Fabrication of optical microcavities with femtosecond ...

Fabrication of optical microcavities with femtosecond laser pulses Jintian Lin 1, Jiangxin Song 1, Jialei Tang 1, Wei Fang 2, †, Koji Sugioka 3, Ya Cheng 1, ‡ State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, China 2 State Key Laboratory of Modern Optical Instrumentation, Department of Optical Engineering, Zhejiang University, Hangzhou 310027, China 3 RIKEN-SIOM Joint Research Unit, RIKEN, Hirosawa 2-1, Wako, Saitama 351-0198, Japan 1

ABSTRACT We report on fabrication of three-dimensional (3D) high-quality (Q) whispering-gallery-mode microcavities by femtosecond laser micromachining. The main fabrication procedures include the formation of on-chip freestanding microdisk through selective material removal by femtosecond laser pulses, followed by surface smoothing processes (CO2 laser reflow for amorphous glass and focused ion beam (FIB) sidewall milling for crystalline materials) to improve the Q factors. Fused silica microcavities with 3D geometries are demonstrated with Q factors exceeding 106. A microcavity laser based on Nd:glass has been fabricated, showing a threshold as low as 69µW via free space continuous-wave optical excitation at the room temperature. CaF2 crystalline microcavities with Q factor of ~4.2×104 have also been demonstrated. This technique allows us to fabricate 3D high-Q microcavities in various transparent materials such as glass and crystals, which will benefit a broad spectrum of applications such as nonlinear optics, quantum optics, and bio-sensing. Keywords: femtosecond laser, direct writing, three dimensional, microcavity, glass, crystal

1. INTRODUCTION Whispering-gallery-mode (WGM) microcavities, which confine light via continuous total internal reflection along a smooth equatorial boundary between the dielectric cavity and surrounding, exhibit very high Q factors and small mode volumes. The long photon lifetime and strong spatial confinement make them excellent candidates for low threshold nonlinear optics1-3, quantum physics4-6, low threshold lasing7-8, and biosensing9-10. Today, most on-chip WGM microcavities, such as microdisks11, microtoroids12, and deformed microcavities13 are fabricated based on planar lithographic approaches. Although the planar lithography is rapid and cost effective, it is difficult to form three-dimensional (3D) microcavities directly. The lithographic techniques also frequently rely on selective material removal by chemical etching, putting a limit on choices of materials. These difficulties make obstacles for many applications that prefer to use out-of-plane light coupling or to employ substrate materials of special linear and nonlinear optical properties. Therefore, there is a need to develop new strategies to overcome these problems. Recently, femtosecond laser micromachining has been proved as a promising solution for fabrication of flexible 3D microstructures, such as microoptics14-15, microfluidics16-20, and polymer-based microcavities21-23. Dielectric materials, such as crystals and amorphous glass materials, can provide wide transparent window and low intrinsic absorption loss, resulting in making themselves ideal materials for high-Q microcavity applications. Although femtosecond laser micromachining has shown great flexibility in fabrication of various kinds of 3D microstructures directly in transparent materials, it has not been considered to be used for fabricating optical microcavities until recently. The major difficulties include fabrication resolution, surface smoothness, and efficient removal of materials in a space-selective manner. To address these problems, we have recently developed an approach for fabricating 3D microcavities in both amorphous glass24-25 and crystalline materials26 using a femtosecond laser, followed by surface smoothing processes such as CO2 laser reflow for amorphous glass materials and focused ion beam (FIB) sidewall milling for crystalline materials. In this paper, we show that, based on this approach, we are able to fabricate high-Q microcavities with complex 3D geometries and configurations in fused silica, microcavity lasers with low threshold in Nd:glass, and high-Q microcavities in crystalline materials such as CaF2 24-26. † ‡

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Laser Resonators, Microresonators, and Beam Control XVI, edited by Alexis V. Kudryashov, Alan H. Paxton, Vladimir S. Ilchenko, Lutz Aschke, Kunihiko Washio, Proc. of SPIE Vol. 8960, 89601A · © 2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2041353 Proc. of SPIE Vol. 8960 89601A-1 Downloaded From: http://spiedigitallibrary.org/ on 04/09/2014 Terms of Use: http://spiedl.org/terms

2. THREE DIMENSIONAL MICROCAVITIES IN GLASS Since the demonstration of the first microdisk lasers by Levi et. al., out-of-plane light output from microdisk lasers is mainly achieved by generating vertical emissions with additional grating structures either at the boundaries27 or on the top surfaces28 of the microdisks. Here, we show that 3D high-Q microcavities with either arbitrary tilting angles or non-uniform heights can be fabricated directly in fused silica using femtosecond laser direct writing, enabling out-of-plane light coupling at arbitrary angles with respect to the substrate24. la)

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Figure 1 (a) Procedures of fabrication of 3D microcavity by femtosecond laser direct writing. Optical microscope images of a tilted fused silica microdisk fabricated by femtosecond laser micromachining and HF wet etching (b) before and (c) after CO2 laser annealing. Insets in (b) and (c): top views of the microcavity. (d) SEM images of the cavity, and (e) SEM image of two microtoroidal cavities with different heights after CO2 laser annealing.

The process flow of fabrication mainly consists of: (1) femtosecond laser direct writing followed by selective chemical etching of the irradiated areas to create the freestanding microdisk structures, and (2) reflow of the silica cavities by CO2 laser annealing to improve the quality factors, as shown in Fig. 1(a). The fused silica glass was first scanned by a tightly focused femtosecond laser pulses for selectively modifying a predesigned area, and then glass material in the modified area was preferentially removed by wet chemical etching16. The femtosecond laser pulses with a pulse energy of ~0.2 μJ was tightly focused 100× objective with a numerical aperture (NA) of 0.9 into the glass sample, which was fixed on the XYZ translation stage with resolution of 1 μm. A layer-by-layer annular scanning method with the lateral scanning step of 1 μm was adopted to form the 3D microdisk supported by a thin pillar. After the laser exposure, the sample was subjected to a ~20 min etching in a solution of 5% HF diluted with water, leaving an on-chip microdisk. Finally a CO2 laser reflow step was adopted by which the surface tension of the melted glass created a smooth surface. The 3D capability of the femtosecond laser direct writing offers extreme flexibility for building 3D microcavity with different configurations, such as microcavities with controllable tilting angles and variable heights, as shown in Figs. 1(d)-(e). (c) 1.00 0.95

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Figure 2 (a) SEM image of a microcavity with a diameter of ~39 μm parallel to the substrate whose Q factor was examined. (b) An optical micrograph of the microcavity coupled with a fiber taper. (c) Transmission spectrum of the microcavity coupled with the fiber taper. The free spectral range (FSR) of 13.65nm agrees well with the numerical calculation result. The inset shows Lorentzian fit (red solid line) of measured spectrum around the resonant wavelength at 1562.85nm (black dotted line), showing a Q factor of 2.09×106.

Proc. of SPIE Vol. 8960 89601A-2 Downloaded From: http://spiedigitallibrary.org/ on 04/09/2014 Terms of Use: http://spiedl.org/terms

The fiber taper coupling method29 was chosen to measure the Q-factor and characterize the transmission spectrum of the microcavity, as shown in Figs. 2(a) and (b). The resonance transmission spectrum of the fiber taper coupled to the microcavity is presented in Fig. 2(c). The inset of Fig. 2(d) shows an individual WGM located at 1562.85 nm wavelength with a Lorentzian-shaped dip with linewidth of 0.745 pm. The Q factor for the microdisk was then calculated to be 2.09×106, with certain possibilities to be improved in the future by replacing the current motion stage with a higher resolution one. This indicates that femtosecond laser micromachining on fused silica enables fabrication of high-Q WGM microcavities. The above-mentioned method for fabrication of high-Q microcavities24, 30 is intrinsically limited to a few kinds of materials that can be modified by femtosecond laser irradiation for selectively promoting the etch rate at the irradiated region. On the other hand, 3D microcavites can also be fabricated in transparent materials with water-assisted femtosecond laser direct writing, which does not rely on selective chemical etching31. Therefore, this technique can significantly broaden the range of the substrate materials for microcavity applications, as far as a surface smoothing method can be applied for the laser-fabricated microdisks. As the first demonstration, we have fabricated a microcavity laser in a Nd:glass chip, which shows a pump threshold as low as 69 μW under continue-wave (CW) excitation25. (b)

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Figure 3 (a)-(d) Procedures of fabrication of a Nd:glass microcavity by femtosecond laser ablation assisted with water, followed by CO2 laser reflow. (e) Evolution of the microcavity photoluminescence (PL) emission spectrum with increasing pump power. (f) Laser output power as a function of the pump power, showing a lasing threshold of 69 μW. Upper inset in (f): schematic of the laser experimental setup. Lower inset in (f): laser spots on the edges of microcavity captured with the 2D CCD array detector mounted on the spectrometer.

The procedure of fabrication consists of (1) femtosecond laser ablation of the backside of glass material which was in contact with water to form a freestanding microdisk and (2) CO2 laser reflow, as shown in Figs. 3(a-d). During the laser writing, the ablation front always contacted with deionized water to efficiently remove the ablation debris. Consequently, the material at the laser irradiated regions was removed to form the freestanding microdisk by continuously layer-by-layer annular scanning of the focused beam in the region according to the predesigned microdisk structures. Usually, the average roughness of a femtosecond laser ablated surface is on the order of a few tens of nanometer.After the laser writing, the CO2 laser reflow step was adopted again to smooth the surface. In principle, atomic-scale surface smoothness could be achieved with the CO2 laser reflow12. To observe the lasing action, the microcavity was optically excited by a CW laser diode operating at 780 nm. The emission spectra collected from the microcavity at different pump powers are presented in Fig. 3(e). Two bright spots located in the both ends of the equatorial ring were observed, as indicated by arrows in the lower inset of Fig. 3(f). This indicates the nature of WGM emission, since light stored in the cavity could only escape tangentially from the edge of the microcavity. At the lowest excitation power, only a broad-spectrum photoluminescence (PL) emission was detected. As the pump laser power was increased, discrete peaks started to appear in the recorded spectra. The intensity of these periodic peaks increased dramatically when the pump laser intensity exceeds a threshold. Figure 3(f) shows the measured output laser power as a function of pump laser energy, and the lasing action occurred at a pump power of only 69 μW. Considering the mismatch between the pump wavelength and absorption peak of the gain medium (808 nm), the threshold could be further reduced by changing the pump laser source.


3. FABRICAT F TION OF CRYSTAL C LINE MIC CROCAVIT TIES Many nonlinear optical efffects can onlyy be achievedd in crystallin ne materials siince the crysttals can provid de the desiredd w of traansparency, veery low intrinnsic absorptio on, and so onn. optical propeerties such as high nonlineearity, wide window Today, most high-Q crysttal WGM miccrocavities aree fabricated by b mechanicaal polishing32. Meanwhile, integration of m to a few w tens of miccrons into a chhip is challeng ging due to thee small WGM microcavities with diameteers of a few microns d by the meechanical pollishing and the t additionaal small sizes of the structtures that aree difficult too be achieved t issue, herre we demonsstrate fabricatiion of on-chipp post-assembliing process reequired. As a first attempt to overcome this CaF2 microdiisk resonators using femtosecond laser micromachinin m ng26.

Figure 4 Procedures of o fabrication of CaF2 microdiisk by femtoseccond laser ablattion assisted byy water and folllowed by FIB milling. m

Commerciallyy available Z-cut Z CaF2 suubstrates withh a thicknesss of 1mm weere used as the material platforms forr fabrication off the microdisks. The proccedures for faabrication of CaF C 2 microdissk resonators are: (1) femttosecond laserr ablation of a CaF2 substraate immersed in water from its backsid de to form a freestanding disk-shaped microstructure m e similarly to thhe case of Ndd:glass; and (22) smoothing of o the sidewaall of the disk--shaped microostructure by FIB F milling too create the ultrra-smooth siddewall of the microdisk, m as illustrated in Fig. 4. The feemtosecond laaser parameters and writingg conditions arre the same ass those describbed in Section 3, except th hat the laser average a powerr was adjusted to be 1mW W, which is nearr the ablation threshold t for CaF2. Ablatioon with a femttosecond laserr operated neaar the ablation n threshold cann enable high resolution r fabrrication. The nonlinear natture of interacction between femtosecondd laser and tran nsparent CaF2 can further ennhance the ressolution. Afterr the laser wriiting, the SEM M image of thee fabricated m microdisk with h a diameter of 88 μm showss a rough surfaace similarly to t that of glasss disk shown in i Fig. 1(b) (ddata not shownn), which musst be improvedd for promotingg the Q factoor. However, for f most crysstalline materiials, CO2 laseer surface refllow is not app plicable33. Forr example, in the t current exxperiment, CaF F2 is transpareent to CO2 lasser beam. Eveen some crysttalline materiaals can absorbb CO2 laser, thee surface therm mal reflow woould create raandomly orien nted crystallitees due to rapidd heating and cooling whichh act as scatterring centers. Previously, FIB F has been employed to o shape the perimeters p of microdisks by b milling thee edges34-36. In reference to these t previous investigationns, in our currrent work, FIIB was used too mill the edg ge of the CaF2 mooth sidewaalls. Figure 5((a) shows an SEM S image of o a microdiskk with a diam meter of 79 μm m microdisk to attain ultra-sm b treated with w FIB millin ng. fabricated on CaF2 substratte which had been 65

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Figure 5 (a) SEM imagges of a CaF2 miccrodisk after FIB B milling. (b) Op ptical micrograpph of the microodisk coupled with w a fiber tapper (top view). (c) SEM imagee of another miicrodisk (side view), v showing the tapered sideewall. (d) Loreentzian fit (red solid line) of measured m spectrrum around the resonant waveelength at 1546..51nm (blue dootted line), show wing a Q factorr of 4.2×104.


Because the refractive index of the CaF2 is close to that of a silica optical fiber, the fiber taper coupling method was chosen to measure the Q-factor and characterize the transmission spectrum of the CaF2 microdisk, as shown in Fig. 5(b)29. It is worthy to mention that the sidewall of the microdisk is not completely vertical but with a tapered angle of 3.8˚, as evidenced in Fig. 5(c). Such taper results from the FIB milling which employs a focused ion beam with a divergence angle. Currently, the taper causes the major limiting factor on the Q-value of the CaF2 microcavity. This is because of the fact that with such a taper, the optical modes will tend to be distributed near the bottom surface of the microdisk. Unfortunately, the bottom surface formed by femtosecond laser micromachining was quite rough, because this area is not accessible by FIB milling. This problem can possibly be solved by improving the FIB milling technique to reduce or even eliminate the tapering effect or by improving the bottom surface quality of the microdisks with coating or chemical polishing. More investigations are required to test these methods. Figure 5(d) shows an individual WGM measured near 1546.51 nm wavelength with a Lorentzian-shaped dip. The linewidth obtained by a Lorentzian fitting is 36.74 pm, as shown by the red curve in Fig. 5(d). The Q factor for the microdisk is then calculated to be 4.2×104. The result also suggests that femtosecond laser micromachining followed by FIB milling can be extended for fabrication of the high quality microdisks in various kinds of crystalline materials, as FIB milling is an ideal tool for polishing hard materials.

4. CONCLUSION To summarize, we have demonstrated fabrication of 3D microcavities using femtosecond laser direct writing. We have shown that this technique allows us to form microcavities with complex 3D geometries and configurations. The materials that can be used for such applications include glass (either passive or active glass materials) and crystals. We demonstrated a microcavity laser in a Nd:glass substrate, which showed a pump threshold as low as ~69 μW. A high-Q optical cavity with a diameter of ~80 μm has also been formed on CaF2 substrate, and there is no difficulty in further reducing the cavity size. Despite its short history of fabricating high-Q microcavities based on femtosecond laser direct writing, the results achieved so far are encouraging us to further proceed. The flexibility of this technique offers potential for fabricating high-Q microcavities in a variety of transparent materials with 3D geometries. This will benefit a broad range of applications including nonlinear optics, quantum information processing, high-sensitivity bio-sensing, etc.

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