UHQ Disk Resonators

UHQ disk resonators are silicon-chip-based devices and are fabricated using only conventional semiconductor processing methods. They attain Q factors close to 1 billion and offer new opportunities for precise control of free-spectral-range (FSR) as well as resonator dispersion. These devices and their variants are the baseline resonator in all of our current resonator work. Moreover, the process method developed for these resonators lends itself to ULL waveguide fabrication.

An over-riding goal of much electronics and photonics research is device integration onto chip platforms. Silicon photonics, for example, is a research area directed towards transfer of device functions that exist in discrete (stand-alone) form to silicon wafers for integration with other photonic and electrionic devices. Full integration of devices can enable remarkable system-on-a-chip complexity and open-up entirely new applications.

Part of our group's work has been to bring ultra-high-Q device research into the realm of integration. We introduced the microtoroid resonator in 2003 as the first UHQ optical resonator fabricated on a semiconductor chip [1]. Its Q factor of 100 million represented about a 10,000-fold increase in Q factor for chip-based devices. The microtoroid is now widely used in micro-cavity research across the world and continues to provide access to amazing physics.

Figure 1: Disk resonator spectral scan for Q measurement.

UHQ disk resonators [2] add a suite of new control features to resonator research that have not been possible using microtoroids. Precise control of diameter is possible which translates into very accurate control of the free spectral range (FSR). This has enabled high performance Brillouin lasers by allowing the precise match of FSR to the Brillouin shift frequency. In addition to diameter control, these resonators can be fabricated at a very wide range of diameters and making possible operation of frequency microcombs with both high efficiency and at detectable rates [3]. Also, the actual shape of the disk resonator can be controlled during fabrication so as to introduce a geometrical component of dispersion that can be used to compensate for the underlying material dispersion of the silica resonator material. We are using this capability to create spectrally flat resonators for high performance microcombs. Fabrication of the UHQ disk resonators uses special, high purity oxide films prepared from float zone silicon combined with additional steps to remove residual water in the wet-grown oxide. Moreover, the etching process has been optimized to provide a highly polished silica surface. The highest performance devices can attain Q factors of nearly 1 billion [2]. The data in figure 1 show a spectrum created by coupling a laser to a disk resonator using a tapered fiber coupler and then scanning the laser through a disk resonant frequency. The linewidth of the resonance determines the Q factor. Also shown is a sinusoidal curve produced by an interferometer and used for frequency calibration. The data in figure 2 show how FSR can be accurately controlled by proper design of a CAD file used to create a lithographic mask. The mask is used to define the shape of the disk resonator in fabrication. The data show a standard deviation of 2.4 MHz for an FSR of about 10.8 GHz.

Figure 2: Process control of Disk resonator FSR.

The linewidth of the Brillouin amplification process is about 50 MHz so this means that all devices on a chip lase with very high efficiency.

D. K. Armani, T. J. Kippenberg, S. M. Spillane and K. J. Vahala, "Ultra-high-Q toroid microcavity on a chip," Nature 421, 925-928 (2003)
Hansuek Lee*, Tong Chen*, Jiang Li*, Ki Youl Yang, Seokmin Jeon, Oskar Painter and Kerry J. Vahala, "Chemically etched ultrahigh-Q wedge-resonator on a silicon chip," Nature Photonics 6, 369–373 (2012)
Jiang Li, Hansuek Lee, Tong Chen, and Kerry J. Vahala, "Low-pump-power, low-phase-noise, and microwave to millimeter-wave repetition rate operation in microcombs," Phys. Rev. Lett. 109, 233901 (2012)