Free-Space optomechanics with light-emitting materials
Project number: 897148
The main goal of the action is to study optomechanical crystals in geometries beyond 1-dimensional nanocrystal beams, which are the standard when probing with a tapered fiber. In addition, the action aimed to incorporate optically active materials into these optomechanical crystal, as a means of probing photonic modes from free space and phononic modes in the material. This work eventually moved towards the direction of developing phononic circuitry and fundamental optomechanical effects, ultimately focusing on the generation and modulation of phononic signals.
Design and optimization
The fellow has gained significant experience and mastery of the optomechanical setup at the host institution, in which a tapered fiber loop is brought into contact with an optomechanical structure allowing light to evanescently couple into guided modes. In doing so, optical and mechanical modes can be probed. This technique is one of the standard approaches to optomechanics. This setup consists on a tuneable diode laser with telecom wavelength emission, a photodiode, and electronic signal analyzer. Fiber optics is used to connect the different elements.
Summary of designs explored in the present action. a) Part of a Scanning electron micrograph (SEM) depicting a W1 circle air-slot photonic crystal waveguide in a suspended silicon membrane.1 b) SEM image of a shamrock phononic crystal in a suspended silicon membrane.2 The inset shows the unit cell of the crystals. c) SEM of a shamrock-inspired optomechanical crystal fabricated in a suspended silicon membrane.3 The insets show the orientation and unit cell). d) Part of the SEM of a quantum valley Hall photonic waveguide with bearded interface. The inset below it depicts a waveguide coupled to mode adapters and grating couplers.
The fellow also took advantage of his existing fabrication experience to fabricate preliminary topological designs at the host institution. Dr Ng also fabricated samples that contained arrays of periodic holes to test diffraction in Brillouin light scattering spectroscopy, which should lead to a publication. Fabrication of optomechanical crystals in InP/InGaAsP were in the end not possible, as the proposed center C2N at the Université Paris-Saclay in France experienced delays in re-opening of their cleanroom.
Scanning electron microscope images of fabricated samples. a) Fabrication of a quantum valley Hall (QVH) topological waveguide with bearded interfaces in GaAs at NBI. b) Structure fabricated by the fellow at the host institution of QVH topological waveguides with bridge interfaces in Si.
Band structure of a shamrock waveguide showing confinement. (a) Band structure for the center waveguide region (blue outline) and the surrounding acoustic shield region (yellow outline). Z-symmetric modes (blue) and z-antisymmetric modes (red) appear, but the tapered fiber loop only allows optomechanical coupling to the z-symmetric modes. (b) Resulting mechanical spectra (bottom) when different optical resonances are driven, as indicated in the representative optical spectra (top). Mechanical Fabry-Perot fringes can be observed, and different modes can be made to lase.
1) Arregui, G., Ng, R.C., et al. “Cavity optomechanics with Anderson-localized optical modes.” Physical Review Letters (2021) arXiv:2110:11005 (Accepted)
Here we demonstrated that inherent unavoidable fabrication imperfections, which are traditionally considered a hindrance, can actually be exploited for optomechanics. Any disorder or imperfections leads to scattering. However, this multiple scattering cause Anderson-localization, or spatially localized modes with extremely high quality factors. By driving these modes, in-plane mechanical motion can be efficiently transduced. This was done in a 2D optomechanical crystal.
2) Madiot, G.,* Ng, R.C.,* et al. “Optomechanical generation of coherent GHz vibrations in a phononic waveguide.” (2022) arXiv:2206.06913 (Under Review – Physical Review Letters)
Here we demonstrated a GHz phonon source using a 2D optomechanical crystal, resulting from exploitation of Anderson-localized mode. We demonstrated a lasing guided phonon mode at 6.8 GHz by using phononic shielding and a center air-slot along the waveguide axis, which allowed for spatial localization within a desired region with enhanced optomechanical coupling. As this is a 2D design, such a phonon could be out-coupled to other on-chip architectures.
3) Florez, O., Arregui, G., Albrechtsen, M., Ng, R.C., et al. “Engineering nanoscale hypersonic phonon transport.” Nature Nanotechnology, 17, 947 (2022)
Here we demonstrated the existence of a room temperature mechanical gap in a shamrock- based phononic crystal. This crystal possesses a 5.3 GHz wide band gap centered at 8.4 GHz. A waveguide was built by combining two phases of the crystal. Guided modes of both the crystal and the waveguide allowed for experimental mapping of the dispersion relation obtained from BLS experiments. In the GHz regime, only partial and narrow mechanical gaps have been shown, generally in less flexible fabrication methods such as self-assembly of colloidal crystals. Furthermore, the control and guiding of mechanical waves at these frequencies has been difficult to achieve and measure, typically relying on complex systems. Our method provided direct, experimental evidence of the existence of a mechanical guided mode at almost 10 GHz.
4) Ng, R.C., et al. “Excitation and detection of acoustic phonons in nanoscale systems.” Nanoscale, 14,13428-13451 (2022)
Here we reviewed the various experimental methods available for exciting and detecting acoustic phonons. In the acoustic regime, especially up to the GHz, phonon excitation, manipulation, and detection is quite challenging. However, the proper understanding and techniques to work with phonons in this regime are key to next generation phononic technological applications.
5) Ng, R.C.,* Nizet, P.,* et al. “Phononic comb modulation in a bimodal optomechanical laser.” (2022) arXiv:2210:16370 (Under Review - Nature Communications)
Here we discovered and explored new, previously unexplored dynamical regimes. We developed a platform in which two mechanical modes that are very far in frequency (MHz vs GHz) are able to indirectly couple to one another by coupling through an optical mode. Mechanical lasing of each of the mechanical modes leads to a formation of a GHz mechanical frequency comb. These results suggest the ability to control multiple mechanical degrees of freedom via a single optical mode.
6) Florez, O., Chavez-Angel, E., Sotomayor-Torres, C.M., and Ng, R.C. “A nanoscale optomechanical topological insulator.” (In Prep, Target: Physical Review Letters)
Here we presented a design which simultaneously exhibits both a topological mechanical and topological optical guided mode via the quantum spin Hall effect. In other words, the design is a topological optomechanical insulator. We demonstrate how both topological and ordinary insulators can be obtained in both the optics and mechanics by a simple tuning of the geometric parameters.
7) Esteso, V., Duquennoy, R., Hilke, M., Ng, R.C., et al. “Quantum thermometry with single molecules in portable nanoprobes.” (In Prep, Target: Physical Review X)
Here we demonstrated the use of organic light-emitting molecules as thermometers at cryogenic temperatures. Currently, the only available techniques to measure cryogenic temperatures are electronic and highly invasive. Conversely, all-optical techniques usually are limited down to about 50 K. This technique allows for local mapping of cryogenic temperatures, and could enable studies of new mechanical regimes at low temperatures.
Conferences and Workshops
1) Nanolito 2021 (invited; Salamanca, Spain). “3D fabrication via two-photon lithography direct laser writing” July 2021.
2) SPIE Optics+Photonics (oral; San Diego, California, USA). “A cavity optomechanical platform for GHz phonon amplification via Anderson-localized optical modes.” August 2022.