This research aims to create interfaces between light and electronics, through their common interaction with a mechanical element. Such interfaces can be used to integrate quantum photonic systems with quantum superconducting circuits in future quantum information devices, for improved on-chip clocks and receivers for mobile communications that benefit from laser control and measurement, and for scalable photonic circuitry and photonic links in next generation computer chips, among other applications.

Recent work on this topic includes:

High bandwidth on-chip capacitive tuning of microtoroid resonators

“High bandwidth on-chip capacitive tuning of microtoroid resonators,” [Optics Express, 24, 20400, 2016].

Capacitive tuning mechanism

In this work, gold electrodes patterned onto the microtoroid resonator (see above) allow for rapid capacitive tuning of the optical whispering gallery mode resonances while maintaining their ultrahigh quality factor, enabling applications such as efficient radio to optical frequency conversion, optical routing and switching applications. A voltage bias applied between the electrodes leads to an attractive capacitive force, which radially strains the device, reducing its size and shifting its optical resonances (see right). This capacitive actuation approach presents several advantages. First it allows for very fast electrical tuning of the resonances reaching up to the tens of MHz range—several orders of magnitude faster than previously demonstrated schemes for microtoroids—while maintaining the ultra-high Q nature of the resonances. Moreover, capacitive tuning requires minimal power expenditure as there is no current flow between the electrodes in the steady state, unlike thermal tuning schemes.

Read more here:

Free spectral range tuning of a microresonator

3D rendering of a free spectral range electrically tunable high quality on-chip microcavity. Rendering source file can be downloaded here.
Opt. Express, vol. 26, pp. 33649–33670, Dec. 2018.

The ability to tune a resonator by an optical linewidth enables optical modulation, as described above. A much more stringent criterion corresponds to the ability to tune a resonator by an entire free spectral range (FSR). FSR-tuning allows resonance with any source or emitter within the material’s transparency range, or between any number of networked microcavities (see animation below). This is typically hard to achieve in miniature resonators due to the very large strains (for strain tuning) or temperatures (for thermal tuning) required. We achieve this here through the use of a double-disk architecture which affords:

  • Larger mechanical compliance for out-of-plane mechanical motion compared to the radial strain used in our previous work [Optics Express, vol. 24, p. 20400, 2016.] (i.e. easier to bend a ruler than compress it along its length),
  • Higher capacitive force due to more interdigitated electrodes spaced closer together,
  • Larger optomechanical coupling rate ∂ω/∂x enabled by the double-disk architecture.

With this approach, we demonstrate an on-chip high quality microcavity with high Q resonances that can be electrically tuned across a full free spectral range (FSR) -see video below- with low voltages and sub-nanowatt power consumption.

Free Spectral Range (FSR) tuning of an optical microcavity. Once a WGM resonator can be tuned by an FSR, it can be made resonant with any wavelength within its material transparency range (blue shading).
Scanning electron micrograph of a fabricated electrically tunable double-disk device. [Opt. Express, 26, 33649, 2018]

Optical spectrum of an electrically tunable double-disk whispering gallery mode resonator, repeatedly updated as the voltage bias applied to the integrated electrodes is varied.
The strong compliance (and associated large optical tuning range) afforded by the double-disk resonator architecture can be visualized with this video taken inside an SEM (scanning electron microscope). Upon increasing the magnification, both silica disks become negatively charged by the electron beam (as they are poor electrical conductors) and repel each other, increasing their separation.

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Injection-locking of optomechanical resonators

“Injection locking of an electro-optomechanical device,” [Optica, 4, 1196-1204, 2017] – 3D rendering file available here.

Visualization of the device’s radial breathing mode, driven by the optical field and injection locked through the integrated electrodes.

The same electrodes used for tuning of the WGM resonator’s optical resonances (see above), can be used to injection-lock the device’s mechanical resonances, i.e. provide the spontaneous locking of the mechanical vibrations to an external drive tone provided by the integrated electrodes.
(This technique is often used in the optical domain in order for a high-power, noisy laser to acquire the spectral characteristics of a low-power, low-noise seed laser, read more here).
We employ this technique to suppress the drift in the optomechanical oscillation frequency, strongly reducing phase noise by over 55 dBc/Hz at 2 Hz offset and tune the oscillation frequency by more than 2 million times its narrowed linewidth, enough to overcome fabrication-induced mechanical frequency variability. We also show how our approach may enable control of the optomechanical gain competition between different mechanical modes of a single resonator.

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Optically tunable photoluminescence and upconversion lasing on a chip

The ability to tune the wavelength of light emission on a silicon chip is important for scalable photonic networks, distributed photonic sensor networks and next generation computer architectures. Here we demonstrate light emission in a chip-scale optomechanical device, with wide tunablity provided by a combination of radiation pressure and photothermal effects.

Green upconversion lasing in a double disk resonator

To achieve this, we develop an optically active double-disk optomechanical system through implantation of erbium ions. We observe frequency tuning of photoluminescence in the telecommunications band with a wavelength range of 520 pm, green upconversion lasing with a threshold of 340±70 μW, and optomechanical self-pulsing caused by the interplay of radiation pressure and thermal effects. These results provide a path towards widely-tunable micron-scale lasers for photonic networks.

Optical microscope top-view displaying optical actuation of a double disk structure. As the laser is repeatedly swept at high power accross an optical resonance of the device, the radiation pressure force acting between the disks modifies their physical separation, as evidenced by the changing colors in the interference pattern.
Green upconversion lasing of a erbium-doped double-disk on a silicon chip (bright green light emanating from the bottom of the chip. The coupling optical fiber is faintly visible in the lower left hand corner).

Optical microscope sideview, showing the intermittent green laser emission in the regime of optomechanical self-pulsing.

Read more here:


See also: PhD thesis by Christiaan Bekker (2020)

We gratefully acknowledge funding from:

Australian Research Council
Lockheed Martin
ARC Centre of Excellence for Engineered Quantum Systems