Complex optomechanics

After many years exploring the role of disorder and imperfection in systems used in quantum photonics, I turned my attention to the field of optomechanics. The nanostructures used in photonics are sufficiently small to become highly sensitive to thermal fluctuations and mechanical motion at room temperature. Thermal energy naturally activates vibrations around frequencies from hundreds of MHz to several GHz, creating a rich interplay between optical and mechanical degrees of freedom.

While phonons are often regarded as detrimental for quantum photonics because they induce decoherence and spectral broadening, my interest has focused on understanding and exploiting the interaction between telecom-wavelength photons and thermally activated mechanical motion in complex nanophotonic systems. In particular, I investigate how structural disorder, nonlinear dynamics, and dissipation shape the interaction between light and vibrations in optomechanical cavities and waveguides.

Fig. 1. Complex nonlinear optomechanics

Disorder as a resource in optomechanics

Any photonic or optomechanical nanostructure is affected by some degree of imperfection due to the unavoidable finite tolerance of the fabrication process. In this line of research, we investigate the role and limitations imposed by disorder for the photon-phonon interaction, while also exploring how disorder itself can become a physical resource to enhance optomechanical coupling in complex systems.

We have explored Anderson-localized cavity optomechanics in suspended silicon photonic-crystal waveguides with slotted line defects. Inherent fabrication imperfections induce sufficient multiple scattering to realize Anderson localization of optical modes. The introduction of an air slot strongly enhances electromagnetic confinement and increases the interaction between light and in-plane mechanical motion.

The resulting tightly confined Anderson-localized modes can be driven into regimes of mechanical amplification and self-sustained phonon lasing through optomechanical backaction. These systems exhibit remarkably rich nonlinear dynamics emerging from the interplay between radiation pressure, two-photon absorption, free-carrier dispersion, and thermo-optic effects operating across multiple timescales.

At high optical powers, silicon optomechanical cavities display strong nonlinear behavior dominated by two-photon absorption, free-carrier dynamics, and thermo-optic nonlinearities. The competition between free-carrier-induced blueshifts and thermo-optic redshifts can drive the cavity into self-pulsing regimes, where the intracavity field oscillates periodically even under continuous-wave excitation. These self-sustained oscillations correspond to stable nonlinear limit cycles involving optical, thermal, electronic, and mechanical degrees of freedom.

The nonlinear modulation of the intracavity field generates time-dependent radiation-pressure forces that coherently drive the mechanical resonator. This mechanism enables mechanical amplification and self-sustained phonon lasing even in the presence of disorder-induced scattering and losses. Beyond conventional dynamical backaction, these systems exhibit hybrid nonlinear dynamics where optical nonlinearities, thermal relaxation, free-carrier dynamics, and mechanical motion become strongly intertwined.

Fig. 2. Shamrock optomechanical crystals

We also design photonic and phononic band structures to realize strong confinement and manipulation of hypersonic phonons at GHz frequencies. In particular, we developed shamrock phononic crystals and waveguides supporting large hypersonic bandgaps and guided mechanical modes experimentally characterized through Brillouin light scattering spectroscopy. These systems provide a platform to explore phononic transport, localization, topology, and optomechanical interactions in complex bosonic systems.

Related publications

Light-motion interaction in disordered nanostructures
Guillermo Arregui Bravo (2021)

Brillouin scattering in bosonic systems
Omar Enrique Florez Peñaloza (2023)

Optomechanical computing and nonlinear dynamics

The nonlinear dynamics emerging in optomechanical cavities are not only relevant from the perspective of cavity optomechanics and phonon lasing, but also provide a platform for physical information processing. In these systems, radiation pressure, two-photon absorption, free-carrier dynamics, and thermo-optic effects interact across multiple timescales, producing self-pulsing instabilities, coherent phonon lasing, synchronization, and dynamical attractors.

More recently, we have explored how these nonlinear optomechanical dynamics can be exploited for reservoir computing. An optomechanical oscillator undergoes a Hopf bifurcation separating two regimes with very different information-processing capabilities: a stochastic Brownian-motion regime below threshold and a coherent self-sustained oscillation regime above threshold.

Above threshold, the system evolves toward a stable optomechanical limit cycle that simultaneously provides nonlinearity, fading memory, and reproducible dynamics. By weakly perturbing this attractor with an input signal, the cavity itself can operate as a physical reservoir computer without requiring external feedback loops.

Using a single chip-integrated optomechanical cavity with time multiplexing and virtual nodes, we demonstrated nonlinear function reconstruction, prediction of chaotic Mackey–Glass time series, and spoken-digit classification. In contrast to many photonic reservoir architectures, the nonlinearity, memory, and dynamical timescale all emerge from the same physical interaction inside the cavity itself.

The intrinsic processing speed of the reservoir is determined by the mechanical resonance frequency. In the present devices, this corresponds to frequencies near 0.4 GHz and nanosecond-scale dynamics, while related optomechanical and nanomechanical systems provide a route toward multi-GHz and potentially sub-terahertz physical computing platforms.

Fig. 3. Optomechanical reservoir computing through nonlinear dynamics

Related publications

Computing with the complex nonlinear dynamics of an optomechanical oscillator
Shulamit Edelstein, Marcos Menéndez, Bingrui Lu, Babak Vosoughi Lahijani, Cefe López, Miguel C. Soriano, Søren Stobbe, Pedro David García.

Invited talk

I summarize many of the results of this research line in this talk.