Ultrahigh-Q (UH-Q) microtoroidal silica resonators represent a distinct class of optical microring resonators with Qs in excess of 100 million. The UH-Q operation makes these devices very attractive for applications where low loss, large circulating power, small mode volume and narrow-linewidth are required. Employing UH-Q microtoroids can dramatically improve the performance of multi-pole add-drop optical filters and resonator based optical biosensors, enable the design of low threshold and narrow linewidth lasers and facilitate cavity QED experiments. Unfortunately the special fabrication process of these microtoroids limits the range of possible integration choices for these resonators. Specifically, the diameter of the microdisk shrinks down as it reflows to its final toroidal shape and after fabrication, microtoroids are physically perched atop a silicon pillar. So those photonic devices that rely on coupling of these resonators to an integrated waveguide or their mutual coupling cannot be realized. Here we describe techniques that may enable fabrication of a new class of photonic devices based on free UH-Q microresonators. Preliminary results show that by employing simple techniques we can detach the microtoroid from the silicon pillar without any damage to the microtoroid structure and maintain their quality factor above 10 million.
After the conventional fabrication process, the silica microtoroid is perched atop a 20-40 mm silicon pillar. To enable safe detachment of the silica microtoroid we reduce the diameter of the silicon pillar down to about 2-4 mm through a second XeF2 dry etching process (Fig. 1(a)). We have designed and fabricated special tools called ‘microforks’ for detachment and transfer of the microtoroids.
Fig. 1(b) is the top view photograph of a microtoroid detached from the silicon pillar using the microfork .The detachment from the pillar doesn’t damage the microring structure and only leaves a small hole (4-6 mm diameter) in the center of the silica membrane (later this hole is used for accurate positioning of the microtoroid). Fig. 2(c) shows five, free microtoroids that are positioned side-by-side on the silicon substrate. These microtoroids are positioned using the Tungsten probe (shown in the bottom inset) that was controlled by a manual 3-D a nanopositoning system. The diagram in the top inset shows how the probe (that has a footprint of 3 mm) controls the microtoroid by insertion into the central hole.
Standard fiber-taper coupling techniques are used to measure the optical quality factor of the free microtoroid. We have designed a special platform that facilitates the fiber-taper coupling to a free microtoroid. This platform consists of a 2 mm thick silica membrane supported by a silicon pedestal (about 30 mm thick). Fig. 2(a) shows a schematic diagram of the coupling configuration). Figure 2(b) is the photograph of a free microtoroid on silica substrate coupled to a fiber-taper. Before detachment the measured quality factor (Qtot) of the microtoroid was about 5x107 corresponding to an unloaded quality factor (Q0) of 5.23x107. After detachment the measured quality factor (Qtot) is about 2.95x107 corresponding to an unloaded quality factor of 3.08x107. The coupling factor is set to small values to minimize the loading effect of the fiber-taper. Although the optical-Q of the free microtoroid is degraded (~40%) the final Q is still above 10 million. We believe the Q-degradation is mainly caused by contamination (since the whole process is performed in the lab environment and not in a clean room). Note that due to the sidewall curvature and thickness of the microtoroid cavity the optical mode does not interact with the substrate. This is an important feature that enables the integration of these resonators on almost any substrate. Currently, in all integrated microring resonators, the lateral optical confinement is provided by the index contrast rather than sidewall curvature consequently they must be fabricated on substrates with lower refractive index than the ring itself. This requirement sets a major limit on the variety of platforms and material systems that can be used in microring structures.
We have demonstrated a practical technique for fabrication of free UH-Q microtoroids. Free microtoroids can add the advantages of UH-Q operation to a variety of photonic devices. Beyond enabling the free microtoroid technology, our approach introduces the concept of fabrication and manipulation of free microresonators to photonic technology. We believe the extra degree of freedom offered by this approach can significantly increase the variety of possible configurations for photonic devices and improve their performance.
More information can be found in the following papers:
M. Hossein-Zadeh and K. J. Vahala
"Free Ultra-high-Q microtoroid: a tool for designing photonic devices"
Optics Express, Volume 15, issue 1, January 2007.
D. K. Armani, T. Kippenberg, S. M. Spillane and K. J. Vahala
"Ultra-high-Q toroid microcavity on a chip"
Nature, vol. 421, pp. 925-929, 27 February 2003.