinear devices operating at low pulse energies. Photonic waveguides have been used for carrier-offset frequency, $f_{0}$, detection and self-referencing of frequency combs using picojoule-level pulses Carlson et al. (2017); Okawachi et al. (2018, 2017); Waldburger et al. (2019).

Optical frequency combs are phase-stabilized mode-locked lasers with broadband spectra of discrete, narrow optical lines spaced by the $f$rep Diddams et al. (2000); Jones et al. (2000); Udem et al. (2001); Cundiff and Ye (2003). These are excellent sources for precision metrology and spectroscopy applications Schliesser et al. (2012) where octave spanning, high-coherence, mid- and longwave-infrared (IR) spectral coverage are desired Sorokina and Vodopyanov (2003); Ebrahim-Zadeh and Sorokina (2008). Such sources access the molecular ro-vibrational states with resonant absorption lines unique to each molecule within the mid-IR (3–5 µm) Sell et al. (2008); Ycas et al. (2018); Muraviev et al. (2018); Lind et al. (2018) and longwave-IR molecular fingerprinting region (6–20 µm) Timmers et al. (2018); Kowligy et al. (2019). These advantages enable coherent probing of multiple ro-vibrational transitions in gas and condensed phases with an unprecedented frequency accuracy. Such lasers, implemented in the form of low-cost on-chip platforms, have applications as portable and mobile sensors for in-lab analysis, and fieldable trace chemical monitoring Sinclair et al. (2014).

In this paper, we utilize fully air clad suspended-Si nanophotonic waveguides Chiles et al. (2013); Kou et al. (2018) for longwave-IR frequency comb generation. Si benefits from a broad transparency window of 1.1–8.5 µm and a high nonlinear index, $n_{2}$, ($\sim 100\times$ that of silica) Kim et al. (1994); Gai et al. (2013). The mature Si infrastructure enables reliable fabrication of versatile, low-loss waveguides suitable for on-chip efficient $\chi^{(3)}$ nonlinear processes. When pumped with 100 pJ, 85 fs pulses at 3.10 µm, we generate IR spectra with 1–2 mW average powers and 2.1-octave bandwidth, spanning 2.0–8.5 µm (1170–5000 ${\mathrm{cm}}^{-1}$). Using the suspended-Si waveguides, we construct a dual-comb spectroscopy (DCS) setup for gas and liquid-phase sensing. Different waveguide widths enable measuring \sim$$\mathrm{missing}{[}\mathrm{meaning}\mathrm{=}{2.7E5}\mathrm{]}{2.7\text{\times}{10}^{5}} comb-lines that span the spectral range of 4.8–8.5 µm (2100–1170 ${\mathrm{cm}}^{-1}$), providing 100 MHz (0.0033 ${\mathrm{cm}}^{-1}$) spectral resolution. The broadest measured simultaneous bandwidth spans 4.8–8.0 µm with $\mathrm{missing}{[}\mathrm{meaning}\mathrm{=}{2.5E5}\mathrm{]}{2.5\text{\times}{10}^{5}}$ comb-lines. We study atmospheric water absorption with the 100 MHz comb-line resolution and broadband absorption spectra of liquid-phase methanol and isopropanol. The smooth waveguide spectrum enables broadband baseline correction to realize excellent agreement with existing Fourier transform infrared (FTIR) measurements.

This work improves the current state-of-the-art on-chip mid-IR generation technologies Lau et al. (2014); Kuyken et al. (2015); Singh et al. (2015); Yu et al. (2017, 2018); Nader et al. (2018); Hickstein et al. (2017); Kowligy et al. (2018); Guo et al. (2018); Sinobad et al. (2018); Grassani et al. (2019) to realize picojoule-scale pulse energy and extends the spectral bandwidth to longwave-IR region. This is the first demonstration, to the best of our knowledge, of an on-chip IR frequency comb with optical bandwidth spanning 2.0–8.5 µm, milliwatt scale average power, and a measured DCS figure-of-merit (FOM) equivalent to state-of-the-art mid-IR systems.

II Waveguide Design and Fabrication

Si-photonics is predominantly focused on telecom applications based on the Si-on-insulator (SOI) material platform. It is, however, challenging to realize high performance nonlinear devices, mainly due to two-photon absorption (TPA) when waveguides are pumped below the TPA cutoff of 2.2 µm. In addition, the SiO2-cladding has absorption at wavelengths $>$3.5 µm, limiting the utility of this platform for mid- and longwave-IR applications Lau et al. (2014); Kuyken et al. (2015); Yu et al. (2017). A number of alternative approaches have been investigated based on modified cladding materials to reduce IR absorption in conjunction with longer-wavelength pump sources to eliminate the TPA. In particular, Si-on-sapphire waveguides pumped above the TPA cutoff Shankar et al. (2013); Singh et al. (2015); Nader et al. (2018) have shown promising results for mid-IR applications, but with bandwidths limited to 6 µm, due to the sapphire absorption. Recently, SiGe-on-Si Sinobad et al. (2018) waveguides have been introduced as a new platform for spectral broadening reaching 8.5 µm. The SixGey/Si interface with 0.16 index contrast, however, limits the geometrical dispersion engineering.

We design and fabricate suspended-Si waveguides based on 700 nm thick fusion-bonded Si membranes Chiles et al. (2013) for supercontinuum generation (Fig. 1a, b). Removing the absorptive cladding eliminates the need for large cross-section waveguides to achieve low propagation losses in the IR Miller et al. (2017) and enables accessing the full transparency of Si. Moreover, this platform enables group-velocity-dispersion (GVD) engineering of the waveguides to realize application-tailored spectra through coherent dispersive wave generation. The bonded Si membrane is provided by a SOI wafer and the air trenches underneath the waveguides (Fig. 1b) are etched in a blank Si wafer prior to bonding (see supplement 1 for the fabrication details). The dimensions of the trenches are designed to avoid leakage losses of the generated longwave-IR light. The waveguides are formed by partial etching of the Si-membrane with etch depth of 390 nm, leaving a slab thickness of 310 nm to achieve the desired dispersion profiles for our nonlinear processes.

We implement floating fork-shaped couplers for efficient input (Fig. 1c, 1d) and output (Fig. 1e, 1f) coupling between the waveguide chip and the free space mode. These are designed with two symmetrical arms to couple the free space mode to two points at the coupler tips. Such geometry enables fast compression of the optical mode into the waveguide, achieving high coupling efficiency and adiabatic operation in much shorter length-scales, compared to the conventional inverse tapers. Moreover, the floating structure enables controlled waveguide mode expansion in both horizontal and vertical directions. The widths of the coupler tips, along with their center-to-center gap can be designed to set the mode-field-diameter of the expanded mode for optimized mode-matching to free space. The compactness of the couplers make them highly desirable for suspended waveguide platforms, where mechanical stability places strong constraints on the dimensions of edge couplers.

We form the couplers by full etching the 700 nm thick suspended-Si membrane. To reduce scattering losses, a Bézier-type curvature is used to define the shape of the arms, while the widths of both arms are tapered from the tip to the point where they merge. The widths of the input and output taper tips are chosen as 440 nm, and 1.4 µm and they taper to 440 nm and 900 nm in a floating length of 10 µm, preserving the adiabatic operation of the couplers. The center-to-center gaps are also designed as 1.86 µm and 2.2 µm for input and output couplers, respectively. We measure the best input coupling efficiency of 1.50$\pm$0.13 dB/coupler at the 3.10 µm pump wavelength and design the output couplers for broadband operation in 6.0–8.5 µm range with 3 dB efficiency.

The high core-cladding index contrast of 3.4:1.0 in suspended-Si waveguides enables versatile geometrical GVD engineering with flat anomalous dispersion at the pump wavelength. The calculated GVD of the waveguides is presented in Fig. 1g as a function of wavelength for different waveguide widths. The plotted range of widths have anomalous dispersion at the pump wavelength, making the waveguides suitable for soliton fission and broadband coherent supercontinuum generation. Moreover, the long wavelength zero-dispersion wavelength (ZDW) can be tailored from 3.5 µm to 5.0 µm by varying the waveguide width, providing the phase matching condition for lithographically engineered dispersive wave generation in the longwave-IR.

For supercontinuum generation (SCG) experiments, we use a mid-IR laser with 85 fs pulses centered at 3.10 µm operating with 100 MHz repetition rate (pump spectrum in Fig. 2a). This source is based on a 1550 nm Er:doped oscillator and difference-frequency-generation (DFG) in a periodically-poled lithium niobate (PPLN) crystal Cruz et al. (2015). We couple the free space beam of the 3.10 µm pump laser to the TE0 mode of the waveguides using a mid-IR Ge28Sb12Se60 aspheric lens with numerical aperture of 0.56. The output is collected using a 0.82 numerical aperture lens and monitored with an InSb camera to optimize the alignment. The output lens is aligned to maximize the coupling for the longwave section of the spectrum at wavelengths $>$5.0 µm and the collected supercontinuum spectra are recorded with an FTIR (Fig. 2a).

Pumping the waveguides at 3.10 µm avoids TPA, and the nonlinear FOM increases by a factor of 4 compared with the TPA-limited value. This is a metric to calculate the trade-off between the nonlinearity of the medium, i.e., waveguides, and the nonlinear absorption. This parameter is defined as $n_{2}/\lambda\beta\textsubscript{TPA}$ and $n_{2}/\lambda I\beta\textsubscript{3PA}$ for TPA and three-photon-absorption (3PA), respectively Gholami et al. (2011). Here $\lambda$ is the pump wavelength and $I$ is the light intensity inside the waveguide. The TPA and 3PA nonlinear absorption coefficients are $\beta\textsubscript{TPA}=$0.65$~{}$\mathrm{c}\mathrm{m}\mathrm{/}\mathrm{G}\mathrm{W} Bristow *et al.* ([2007](#bib.bib39)) and $\beta\textsubscript{3PA}=$1.75\text{\times}{10}^{-3}$~{}$\mathrm{c}\mathrm{m}^{3}\mathrm{/}\mathrm{G}\mathrm{W}^{2} Gai et al. (2013) at 1550 nm and the pump wavelength, respectively. The value $2.5\text{\times}{10}^{-5}$ $\mathrm{c}\mathrm{m}^{2}\mathrm{/}\mathrm{G}\mathrm{W}$ is used for the nonlinear index, $n_{2}$ Gai et al. (2013).

We present the calculated and measured supercontinuum spectra of different waveguide widths with 100 pJ pulse energy coupled into the waveguide in Fig. 2b and Fig. 2c, respectively. The measured output optical powers of the waveguides range 1–2 mW (average power), depending on the waveguide widths. The theoretical supercontinuum is calculated by solving the generalized nonlinear Schrödinger equation (gNLSE) Lin et al. (2007); Nader et al. (2018). The measured spectra are in excellent agreement with the simulations, enabling full control over the geometrical dispersion design parameters. For the waveguide widths of 1.0–3.0 µm, the supercontinuum contains a dispersive wave that is lithographically tailored from 5.0–8.5 µm, as predicted by the engineered long wavelength ZDW (Fig. 1g). Note that the narrowing of the longwave-IR dispersive wave bandwidth, above 8.0 µm, is consistent with the absorption by Si phonon modes, not represented in our model. For wider waveguides (3.0–4.0 µm widths) the broad and flat anomalous dispersion profiles with near-zero values result in broad supercontinuum generation covering more than an octave. The broadest bandwidth is measured for the waveguide width of 3.10 µm and it spans 2.0–7.5 µm. This broadband spectrum of comb-lines is suitable for spectroscopic study of a wide range of gas, liquid and solid phase samples.

III Dual-comb spectroscopy

Frequency comb lasers have been extensively utilized for spectroscopy applications in the infrared. This includes complementing existing Fourier-transform spectrometers Mandon et al. (2009); Spaun et al. (2016); Bjork et al. (2016); Changala et al. (2019) as well as the development of alternative spectroscopy schemes such as direct frequency comb spectroscopy (DFCS) with a highly dispersive etalon Diddams et al. (2007); Nugent-Glandorf et al. (2012); Fleisher et al. (2014); Iwakuni et al. (2019), electro-optic sampling of the electric field Bartels et al. (2007); Sell et al. (2008); Kowligy et al. (2019), and dual-comb spectroscopy (DCS) Keilmann et al. (2004); Coddington et al. (2016); Kara et al. (2017); Timmers et al. (2018); Weichman et al. (2019).

DCS is an implementation of Fourier transform spectroscopy in which the interference of two frequency combs with slightly different repetition rates, $f$rep and $f$rep+$\delta f$rep, maps the optical spectrum to the radio frequency domain Coddington et al. (2016). Depending on the configuration, one or both combs pass through a sample and then interfere on a photodetector. In the time domain, an interference pattern is recorded whenever the pulses of the two combs overlap in time, generating a repetitive interferogram with period of 1/$\delta f$rep. The Fourier transform of this signal results in the optical spectrum of the dual-comb system with the imprinted sample absorption.

We use our waveguides in a dual-comb setup to demonstrate the coherence of the waveguide-generated light for DCS applications. The second comb in our DCS experiment is based on an Er:doped oscillator and intrapulse DFG in an orientation-patterned GaP (OP-GaP) crystal Timmers et al. (2018), generating $\sim$300 µW of optical power in the wavelength range of 4–17 µm. To better match the spectral bands, we add a short pass filter (MgF2 window) to the beam path of the OP-GaP comb, limiting its long wavelength bandwidth to <9 µm. In addition, we also place a 4.5 µm long pass filter in the combined beam path of the two combs. The addition of these filters limits the spectral bandwidth of the dual-comb system to 4.5–8.5 µm. The seed 1550 nm light of both combs are self-referenced for $f_{0}$ stabilization using conventional f-to-2f interferometers. In addition, using a 1550 nm cavity-stabilized continuous-wave laser, we stabilize repetition rates (Fig. 2a) at the $\sim$100 nHz level (at 1 s) with $\delta f_{rep}$ = 50 Hz Timmers et al. (2018). This results in the dual-comb interferogram periodicity of 20 ms. We present a sequence of five interferograms in Fig. 3a, measured in a 100 ms acquisition time window using the output of the 2.95 µm wide waveguide. The low amplitude noise and the high mutual coherence of the stabilized lasers enable us to achieve the estimated time-domain signal-to-noise ratio (SNR) of 1200 after averaging 16384 frames, corresponding to 27 minutes. Figure 3b presents a 80 µs window of one of the interferograms. The large oscillation near t = 0 s is the center-burst representing the spectral envelop of the dual-comb system. The trailing oscillations are molecular free-induction-decay Coddington et al. (2010) signatures of the atmospheric water absorption centered at $\sim$6.25 µm ($\sim$1600 ${\mathrm{cm}}^{-1}$).

We retrieve the optical spectrum by calculating the Fourier-transform of the time-domain interferogram. Using three different waveguide widths of 3.35 µm, 2.95 µm, and 2.80 µm (Fig. 3c) our dual-comb setup covers the infrared range of 4.8–8.5 µm (2100–1170 ${\mathrm{cm}}^{-1}$) with spectral resolution of 100 MHz (0.0033 ${\mathrm{cm}}^{-1}$). The broadest simultaneous bandwidth is measured using the 3.35 µm wide waveguide and it spans 4.8–8.0 µm (1250–1200 ${\mathrm{cm}}^{-1}$), containing $\mathrm{missing}{[}\mathrm{meaning}\mathrm{=}{2.7E5}\mathrm{]}{2.7\text{\times}{10}^{5}}$ comb-lines. The highest spectral SNR is obtained using the 2.95 µm wide waveguide, while the waveguide width of 3.35 µm has the lowest measured SNR due to the broad bandwidth and the lower optical power per comb-line generated by this device. For the averaging time of $\tau$ = 330 s, we estimate the highest obtained SNR to be 180 over the 374 ${\mathrm{cm}}^{-1}$ spectral bandwidth. Normalized to a one second averaging time, we get $SNR$ = $10$ $\sqrt{Hz}$ at 100 MHz comb-line resolution. Having a total of $M$ = 112,200 comb-lines, we calculate our DCS FOM as $M\times SNR~{}=$ $1.1\text{\times}{10}^{6}$ $\sqrt{Hz}$ Newbury et al. (2010). This is similar to the previously reported value for a dual-comb setup with two identically designed infrared frequency combs based on intrapulse DFG in OP-GaP crystals Timmers et al. (2018), confirming the coherence of the nonlinear processes in the Si waveguides. Figure 3d presents zoomed-in views of the three spectra in Fig. 3c, emphasizing the resolved individual comb-lines with the 100 MHz spacing. Here the comb-lines are resolved with 20 dB, 25 dB, and 30 dB extinction ratios for waveguide widths of 3.35 µm, 2.80 µm, and 2.95 µm, respectively.

We utilize the waveguide-based dual-comb setup to study atmospheric water vapor from 1200–1600 ${\mathrm{cm}}^{-1}$. Such analysis enables monitoring of controlled lab environment through the study of atmospheric pressures and water concentrations, measured in terms of volume-mixing ratio. Figure 4a presents the measured atmospheric water absorbance with 100 MHz comb-line resolution. Data, presented in red, is compared to the HITRAN database Gordon et al. (2017), presented in blue and reflected about x-axis. We define absorbance as $A=\mathrm{log_{10}}(I/I_{0})$, where $I_{0}$ and $I$ are the calculated spectral baseline (Supplement 1) and the measured spectrum, respectively. Our dual-comb setup has an atmospheric beam path of $\sim$2 $\mathrm{m}$, resulting in many saturated absorbance peaks. Hence, we only present and analyze the absorption data in three unsaturated regions of 1258–1272 ${\mathrm{cm}}^{-1}$, 1340–1373 ${\mathrm{cm}}^{-1}$, and 1588–1608 ${\mathrm{cm}}^{-1}$. The data in these ranges are measured using 2.80 µm, 2.95 µm, and 3.35 µm wide waveguides, respectively.

We calculate the fit residuals as data minus the HITRAN model (Fig. 4b). The calculated root-mean-square (RMS) of the residuals is 0.007 for the measurement ranges of 1258–1272 ${\mathrm{cm}}^{-1}$ and 1342–1373 ${\mathrm{cm}}^{-1}$, emphasizing the excellent agreement between the DCS and HITRAN. This value increases for the 1588–1680 ${\mathrm{cm}}^{-1}$ range to 0.02 due to the lower optical power. Such excellent agreement between DCS measurements and the HITRAN database enables us to estimate the in-lab atmospheric pressure and volume-mixing ratio of the water content as 817$\pm$41 mbar and 0.016$\pm$0.002, respectively. These values agree well with the typical recorded values for the city of Boulder due to the area’s higher elevation. A detailed explanation of this estimation is provided in Supplement 1.

To emphasize our high DCS figure-of-merit, excellent frequency resolution and low fitting uncertainty, we highlight three absorbance features in Fig. 4a and present their zoomed-in windows in Fig. 4c. The high DCS FOM enables us to detect absorbance features of 10-2 levels (Fig. 4c, panel I) which is similar to the levels achieved with FTIR spectrometers. Moreover, the fine comb-tooth spacing enables resolving narrow linewidth features such as presented in Fig. 4c, panel II, with FWHM $\approx$ 1.2 GHz (0.04 ${\mathrm{cm}}^{-1}$). In a conventional FTIR spectrometer, a delay range of 3.0 m would be required to achieve the demonstrated 100 MHz spectral resolution. Despite the increased residual in 1588–1608 ${\mathrm{cm}}^{-1}$ range, the measurement still agrees well with HITRAN (Fig. 4c, panel III). We can estimate the in-lab atmospheric pressure and water concentration by only comparing the HITRAN model to this data set. In this case, the atmospheric pressure and water content are calculated within 1.4% and 3% of the values estimated using the data from all waveguide widths, respectively.

We also measure the absorption spectrum of liquid phase alcohols, leveraging the broadband and smooth waveguide spectra. Characterization of broad absorbers require spectral envelope stability over a broadband region to enable proper baseline measurement and subtraction Ycas et al. (2018). The stability of our supercontinuum-generated frequency comb enables coherent averaging and measurement of large absorbance values. Moreover, to eliminate the effects of long term, few minutes timescale, drifts we perform simultaneous measurement of the sample and reference spectra. In such a scheme, we use two liquid-nitrogen cooled mercury-cadmium-telluride (MCT) detectors at the two sides of the 50/50 beamsplitter that combines the infrared combs (Fig. 5a). The combined beams pass through a sample cell in one of the arms before being detected. In the other arm the beam is sent directly to the detector as the reference measurement. Care is taken to have equal beam paths between the two arms to minimize the residual atmospheric absorption features after the baseline subtraction.

We choose isopropanol and methanol for liquid-phase spectroscopy because they are widely used for scientific and industrial applications. For our DCS demonstration we use a 15 µm thick liquid-sample cell to minimize the interaction length with the liquid and avoid saturated absorption. In liquid-phase spectroscopy the absorbance lines are broadened to form a continuous spectrum and 1 ${\mathrm{cm}}^{-1}$ spectral resolution is sufficient to resolve the features. We perform the liquid-phase DCS experiments with a resolution of 0.67 ${\mathrm{cm}}^{-1}$, achieved via temporal apodization of the dual-comb interferogram to a 100 µs window. Fig. 5b presents the sample and reference spectra of methanol (top panel) and isopropanol (bottom panel), measured using different waveguide widths for each sample.

We compare our baseline subtracted (Supplement 1) DCS data to measurements performed using a commercial FTIR operating with 1 ${\mathrm{cm}}^{-1}$ resolution in Figs. 5c and Figs. 5d for methanol and isopropanol, respectively. The DCS data is in excellent agreement with the FTIR spectrum, yielding a RMS residual value of 0.02. We note that the calculated residual levels are only limited by the uncorrected atmospheric water absorption contaminating the DCS data. Such agreement enables accurate analysis of the DCS spectra to assign the measured absorbance features to different molecular vibrational transitions (Supplement 1).

While demonstrated here for well-known alcohols, the bright waveguide-generated light and dual-comb measurement techniques we employ can be widely applied to other samples. The frequency range of 1200–1600 ${\mathrm{cm}}^{-1}$ enables access to C—H and O—H bending functional groups. This region of the infrared provides stronger integrated absorption intensities and lower ro-vibrational density of states, when compared with C—H stretching functional groups at 2000–3000 ${\mathrm{cm}}^{-1}$ (3–5 µm).

IV Conclusion and Summary

We introduced suspended-Si waveguides as a versatile nonlinear photonic platform for spectral engineering of frequency combs across the mid- and longwave-IR. Our waveguide-generated comb light covers the optical bandwidth of 2.0–8.5 µm with milliwatt-scale average power enabling access to the molecular fingerprinting region. Here, we leverage careful dispersion engineering and mature fabrication to demonstrate efficient photonic-chip based spectral engineering across a 115 THz bandwidth with a 100 pJ pump pulse energy. To the best of our knowledge, this is the first demonstration of nanophotonic-based frequency combs with such broadband spectra and milliwatt-scale average powers.

To demonstrate the coherence of the waveguide-generated light and its utility for DCS applications, we utilized our nonlinear devices in a dual-comb setup. We have achieved a DCS FOM of $1.1\text{\times}{10}^{6}$ $\sqrt{Hz}$ which is competitive with current state-of-the-art systems operating in the same spectral region. The smoothness of the generated spectra and our fine comb-tooth spacing of 100 MHz (0.0033 ${\mathrm{cm}}^{-1}$) enables probing of gas-phase narrow linewidth absorbers as well as condensed-phase samples with broad absorption features. The DCS data have excellent agreement with the standard databases and FTIR measurements within less than 5% error.

Moreover, using the 2.80 µm wide waveguide as an example, $\sim$20% of the optical power at 8 µm extends into the air trenches underneath the waveguide core. Such an easy, on-chip access to the longwave-IR optical power presents the opportunity to integrate these devices with chip-based chemical delivery systems like microfluidic channels. We envision the integration of this platform with high repetition rate laser frequency combs and chemical delivery schemes for on-chip, parallel, multichannel in situ chemical studies and reaction monitoring.

Funding:

This work is supported by NIST and the Defense Advanced Research Projects Agency (DARPA), Defense Sciences Office (DSO) under the SCOUT program.

Acknowledgements:

We thank Travis Autry, Fabrizio R. Giorgetta, Esther Baumann, Jeffrey Shainline, and David Carlson for useful discussions and inputs on the manuscript. F.C.C. acknowledges funding from Fapesp (Grant # 2018/26673-5). This is a contribution of NIST, an agency of the US government, not subject to copyright. Product disclaimer: Any mention of commercial products is for information only; it does not imply recommendation or endorsement by NIST.