Silica microtoroid resonator sensor with ... - OSA Publishing

Silica microtoroid resonator sensor with monolithically integrated waveguides Xiaomin Zhang,1 and Andrea M Armani1,2,* 1

Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089, USA 2 Ming Hsieh Department of Electrical Engineering-Electrophysics, University of Southern California, Los Angeles, California 90089, USA * [email protected]

Abstract: Due to their wide operating range, silica toroidal whispering gallery mode microresonators have enabled numerous applications from fundamental physics to lasing and sensing. However, the integration of a waveguide with these microresonators has not been achieved which limits their integration with additional on-chip components. Here, we demonstrate a novel approach for monolithically integrating a silica microtoroid with an on-chip waveguide to form a fully integrated microtoroid-waveguide system with quality factors in excess of 4 million. Similar to the conventional toroidal cavities, power-independent operation is demonstrated. UV and temperature sensing experiments are also performed using the monolithically integrated microtoroid-waveguide system. ©2013 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (230.5750) Resonators; (140.3948) Microcavity.

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#195240 - $15.00 USD Received 5 Aug 2013; revised 15 Sep 2013; accepted 17 Sep 2013; published 26 Sep 2013 (C) 2013 OSA 7 October 2013 | Vol. 21, No. 20 | DOI:10.1364/OE.21.023592 | OPTICS EXPRESS 23592

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1. Introduction High quality factor (Q) whispering gallery mode optical microcavities can confine and store light for tens to several hundreds of nanoseconds, allowing the build-up of high intensity optical fields. As a result of this performance, they have impacted fundamental physics and biological studies [1-5] and are frequently a critical component in optical communications systems, performing the role of an optical buffer or a laser.[6-9] These numerous applications necessitate the development of a fully integrated high-Q platform, with both the microcavity and the waveguide on the same substrate, with a wide operational range. The quality factor of the device is controlled by numerous intrinsic loss mechanisms, such as the scattering of photons from surface defects and material loss, as well as extrinsic loss mechanisms, such as coupling loss.[10] Therefore, to maximize the performance of the device, it is necessary to minimize potential loss mechanism. Silica and Fluoride-based ultra-


high-Q cavities have successfully tackled this challenge by optimizing both the device fabrication process and the coupling mechanism.[11-13] For example, by using a laser reflow approach or a diamond polishing method, surface scattering is significantly reduced. When this fabrication method is combined with a low-loss (high efficiency) evanescent coupling approach, such as a tapered optical fiber or a prism coupler, it is possible to achieve ultra-high Q factors.[12, 14] However, because these coupling methods are not integrated on a silicon substrate, the development of multiplexed systems is limited. Currently, all high Q whispering gallery mode microcavities which are monolithically integrated with a waveguide on a silicon substrate are limited by surface defects which arise from the fabrication process,the inherent material loss of the device or the bending loss.[6, 15-20] Additionally, because they are fabricated from silicon or silicon-based materials like silicon nitride, their high-Q performance is not constant from the visible through the near-IR. One device, the silica microtoroidal cavity, solved both of these problems. By employing a laser-based reflow process, the lithographically induced defects are removed, and the material loss of silica is inherently low from the visible through the near-IR.[11] However, because the reflow process reduces the diameter of the device by several microns, the integration with an on-chip fabricated waveguide has not been demonstrated. Additionally, to minimize the coupling loss, the optical field profile (propagation constants) should match that of the toroidal cavity, which places geometrical restrictions on the type of waveguides which can be used. For this reason, researchers have relied on tapered fiber waveguides, which are not fabricated directly on the silicon substrate. Recently, suspended silica waveguides integrated on a silicon wafer were developed. Although several different geometries (eg trapezoids, cylinders) were developed nearly simultaneously, in order to have efficient energy transfer from the waveguide to the toroid, a cylindrical shape is ideal as it improves the coupling efficiency between the waveguide and the resonant cavity.[14, 21-24] The cylindrical devices demonstrated relatively low optical loss and relied on conventional lithographic fabrication methods, laying the groundwork for a fully integrated device. Here, we employ a combination of top down and bottom-up fabrication methods, allowing a fully integrated microtoroid-waveguide system with Q> 3 million at both 1300nm and 1550nm to be fabricated. The basic optical properties and power-dependent behavior are characterized. We also perform temperature sensing and UV sensing experiments. 2. Device fabrication The fundamental challenge which researchers faced when trying to monolithically integrate a toroidal resonant cavity with an on-chip waveguide is that the reflow process reduces the diameter of the device by several microns as the silica melts towards the pillar.[11] As such, if a waveguide is patterned adjacent to a disk and the disk is reflowed, the coupling gap will increase by several microns. Because a coupling gap should be sub-wavelength (sub-micron), this significant change created a fundamental barrier to integrating a toroid with a waveguide using the conventional toroid fabrication method. However, if a thicker oxide is used, then a more complex reflow mechanism is possible. Instead of a simple horizontal reflow collapse, in which the silica reflows radially inward, the silica can be patterned such that it collapses in both the vertical and horizontal directions simultaneously. Depending on the ratio of the horizontal and vertical collapse, which is controlled by the thickness of the oxide, the diameter of the device can stay the same, decrease or increase (Fig. 1). More importantly, because the collapse can be designed such that the final gap is smaller than the initial gap, it also enables lower resolution lithographic methods to be used. Therefore, this approach opens the possibility for integrating a toroid with an on-chip waveguide.


Fig. 1. Renderings and schematics of waveguide and toroid structure (a)/(c) before and (b)/(d) after the laser reflow process. The dotted lines in the renderings indicate the cross sections drawn in the schematics. Purple is silica and gray is silicon. In the present work, h=12μm, a=8μm, gap=3μm D=70μm, and d=10μm. After reflow, the initial 3micron gap was decreased to sub-micron.

An overview of the fabrication process is shown in Figs. 2(a)-(d). The fabrication is composed of four major steps, starting with a thick (12μm) thermal oxide (wet) on a 300μm silicon wafer (WRS Materials), and it involves a pair of photolithographic patterning and advanced silicon dioxide etching (AOE) processes (Step 1 and Step 2), XeF2 etching of silicon (Step 3) and a final CO2 laser reflow (Step 4). As indicated in Fig. 2, the waveguide is slightly bent. This distortion pushes the optical field towards the toroid, relaxing the coupling tolerances.

Fig. 2. Overview of the fabrication process flow for the integrated microtoroid-waveguide system. (a) Step 1 and (b) Step 2 define the waveguide and ring structures in the 12μm thermal oxide layer. (c) Step 3 undercuts the oxide, elevating it off of the silicon substrate, and (d) Step 4 is the laser reflow process. Between each step, there are a series of sample cleaning procedures, including oxygen plasma, Chrome removal and piranha cleaning.

The first step is to pattern and etch approximately 3μm of thermal oxide (Fig. 3(a)). This 3μm layer will form the support membrane of the waveguide and the disk in the final structure. Chrome (Cr) serves as an etching mask in the dry etching process. This 200nm Cr layer is deposited using e-beam evaporation. Then photolithography with AZ5214 photoresist and chrome wet etching are performed to define the pattern shown in Figs. 3(a) and (b). After the chrome wet etching, the photoresist is removed, leaving only the chrome mask for the subsequent oxide dry etching. Advanced oxide etching system (AOE) is then used to etch the silicon dioxide.[25] The etching rate is approximately 300nm/min using CHF3 as the process


gas. The chrome mask is removed using chrome etchant after the AOE etching is complete. Piranha cleaning is used to remove any residue produced by AOE etching. In preparation for the second etch step, approximately 500 nm of chrome is deposited using e-beam evaporation to serve as the etching mask. Fig. 3(c) and Fig. 3(d) show the SEM images of the deposited Cr on waveguide. The thickness of Cr on the horizontal surface is 500nm; however, the thickness of Cr on the sidewall is only 50nm. These images also show the uniformity of the AOE process in the first step.

Fig. 3. Step 1 in the fabrication process. (a) Pov-Ray rending of Step 1. (b) Optical microscope image showing the result of patterning and etching down 3 μm. (c)/(d)SEM images showing the result of the second Cr deposition. The thickness of Cr on the flat surface is 500nm; however, the thickness of Cr on the sidewall of is only around 50nm.

The second step is to pattern and etch the remaining 9μm of thermal oxide to form the ring and waveguide structures (Figs. 4(a) and (b)). At the end of this step, the width of the ring and waveguides is 8μm and the gap between the ring and the adjacent waveguide is 3μm. A photolithography step with AZ4620 photoresist and chrome wet etching is used to define the pattern. In this step, the thicker AZ4620 photoresist is used in place of the thin photoresist AZ5214 to protect the vertical silica interface from the first step AOE etching as the 50nm Cr layer on the vertical sidewall is not sufficient for 9μm of thermal oxide etching. This protection cap is shown in Figs. 4 (c) and (d). After the Cr etch step, there is a slight undercut of Cr which is normal for a wet etching process. When the chrome wet etching is finished, the AOE process is used to etch the remaining 9μm of thermal oxide, exposing the silicon substrate.

Fig. 4. Step 2 in the fabrication process. (a) Pov-Ray rending of Step 2. (b) Optical microscope image showing the top view of the waveguide and ring structures after the second AOE etching. The width of the ring and the bent waveguide are 8μm. The gap between the silica waveguide and the ring is 3μm at this point. (c)/(d) SEM images showing the second photolithography with AZ4620 photoresist protective coating and the Cr wet etching. There is some undercut of the Cr which is normal for wet etching.

When this pair of steps is finished, a series of cleaning procedures are performed. O2 plasma stripping is used to remove any photoresist residue, followed by chrome etching to remove the residual chrome mask. Piranha cleaning is then used to remove any remaining residue on the surface of the sample. The sample is also dipped in buffered oxide etchant (BOE) to change the surface of the sample to hydrophobic. The wafer is then cut into single devices with both ends of waveguide cleaved to form the input and output injection ports. Subsequently, xenon difluoride (XeF2) is used to etch the silicon, undercutting the waveguide and ring structures until the outer boundary of the silicon pillar is right at the inner boundary of the ring (Step 3, Fig. 5(a)). This etch depth optically isolates the low refractive index device from the high refractive index substrate (Figs. 5(b)-(d)). XeF2 is ideal for this step as it is extremely selective for silicon over silica and has an extremely uniform etch rate. During this process, it is critical to etch to the inside of the ring, as indicated in Fig. 1(c).


Fig. 5. Step 3 in the fabrication process. (a) Pov-Ray rending of Step 3. (b)/(c)/(d) SEM images showing the XeF2 etching results of the side view of the coupling region of the ring and waveguide, the entire device and the top view of the device.

The last step is the carbon dioxide (CO2) laser reflow of the structure (Figs. 6(a)-(c)). A CO2 laser is selected as it reflows the silica device without affecting the underlying silicon substrate. The utilization of the CO2 reflow process allows the device to achieve an ultrasmooth surface, reducing the surface defects which arise from the lithographic and etching steps. However, unlike in previous CO2 laser reflow processes[11], because the h/a ratio is greater than one, after the reflow process, the diameter of both the toroid and reflowed waveguides increase due to the collapse of the ring and waveguide structures towards each other. Therefore, the critical sub-micron gap which enables high efficiency evanescent coupling between the waveguide and toroid is achieved. Fig. 6(b) shows an SEM image of the endface of the reflowed waveguides, and Fig. 6(c) shows a SEM image of the fully integrated microtoroid-waveguide system. The microtoroid and the waveguide are coplanar. The major and minor diameters of the microtoroid are approximately 70μm and 10μm, respectively. The diameter of the waveguide is approximately 10μm as well, and the radius of curvature in the coupling region is 20μm. Given the dimensions of the waveguide, the waveguide is multimode.[21] Several devices are made with different waveguide radius of curvatures, ranging from 20μm to 100μm.

Fig. 6. Step 4 in the fabrication process. (a) Pov-Ray rending of Step 4 showing the system in operation with the light from the waveguide coupled into the microtoroid. (b) The endface of the waveguide after the reflow process. (c) The microtoroid and the waveguide are coplanar. The major and minor diameters of the microtoroid are around 70μm and 10μm, respectively. The waveguide also has a diameter of approximately 10μm. The radius of curvature of the waveguide in the coupling region is 20μm. Both the microtoroid and the waveguide have ultrasmooth surfaces resulting from the CO2 laser reflow process. The gap between the waveguide and the microtoroid is reduced to ~500nm because the waveguide and toroid collapse during the reflow process and reflow towards each other.

There is an optimum value of CO2 laser power at which the gap is smallest; at lower power, the device will not thoroughly reflow, and at higher power, the device will over reflow. In either case, the gap will be larger than the optimum value. In the reflow set-up used in the present work, this power is approximately 30W distributed over an approximately 300μm diameter spot size. As this laser output power is dependent on the specific components used the reflow set-up which induce loss, the details of the reflow set-up used here, including components and a video of the set-up, are in reference. [26] 3. Device characterization Several different device characterization measurements are performed. The quality factor and free spectral range of the devices are characterized at both 1300nm and 1550nm. Additionally, a quality factor stability test is performed, measuring the Q daily for over 7 days. The behavior at high input powers is also measured. 3.1 Test and measurement set-ups


The same basic testing set-up is used for both sets of characterization experiments with minor changes (Fig. 7). A pair of single mode, narrow linewidth tunable lasers (Velocity series, Newport) centered at 1330nm and at 1550nm are coupled into and out of the waveguide injection ports using a pair of single mode lensed fibers which are aligned using motorized nano-positioning stages. As the laser is scanned over a series of wavelengths, the output signal is recorded on a high speed digitizer/oscilloscope (National Instruments). This signal is recorded using a LabView program. When measuring Q spectra, the laser scan rate and scan range are optimized to reduce any thermal effects. The experimentally measured or loaded quality factor is determined by fitting a single resonance to a Lorentzian and using the expression Q=λ/δλ, where λ is the wavelength and δλ is the full width at half maximum[11].

Fig. 7. The testing set-up. The tunable laser is coupled into and out of the waveguide using a pair of lensed optical fibers. The testing set-up is controlled using a computer with a series of integrated PCI cards (National Instruments) which are controlled with LabView. The laser scan rate and range are controlled using either a function generator (PCI Func Gen Card) or the general laser communication port (PCI GPIB Card). The transmission signal is recorded on the high speed digitizer/oscilloscope card (PCI Digitizer Card). For the high power measurements, an erbium doped fiber amplifier (EDFA) is integrated inline.

To determine the effect of circulating power on the resonant wavelength and lineshape, the 1550nm tunable laser is connected to an EDFA to amplify the power. The output from the EDFA is connected to the input lensed fiber. The power from the EDFA can be increased manually. To determine the circulating power in the microtoroid, the insertion loss and the percent of coupling are taken into account. 3.2 Optical device properties Representative resonance spectra with the Lorentzian fit at 1300nm and 1550nm are shown in Fig. 8. The full width half maximum from the fit at 1330nm is 3.1×10-4nm, yielding a loaded Q value of 4.3×106. The full width half maximum from the fit at 1550nm is 4.9×104 nm, yielding a loaded Q value of 3.2×106. Based on the wavelength trends (coupling decreasing with decreasing wavelength) and the gap size, the integrated system is operating in the under-coupled regime in both measurements. Critically-coupled and over-coupled regimes could be realized by starting with a smaller gap in the initial mask or by varying the height and width ratio. This measurement is repeated on several different devices, varying the waveguide radius of curvature from 20μm to 100μm. All devices yielded Q values in excess of 3 million (3×106). Additionally, after storing the devices in ambient environments for over a week (room atmosphere and temperature), the Q values and spectra are measured again and no statistically significant change occurred (Q values fluctuated by +/-3%). Because of the volume of devices which are fabricated, numerous wafers are used, and no wafer dependence on the Q is observed. Therefore, these values are highly reproducible. The free spectral range (FSR) of an optical cavity is the separation between two sequential resonant wavelengths, and it can be approximated using the following expression: ΔλFSR=λ2/(Lcavneff)=λ2/(πDneff), where ΔλFSR is the FSR, λ is the wavelength, Lcav is the length of the cavity, neff is the effective refractive index, and D is the cavity diameter. The neff is determined using COMSOL Multiphysics finite element method simulations. Based upon the materials and geometry used in the present system, an FSR of 5.49nm is expected at 1300nm.


The experimentally measured FSR at 1300nm is 5.53nm, which is in excellent agreement with the theoretical prediction.

Fig.8. Representative quality factor measurements of the integrated microtoroid-waveguide system with Lorentzian fits. (a) At 1300nm, the full width at half maximum (δλ) from the fit is 3.1×10-4, yielding a loaded Q value of 4.3×106. (b) At 1550nm, the full width half maximum (δλ) from the fit at 1550nm is 4.9×10-4, yielding a loaded Q value of 3.2×106.

3.3 Power dependence One of the advantages of using silica over other material systems is its relative immunity to non-linear behaviors, even at high input powers. Specifically, even with high circulating powers, the performance of the device, as measured by the quality factor and the resonant wavelength, is extremely stable. The primary mechanism which changes the device behavior at high input powers is based on the thermo-optic effect, in which optical power is converted to heat which induces a refractive index change. [27] However, because the thermo-optic coefficient of silica is very low, the overall effect on the silica devices is also small. As a result, high optical powers can be coupled into the cavity with minimal impact on the resonant linewidth (Q) and wavelength.

Fig.9. As the input power is increased, the resonant wavelength slightly shifts. However, because of the low thermo-optic coefficient of silica, several hundred microwatts of input power, which corresponds to several watts of circulating power, are needed to induce a multilinewidth shift.

Fig. 9 shows the power dependent wavelength response of the device in terms of both input power (bottom x-axis) and circulating power (Pcirc) (top x-axis). The resonant wavelength increases linearly in direct proportion to the input power, indicating that thermal effects are solely responsible for the change. It is important to note that even with a change in circulating power of 10W, which corresponds to a Pcirc increase from 9W to 19W, the change


in the resonant wavelength is small, approximately 5pm, which corresponds to 10 cavity linewidths. This level of stability is highly desirable in applications such as add drop filters and buffers. Devices made from other material systems such as silicon will exhibit much larger non-linear shifts at these input powers. For example, because of the large thermal optic coefficient of silicon in relation to silica, the change in resonant wavelength shift will be nearly 10 times larger. 4. Temperature and UV sensing demonstrations To demonstrate the utility of this device in sensing applications, two different sensing experiments are performed. First, the device is used to detect small changes in temperature in the environment, and then the device is used to detect changes in UV intensity. While the need for high resolution temperature detection has numerous applications ranging from laser stabilization to industrial process control, a need for UV detection is not immediately apparent. However, under and over UV exposure has been linked to both psychological and physical diseases.[28] Therefore, an accurate method to monitor UV exposure is of interest. 4.1 Detection mechanism and testing set-up Because the specific location of the resonant wavelength is determined by both the geometry of the cavity and the device’s material properties, changes to either value will induce a resonant wavelength shift. This effect is summarized in the below expression:

Δλ / λ = Δn / n + ΔR / R.

(1) where λ is the resonant wavelength, Δλ is the change in resonant wavelength, n is the refractive index, Δn is the change in refractive index, R is the radius of the device, and ΔR is the change in radius. In the present work, the primary detection signal is due to a refractive index change caused by the thermo-optic effect. Therefore, the above expression can be simplified to [3, 27]:

Δλ = λ0 (ε +

dn / dT ) ΔT . n

(2)

where ΔT is the change in temperature and ε is the thermal expansion coefficient. For fused silica, dn/dT is 1.2E-5 C-1 and ε is 0.55E-6 C-1 at room temperature [29]. However, because the optical field is not entirely confined within the device, the above expression provides an upper bound for the measurement. It is worth noting that while other materials such as silicon or polymers have larger dn/dT’s, and therefore can provide larger temperature detection signals, as a result of the relative immunity to environmental noise (eg electrical fields), the background noise level of a silica-based sensor is very low. Therefore, the overall signal to noise (SNR) is improved.

Fig. 10. The testing set-ups for the detection experiments. (a) The cylindrical heater is integrated directly under the integrated resonator, and the thermocouple is adjacent to the sample. (b) The UV lamp is position directly above (13mm gap) the resonant cavity.


To perform the detection experiments, the same testing set-up to the one shown in Fig. 7 is used with two modifications. For the temperature experiments, the sample stage is replaced with one that has an integrated heater and thermocouple (Fig. 10(a)). Specifically, the heater (Omega CSH-102100/120 V) is embedded into the sample holder directly under the device. Silver conductive epoxy (MG Chemicals) is used between the heater and the sample holder to ensure effective heat transfer, to minimize heat lost to the environment and to hold the heater in place. A fast response thermocouple sensor (Omega SA1XL) is attached immediately adjacent to the optical device to accurately read the temperature in real time. The response time for the thermocouple sensor is less than 0.15 s. Both the heater and the sensor are connected to a benchtop controller (Omega CSC32 series) to control and read the temperature. To perform the UV detection experiments, a 385 nm LED UV lamp with an integrated 5mm lightguide is used (Bluewave LED DX-1000) (Fig. 10(b)). The lightguide can deliver up to 15W/cm2 in a high intensity spot. The intensity is adjustable from 1% to 100% of the total power using a digital controller. During the present series of experiments, a sub-set of this range is used (54mW/cm2 to 100mW/cm2). The lightguide is mounted vertically 13mm above the device, and the power delivered to the device is measured using a UV power meter. In both sets of experiments, a 1550nm tunable laser connected to a lensed optical fiber is used to couple light into the cavity. The cavity Q of the device is measured as described previously. The change in the resonant wavelength is tracked and recorded using a customized LabView peak tracking program. Both the transmission and the resonant wavelength are simultaneously recorded. By monitoring both values, fluctuations in coupling can also be detected. To determine the background noise level in the system and calculate the signal to noise level, a baseline signal is taken for three minutes, and the signal fluctuation is fit to a Gaussian (normal). The noise level is set as the value which is 3σ from the center of the Gaussian. This value contains all noises in the system, and it is compared to the cavity linewidth of the device. The cavity linewidth is typically considered the detection limit, unless complex algorithms or detection schemes, such as heterodyning are used [30, 31]. Additionally, by setting the signal to noise level at 2, a theoretical detection limit is calculated. 4.1 Temperature Representative results from the temperature sensing experiments are shown in Fig. 11. As indicated in Fig. 11(a), the temperature increase increment is changed from 0.5 oC to 1.7 oC to check the linearity of the sensor. The slight dip at the end of equilibrium of each increment is due to cooling while the next temperature increment is manually set on the digital controller. However, after this initial dip, the resonant wavelength shift is easily identifiable and stable. It is important to note that the exposure to the temperature increase does not change the linewidth shape (or Q), as the circulating power within the cavity is not changing. A histogram of the noise distribution in the measurement is shown in the inset of Fig. 11(a). This measurement is taken with a ΔT of 0.5 oC. As such, it includes laser fluctuations, detector noise, and resonator noise as well as noise from the temperature stage. However, even with these numerous potential sources of noise, the total noise (3σ) in the measurement is 0.4952pm. The loaded Q of the device is 3.6x106 at 1550nm, which corresponds to a linewidth of 0.431pm. Therefore, this noise level nearly approaches the theoretical limit. As shown in Fig. 11(b), the resonant wavelength shift increases linearly as the temperature is changed, and the slope is 14.6pm/°C. Using this value, we can calculate a theoretical sensitivity limit which incorporates both the noise and the sensing performance of the device. Specifically, by setting the signal to noise value at 2, the minimum ΔT which is detectable is 0.0678 oC. This value is particularly impressive as it assumes no noise correcting algorithms or subsequent data processing has been used.[32, 33] Assuming that the dominant contribution to this shift is the change in refractive index, the slope is directly proportional to the dn/dT of the material and corresponds to a dn/dT value of


1.27 ×10-5 °C, which is in excellent agreement with the established dn/dT value of silica (1.2×10-5 °C). [29]

Fig. 11. Temperature sensing experimental results. (a) Sensor response when the temperature is increased. Inset: The histogram from the noise measurement with a Gaussian fit. (b) The results from part (a) are re-plotted to highlight the relationship between the resonance shift and the temperature change. The solid line is the linear fit.

4.2 UV sensing While the detection of a temperature change is clearly based on the thermo-optic effect, it is not immediately apparent that a similar mechanism is responsible for the detection of UV light. However, because of the high material absorption (α) of silica in the UV range and the heat capacity of silica (c), the same mechanism applies. Specifically, the temperature change (ΔT) upon exposure to UV light can be analytically described by: ΔT=Eαl/mc, where c is 703 J/kgC, α is 3.262m-1 at 385nm, l is the absorption length, m is the mass of the device, and E is the energy from the UV source (E=UV power · exposure time · device surface area).[34] Both l and m are determined by the specific toroid dimensions while E is an experimentally controlled parameter. For example, based on the previous expression, a 15 minute exposure at 100mW/cm2 would result in a resonant wavelength shift of 44pm at 1550nm. Fig. 12(a) shows the sensor response to several different UV intensities, increasing from 54mW/cm2 to 100mW/cm2 and then decreasing back to 54mW/cm2. For each cycle, we expose the integrated toroid system and the UV on for 15minutes, then turn the UV off and wait for the resonant wavelength to recover. The device response has a characteristic slope (Δλ/UV power) of 0.389pm/mWcm-2. The sensor system exhibited very little hysteresis, and the wavelength completely returned to its original position after all 5 cycles. This result indicates that the UV sensing response produced by the microtoroid-waveguide system is reproducible, and its sensing performance is not degraded due to UV-induced damage to the silica. Fig. 12(b) is an enlarged version of the highest intensity signal (100mW/cm2). Once the UV light is turned on, the resonant wavelength undergoes a large shift and then stabilizes. When the UV is turned off, the resonant wavelength recovers quickly. Additionally, the total shift of 39pm is in good agreement with the predicted value of 44pm. The slight difference between the theoretical prediction and the experimental result is most likely because not all of the UV energy is converted to heat. Specifically, some of the light could be scattered off of the silica surface.[35] In this measurement, the total noise (3σ) with no UV exposure (black line) is 0.5587pm (Fig. 12(b), inset). This value is comparable to the device linewidth. However, as the device is exposed to the higher intensity UV light, the linewidth slightly broadens. Therefore, the noise also slightly increases. However, because the signal also increases, the overall device performance is not affected. By combining the device response value and the noise level, we can calculate a theoretical sensitivity limit. Setting the signal to noise threshold at 2 and assuming the experimental


conditions in the present work (15 minute exposure time), the minimum UV value which is detectable is 2.87mW/cm2. However, by increasing either the device size or the exposure time, lower intensities of UV could be detected.

Fig. 12. UV sensing experimental results. (a) Sensor response with several different exposure UV powers, increasing from 54mW/cm2 to 100mW/cm2 and then decreasing to 54mW/cm2 (b) The characteristic UV sensing curve showing both the forward and reverse response at 100mW/cm2. The resonance undergoes a large, rapid wavelength shift once the UV is turned on. When the UV is turned off, the resonant wavelength returns to its original value. Inset: The histogram from the noise measurement with a Gaussian (normal) fit.

5. Conclusions In conclusion, a monolithically integrated silica microtoroid-waveguide system is successfully demonstrated. Because the system is fabricated from silica, the high-Q factors will be maintained from the visible through the near-IR, paving the way for high performance, fully integrated photonic systems which operate across a wide range of wavelengths. The device has an extremely well-defined free spectral range which agrees very closely with theoretical calculations. Finally, as a result of the low thermo-optic coefficient of silica, the resonant wavelength is extremely stable, even at high input powers and circulating intensities, and experiences a predictable, linear increase. Additionally, sensing experiments are performed using the microtoroid-waveguide system. Specifically, the system is able to detect temperature changes, and the wavelength shift changes linearly as the temperature increases. This system has a characteristic UV sensing response, and the signal produced is stable and is not degraded by UV exposure. This combination of high Q and wavelength flexibility will enable this monolithically integrated system to accelerate research in the fields of systems biology and biodetection [36-38], optical computing and telecommunications[6, 39], and fundamental physics [40]. Acknowledgments This work was supported by the Office of Naval Research [N00014-11-1-0910].
