Tuning of optical microcavities

Background

This work focusses on the ability to modify—dynamically or not—the resonance wavelength of optical microcavities. For instance optical whispering gallery mode (WGM) resonators possess a discrete set of optical resonances, for which the optical path length corresponds to an integer multiple of the free space wavelength, enabling light to interfere constructively with itself upon completion of a round-trip (see below).

WGM_spectrum
Left: geometric optics representation of a WGM with resonance condition; Middle: Field distribution; WGM resonator spectrum, showing a discrete set of resonances separated by a free spectral range.

The ability to tune these optical resonators enables many applications, including:

  • Reconfigurable optical circuits

We can illustrate this functionality using the example of a simple add/drop filter (see below).  This device consists of an optical resonator coupled to two optical waveguides. It allows for light to be routed on a chip, with different wavelengths traveling along different paths. The ability to dynamically change the device’s resonances therefore enables in situ reconfiguration of the routing of the photons on the chip.

add_drop_filter
WGM Add drop filter which consists of an optical resonator coupled to two optical waveguides (left). Light which is non-resonant with the device goes straight through (middle), while wavelengths of light which are resonant are ‘dropped’ in another port (right).
  • Optical intensity/phase modulation
animation_optical_modulation
Optical intensity modulation

The mechanism behind optical intensity modulation is illustrated on the right. When the optical resonance wavelength is modulated in time (blue trace), the level of light transmission through the resonator (red dot) fluctuates in time, even for a laser beam of fixed wavelength and intensity (materialized by the dashed black line) as experimentally shown below. This can be used for example to encode information or drive a device. (Positioning the laser on resonance rather than detuned off resonance would switch from intensity modulation to phase modulation). The same force used to alter the resonance wavelength of the cavity can also be used for injection locking of an optomechanical resonator, as discussed below.

  • Spectral alignment of dissimilar resonators
dissimilar_WGM_resonators

A great practical challenge in integrated photonics is to have multiple high Q,  small size resonators interacting with each other. (Small size and high Q are for instance desirable for strong light-matter interaction and dense on-chip packaging).  In the example image to the left, because of a fabrication-induced size mismatch, light injected into the waveguide will interact either with resonator 1 or resonator 2, but not with both simultaneously. The fabrication tolerance ΔR required for two WGM resonators of nominal radius R to share the same resonance wavelength (to within their optical linewidth Δλ) can be estimated using the resonance condition 2πnR=mλ.   This condition implies Δλ/λ=ΔR/R,  and hence the fabrication tolerance is given by ΔR=R/Q, where Q=λ/Δλ is the quality factor of the optical resonances. For two resonators with nominal radius R=1 micron and optical Q=104, this implies the resonators need to be identical to within less than one Angstrom, i.e. down to the level of a single atom.  We have shown how this extremely stringent condition can be achieved through permanent light-assisted etching (read more below). While we have mentioned only WGM resonators here, other resonator geometries, such as photonic crystals, encounter similar constraints.


Some examples of work on this topic are presented below:

  • High bandwidth tuning of microtoroids (Skip to section)
  • Free spectral range tuning of an optical microresonator (Skip to section)
  • Injection locking of optomechanical resonators (Skip to section)
  • Permanent tuning of ensembles of cavities by photoelectrochemical etching (Skip to section)
  • Light-Mediated Cascaded Locking of Multiple Nano-Optomechanical Oscillators (Skip to section)

High bandwidth on-chip capacitive tuning of microtoroid resonators

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

capacitive_actuation_WGM
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) with low voltages and sub-nanowatt power consumption.

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

Read more here:


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.

Read more here:


Permanent tuning through photoelectrochemical etching

3 microdisks
“Scalable high-precision tuning of photonic resonators by resonant cavity-enhanced photoelectrochemical etching,” [Nature Comms, 8, 14267, 2017]
All resonators in the image are nominally identical down to less than one tenth of an atomic monolayer, thanks to the light-assisted etching technique.

Photonic lattices of mutually interacting indistinguishable cavities represent a cornerstone of collective phenomena in optics and could become important in advanced sensing or communication devices. The disorder induced by fabrication technologies has however so far hindered the development of such resonant cavity architectures. Indeed, as detailed above, the fabrication tolerance for two WGM resonators of nominal radius R to share the same resonance wavelength is given by ΔR=R/Q, where Q is the quality factor of the optical resonances. For two resonators with nominal radius R=1 micron and a modest optical Q of 104, this implies the resonators need to be identical to within less than one Angstrom, i.e. down to the level of a single atom. In consequence, two nominally identical cavities, once fabricated, always resonate at distinct wavelengths, precluding collective spectral alignment or resonant interaction with targeted references. This disorder is a major obstacle for the future of nanophotonics, with post-fabrication tuning methods (such as addressing each resonator with a distinct electrical heater) typically limited by complexity and poor scalability.

In this work, we developed a new simple and scalable tuning method for ensembles of microphotonic and nanophotonic resonators, which enables their permanent collective spectral alignment. The method relies on cavity-enhanced photoelectrochemical etching (PEC) in a fluid, where laser light is used to trigger an extremely precise etching process. The presence of light in the microcavity generates carriers leading to the formation of ionic species of the semiconductor, which are then dissolved away (as illustrated in the figure on the right). This forms an ultra precise laser chisel, capable of controlling the dimensions of resonators with a precision at the level of a hundredth of an atomic monolayer.

Beyond its extreme precision, the method can moreover be used to spectrally align a large number of optical microcavities with a single continuous laser sweep, as illustrated below:

animate5.gif
Animation of the PEC tuning process: the tuning laser (vertical black line) is set to a higher wavelength, and gradually blue-shifted. As it enters the first optical resonance, the corresponding disk will be etched by the laser light, reducing its size, and reducing its optical resonance wavelength. Next two disks are etched together by the laser light, and their optical resonances dragged along by the tuning laser. This goes on until all resonators are permanenty spectrally aligned, using a single laser and a single laser wavelength sweep, and without the need having to individually identify each resonator of the set.
PEC_tuning

Read more here:


Light-Mediated Cascaded Locking of Multiple Nano-Optomechanical Oscillators

“Light-Mediated Cascaded Locking of Multiple Nano-Optomechanical Oscillators,” [PRL, 118, 063605, 2017] – 3D rendering file available here.

In the context of spectrally aligning microcavities -without reverting to permanent tuning through photoelectrochemical etching as described above– some amount of fabrication-induced optical disorder can be overcome through thermo-optic effects. This approach relies on the fact that the refractive index of most materials is temperature dependent. As light enters a microcavity, the cavity is heated and its optical resonances are shifted to higher wavelenghts. In this way, using light from a single laser, the optical resonances of many distinct optical resonators can be ‘pushed’ and merged into one another (see paper for more details).

Scanning electron microscope image of arrays of optomechanical resonators (blue) coupled to a common optical waveguide. Light injection and collection is achieved through lensed fiber coupling to the inverted tapers located at the waveguide extremities (see more here).

Using this approach, we fabricate arrays of distant optomechanical resonators which are coupled to a single laser source via a common integrated optical waveguide, as illustrated in the figure on the right. We demonstrate light-mediated frequency locking of these gigahertz optomechanical oscillators—in what constitutes a form of cascaded optical injection locking.

Experimental power spectrum showing the light-mediated frequency locking (synchronization) of three spatially separated optomechanical resonators.
Signature of the frequency locking of two (middle) and three (bottom) distinct optomechanical resonators.

Read more here:

Cascaded optomechanical resonators

Links:


Keywords: Tuning of WGM cavities, optomechanics, injection-locking, frequency tuning of Gallium Arsenide resonators through photoelectrochemical etching, PEC tuning of Whispering Gallery Mode resonators, free spectral range tuning, FSR tuning, capacitive tuning of microcavities, thermal tuning of optomechanical resonators, phase and intensity modulation with on-chip resonators, micro- and nanophotonics, reconfigurable optical circuits, on-chip photonics,  cascaded optomechanical resonators, integrated waveguides, lensed fiber coupling, silica microtoroid, microtoroid tuning, silica double-disk resonator.

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