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Abstract
Waveguide Raman spectroscopy uses the evanescent field outside a waveguide to probe the analyte on the surface of the chip, permitting to selectively study thin films or nanostructures on top of the waveguide while benefiting from the long iteration path of the excitation with the analyte. Both the polarization of the excitation mode as well as the refractive index contrast of the waveguide platform play an important role in the Raman excitation process as well as the coupling efficiency of the generated Raman signal back into the waveguide. In this article, we characterize three waveguide platforms of different refractive index contrasts for waveguide Raman, namely Al2O3, Si3N4 and TiO2 on SiO2. Toluene was used as a test analyte. Both background and analyte were measured for quasi- transverse electric (quasi-TE) and quasi- transverse magnetic (quasi-TM) modes. TM modes generate less background than TE modes due to less confinement of the mode in the waveguide core materials. A combination of Si3N4 and quasi-TM polarization led to the highest SNR in this study.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical sensors and fiber optic sensors have some inherent advantages compared with their electrical counterparts: they can provide high selectivity, immunity to electromagnetic interference (EMI), a wide dynamic range, easy multiplexing and the possibility to be deployed in hostile environments thanks to their electrical and chemical inertness [1,2]. By measuring intensity, phase and/or refractive index changes, optical sensors can sense a wide variety of parameters, including temperature, pressure, vibration, and presence of specific chemical species [1,3]. In the last few decades, optical sensors have been integrated on-chip leveraging existing semiconductor fabrication technologies. Integrated optical sensors not only enable miniaturization of system and mass production but also lead to more robust sensors. Since their conception, integrated optical sensors have attracted a great deal of interest [4–11]. However, the detection of different analytes is restricted to the availability of suitable capture layers, making the detection of unknown analyte challenging.
Raman spectroscopy is a useful analytical tool to identify molecules in a given analyte in a label-free and nondestructive manner. It is compatible with a wide range of samples in liquid, solid or gas phase. Raman spectroscopy has a wide range of applications in analytical chemistry, food science [12], pharmaceutical industry [13], art and archaeology [14], biomedical studies and in the clinic [15,16]. Several studies have demonstrated the detection of single or multiple nanoparticles using Raman spectroscopy combined with optical tweezers [17–20]. Raman spectroscopy has also been used to study thin surface layers [21]. Advanced techniques, such as surface-enhanced Raman spectroscopy, allow collecting Raman spectrum from thin films on a surface or nanoparticles [22,23].
A Raman spectrum with a high signal-to-noise ratio (SNR) is required to separate the signal of a nanostructure or monolayer from the surrounding bulk background signals. In a free-space optical system, the resolution of the Raman microscope cannot be better than the diffraction limit, which sets a lower limit on the total probed volume. When a large surface must be studied, given that the volume scales with the fourth power of the numerical aperture, the surface-to-volume ratio becomes increasingly unfavorable. To overcome these limitations, waveguide Raman (WGR) spectroscopy has been proposed. WGR uses the evanescent field above a waveguide to probe an analyte in close proximity to the waveguide. The interaction length with the analyte can be increased by increasing the length of the waveguide being just limited by the propagation losses. WGR reduces the collection time 10-100 fold to achieve the same Raman signature as a conventional Raman microscope [24].
Since WGR was first demonstrated by P. O’Connor and J. Tauc in 1978 [25,26], WGR spectroscopy has proven to be a useful technique to study different analytes such as gas-phase molecules [27], thin films [28] and microparticles [29]. However, waveguide fabrication technologies were a major bottleneck that prevented WGR from showing its advantages. Leveraging the advances in microfabrication technologies for integrated photonics, Dhakal et al. successfully demonstrated WGR detection of isopropyl alcohol (IPA) in transmission mode using Si3N4 waveguides and 785 nm excitation [24]. Since then, WGR has been shown to be a promising technique to investigate thin films and different liquid and gas-phase molecules [27,30–33]. Silicon nitride (Si3N4) has been mainly used as a waveguide core material [27, 30, 33, 34]. Si3N4 has a high refractive index, which increases the coupling efficiency of the generated Raman signal. Although some Si3N4 waveguides showed a low fluorescence background [34,35], many Si3N4 waveguides have a relatively high fluorescent background in the near-infrared [36,37]. This inherent background can limit the SNR of the Raman signals, which has motivated the search for other high refractive index materials. Evans et al. studied WGR in titanium oxide (TiO2) waveguides [38]. Raza et al. compared four high refractive index contrast materials typically used in integrated photonics, namely aluminum oxide (Al2O3), silicon nitride (Si3N4), titanium oxide (TiO2) and tantalum oxide (Ta2O5) [39]. In their study, the relatively low refractive index contrast of Al2O3 led to poor Raman coupling efficiency, while the elevated background and high propagation losses of TiO2 led to poor SNR. Their study contained only TE polarization.
In this study, we compare the performance of Al2O3, Si3N4, and TiO2 for both TE and TM polarization to find the most suitable platform for on-chip Raman spectroscopy. According to FDE simulations, the TM mode supported by the designed waveguides concentrates the field outside the core of the waveguide. Therefore, the minimization of the background is expected. At the same time, the higher intensity at the core-cladding boundary enhances the coupling of the emitted Raman photons back into the waveguide core. The background of each waveguide platform is measured for both TE and TM excitation polarization. Toluene was used as the analyte in our experiments due to its characteristic Raman signature.
2. Experiment
2.1 Design considerations for waveguide-integrated Raman spectroscopy
Channel waveguides were designed for the three materials in this study for operation in both the TE and TM polarizations. Channel waveguides were selected because this geometry maximizes the contact area with the analytes of interest when compared with other structures such as slab waveguides or rib waveguides. Furthermore, channel waveguides support smaller bending radii when compared with the previously mentioned alternatives, which is advantageous to produce long spiral waveguides that increase the interaction length with the analyte while occupying a small surface area on the chip.
Lumerical MODE solutions was used to perform mode calculations for the different waveguide structures (Fig. 1). In the simulations, the refractive indices of each material were calculated using the Cauchy’s equation given by
Fig. 1. Mode profiles of the three waveguide platforms under study. The simulations were performed with toluene as upper cladding. The dimensions of the simulated waveguides are Al2O3: 1.2 µm ${\times}$ 346 nm, Si3N4: 1.2 µm ${\times}$ 127 nm and TiO2: 1.2 µm ${\times}$ 181 nm.
Table 1. Table of Cauchy coefficients for the waveguide core materials.
Table 2. Dimension of the three waveguides used for the experiments.
2.2 Waveguide fabrication
A summary of the fabrication flow to produce LPCVD Si3N4, sputtered Al2O3 and sputtered TiO2 waveguides can be seen in Fig. 2. The detailed process flow is described in Table 5 in Appendix A. The fabrication begins with the deposition of the optical material of the core onto an oxidized (i.e., 8 µm thermal oxide) silicon wafer. Low-pressure chemical vapor deposition (LPCVD) was used to deposit the Si3N4 layer. Al2O3 and TiO2 were deposited by reactive RF and DC magnetron sputtering, respectively. A standard UV lithography (i-line, λ = 365 nm) process follows. First, the substrates were cleaned in nitric acid (HNO3) bath for 10 minutes to eliminate any organic residues, rinsed with a quick dump rinse (QDR) and spin-dried. Then, a dehydration bake and a deposition of an HMDS monolayer using a vacuum oven was performed. A photoresist (Olin OiR 907/17) was deposited by spin coating. A UV mask aligner (Electronic Vision Group, EVG620) was used to transfer the patterns into the resist layer. The development of the structures was done by direct immersion in OPD4262 developer. Reactive ion etching (RIE) was used to transfer the patterns created by photolithography into the optical layer. After the etching of the optical layer, resist stripping was done in an oxygen plasma to eliminate any remaining resist. Argon/oxygen plasma was used to process the Si3N4 layers and nitrogen/oxygen plasma was used for TiO2 and Al2O3 layers. A SiO2 cladding (3 µm thick) was deposited by PECVD everywhere apart from in the sensing window. Finally, the resulting wafer was diced into individual chips. Before the dicing process, a resist layer was spin-coated (Olin OiR 908-35) and baked in order to protect the optical components. A special optical-grade diamond blade (Microace series 3; Loadpoint Ltd, Swindon, UK) was used during dicing. The resulting chips were stored at this point. Individual chips were extracted and cleaned only just before performing the experiments. Acetone was used to remove the protective resist layer. Then, sonication was done to eliminate particles from the dicing step. The chips were cleaned in nitric acid for 10 minutes to eliminate any organic residues, rinsed using QDR and spin-dried.
Fig. 2. Fabrication process flow to produce Si3N4, Al2O3 and TiO2 waveguides. The different materials used have been indicated with multiple colors.
2.3 Experimental setup
Raman experiments were conducted using the Raman measurement setup described in Fig. 3. A 785 nm pump beam is generated by a laser diode (LD-785-SE400; Thorlabs Inc., Newton, NJ), mounted on a temperature-controlled laser diode mount (TCLDM9; Thorlabs Inc.). The laser is driven by a diode controller (LDC-3722; ILX Lightwave, Bozeman, MT). To achieve a stable laser output, we operated the laser at a driving current of 260 mA and a temperature set-point of 25 °C. We controlled the laser power with a half-wave plate and a polarizing beamsplitter. The pump beam was filtered by a laser clean-up filter (LL01-785; Semrock Inc., Rochester, NY) and then polarized by a half-wave plate to excite either the TE00 or TM00 mode of the optical waveguide. The polarized pump light was coupled into the waveguide with a microscope objective lens (40x/0.60; Nachet, France). To optimize coupling, the Rayleigh scattering from the waveguide was monitored by a CMOS camera (BFLY-U3-23S6M-C; Point Grey Research Inc., Richmond, Canada) from the top of the waveguide. The same objective lens was used to collect the light in a backscattering configuration. The collected light passed through a dichroic mirror (LPD02-785RU-25; Semrock Inc.) and long-pass filter (LD02-785RU-25; Semrock Inc.). It was then dispersed by a spectrograph with a 300 grooves/mm grating (IsoPlane 160; Princeton Instruments, Trenton, NJ, USA). The dispersed light was recorded by a 1002 × 1004 EM-CCD camera (Andor iXon DV885JCS-VP; Andor Instruments, Belfast, UK) cooled to -70℃ to reduce the dark noise generated by the imaging sensor. For each measurement, we utilized 100 sec integration time and 36 mW of pump power. A long accumulation time was used to maximize the SNR. The spectrum was integrated vertically (i.e., 1004 lines) to create a line spectrum of 1002 pixels.
Fig. 3. Schematic of the Raman measurement setup. Left top shows the light propagating in a Si3N4 waveguide (scale bar is 300 µm); left bottom shows a chip mounted on the Raman microscope. The pump light is coming in the horizontal direction and coupling is monitored in the vertical direction.
3. Results and discussion
According to the simulation, the Si3N4 waveguide and the TiO2 waveguide can guide higher-order modes (Si3N4: 2 TE and 2 TM, TiO2: 4TE and 3TM). However, the coupling efficiency of the pump light into the higher modes is more than three orders of magnitude smaller than the efficiency of the fundamental TE or TM modes. Thus, the effect of higher modes in the Si3N4 and TiO2 waveguides is considered negligible.
All waveguide core materials have their own intrinsic background. The origin of the background can be Raman or fluorescence. The intrinsic background is a limiting factor for the detection of Raman signal from a low-concentration analyte. A high background signal reduces the SNR achievable in the Raman spectrum. To compare the inherent background of the selected core materials in this study, air cladded waveguides were used first, although it is important to note that the optical mode will have a larger overlap with the SiO2 under cladding due to the lower refractive index of the air with respect to toluene. Al2O3, Si3N4, and TiO2 channel waveguides were fabricated following the process flow described in Section 2; the dimension of each waveguide is specified in Table 2.
We performed experiments with p-polarized (parallel to the chip surface) light and s-polarized light (vertical to the chip surface) to excite the waveguide TE and TM modes, respectively. The data collected with the EMCCD camera consisted of arrays of 1002 × 1004 pixels. Two of these pixels were defected, namely pixel (648,232) and (946,630), which introduced sharp peaks at 1130 cm-1 and 1624 cm-1. The values of those pixels were removed from the data by replacing them with the mean value of the adjacent pixels. Apart from removing those artifacts caused by the CCD itself, the data is presented as collected without any further signal processing, such as smoothing or background subtraction. To compare the waveguide Raman spectra from the three different types of waveguides, the Raman spectra were normalized and scaled. Firstly, the spectra were normalized by the calculated optical power coupling efficiency of the waveguides. Given the small waveguide dimensions and subsequent small mode size, any trial to experimentally determine the coupling efficiency of the waveguides used in the experiments led to imprecise/unreliable results. Thus, the calculated coupling efficiency (i.e., mode overlap calculation using the Lumerical MODE solutions software) was used for the normalization of the results. The normalization was followed by amplitude scaling based on the saturation length of the waveguides [39]. Although the length of all waveguides is equal (∼1.4 cm), the effective waveguide length for WGR can be different due to the different propagation loss of the waveguides. For the backscattering WGR measurement, a ratio of the Stokes signal power and the pump power can be written as
The propagation losses of all waveguides were measured with a toluene cladding for TE and TM polarization since there is a polarization dependence on the propagation loss, and it is shown in Table 3. The propagation loss was measured from the top of the waveguide [40]. The 785 nm light was coupled into the waveguide, and the image of the scattered light along the waveguide was taken from the top using a camera (BFLY-U3-23S6M-C; Point Grey Research Inc., Richmond, BC, Canada). The intensity values along the waveguide were collected from the image. An exponential function fitting to the intensity values was performed to extract the propagation loss α. Figure 4(A) shows the image of a Si3N4 waveguide guiding the 785 nm pump beam. Yellow dots in Fig. 4(A) represent data collection points. Figure 4(B) shows the collected intensity along the waveguide (blue dots) and an exponential function fitting (red line). Table 3 shows huge uncertainties. It is because the method that we use is based on scattering. There are many scattering points that present a much more intense signal than the average scattering along the waveguide. It limits the accuracy of the measurement. These scattering points can also contribute to creating additional background signals.
Fig. 4. (A) shows an image of a Si3N4 waveguide guiding 785 nm light. (B) represents the intensity data collected from the image of the waveguide. The intensity data were collected along the waveguide; yellow dots show location where the intensity data is collected. The polarization of the input beam is not controlled in this measurement.
Table 3. Propagation losses of the different waveguides measured with a toluene cladding (n=1.48).
The mode profiles for the TE and TM modes propagating in the different materials considered in this study will also have an influence on the measured signal intensity. Raza et al. demonstrated that the effective modal area, together with the refractive index of the core material, has a direct influence on the resulting integrated Raman intensity at the input of a photonic waveguide [39].
Figure 5 shows the background spectrum collected from the Al2O3, Si3N4, and TiO2 waveguides. An incident pump power of 36 mW was utilized. The Raman background was probed for both TE [Fig. 5(A)] and TM polarization [Fig. 5(B)]. The Y-axis represents the counts on the EMCCD camera after 100 sec integration time. In the case of the Al2O3 waveguide, the mode profiles of both TE and TM modes exhibit similar overlap with the core region (∼0.656), as can be seen in Fig. 1. A similar Raman background is, therefore, expected. Furthermore, the higher coupling efficiency of the thicker Al2O3 waveguides can also explain the higher intensity of the measured background. When the TE mode of the Al2O3 waveguide was excited, we observed ripples in the signal. The spectral periodicity of ∼6 nm indicates some spurious reflection over a distance of 30 µm. Since this is not related to the thickness of the waveguide (tAl2O3 = 346 nm) or another relevant physical effect, we can ignore these ripples. Although the Si3N4 waveguide is known for a broad fluorescence background in the near-infrared region [36,37], the TM mode gave little fluorescence background and the baseline above 700 cm-1 is quite flat. While the TE mode excites a band between 360–870 cm-1, the TM mode has less background below 500 cm-1. A great reduction of the background signal is observed for the TM polarization in the TiO2 waveguide, especially below 1000 cm-1. The reduction originates from a lower overlap of the TM mode with the waveguide core. All the waveguides show a sharp peak around 520 cm-1. This location corresponds to a Raman peak of the Si-Si bond of the substrate.
Fig. 5. Inherent background signals for the three waveguide platforms in two different fundamental mode; (A) TE and (B) TM. Dotted vertical lines represent the position of Raman peaks of toluene.
After the background measurements, an analyte was probed by the waveguides for TE and TM polarization. For this experiment, 4 µl of toluene was added onto the sensing window. To avoid the collection of Raman signal from the toluene droplet directly, we placed the droplet ∼2–3 mm from the end facet of the waveguide and on top of the spiral. In addition, the 90 deg bend of the input waveguide into the spiral further minimizes the direct excitation of Raman from the toluene by stray light. To further ensure that the collected Raman signal originates solely from the waveguide, we restricted the uncoupled stray light by a physical block placed in very close proximity above the input waveguide. The camera was set to continuously record the Raman spectra using a low integration time (1 sec) to monitor spectral changes over time as the toluene was added to the sensing window. Several Raman peaks appeared immediately after adding toluene onto the chip. Again, an incident pump power of 36 mW was used in all experiments. After the toluene measurement, the sharp artifacts at (648, 232) and (964, 630) pixels were removed. The data was then scaled by the calculated input coupling efficiency and saturation length, as explained above.
The collected Raman spectra of toluene are shown in Fig. 6. Clear peaks near 520 cm-1, 786 cm-1, 1003 cm-1, 1030 cm-1 and 1211 cm-1, which are distinct Raman signatures of toluene, are observed for the three waveguides in both polarizations. Both TE and TM mode show the presence of the Raman in the spectra from all three platforms. The peak at 520 cm-1 is hard to distinguish as it overlaps with the silicon peak. As expected, the Raman signatures measured under TM excitation exhibit higher SNR than their TE counterparts. This is due to the higher overlap of the excitation field with the toluene as well as a lesser background. Table 4 shows the SNR of the 1003 cm-1 peaks of the three material platforms for both TE and TM polarization. The SNR ratio was calculated based on the photon count of the peak and the averaged photon count of the bottom of the selected peak [30]. The SNR can be written as
where Csig is the photon count of the Raman peak (1003 cm-1) and Cbg is the averaged photon count of the baseline of the selected Raman peak. The calculation also shows higher SNR Raman signal was collected with TM polarization than TE polarization. Contrary to what it was expected, the SNR obtained from the TiO2 waveguide in TM configuration was about 10-fold lower than for the other two materials, despite the higher refractive index contrast (i.e., the higher coupling efficiency of Raman signal back into the waveguide) and similar (or even lower) background that the other two material platforms. The lower coupling efficiency of the pump into this waveguide can explain the lower performance of the TiO2 waveguides. Improving the coupling efficiency of the pump by developing mode size converters and reducing the propagation losses of the TiO2 in the near-infrared (i.e., 785-1000 nm range) would improve this platform. The deposition of a fully amorphous TiO2 without any anatase nanocrystals would help in further reducing the Raman background from the TiO2 waveguides.Fig. 6. Raman spectra of toluene obtained on the three waveguide platforms. Panel (A) shows Raman spectra for TE and panel (B) shows spectra for TM. Transparent purple curve represents Raman spectrum of bulk toluene collected by same experimental setup.
Table 4. SNR of Raman spectra of three waveguide materials. The SNR was calculated by the photon count at 1003 cm-1 and the averaged photon count at the bottom of the Raman peak.
4. Conclusion
In summary, we designed and fabricated waveguide chips using three different materials, Al2O3, Si3N4, and TiO2, to compare their performance for WGR. The waveguide chips containing sensing devices consisting of 14 mm long spirals were fabricated using standard microfabrication methods. Air and toluene cladded spiral waveguides were measured to collect the inherent background and Raman signal from the analyte. The Raman background from the core of the waveguides was reduced by using TM polarization. This effect is ascribed to the smaller overlap of the TM mode with the waveguide core material. Although the higher refractive index contrast of the TiO2 waveguides increases the coupling efficiency of the Raman signal back into the waveguide core, the lower coupling efficiency of pump light into the waveguide led to a smaller SNR for the TiO2 platform. For the Al2O3 and Si3N4 waveguides, TM polarization led to a lower background and obtained Raman spectra of toluene with a higher SNR. In this comparison, we conclude that the combination of a Si3N4 waveguide and a vertically polarized pump beam is best for the WGR in our observation. This combination showed the highest SNR Raman spectra of the analyte as well as less background generation and a reasonably flat baseline.
Appendix A
Table 5. A table of the fabrication processes and the recipe for the Al2O3, Si3N4 and TiO2 waveguides. All fabrication was performed at the NanoLab in University of Twente, the Netherlands.
Funding
Nederlandse Organisatie voor Wetenschappelijk Onderzoek (STW-13328, STW-14197).
Acknowledgments
The authors would like to thank the user committee of the WaterPrint project for their useful suggestions during the project meetings. The authors also thank Ewoud Vissers for his contribution to the development of the fabrication process for TiO2 layer during his master project and Sergio A. Vázquez-Córdova for the ellipsometric characterization of Si3N4, Al2O3 and thermal SiO2 layers to extract the Cauchy coefficients of these materials.
The other authors of this paper dedicate this work to our colleague and co-author Yean-ShengYong who, sadly, deceased shortly after submission of this paper.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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