Statistical theory of a quantum emitter strongly coupled to Anderson-localized modes

Cavity Optomechanics with Anderson-Localized Optical Modes

Confining photons in cavities enhances the interaction between light and matter. In cavity optomechanics, this enables a wealth of phenomena ranging from optomechanically induced transparency to macroscopic objects cooled to their motional ground state. Previous work in cavity optomechanics employed devices where ubiquitous structural disorder played no role beyond perturbing resonance frequencies and quality factors. More generally, the interplay between disorder, which must be described by statistical physics, and optomechanical effects has thus far been unexplored. Here, we demonstrate how sidewall roughness in air-slot photonic-crystal waveguides can induce sufficiently strong backscattering of slot-guided light to create Anderson-localized modes with quality factors as high as half a million and mode volumes estimated to be below the diffraction limit. We observe how the interaction between these disorder-induced optical modes and in-plane mechanical modes of the slotted membrane is governed by a distribution of coupling rates, which can exceed g_{o}/2π∼200  kHz, leading to mechanical amplification up to self sustained oscillations via optomechanical backaction. Our Letter constitutes the first steps towards understanding optomechanics in the multiple-scattering regime and opens new perspectives for exploring complex systems with a multitude of mutually coupled degrees of freedom.

Single Virus Detection on Silicon Photonic Crystal Random Cavities

On-chip silicon microcavity sensors are advantageous for the detection of virus and biomolecules due to their compactness and the enhanced light-matter interaction with the analyte. While their theoretical sensitivity is at the single-molecule level, the fabrication of high quality (Q) factor silicon cavities and their integration with optical couplers remain as major hurdles in applications such as single virus detection. Here, label-free single virus detection using silicon photonic crystal random cavities is proposed and demonstrated. The sensor chips consist of free-standing silicon photonic crystal waveguides and do not require pre-fabricated defect cavities or optical couplers. Residual fabrication disorder results in Anderson-localized cavity modes which are excited by a free space beam. The Q ≈10⁵ is sufficient for observing discrete step-changes in resonance wavelength for the binding of single adenoviruses (≈50 nm radius). The authors' findings point to future applications of CMOS-compatible silicon sensor chips supporting Anderson-localized modes that have detection capabilities at the level of single nanoparticles and molecules.

Enhancement of the spontaneous emission rate of Rhodamine 6G molecules coupled into transverse Anderson localized modes in a wedge-type optical waveguide

In this paper, the dynamics of the spontaneous emission rate of Rhodamine 6G dye molecules, coupled into disorder-induced optical cavities in a scattering medium, is investigated by a time-resolved spectroscopic technique. The system is a wedge-type wave-guiding system formed by a polymer with randomly positioned air inclusions. The scattering of light in the medium induces transverse Anderson localization, which gives rise to quasi-optical modes or Anderson-localized cavities. The presence of these modes strongly enhances the decay emission of the emitters. The waveguide is fabricated by a conventional fiber drawing technique inside a fused silica micro-rod. Localized optical modes are observed to appear in the form of sharp spectral resonance peaks at various frequencies throughout the photoluminescence spectrum of the dye molecules. The spontaneous emission rate of the molecules on resonance with the localized modes is measured to enhance by a factor of up to 6.8, which elucidates that the transverse Anderson localization enables an efficient way to alter the spontaneous emission rate of quantum emitters in an optically asymmetric simple wedge-type photonic waveguide, offering a moderate alternative to highly engineered sophisticated light-wave devices.

Measurement of the profiles of disorder-induced localized resonances in photonic crystal waveguides by local tuning

Near the band edge of photonic crystal waveguides, localized modes appear due to disorder. We demonstrate a new method to elucidate spatial profile of the localized modes in such systems using precise local tuning. Using deconvolution with the known thermal profile, the spatial profile of a localized mode with quality factor (Q) > 10⁵ is successfully reconstructed with a resolution of 2.5 μm.

Lower bound for the spatial extent of localized modes in photonic-crystal waveguides with small random imperfections

Light localization due to random imperfections in periodic media is paramount in photonics research. The group index is known to be a key parameter for localization near photonic band edges, since small group velocities reinforce light interaction with imperfections. Here, we show that the size of the smallest localized mode that is formed at the band edge of a one-dimensional periodic medium is driven instead by the effective photon mass, i.e. the flatness of the dispersion curve. Our theoretical prediction is supported by numerical simulations, which reveal that photonic-crystal waveguides can exhibit surprisingly small localized modes, much smaller than those observed in Bragg stacks thanks to their larger effective photon mass. This possibility is demonstrated experimentally with a photonic-crystal waveguide fabricated without any intentional disorder, for which near-field measurements allow us to distinctly observe a wavelength-scale localized mode despite the smallness (~1/1000 of a wavelength) of the fabrication imperfections.

High-Q coupled resonances on a PhC waveguide using a tapered nanofiber with high coupling efficiency

We experimentally demonstrate high-Q cavity formation at an arbitrary position on a silicon photonic crystal waveguide by bringing a tapered nanofiber into contact with the surface of the slab. An ultrahigh Q of 5.1 × 10(5) is obtained with a coupling efficiency of 39%, whose resonant wavelength can be finely tuned by 27 pm by adjusting the contact length of the nanofiber. We also demonstrate an extremely high coupling efficiency of 99.6% with a loaded Q of 6.1 × 10(3). We show that we can obtain a coupled resonances, which has the potential to be used for slow light generation.

Statistical measurements of quantum emitters coupled to Anderson-localized modes in disordered photonic-crystal waveguides

We present a statistical study of the Purcell enhancement of the light emission from quantum dots coupled to Anderson-localized cavities formed in disordered photonic-crystal waveguides. We measure the time-resolved light emission from both single quantum emitters coupled to Anderson-localized cavities and directly from the cavities that are fed by multiple quantum dots. Strongly inhibited and enhanced decay rates are observed relative to the rate of spontaneous emission in a homogeneous medium. From a statistical analysis, we report an average Purcell factor of 4.5 ± 0.4 without applying any spectral tuning. By spectrally tuning individual quantum dots into resonance with Anderson-localized modes, a maximum Purcell factor of 23.8 ± 1.5 is recorded, which is at the onset of the strong-coupling regime. Our data quantify the potential of Anderson-localized cavities for controlling and enhancing the light-matter interaction strength in a photonic-crystal waveguide, which is of relevance for cavity quantum-electrodynamics experiments, efficient energy harvesting and random lasing.

Light transport and localization in two-dimensional correlated disorder

Structural correlations in disordered media are known to affect significantly the propagation of waves. In this Letter, we theoretically investigate the transport and localization of light in 2D photonic structures with short-range correlated disorder. The problem is tackled semianalytically using the Baus-Colot model for the structure factor of correlated media and a modified independent scattering approximation. We find that short-range correlations make it possible to easily tune the transport mean free path by more than a factor of 2 and the related localization length over several orders of magnitude. This trend is confirmed by numerical finite-difference time-domain calculations. This study therefore shows that disorder engineering can offer fine control over light transport and localization in planar geometries, which may open new opportunities in both fundamental and applied photonics research.

Random nanolasing in the Anderson localized regime

The development of nanoscale optical devices for classical and quantum photonics is affected by unavoidable fabrication imperfections that often impose performance limitations. However, disorder may also enable new functionalities, for example in random lasers, where lasing relies on random multiple scattering. The applicability of random lasers has been limited due to multidirectional emission, lack of tunability, and strong mode competition with chaotic fluctuations due to a weak mode confinement. The regime of Anderson localization of light has been proposed for obtaining stable multimode random lasing, and initial work concerned macroscopic one-dimensional layered media. Here, we demonstrate on-chip random nanolasers where the cavity feedback is provided by the intrinsic disorder. The strong confinement achieved by Anderson localization reduces the spatial overlap between lasing modes, thus preventing mode competition and improving stability. This enables highly efficient, stable and broadband wavelength-controlled lasers with very small mode volumes. Furthermore, the complex interplay between gain, dispersion-controlled slow light, and disorder is demonstrated experimentally for a non-conservative random medium. The statistical analysis shows a way towards optimizing random-lasing performance by reducing the localization length, a universal parameter.

Strongly coupled slow-light polaritons in one-dimensional disordered localized states

Cavity quantum electrodynamics advances the coherent control of a single quantum emitter with a quantized radiation field mode, typically piecewise engineered for the highest finesse and confinement in the cavity field. This enables the possibility of strong coupling for chip-scale quantum processing, but till now is limited to few research groups that can achieve the precision and deterministic requirements for these polariton states. Here we observe for the first time coherent polariton states of strong coupled single quantum dot excitons in inherently disordered one-dimensional localized modes in slow-light photonic crystals. Large vacuum Rabi splittings up to 311 μeV are observed, one of the largest avoided crossings in the solid-state. Our tight-binding models with quantum impurities detail these strong localized polaritons, spanning different disorder strengths, complementary to model-extracted pure dephasing and incoherent pumping rates. Such disorder-induced slow-light polaritons provide a platform towards coherent control, collective interactions, and quantum information processing.

Strong coupling to two-dimensional Anderson localized modes

We use a scattering formalism to derive a condition of strong coupling between a resonant scatterer and an Anderson localized mode for electromagnetic waves in two dimensions. The strong coupling regime is demonstrated based on exact numerical simulations, in perfect agreement with theory. The strong coupling threshold can be expressed in terms of the Thouless conductance and the Purcell factor. This connects key concepts in transport theory and cavity quantum electrodynamics, and provides a practical tool for the design or analysis of experiments.

Spectrum broadening of high-efficiency second harmonic generation in cascaded photonic crystal microcavities

An effective approach is proposed to broaden the spectrum of high-efficiency second harmonic generation in a one-dimensional photonic crystal based on the cascaded structure. By controlling the thickness of the joint layer, it is possible to realize a flat-top or quasiflat-top impurity band centered at the fundamental wavelength due to mode splitting effect in coupled cavities. Simulation results reveal that the spectrum of generated second harmonic exhibits a hump-like or multi-peak profile with wavelength tuning. It is a salient feature that the spectral stability of efficiency enhancement could be greatly improved compared to the conventional Lorentzian profile while maintaining an ultrahigh Q factor. Such merit of spectra re-shaping can significantly relax the stringent requirements for ultrahigh-Q microcavities in practical applications, e.g. fabrication inaccuracy, thermal variation, and wavelength detuning.

Nonuniversal intensity correlations in a two-dimensional Anderson-localizing random medium

Complex dielectric media often appear opaque because light traveling through them is scattered multiple times. Although the light scattering is a random process, different paths through the medium can be correlated encoding information about the medium. Here, we present spectroscopic measurements of nonuniversal intensity correlations that emerge when embedding quantum emitters inside a disordered photonic crystal that is found to Anderson-localize light. The emitters probe in situ the microscopic details of the medium, and imprint such near-field properties onto the far-field correlations. Our findings provide new ways of enhancing light-matter interaction for quantum electrodynamics and energy harvesting, and may find applications in subwavelength diffuse-wave spectroscopy for biophotonics.