arXiv subjects
Publications and source records attributed to Thiago P. M. Alegre.
We present a method for resolving spatial mode overlaps in coupled microresonators based on Kerr and thermal cross-phase modulation. Through a pump-probe setup, we measure experimental overlap in a three-ring resonator with good agreement with analytical theory. Our technique can be generalized for describing nonlinear interactions in more complex multi- and coupled-mode systems.
We report on the experimental demonstration of cross-polarization backward stimulated Brillouin scattering (BSBS) in lithium niobate on insulator (LNOI) waveguides. Performing polarization-sensitive pump and probe measurements, we captured both intra- and intermodal scattering between counterpropagating fundamental optical modes. Remarkably, cross-polarization scattering achieved SBS gains that exceeded $G_{B}=$80m$^{-1}$W$^{-1}$. This substantial gain not only broadens the utility of polarization in SBS but also paves the way for high-performance devices, including ultranarrowband lasers, robust broadband nonreciprocal devices, RF filters, and microwave-to-optical converters.
Integrated optomechanical systems are one of the leading platforms for manipulating, sensing, and distributing quantum information. The temperature increase due to residual optical absorption sets the ultimate limit on performance for these applications. In this work, we demonstrate a two-dimensional optomechanical crystal geometry, named \textbf{b-dagger}, that alleviates this problem through increased thermal anchoring to the surrounding material. Our mechanical mode operates at 7.4 GHz, well within the operation range of standard cryogenic microwave hardware and piezoelectric transducers. The enhanced thermalization combined with the large optomechanical coupling rates, $g_0/2π\approx 880~\mathrm{kHz}$, and high optical quality factors, $Q_\text{opt} = 2.4 \times 10^5$, enables the ground-state cooling of the acoustic mode to phononic occupancies as low as $n_\text{m} = 0.35$ from an initial temperature of 3 kelvin, as well as entering the optomechanical strong-coupling regime. Finally, we perform pulsed sideband asymmetry of our devices at a temperature below 10 millikelvin and demonstrate ground-state operation ($n_\text{m} < 0.45$) for repetition rates as high as 3 MHz. Our results extend the boundaries of optomechanical system capabilities and establish a robust foundation for the next generation of microwave-to-optical transducers with entanglement rates overcoming the decoherence rates of state-of-the-art superconducting qubits.
We numerically demonstrate that Lithium Niobate on Insulator (LNOI) waveguides may support confined short-wavelength surface acoustic waves that interact strongly with optical fields through backward stimulated Brillouin scattering in both $Z$ and $X$-cut orientation. We conduct fully anisotropic simulations that consider not only moving boundary and photoelastic forces, but also roto-optic forces for the Brillouin interaction. Our results indicate that photoelasticity dominates the Brillouin gain and can reach as high as $G_{B}/Q_{m}$ = 0.43 W$^{-1}$m$^{-1}$ in standard ridge waveguides
Experimental exploration of synchronization in scalable oscillator micro systems has unfolded a deeper understanding of networks, collective phenomena, and signal processing. Cavity optomechanical devices have played an important role in this scenario, with the perspective of bridging optical and radio frequencies through nonlinear classical and quantum synchronization concepts. In its simplest form, synchronization occurs when an oscillator is entrained by a signal with frequency nearby the oscillator's tone, and becomes increasingly challenging as their frequency detuning increases. Here, we experimentally demonstrate entrainment of a silicon-nitride optomechanical oscillator driven up to the fourth harmonic of its 32 MHz fundamental frequency. Exploring this effect, we also experimentally demonstrate for the first time a purely optomechanical RF frequency divider, where we performed frequency division up to a 4:1 ratio, i.e., from 128 MHz to 32 MHz. Further developments could harness these effects towards frequency synthesizers, phase-sensitive amplification and nonlinear sensing.
Colors observed in nature are very important to form our perception of an object as well as its design. The desire to reproduce vivid colors such as those found in birds, fishes, flowers and insects has driven extensive research into nanostructured surfaces. Structural colors based on surface plasmon resonance (SPR) have played an important role in this field due to its high spatial resolution. Creating vivid color reflecting surfaces is still a major challenge and could revolutionize low power consumption image displays. Here we combine diffraction and plasmonic effects to design bright and vivid-color reflecting surfaces. The periodic reflection patterns are designed through genetic algorithm optimization and are defined in aluminum coated silicon chips.
Elastic dissipation through radiation towards the substrate is a major loss channel in micro- and nanomechanical resonators. Engineering the coupling of these resonators with optical cavities further complicates and constrains the design of low-loss optomechanical devices. In this work we rely on the coherent cancellation of mechanical radiation to demonstrate material and surface absorption limited silicon near-field optomechanical resonators oscillating at tens of MHz. The effectiveness of our dissipation suppression scheme is investigated at room and cryogenic temperatures. While at room temperature we can reach a maximum quality factor of 7.61k ($fQ$-product of the order of $10^{11}$~Hz), at 22~K the quality factor increases to 37k, resulting in a $fQ$-product of $2\times10^{12}$~Hz.
The simultaneous control of optical and mechanical waves has enabled a range of fundamental and technological breakthroughs, from the demonstration of ultra-stable frequency reference devices to the exploration of the quantum-classical boundaries in laser-cooling experiments. More recently, such an opto-mechanical interaction has been observed in integrated nano-waveguides and microcavities in the Brillouin regime, where short-wavelength mechanical modes scatters light at several GHz. Here we engineer coupled optical microcavities spectra to enable a low threshold excitation of mechanical travelling-wave modes through backward stimulated Brillouin scattering. Exploring the backward scattering we propose microcavity designs supporting super high frequency modes ($\sim25$ GHz) an large optomechanical coupling rates ($g_0/2π\sim 50$ kHz).