All-optical radiofrequency modulation of Anderson-localized modes

All-optical modulation of light relies on exploiting intrinsic material nonlinearities. However, this optical control is rather challenging due to the weak dependence of the refractive index and absorption coefficients on the concentration of free carriers in standard semiconductors. To overcome this limitation, resonant structures with high spatial and spectral confinement are carefully designed to enhance the stored electromagnetic energy, thereby requiring lower excitation power to achieve significant nonlinear effects. Small mode-volume and high quality (Q)-factor cavities also offer an efficient coherent control of the light field and the targeted optical properties. Here, we report on optical resonances reaching Q - 10^5 induced by disorder on novel photonic/phononic crystal waveguides. At relatively low excitation powers (below 1 mW), these cavities exhibit nonlinear effects leading to periodic (up to - 35 MHz) oscillations of their resonant wavelength. Our system represents a test-bed to study the interplay between structural complexity and material nonlinearities and their impact on localization phenomena and introduces a novel functionality to the toolset of disordered photonics

Thermal noise and optomechanical features in the emission of a membrane-coupled compound cavity laser diode

We demonstrate the use of a compound optical cavity as linear displacement detector, by measuring the thermal motion of a silicon nitride suspended membrane acting as the external mirror of a near-infrared Littrow laser diode. Fluctuations in the laser optical power induced by the membrane vibrations are collected by a photodiode integrated within the laser, and then measured with a spectrum analyzer. The dynamics of the membrane driven by a piezoelectric actuator is investigated as a function of air pressure and actuator displacement in a homodyne configuration. The high Q-factor ($\sim 3.4\cdot 10^4$ at $8.3 \cdot 10^{-3}$ mbar) of the fundamental mechanical mode at $\sim 73$ kHz guarantees a detection sensitivity high enough for direct measurement of thermal motion at room temperature ($\sim 87$ pm RMS). The compound cavity system here introduced can be employed as a table-top, cost-effective linear displacement detector for cavity optomechanics. Furthermore, thanks to the strong optical nonlinearities of the laser compound cavity, these systems open new perspectives in the study of non-Markovian quantum properties at the mesoscale

Probing the Spontaneous Emission Dynamics in Si-Nanocrystals-Based Microdisk Resonators

As a possible cavity quantum electrodynamical system, unlike III-V quantum dots, Si-NCs are not considered ideal emitters for emission rate enhancement observations (Purcell effect). Here, we report on direct measurements of spontaneous emission rate enhancement of Si-NCs embedded in a whispering-gallery mode resonator at room temperature. Using time-resolved microphotoluminescence experiments, we demonstrate important lifetime reductions ( ∼ 70 % ) for Si-NCs coupled to cavity modes with respect to uncoupled ones. Comparing experiments with the theoretical Purcell enhancement in a bad emitter regime, we estimate effective linewidths of ∼ 10     meV through which Si-NC emitters are coupled to cavity photons. Finally, our study provides an alternative method for the estimation of subnatural linewidths of quantum dots at room temperature

Microwave oscillator and frequency comb in a silicon optomechanical cavity with a full phononic bandgap

Cavity optomechanics has recently emerged as a new paradigm enabling the manipulation of mechanical motion via optical fields tightly confined in deformable cavities. When driving an optomechanical (OM) crystal cavity with a laser blue-detuned with respect to the optical resonance, the mechanical motion is amplified, ultimately resulting in phonon lasing at MHz and even GHz frequencies. In this work, we show that a silicon OM crystal cavity performs as an OM microwave oscillator when pumped above the threshold for self-sustained OM oscillations. To this end, we use an OM cavity designed to have a breathing-like mechanical mode at 3.897 GHz in a full phononic bandgap. Our measurements show that the first harmonic of the detected signal displays a phase noise of ≈−100 dBc/Hz at 100 kHz. Stronger blue-detuned driving leads eventually to the formation of an OM frequency comb, whose lines are spaced by the mechanical frequency. We also measure the phase noise for higher-order harmonics and show that, unlike in Brillouin oscillators, the noise is increased as corresponding to classical harmonic mixing. Finally, we present real-time measurements of the comb waveform and show that it can be fitted to a theoretical model recently presented. Our results suggest that silicon OM cavities could be relevant processing elements in microwave photonics and optical RF processing, in particular in disciplines requiring low weight, compactness and fiber interconnection

Vertical coupling of laser glass microspheres to buried silicon nitride ellipses and waveguides

We demonstrate the integration of Nd3+ doped Barium-Titanium-Silicate microsphere lasers with a Silicon Nitride photonic platform. Devices with two different geometrical configurations for extracting the laser light to buried waveguides have been fabricated and characterized. The first configuration relies on a standard coupling scheme, where the microspheres are placed over strip waveguides. The second is based on a buried elliptical geometry whose working principle is that of an elliptical mirror. In the latter case, the input of a strip waveguide is placed on one focus of the ellipse, while a lasing microsphere is placed on top of the other focus. The fabricated elliptical geometry (ellipticity=0.9) presents a light collecting capacity that is 50% greater than that of the standard waveguide coupling configuration and could be further improved by increasing the ellipticity. Moreover, since the dimensions of the spheres are much smaller than those of the ellipses, surface planarization is not required. On the contrary, we show that the absence of a planarization step strongly damages the microsphere lasing performance in the standard configuration.Comment: 10 pages, 4 figure

Whispering-gallery modes and light emission from a Si-nanocrystal-based single microdisk resonator

We report on visible light emission from Si-nanocrystal based optically active microdisk resonators. The room temperature photoluminescence (PL) from single microdisks shows the characteristic modal structure of whispering-gallery modes. The emission is both TE and TM-polarized in 300 nm thick microdisks, while thinner ones (135 nm) support only TE-like modes. Thinner disks have the advantage to filter out higher order radial mode families, allowing for measuring only the most intense first order modal structure. We reveal subnanometer linewidths and corresponding quality factors as high as 2800, limited by the spectral resolution of the experimental setup. Moreover,we observe a modification of mode linewidth by a factor 13 as a function of pump power. The origin of this effect is attributed to an excited carrier absorption loss mechanism.Comment: 5 pages, 5 figure

Broadband Dynamic Polarization Conversion in Optomechanical Metasurfaces

Artificial photonic materials, nanofabricated through wavelength-scale engineering, have shown astounding and promising results in harnessing, tuning, and shaping photonic beams. Metamaterials have proven to be often outperforming the natural materials they take inspiration from. In particular, metallic chiral metasurfaces have demonstrated large circular and linear dichroism of light which can be used, for example, for probing different enantiomers of biological molecules. Moreover, the precise control, through designs on demand, of the output polarization state of light impinging on a metasurface, makes this kind of structures particularly relevant for polarization-based telecommunication protocols. The reduced scale of the metasurfaces makes them also appealing for integration with nanomechanical elements, adding new dynamical features to their otherwise static or quasi-static polarization properties. To this end we designed, fabricated and characterized an all-dielectric metasurface on a suspended nanomembrane. Actuating the membrane mechanical motion, we show how the metasurface reflectance response can be modified, according to the spectral region of operation, with a corresponding intensity modulation or polarization conversion. The broad mechanical resonance at atmospheric pressure, centered at about 400 kHz, makes the metasurfaces structure suitable for high-frequency operation, mainly limited by the piezo-actuator controlling the mechanical displacement, which in our experiment reached modulation frequencies exceeding 1.3 MHz

Mechanical oscillations in lasing microspheres

We investigate the feasibility of activating coherent mechanical oscillations in lasing microspheres by modulating the laser emission at a mechanical eigenfrequency. To this aim, 1.5%Nd3+:Barium-Titanium-Silicate microspheres with diameters around 50 μm were used as high quality factor (Q>106) whispering gallery mode lasing cavities. We have implemented a pump-and-probe technique in which the pump laser used to excite the Nd3+ ions is focused on a single microsphere with a microscope objective and a probe laser excites a specific optical mode with the evanescent field of a tapered fibre. The studied microspheres show monomode and multi-mode lasing action, which can be modulated in the best case up to 10 MHz. We have optically transduced thermally-activated mechanical eigenmodes appearing in the 50-70 MHz range, the frequency of which decreases with increasing the size of the microspheres. In a pump-and-probe configuration we observed modulation of the probe signal up to the maximum pump modulation frequency of our experimental setup, i.e., 20 MHz. This modulation decreases with frequency and is unrelated to lasing emission, pump scattering or thermal effects. We associate this effect to free-carrier-dispersion induced by multiphoton pump light absorption. On the other hand, we conclude that, in our current experimental conditions, it was not possible to resonantly excite the mechanical modes. Finally, we discuss on how to overcome these limitations by increasing the modulation frequency of the lasing emission and decreasing the frequency of the mechanical eigenmodes displaying a strong degree of optomechanical coupling