April 1, 2014 / Vol. 39, No. 7 / OPTICS LETTERS
1841
Inverted-wedge silica resonators for controlled and stable coupling Fang Bo,1,2 Steven He Huang,1 Şahin Kaya Özdemir,1,3 Guoquan Zhang,2 Jingjun Xu,2 and Lan Yang1,4 1
Electrical and Systems Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, USA 2 The MOE Key Laboratory of Weak Light Nonlinear Photonics, TEDA Applied Physics Institute and School of Physics, Nankai University, Tianjin 300457, China 3
e-mail: [email protected] 4 e-mail: [email protected]
Received November 20, 2013; revised February 11, 2014; accepted February 18, 2014; posted February 21, 2014 (Doc. ID 201564); published March 21, 2014 Silica microresonators with an inverted-wedge shape were fabricated using conventional semiconductor fabrication methods. The measured quality factors of the resonators were greater than 106 in 1550 nm band. Controllable coupling from undercoupling to the overcoupling regime through the critical coupling point was demonstrated by horizontally moving a fiber taper while in touch with the top surface of the resonator. The thin outer ring of the resonator provided a support for the fiber taper leading to robust stable coupling. © 2014 Optical Society of America OCIS codes: (140.3945) Microcavities; (140.4780) Optical resonators; (230.5750) Resonators. http://dx.doi.org/10.1364/OL.39.001841
Whispering-gallery-mode (WGM) microresonators [1–4] with high quality factors Q are of importance in studying fundamental physics and in developing practical applications. They have been used successfully to fabricate devices for optical communication [2–4], to study nonlinear optics at low power levels [2–4], to demonstrate strong light-atom coupling [5], to achieve single nanoparticle [6–8] and protein detection [9], to study cavity optomechanics [10], and they have been proposed as a platform to realize quantum nondemolition measurements [11]. This widespread use of WGM resonators stems from their ability to confine light within a small mode volume for extended durations of time via continuous total internal reflection which helps to enhance the light–matter interactions significantly. In the past decade, there has been an increasing interest in the studies of WGM microresonators thereof WGM resonators of different geometries (e.g., sphere, disk, ring, toroid, bottle) and materials (e.g., silica, silicon, silicon nitride) have been proposed and fabricated [4]; each having its own advantages and disadvantages over the others. For example, silica microtoroids [12] have Q of the order of 108 thanks to the significantly reduced scattering losses due to surface-tension-induced smooth surface as the result of a reflow process using a CO2 laser [12]. However, the CO2 laser reflow process is not compatible with conventional semiconductor processing, and during this process the size of the device shrinks making it difficult to have precise control on the size of the resonator. Microdisk resonators, on the other hand, have lower Q than microtoroids due to higher scattering losses. Absence of the final reflow process in microdisk fabrication allows for the fabrication of resonators of desired sizes as defined by the mask used in photolithography process. This, in turn, makes it possible to fabricate coupled microdisk resonators with fixed coupling distances [13], which is impossible for the case of microtoroids. Recently, silica disk resonators with a wedge shape, referred to as a wedge resonator, have been fabricated by optimizing the fabrication process of microdisk resonators. Wedge resonators of millimeter sizes with Q over 0146-9592/14/071841-04$15.00/0
108 have been reported [14]. These resonators achieve such high-Q values without the need for CO2 laser reflow, and due to their larger size they are useful in the studies of stimulated Brillouin scattering [14] and optical combs [15]. Evanescent wave couplers, such as prisms, waveguides, or tapered fibers, have been used to couple light in and out of WGM resonators. With fiber tapers, coupling efficiency of 99% has been reported in experiments with high-Q resonators [16]. However, maintaining a stable and accurately controlled coupling between the fiber taper and the resonator is challenging in the field outside the controlled laboratory environment. For example, airflow, mechanical vibrations, and thermal fluctuations may easily perturb the coupling conditions and thus affect the transmission spectra of the resonator. Reflowed silica side walls or nanofork structures close to a resonator have been fabricated to form a support for the tapered-fiber coupler, providing mechanical stability and robust coupling conditions [17,18]. In an alternative approach, resonators with different shapes, such as the one referred to as the “octagonal silica toroidal microcavity” [19], are fabricated to allow controllable coupling by changing the contact point between the resonator and the fiber taper. Since the fiber taper is in contact with the resonator, coupling is robust to external perturbations. In this work, we report a new type of high-Q silica resonator fabricated by modifying the fabrication processes of microdisk and wedge resonators. We refer to this resonator as an inverted-wedge resonator because the top surface of this resonator is larger than the bottom surface, in contrast to that of the wedge resonators [14] (Figs. 1 and 2). The resonator was fabricated using conventional semiconductor fabrication processes. Benefiting from its unique geometry with a flat top surface having a thin outer ring, robust critical coupling between a tapered fiber and a resonator was experimentally demonstrated with the outer ring serving as a support for the tapered fiber. The coupling strength was continuously tuned from undercoupling to overcoupling © 2014 Optical Society of America
1842
OPTICS LETTERS / Vol. 39, No. 7 / April 1, 2014 Wedge
1 Mask with PR
2. Etch SiO2 with HF
3. Remove PR
4. Etch Si with XeF2 Inverted wedge
t2 t1
t2 t1
t2 t1 Inverted wedge with a ring
3. Etch Si with XeF2
0
1841
Inverted-wedge silica resonators for controlled and stable coupling Fang Bo,1,2 Steven He Huang,1 Şahin Kaya Özdemir,1,3 Guoquan Zhang,2 Jingjun Xu,2 and Lan Yang1,4 1
Electrical and Systems Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, USA 2 The MOE Key Laboratory of Weak Light Nonlinear Photonics, TEDA Applied Physics Institute and School of Physics, Nankai University, Tianjin 300457, China 3
e-mail: [email protected] 4 e-mail: [email protected]
Received November 20, 2013; revised February 11, 2014; accepted February 18, 2014; posted February 21, 2014 (Doc. ID 201564); published March 21, 2014 Silica microresonators with an inverted-wedge shape were fabricated using conventional semiconductor fabrication methods. The measured quality factors of the resonators were greater than 106 in 1550 nm band. Controllable coupling from undercoupling to the overcoupling regime through the critical coupling point was demonstrated by horizontally moving a fiber taper while in touch with the top surface of the resonator. The thin outer ring of the resonator provided a support for the fiber taper leading to robust stable coupling. © 2014 Optical Society of America OCIS codes: (140.3945) Microcavities; (140.4780) Optical resonators; (230.5750) Resonators. http://dx.doi.org/10.1364/OL.39.001841
Whispering-gallery-mode (WGM) microresonators [1–4] with high quality factors Q are of importance in studying fundamental physics and in developing practical applications. They have been used successfully to fabricate devices for optical communication [2–4], to study nonlinear optics at low power levels [2–4], to demonstrate strong light-atom coupling [5], to achieve single nanoparticle [6–8] and protein detection [9], to study cavity optomechanics [10], and they have been proposed as a platform to realize quantum nondemolition measurements [11]. This widespread use of WGM resonators stems from their ability to confine light within a small mode volume for extended durations of time via continuous total internal reflection which helps to enhance the light–matter interactions significantly. In the past decade, there has been an increasing interest in the studies of WGM microresonators thereof WGM resonators of different geometries (e.g., sphere, disk, ring, toroid, bottle) and materials (e.g., silica, silicon, silicon nitride) have been proposed and fabricated [4]; each having its own advantages and disadvantages over the others. For example, silica microtoroids [12] have Q of the order of 108 thanks to the significantly reduced scattering losses due to surface-tension-induced smooth surface as the result of a reflow process using a CO2 laser [12]. However, the CO2 laser reflow process is not compatible with conventional semiconductor processing, and during this process the size of the device shrinks making it difficult to have precise control on the size of the resonator. Microdisk resonators, on the other hand, have lower Q than microtoroids due to higher scattering losses. Absence of the final reflow process in microdisk fabrication allows for the fabrication of resonators of desired sizes as defined by the mask used in photolithography process. This, in turn, makes it possible to fabricate coupled microdisk resonators with fixed coupling distances [13], which is impossible for the case of microtoroids. Recently, silica disk resonators with a wedge shape, referred to as a wedge resonator, have been fabricated by optimizing the fabrication process of microdisk resonators. Wedge resonators of millimeter sizes with Q over 0146-9592/14/071841-04$15.00/0
108 have been reported [14]. These resonators achieve such high-Q values without the need for CO2 laser reflow, and due to their larger size they are useful in the studies of stimulated Brillouin scattering [14] and optical combs [15]. Evanescent wave couplers, such as prisms, waveguides, or tapered fibers, have been used to couple light in and out of WGM resonators. With fiber tapers, coupling efficiency of 99% has been reported in experiments with high-Q resonators [16]. However, maintaining a stable and accurately controlled coupling between the fiber taper and the resonator is challenging in the field outside the controlled laboratory environment. For example, airflow, mechanical vibrations, and thermal fluctuations may easily perturb the coupling conditions and thus affect the transmission spectra of the resonator. Reflowed silica side walls or nanofork structures close to a resonator have been fabricated to form a support for the tapered-fiber coupler, providing mechanical stability and robust coupling conditions [17,18]. In an alternative approach, resonators with different shapes, such as the one referred to as the “octagonal silica toroidal microcavity” [19], are fabricated to allow controllable coupling by changing the contact point between the resonator and the fiber taper. Since the fiber taper is in contact with the resonator, coupling is robust to external perturbations. In this work, we report a new type of high-Q silica resonator fabricated by modifying the fabrication processes of microdisk and wedge resonators. We refer to this resonator as an inverted-wedge resonator because the top surface of this resonator is larger than the bottom surface, in contrast to that of the wedge resonators [14] (Figs. 1 and 2). The resonator was fabricated using conventional semiconductor fabrication processes. Benefiting from its unique geometry with a flat top surface having a thin outer ring, robust critical coupling between a tapered fiber and a resonator was experimentally demonstrated with the outer ring serving as a support for the tapered fiber. The coupling strength was continuously tuned from undercoupling to overcoupling © 2014 Optical Society of America
1842
OPTICS LETTERS / Vol. 39, No. 7 / April 1, 2014 Wedge
1 Mask with PR
2. Etch SiO2 with HF
3. Remove PR
4. Etch Si with XeF2 Inverted wedge
t2 t1
t2 t1
t2 t1 Inverted wedge with a ring
3. Etch Si with XeF2
0
