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Titanium-enhanced Raman microcavity laser Nishita Deka,1,† Ashley J. Maker,2,† and Andrea M. Armani1,2,* 1
Ming Hsieh Department of Electrical Engineering-Electrophysics, University of Southern California, Los Angeles, California 90089, USA 2 Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089, USA *Corresponding author:
[email protected] Received November 13, 2013; revised January 15, 2014; accepted January 23, 2014; posted January 24, 2014 (Doc. ID 201361); published March 5, 2014 Whispering gallery mode microcavities are ideally suited to form microlaser devices because the high circulating intensity within the cavity results in ultralow lasing thresholds. However, to achieve low-threshold Raman lasing in silica devices, it is necessary to have quality factors above 100 million. One approach to circumvent this restriction is to intercalate a sensitizer into the silica, which increases the Raman gain. In the present work, we demonstrate a Raman laser based on a titanium sensitized silica solgel coated toroidal microcavity. By tuning the concentration of the Ti, the Raman efficiency improves over 3× while maintaining sub-mW thresholds. © 2014 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (130.3130) Integrated optics materials; (140.3550) Lasers, Raman; (160.6060) Solgel. http://dx.doi.org/10.1364/OL.39.001354
Ultrahigh quality factor (Q) whispering gallery mode optical cavities have exceptionally long photon lifetimes, which result in high build-up intensities or circulating powers within the cavity. As a result, these devices are frequently used as the fundamental element in a microlaser system. For example, microlasers based on whispering gallery mode optical cavities have been demonstrated using numerous approaches, ranging from directly embedding a gain medium within the cavity to leveraging the inherent nonlinearities of the cavity material [1–9]. These devices have been used in a wide variety of applications, such as single virus detection and the creation of frequency combs [10,11]. When designing microlasers, it is especially important to consider the optical field intensity and the gain coefficient [5–7]. The optical field intensity is directly related to the Q of the cavity and the optical mode volume, while the gain coefficient is determined by the specific lasing medium and excitation approach. These parameters can balance each other out. In other words, a higher Q cavity can compensate for a weaker gain coefficient. It is precisely this balance which has enabled the development of ultralow threshold Raman lasers based on silica high-Q whispering gallery mode resonant cavities [1,2]. An alternate approach is to incorporate secondary materials into the microcavity which act as sensitizers, increasing the efficiency of excitation [12–14]. Using sensitizers to improve excitation efficiency and reduce lasing threshold is a common approach in the fabrication of rare-earth microlasers. For example, a Yb-Er codoped microlaser has a significantly lower lasing threshold than an Er microlaser because the presence of the excited Yb increases the efficiency of the Er lasing [12–14]. However, while rare-earth sensitizers have been extensively studied, similar attention has not been given to employing sensitizers in silica solgel Raman lasers, in which the silica is the gain material and not simply the host material. One proven sensitizer is titanium (Ti). By analyzing the Raman spectra of various glasses containing Ti and other metals, researchers have found that adding metals such 0146-9592/14/061354-04$15.00/0
as titanium can increase the polarizability of bonds in glass networks. This increase in polarizability can significantly enhance nonlinear effects, such as Raman lasing [15]. For example, the addition of heavy metal oxides including TiO2 has been shown to improve the Raman gain by over 100× in tellurite glasses [15,16]. Following this approach, adding titanium could improve polarizability and enhance Raman behavior in silica glasses as well. However, despite this enhancement, a Ti-silica microcavity Raman laser has not been demonstrated previously. One of the challenges is the synthesis of a Ti-silica material that is compatible with standard lithographic processes. Specifically, the presence of the Ti results in nano/microcracks and a semi-porous film that etches nonuniformly. Recently, this challenge was solved with the creation of a hybrid microcavity device. This structure consists of a primary toroidal microcavity fabricated from thermal oxide with a conformal Ti-silica coating [Figs. 1(a) and 1(b)]. In this Letter, we demonstrate a Ti-enhanced Raman microcavity laser. Although the presence of the Ti does slightly decrease the cavity Q, it significantly increases the Raman gain coefficient. As a result, the overall lasing performance of the device is improved. Therefore, this approach provides a route to use lower Q cavities to achieve ultralow threshold Raman lasers. The silica toroidal microcavities are fabricated from 2 μm of thermal oxide on a silicon wafer using a
Fig. 1. Solgel-coated toroidal optical cavity. (a) Rendering of toroidal optical microcavity, indicating the location of the Tisilica coating and optical field. (b) Scanning electron micrograph of the solgel-coated toroidal microcavity. © 2014 Optical Society of America
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combination of photolithography, buffered oxide and xenon difluoride etching, and a carbon dioxide laser reflow process [17]. The process is optimized to fabricate devices with 30 2 μm major diameters and 8 μm minor diameters. After the toroids are fabricated, the Ti-doped solgel silica layer is synthesized as previously detailed using methyltriethoxysilane (MTES) (Alfa Aesar, 98%) as the precursor in the acid-catalyzed hydrolysis/condensation reaction [18]. The titanium is added in the form of titanium butoxide at four different molar ratios of titanium to silica (2, 5, 8, and 10 mol. % Ti in the starting solution). To accurately isolate the role of the titanium, a solgel is synthesized using the same procedures without the addition of Ti and is labeled as the 0 mol. % coating. It is important to note that these solgel silica coatings have different optical and material properties compared to thermally grown silica, resulting in different device behavior. Therefore, the presence of the coating is necessary in order for the device to act as an accurate control. Once the solgel materials are produced, they are spincoated onto the toroid devices and bare silicon control wafers (7000 rpm) and then annealed at 1000°C for 1 h in a tube furnace. These procedures result in ∼177 nm thick coatings with refractive indices which steadily increase from 1.458 (for 0 mol. % Ti) to 1.514 (for 10 mol. % Ti) at 765 nm, as measured by spectroscopic ellipsometry on the control wafers [19]. As a control, the Raman spectra of the doped and undoped thin films on the control wafers are measured at 532 nm with a laser intensity of 200 mW. The polymerization degree (I p ) is then determined by dividing the area of the ∼500 cm−1 peak by the ∼1000 cm−1 peak area within a given spectra. This analysis can quantitatively describe the glass nanostructure and composition [20]. Using this approach, we observe that I p increases from 3.55 for the undoped film to 5.26 for the 10 mol. % Ti film, which indicates that the titanium can change the silica bonds slightly and cause a stronger Raman response. One of the key parameters of a laser is the lasing threshold. In Raman lasers, this metric is proportional to P thres ∝ n2eff Vm ∕gQ2 , where Vm is the optical mode volume, neff is the effective refractive index, and g is the Raman gain coefficient [21]. To more quantitatively explore the effect of the refractive index increase on the optical field distribution (Vm ), finite element method (FEM) simulations of the device are performed in COMSOL Multiphysics. Following procedures developed by Oxborrow and Cheema, the optical field distribution, mode volume, and effective refractive index can be theoretically calculated for microtoroid cross sections with the different coatings [22,23]. To closely match the actual fabricated devices, the toroid major (minor) diameter is set to 30 (8) μm, with a 177 nm thick coating. The experimentally measured refractive indices are used for the coatings. The refractive index of the silica toroid is 1.45. A mesh size of 8.88 × 10−4 μm2 is used, and the fundamental mode solution closest to the 765 nm pump wavelength is used for subsequent analysis. Using the COMSOL models, the changes in the effective refractive index (neff ) and the effective mode volume (Vm ) upon adding the Ti are determined [24,25]. Both of
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Fig. 2. FEM simulations and analysis. The percentage of light in the film increases as the refractive index or Ti concentration increases. Upper left inset: FEM model of toroidal cavity with 10 mol. % Ti. Lower right inset: optical mode volume (Vm ) as a function of refractive index. As the refractive index increases, the optical field moves into the film, and the total mode volume decreases, resulting in a greater fraction of the optical field residing in the film.
these parameters can contribute to the Raman threshold (P thres ) and lasing wavelength and are calculated from the results in Fig. 2. As the titanium concentration increases from 0 to 10 mol. %, neff increases from 1.34 to 1.39, and the Vm decreases from 80.35 to 77.46 μm3 . However, it is important to note that the Ti is only present in the coating. Therefore, the more relevant consideration is the ∼10% increase in the optical field intensity in the coating (Fig. 2). If we assume that the Ti does not behave as a sensitizer and that the cavity Q is constant, we can predict a relative change in the lasing threshold using the expression for P thres shown previously [21]. Following this analysis, as the titanium concentration increases to 10 mol. %, the Raman lasing threshold power is expected to increase up to fivefold compared to the titanium-free coatings (assuming that the gain is constant). However, this analysis does not account for titanium’s sensitizing effects, such as a potential change in the gain coefficient. To experimentally verify these results, the coated toroidal devices are characterized as follows. Light from a tunable, narrow linewidth laser centered at 765 nm (Velocity series, Newport) is coupled into the toroidal cavity using a tapered optical fiber waveguide. The waveguide is aligned with the cavity using a high-precision 3axis motorized stage. These high-efficiency (low-loss) waveguides are ideal for the present series of experiments as they allow for tunable coupling of light into the cavity from the undercoupled regime to the critically coupled regime. Specifically, to accurately measure the intrinsic Q of the cavity, it is necessary to be in the undercoupled regime. In contrast, to maximize the circulating intensity in the cavity, the device should be critically coupled. To determine the Q, the linewidth approach is used, where Q λ∕Δλ. The spectrum is recorded on a high-speed digitizer, and the linewidth (Δλ) is determined from a Lorentzian fit. The presence of titanium causes the measured quality factor of the coated toroids to decrease slightly from ∼2.4 × 107 for the titanium-free
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coatings to ∼1.1 × 107 for the 10 mol.% coatings. These results agree with previous studies, which show that the Q decreases in Ti-doped silica devices due to titanium’s increased material absorption losses [19]. To characterize the lasing behavior of these devices, the same testing setup as previously described is used, and the pump laser is critically coupled into the coated resonant cavities using a tapered optical fiber. Based on a spectral analysis, the fundamental mode of the cavity is excited. The Raman emission from the coated toroids is detected on a fiber-coupled spectrograph [Andor Shamrock spectrograph (SR-163) with a Newton CCD detector] [26]. Given the grating used in the present series of experiments, the resolution of the spectrograph is 0.76 nm and the maximum count threshold is 65,000. Multiple coated samples are tested in all measurements (N > 3). Additionally, the testing setup is enclosed in a black-out curtain, and the residual signal is normalized out of the measurement. Several characterization measurements are performed on the different devices. First, the threshold of the first Stokes emission peak is determined by changing the amount of power coupled into the resonant cavity and recording the output emission spectra. Second, the coupled power is increased further to generate cascaded Raman. Representative lasing spectra are plotted in Figs. 3(a) and 3(b). At low input power, a single emission peak centered at 796 nm is easily identifiable. It is important to note that, given the 0.76 nm resolution of the spectrograph, it is quite possible that there are multiple lasing lines within this single peak. By plotting the measured emission intensity against input power for the first emission peak, Raman threshold plots can be produced. The threshold graph for a device coated with 10 mol. % Ti is shown in the Fig. 3(a) inset. Fitting a line to the threshold plot allows the lasing threshold and slope efficiency to be determined from the x intercept and slope, respectively. As the input power is increased, higher order lines are generated. Since the free spectral range of the microlaser is ∼4 nm, additional peaks can appear in clusters, such as the pair of peaks near 800 nm [Fig. 3(b)]. However,
Fig. 3. Output emission spectra and lasing thresholds. In both spectra, the 765 nm pump wavelength is shown. (a) Output emission spectra for device coated with 10 mol. % Ti, with lasing peak centered at 796 nm. The inset shows a normalized threshold graph, indicating a threshold value of 117 μW. (b) Cascaded Raman generated by device coated with 10 mol. % Ti. The pump generates lasing at 796 nm, which pumps a cascaded Raman peak at 824 nm. Since the free spectral range of the microlaser is ∼4 nm, multiple peaks can appear in each cascade, such as the pair near 800 nm.
because of the complex nature of the energy transfer that occurs during the cascade, the thresholds of the higher order lines are interdependent. As such, stating a single threshold for these lines would be misleading. Based on the emission versus input power plots, some interesting trends in the efficiency and threshold can be observed as the titanium concentration increases. As can be seen in Fig. 4, the lasing efficiency increases over threefold as the mol. % of Ti in the coating increases. This improvement in efficiency is likely due to the increased interaction of light with the titanium sensitizer. The lasing threshold values also improve initially due to the presence of titanium. Raman threshold values for 2, 5, 8, and 10 mol. % Ti-coated toroids were calculated to be sub-mW (52.6 5.4, 113 9.87, 159.9 11.6, and 117 14.2 μW, respectively). For comparison, a silica toroid with a titanium-free coating had a lasing threshold of 170 21.4 μW. While a change in threshold upon the addition of the Ti is clear, a simple inspection of the above threshold data does not immediately reveal any concentrationdependent trends. Due to the subtle variations between devices that arise from the fabrication procedure and the threshold’s strong dependence on mode volume, refractive index, and Q factor, this relationship is masked. To more easily visualize the dependence of threshold on Ti concentration, the above threshold values are normalized by n2eff Vm ∕Q2 . As can be observed in the Fig. 4 inset, there is a strong correlation between the threshold and the Ti concentration at low concentrations. Specifically, with only 2 mol. % Ti, the threshold experiences a significant decrease, indicating that the Raman gain has increased. However, as the concentration of Ti increases, the threshold increases. This response is predicted from classic laser theory that describes the balance between increasing laser threshold and slope efficiency [27]. Namely, while increasing the gain will improve the slope efficiency, past a certain threshold, it can also increase the transmission loss, which will decrease the lasing threshold. Therefore, there is a delicate balance between these two parameters.
Fig. 4. Lasing efficiency and lasing threshold (inset) as a function of Ti concentration. The lasing efficiency increases as the mol. % of Ti increases. The lasing thresholds are lower with the addition of Ti, when compared to the silica device with a titanium-free coating. The error bars in the inset are smaller than the symbols.
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This decrease in threshold power is significantly different from the up to fivefold increase in lasing threshold predicted by COMSOL. Because the COMSOL models did not account for the sensitizing effects of the titanium, these results validate the original hypothesis that Ti acts as a sensitizer for Raman lasing. Specifically, Ti both decreases the lasing threshold and increases the lasing efficiency. This improvement to the Raman gain offsets the decrease in lasing threshold due to reduced quality factor, enabling Raman lasing with μW thresholds to be achieved in devices with lower quality factors. In conclusion, we have theoretically modeled and experimentally demonstrated a titanium-enhanced Raman microcavity laser integrated on a silicon wafer. By coating a silica microtoroid cavity with a Ti-doped solgel, we achieve lasing thresholds as low as 52.6 μW. Additionally, the lasing efficiency can be increased by increasing the mol. % of Ti in the device coating. Therefore, the addition of titanium allows Raman lasing performance to be significantly enhanced. The ability to use lower Q devices will decrease the fabrication tolerances, enabling a wider range of applications and the development of a fully integrated platform. The authors would like to thank Hari Mahalingam and Prof. William Steier at the University of Southern California for spectroscopic ellipsometry. The authors also thank Rohan Dhall, Shermin Arab, and Prof. Steve Cronin for Raman spectrometry. Ashley Maker is supported by an Annenberg Graduate Research Fellowship. This work was supported by the National Institutes of Health through the NIH Director’s New Innovator Award Program [1DP2OD007391-01]. Additional information is available at http://armani.usc.edu. †These authors contributed equally to this work. References 1. S. M. Spillane, T. J. Kippenberg, and K. J. Vahala, Nature 415, 621 (2002). 2. M. Chistiakova and A. M. Armani, Opt. Lett. 37, 4068 (2012). 3. I. S. Grudinin and L. Maleki, Opt. Lett. 32, 166 (2007). 4. B.-B. Li, Y.-F. Xiao, M.-Y. Yan, W. R. Clements, and Q. Gong, Opt. Lett. 38, 1802 (2013).
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