TiO2 Nanophotonic Sensors for Efficient Integrated Evanescent

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TiO nanophotonic sensors for efficient integrated evanescent-Raman spectroscopy Christopher C. Evans, Chengyu Liu, and Jin Suntivich ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00314 • Publication Date (Web): 14 Jul 2016 Downloaded from http://pubs.acs.org on July 23, 2016

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TiO2 nanophotonic sensors for efficient integrated evanescent-Raman spectroscopy Christopher C. Evans1,2, Chengyu Liu1,3, and Jin Suntivich1,4,* 1

Kavli Institute at Cornell for Nanoscale Science, 2Laboratory for Atomic and Solid State Physics, 3School

of Applied and Engineering Physics, 4Materials Science and Engineering Department, Cornell University, Ithaca, NY 14853, USA. *E-mail: [email protected]

Abstract: Waveguide-based evanescent Raman sensors are an attractive chemical-detection technology due to their compact format, increased signal over micro-Raman spectrometers, and nanoscale surface sensitivity. To improve the device’s performance, herein, we experimentally demonstrate strategies to increase the efficiency of waveguide-based evanescent Raman sensors over the state-of-the-art silicon-nitride devices by more than an order of magnitude. First, we use pumping at visible wavelengths (532 nm versus 785 nm, which is commonly used) to exploit the Raman cross-section’s λ–4 dependence. Second, we use titanium dioxide (TiO2), which combines comparatively low visible background luminescence with a high refractive index to concentrate light near the waveguide’s surface. These visibly pumped TiO2-sensors display >50x more Stokes signal per input pump power over silicon-nitride devices. Lastly, we explore ring-resonators and observe on-resonance Stokes emission with peak rates >30× higher than an equivalent length straight waveguide. We attribute the higher Stokes emission to a combination of pump and emission enhancements and propose using this approach to increase Stokes signal in future devices. —1— ACS Paragon Plus Environment

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Keywords: Raman sensor, titanium dioxide, waveguide, ring resonator, cavity enhancement TOC Graphic

Fully-integrated on-chip Raman sensors are an enabling technology for drug discovery1, medical diagnostics2–4, threat detection5,6, environmental-quality monitoring7 and can provide a portable solution to chemical analysis in remote areas and extreme environments8,9. Recent advances in on-chip laser sources10, spectrometers11,12, and optofluidics13,14 bring such on-chip sensors closely within reach. However, designing the light-chemistry interfacing component to replace bulky microscope objectives is challenging in terms of Raman excitation and collection efficiency, device complexity, which includes guiding of pump and Stokes signal waves15, and surface chemical stability. One promising solution is to utilize the evanescent field outside of a dielectric waveguide for excitation and collection16–24. The index contrast at the waveguide’s surface supports an evanescent field, which allows both the pump and the Stokes modes to interact with the analytes (Fig. 1a). By collecting the Raman signal in this configuration, we can increase the signal of the analytes by increasing the waveguide’s length. In comparison to conventional micro-Raman systems that only collect signal

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within the focal volume (~µm3), this waveguide-based approach collects signal close to the surface (~tens of nanometers) over macroscopic lengths up to several centimeters25,26. Given these advantages, it is desirable to optimize the efficiency of integrated evanescent-Raman sensors to increase the Stokes signal. In this Communication, we experimentally demonstrate theoretical and experimental methods to enhance the efficiency of evanescent-Raman sensors by more than an order of magnitude over existing Si3N4 integrated evanescent-Raman sensors by using alternative materials and shorter wavelengths. In addition, our microchip approach allows us to take advantage of engineered planar photonic cavities (e.g. micro-ring resonators or photonic crystal cavities) that can recirculate pump light to further decrease on-chip pump power requirements while simultaneously enhancing resonant Stokes emission27. We show that the use of a cavity resonator can provide conversion efficiency improvements beyond what can be achieved using length optimization. Our work reveals a path toward scalable, compact, and highly efficient photonic chips, which may have an important impact by incorporating integrated optics with applications in chemistry and life sciences. Integrated evanescent-Raman devices that operate at the visible wavelengths have many advantages over currently-used near-infrared (NIR, i.e. 785 nm) devices20,28. Shorter wavelengths increase the Raman cross section (scaling as λ–4)29, support reducedmode volumes (scaling as λ/n)17,20, and, by pumping close to a molecule’s absorption edge (e.g. at visible wavelengths for red-blood cells30), can achieve enhanced Raman scattering via resonance-Raman scattering. From a design standpoint, the Stokes shift (Δ = − ) is smaller relative to the pump frequency ( ) at visible versus NIR frequencies, thus making it easier to achieve higher pump-Stokes overlap within the


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waveguide, even for large Raman shifts. For example, using a pump wavelength of 785 nm and a 3000 cm–1 shift (i.e. C–H bonds) emits Stokes photons around 1027 nm, while the same Stokes shift using a 532-nm pump emits photons around 633 nm. In addition, visible operation is compatible with high-quality, low-cost, commercially available lasers, and, by maintaining Stokes wavelengths 2.4, see Table 1)3,28,35–37. Furthermore, its wide indirect bandgap38,39, limited fluorescence (see SI), chemical stability40, and biocompatibility41 render TiO2 a highly attractive platform for the sensing of an array of chemical and biological agents, especially in harsh-environment conditions. However, Dhakal’s study only provides a theoretical argument for the use of TiO2. While TiO2’s strong linear and nonlinear refractive indices (approximately 30 times the value of silica) and negative thermo-optic coefficient has received attention for integrated nonlinear optical42–44 and athermal45–49 devices, TiO2 is a relatively new photonic platform, whose waveguides currently exhibit higher losses than more mature platforms such as silicon and silicon nitride38,39. We use a recently reported fabrication process to overcome this challenge and create low-loss amorphous TiO2 waveguides for evanescent-Raman sensors50. Using transverse-magnetic polarization (TM), and green wavelengths (532 nm), we experimentally measure Raman signals with conversion efficiencies >50× higher than existing NIR demonstrations in silicon nitride, which was reported using transverseelectric (TE) polarization and a 785 nm pump20. Lastly, we demonstrate an approach to increase the Stokes signal, beyond simply increasing the waveguide’s length, by using ring resonators. In this separate experiment, we use conventional top-down fabrication methods to develop TiO2 ring resonators and use a tunable 780-nm pump laser to excite on-resonance. Despite the higher losses in these top-down fabricated waveguides, we demonstrate cavity-enhanced evanescent


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Raman scattering with conversion efficiencies within the ring that are >30× higher than an equivalent length straight waveguide. These results provide a roadmap toward miniaturized on-chip Raman sensors with increased sensitivity. To our knowledge, this work provides the first demonstration for a ring resonator approach to integrated evanescent-Raman spectroscopy.

Material

Band gap

n (532 nm)

Cladding

Loss

Reference

Si3N4

5 eV

1.93

unclad

2.5 dB/cm

Ref. 51

TiO2

3.1 eV

2.44

in liquid (nclad = 1.39)

7.5 dB/cm

This work

Table 1: Comparison of the properties of waveguides made from TiO2 vs. Si3N4 Although there have been several proposed definitions of the “efficiency” of evanescent-Raman scattering in the literature20,28, none of these definitions adequately describe the performance of a realistic waveguide (with non-zero loss) in isolation (avoiding the influence of input and output coupling losses). A more appropriate efficiency definition should collectively capture these effects and measure the amount of launched power vs. the Stokes signal at the output. We define this ratio as our experimental efficiency. Our definition uses the power in the Stokes signal (in the forward direction) at the end of the waveguide normalized to the input pump power, given by:

≡ =

.

(1)

Here, !" is the Stokes power at the end of the waveguide of length L, !# 0 is the pump power at the input of the waveguide, is the bidirectional “specific conversion —6— ACS Paragon Plus Environment

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efficiency” of the waveguide (sr), is the density of molecules on the surface (cm–3), is the Raman cross section per molecule (cm2/sr), and !% & is the pump power. Δ( is the difference in guiding loss between the pump and the Stokes beams (for more details, see the Supplementary Information, SI). The factor of ½ accounts for forward-collection geometry used here. The efficiency is different from the existing efficiencies that have been studied in the literature: , , ) , and * . These previous parameters are insufficient to compare experimental waveguide efficiencies. The specific conversion efficiency, , does not include the effect of the molecule (specifically and ), nor does it include the effect of propagation losses. The so-called bidirectional conversion efficiency, , includes the molecule’s contribution, but neglects propagation losses. Often, the parameter ) ≡ !+ /! = is reported, which is the ratio of the transmitted Stokes to transmitted pump power; however, it does not capture the effect of the magnitude of the waveguide’s loss (as it accounts only for the difference in loss, Δ( and is therefore only meaningful for loss-less waveguides). As a result, , , and ) are not suitable for predicting the performance of a realistic waveguide. Among the previously studied efficiencies, the parameter * is most closely related to 20 (see SI). However, * includes input and output coupling losses and as a result, this parameter depends on the quality of the facets20 and coupling methods52, which complicates the assessment of the intrinsic conversion efficiency of the waveguide. We therefore elect to use to quantify the efficiency of the evanescent-Raman waveguide, which addresses all of these concerns. Lastly, as the waveguide can be made arbitrarily long, we will compare between devices at the length that experimentally —7— ACS Paragon Plus Environment

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maximizes the Raman efficiency, which we denote as ./0 . We note that the theoretical maximum should occur near = 1/( using the assumption that Δ( 2 0. We first compare the efficiency of our TiO2 devices to previous demonstrations in silicon nitride that used a 785-nm pump and TE polarization20. We note that although a more recent report has used a 785-nm pump and TM polarization, this paper did not report sufficient data for us to make a proper experimental comparison using or even * 28. We will therefore focus on the comparison to the 785-nm pump and TEpolarization case. We first design a waveguide using a lift-off fabrication procedure50 that enables single TM-like mode operation around 532 nm with low propagation losses (7.5 ± 0.7 dB/cm, see SI) when compared to reported single-mode losses using top-down fabrication methods39. In Fig 1b, we show a scanning-electron microscope (SEM) image of an exemplary lift-off waveguide. The bandwidth (Δ/ at visible wavelengths allows our designed waveguide to exhibit a single TM-like mode from 532 nm (Fig. 1c) to 635 nm (corresponding a Stokes shift of ~3050 cm–1, see SI).

Fig. 1: a) A schematic representation of a waveguide-based evanescent Raman scattering device, showing the waveguide, analyte, and the guided visible pump and Stokes beams. b) Scanning electron microscope (SEM) image of a typical TiO2 waveguide at a 45° tilt fabricated using a dielectric lift-off technique. We show a device that is larger than our experimental waveguides to demonstrate its unique geometry. c) The TM-like mode profile (electric-field intensity) of our waveguide using a pump wavelength of 532 nm, showing the highly surface-localized evanescent field (50% of the signal is generated within the first 22 nm from the —8— ACS Paragon Plus Environment

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surface, see SI). This waveguide is approximately 70-nm high and 500-nm wide (see SI). To compare the efficiency to previous reports20, we experimentally measure with isopropanol using a 0.72-cm long waveguide (we expect a theoretical maximum efficiency for a 0.58-cm long waveguide calculated from our measured losses, see SI). Using 1.3 mW of pump power around 532 nm, we record typical counting rates of 1600 counts/sec/mW above baseline within the 819 cm–1 Raman peak (normalized to the preobjective pump power, see SI). If we account for all sources of coupling loss (chip, fiber, and spectrometer, including the slit), we obtain 0.72 cm to be 8.5 ± 2.1 × 10–10. The experimentally obtained 0.72 cm matches our theoretically calculated value (see SI). Our theoretical model suggests that a slightly shorter waveguide can further increase , with a theoretical maximum efficiency ./0 = 1.1 × 10–9 for a 0.58-cm long waveguide. In comparison to the values in silicon nitride, we use the highest reported experimental conversion efficiency (* ~8.5 × 10–13) that occurred for a 1.6cm long waveguide. Using this value we estimate 1.6 cm to be ~1.5 × 10–11, with a theoretical maximum efficiency of ./0 = 7.2 × 10–11 (using 785-nm light, TE-like polarization, and a 2.1-cm long waveguide)20. Thus, our experimental efficiency is >50× larger than previous experimental values in silicon nitride and an over an order of magnitude greater than the theoretical maximum efficiency using similar geometries with 785-nm light and TE-polarization20. We note that our efficiency is still higher that the efficiency of silicon nitride when using all other efficiency definitions with similar geometries20,28 (see SI).


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We now discuss the origin of the efficiency improvement in TiO2. The first improvement comes from the increase in the local density of modes, (a wavelengthscaled geometrical factor that accounts for the local density of modes, see SI)28. This enhancement, as suggested theoretically by Dhakal et al., is due to the higher index of TiO2 over silicon nitride. We calculate a top-cladding η0 of 0.20 sr, which is 3× and 1.4× higher than the theoretical values in silicon nitride with TE-like20 and TM-like28 polarizations, respectively20. However, this enhancement only makes up part of TiO2’s advantage. The second improvement is to increase the Raman cross section () by using shorter wavelengths, translating to additional factor of ~5× for 532-nm operation over a 785-nm operation. The enhancement from and thus represents over an order of magnitude improvement when compared to previous theoretical results in silicon nitride using 785 nm with TE-like polarization20 and ~7× for a TM-like polarization28. In addition, the limited field within the core increases the signal to core-background noise ratio to 7:1 (see SI). These enhancements, in combination with the low guiding loss of our TiO2 waveguide give rise to the observed efficiency in this work. Beyond having increased signal, wavelength flexibility is another key advantage for TiO2-based systems. To demonstrate this wavelength flexibility, we measure toluene signal within the “fingerprinting window” (700–1300 cm–1) at several pump wavelengths using geometrically scaled waveguides (Fig. 2b). In Fig. 2a, we show typical Raman signal from toluene using a 532-nm TM-like pump. We observe a low and slowly varying background signal that allows us to retrieve the Raman signal using standard baselinesubtraction methods53 (see SI). All peaks are consistent with the reference toluene spectra

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obtained from a commercial Raman microscope in Fig. 2b, irrespective of the pump wavelength. This wavelength flexibility allows us to probe Raman signal at high wavenumbers (e.g. 2800–3650 cm–1 for C-H and O-H bonds) while maintaining compatibility with high-efficiency, silicon-based detectors. Silicon-based detectors have responsivity that typically drops-off near 1000 nm; thus pumping at wavelengths longer than 785 nm results in C-H signal that is beyond the edge of the detector’s bandwidth (a 3100 cm–1 shift from 785 nm corresponds to Stokes signal around 1037 nm). In addition, shorterwavelength operation better ensures pump-signal overlap for large Stokes shifts and avoids the waveguide’s long-wavelength cutoff17,20,28. This wavelength-advantage allows us to probe the Raman spectra of common organic solvents (methanol, isopropanol, and acetone) from 2600–3200 cm–1 using a pump wavelength of 633 nm (Fig. 2c). Our results demonstrate the low-fluorescence and wide-wavelength applicability of TiO2, showing that TiO2 nanophotonic waveguides are an ideal candidate for the lightchemistry interface component in compact integrated Raman spectrometers. We show that our residual background is likely due to uniform Raman scattering within the amorphous waveguide that is straightforward to correct, yet it may limit the sensitivity of this approach. Furthermore, a major advantage of the higher conversion efficiency at visible wavelength is the compatibility with inexpensive, low power lasers; for example, our experiments essentially use a 532-nm laser pointer.


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Fig. 2: Demonstrations of evanescent Raman scattering using TiO2 waveguide devices. a) Collected Raman signal from toluene using a 532nm pump wavelength showing strong Raman scattering and weak background signal (inset: guided green light in a typical 1-cm wide TiO2 chip with liquid analyte on top). b) Comparison of Raman signal of toluene taken using a commercial Raman microscope and our evanescent Raman system at several wavelengths. We adjust the geometry for different operating wavelengths to maintain similar modal profiles to Fig. 1c. c) Using a similar geometrically-scaled waveguide and pumping at 633 nm, we compare different solvents, showing clear differences in the Raman spectra of the C-H bonds.

We now discuss a method to improve the efficiency of an evanescent-Raman sensor beyond the straight-waveguide limit by using ring resonators. Other methods, such as using slot-waveguides to concentrate the mode within the analyte28 or integrating plasmonic nanostructures on top of channel waveguides to exploit surface enhanced Raman scattering (SERS)4,32,54, often exhibit higher losses than channel waveguide, potentially limiting the device’s efficiency. In addition to being compatible with these approaches, optical micro-ring resonators are easily integrated and can simultaneously boost the effective pump-power (using resonant pumping) and increase emission (via the Purcell effect). Here, we demonstrate this strategy for increasing device efficiency using a TiO2 ring-resonator. The increased efficiency in ring resonators can arise from two effects. First, for a high quality-factor (Q-factor) ring resonator, the circulating power ( !89:8 ) in the


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waveguide’s cross-section increases by

;30× more Stokes signal compared to an equivalent-length (314 µm) waveguide (see SI for a detailed calculation). Interestingly, we find that the ring-resonator produces more signal (~8x) than a straight bus waveguide at an optimum length (at = 1/( where = ./0 , see SI). Furthermore, the ring resonator is an order of magnitude shorter than an optimized straight waveguide (314 µm


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vs. 3060 µm, at = 1/( ). Thus, our ring resonator

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produces nearly an order of

magnitude more peak signal than an optimized straight waveguide while using only a footprint of 0.01 mm2. To determine the origin of the enhancement, we plot an extended 3-nitrotoluene data set on a log-plot in Fig. 4c. We observe a uniform enhancement of approximately 6 dB above the baseline for all wavelengths. Next, we estimate whether the enhancement originates from the pump enhancement alone or whether the cavity also increases the emission at the Stokes frequency. For this device, we estimate a pump enhancement factor of 5 (corresponding to 2.5× the power circulating in the ring versus the straight waveguide). As this pump enhancement falls below the signal enhancement, we conclude that the pump enhancement alone cannot explain the increase in the on-resonance Stokes signal. We thus hypothesize that the signal enhancement in the TiO2 ring resonator must come from both the pump enhancement and the increased emission at the Stokes frequency, consistent with the Purcell effect as observed in other studies57. Optimizing the balance between coupling for pump and Stokes waves may increase the efficiency in future devices58.


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Fig. 4: a) Resonantly-enhanced Raman scattering of 3-nitrotoluene around 870 nm (1350.2 cm–1) using a ring resonator with a radius of 50 µm, showing the signal when a 780-nm laser is tuned off and on the ring’s resonance. Each peak corresponds to a resonant mode of the cavity. b) Microscope image of the 3-nitrotoluene experimental setup. The droplet partially covers the bus waveguide, thus, the off-resonance background is mostly due to this bus-waveguide signal. We note the ring location with a dashed line. c) Plotting an extended spectrum on a log-scale, we measure uniform on-resonance enhancement above the baseline of approximately 6 dB. In summary, we report TiO2 nanophotonics whose high refractive index, wide transparency, and low fluorescence enable efficient evanescent Raman scattering. The higher refractive index of TiO2 compared to silicon nitride and the ability to operate at 532 nm versus 785 nm enhance the maximum Raman conversion efficiencies, , by >50× over the previous reports using silicon nitride20. These devices are highly sensitive to molecules near the surface (50% of the signal is generated in 30× higher Stokes signal than an equivalent straight waveguide. We find that the pump enhancement factor alone cannot explain this observation. We therefore propose that the emission of the Stokes wave may be enhanced by the Purcell effect as well. Our work shows the application of exploring new photonic materials to overcome


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challenges in functional photonics, specifically, for creating label-free, low-cost, high performance sensing devices for chemical and life science applications.

Methods

Evanescent-Raman device fabrication Devices for visible and near-infrared measurements were fabricated using a dielectric liftoff procedure. Using a 3-µm thick thermally oxidized silicon wafer, we spin on 260-nm of lift-off resist (LOR, MicroChem Corp.) and 600-nm of positive deep-ultraviolet resist (UV-210, Rohm and Haas Electronic Materials LLC). We expose using a DUV-stepper (λ = 248 nm). Our mask has a series of 8 U-bends with several widths (250–1500 nm in 250 nm increments). Post-exposure baking and development produce variable gaps in our DUV resist corresponding to our mask dimensions and an undercutting of approximately 500 nm. Next, we deposit ~250 nm of amorphous TiO2 using DC reactive sputtering of titanium metal in an oxygen/argon environment. Then, we sonicate in a series of baths— remover-PG, acetone, and isopropyl alcohol—to lift-off the resist and TiO2 not adhered to the substrate, producing the final waveguides. Lastly, we dice and polish our waveguide facets to produce an unclad waveguide for measurement. For 532-nm measurement, we reduce the length of the chip to include only 3.5 U-bends. Cavity-enhanced evanescent resonator fabrication We use standard top-down fabrication procedures to fabricate our ring-resonator devices. We start by depositing a 250-nm thick TiO2 layer using reactive sputtering. Next we use a


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bilayer of ARC (anti-reflective coating) and UV-210 photoresists, and then expose in a deep-ultraviolet stepper (λ = 248 nm). After development, we etch using C4F6 and He cooling. Lastly, we prepare the facets by dicing our wafer and polishing. For Fig. 4, we use a 50-µm radius resonator with a width of 370 nm (bottom width) that has a coupling gap of 450 nm. To match the FSR to the peaks of 3-nitrotoluene, we fabricate and test several chips using different doses during photolithography, thus changing the resulting dimension and FSR. For the power-dependent measurement, we use a similar chip; however, the gap is increased to 585 nm to decrease losses at the Stokes wavelength. Evanescent-Raman measurements We measure evanescent Raman signal at several wavelengths using TiO2 chips fabricated using the lift-off method. We use a 532 nm diode, a HeNe, and an amplified 780-nm distributed-feedback (DFB) laser as our sources. For each source, we use a narrow bandpass filter centered on the laser wavelength. We control the power using a ½waveplate and polarizer set to vertical polarization to excite the transverse-magnetic mode of our waveguides. We couple into and out of our chip using the end-fire method with 0.85 NA objectives. For each measurement, we place a droplet of solution directly on top of the chip, covering the chip. We pass the output through a long-pass filter (550, 650, and 800 nm for 532, 633, and 780 nm pump wavelengths) and couple the light into a fiber collimator attached to a single-mode fiber. Lastly, we use a commercial spectrometer with a TEC cooled (–80° C) camera. We remove the slowly-varying background using the Lieber method53, modified to replace a polynomial fit to a cubic spline to cover the entire range of interest.


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Cavity-enhanced Raman measurements For the cavity-enhanced Raman measurements, we use the same setup as for the evanescent-Raman measurement except with a top-down fabricated chip. For Fig. 4a, we use a 50-µm radius ring with a measured Q of ~5 × 104. We measure 3-nitrotoluene,set the input power to 10.6 mW, and current tune our 780-nm DFB laser on and off resonance using a 10-second acquisition time.

Author Contributions:

CCE and JS conceived of the basic idea for this work. CCE and CL designed and carried out the experiments, and analyzed the results. JS supervised the research. CCE wrote the first draft and all authors contributed to writing of this paper.

Acknowledgments:

We acknowledge Michal Lipson and Chris Phare for providing many insights and helpful discussions. We thank Ryan Badman, Fan Ye, and Michelle Wang for providing silicon nitride films for luminescence testing. We acknowledge support from the Samsung Advanced Institute of Technology (SAIT) through the Global Research Outreach (GRO, Dr. Yongwon Jeong). CCE and CL acknowledge support from the Kavli Institute at Cornell for Nanoscale Science. This work was supported in part by the Cornell Center for — 20 — ACS Paragon Plus Environment

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Materials Research with funding from the NSF MRSEC program (DMR-1120296), and using the Cornell NanoScale Facility, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (Grant ECCS-1542081).

Supporting Information:

Supporting Information Available: Luminescence comparison, experimental setup, liftoff waveguide characterization, Raman efficiency calculations and cavity-enhanced Raman scattering analysis. This material is available free of charge via the Internet at http://pubs.acs.org.


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