We demonstrate a tuneable nanocube-on-mirror Fabry-Perot cavity that combines the record breaking plasmonic confinement of the nanocube-on-mirror (NCoM) system with the high quality factors and tuneability of microcavity systems. We demonstrate selective addressing of individual molecular vibrational lines with robust SERS enhancements on par with those of the seminal NCoM system, reaching sideband resolved SERS at Q/V values above 1 million inverse cubic wavelengths. We envision this as a platform for sideband-resolved molecular optomechanics, polariton chemistry and vibrational strong coupling.
We create multi-mode nano-optomechanical networks in which the interactions between mechanical modes are induced and fully reconfigured through time-modulated radiation pressure forces. We study the nonreciprocal and topological states that emerge from controlled breaking of time-reversal symmetry and Hermiticity in such laser-driven optomechanical metamaterials. We demonstrate unidirectional flow of sound and the emergence of the quantum Hall effect in small networks of nanomechanical resonators. We uncover that broken time-reversal symmetry can influence the thermodynamic efficiency of optomechanical refrigeration. Moreover, we realize the bosonic Kitaev chain; the bosonic counterpart of the fermionic model that famously predicts Majorana zero modes. This establishes a non-Hermitian topological phase in which a unique form of directional amplification emerges as a physical phenomenon that links to the chain’s topological nature. This behavior has intriguing implications for signal processing and enhanced sensing performance.
We study the use of deep neural networks towards the prediction of the optical properties of two-dimensional photonic crystals, as well as their inverse design. We incorporate a rigorous tight-binding model as a known operator in the machine learning algorithm. This physics-informed approach allows the prediction of meaningful model parameters rather than the high-dimensional full response, allowing for an efficient method as well as potential insight in the physical workings of specific designs. We demonstrate a four-order-of-magnitude speedup of prediction of bandstructures and field symmetries over full-field calculations, and proof-of-concept inverse design of photonic crystals with large gaps, flat bands, and Dirac-point degeneracies.
Recent years have seen a strong interest in the possibility to enhance classical and quantum sensing in suitably engineered non-Hermitian multimode systems, displaying e.g. parity-time symmetry or topological phases. The bosonic Kitaev-Majorana chain is a proposed non-Hermitian topological model that is predicted to feature enhanced responsivity to small perturbations, linked to unidirectional amplification. We report an experimental realization of the bosonic Kitaev chain in a nano-optomechanical network, in which two-mode squeezing and beamsplitter interactions between nanomechanical modes are generated through temporally modulated radiation pressure control fields. We demonstrate that this system displays a dramatic sensitivity to boundary conditions, and a unique exponential scaling of the responsivity to a small perturbation with the number of resonators.
The question whether the efficiency of thermodynamic devices such as heat engines and refrigeration is altered through the breaking of time-reversal symmetry has been a topic of significant debate. We experimentally investigate the cooling of nanomechanical resonators in multimode optomechanical devices, in which time-reversal symmetry for thermal fluctuations can be broken through suitable temporal modulation of the radiation pressure control field. We study the resultant nonreciprocal transport of thermal vibrations, and show that the controlled breaking of time-reversal symmetry through synthetic magnetic fields can enhance the cooling performance, yielding lower phonon occupancies than the conventional limit.
We demonstrate a novel paradigm for fiber sensing, based on the combination of an electrical sensor and a fiber-tip electro-optic metasurface. We transfer a semiconductor photodiode on the tip of an optical fiber and measure its current-voltage characteristics using an electro-optic photonic crystal. Any change in the electrical characteristics due to the environment can then be sensed via the fiber, greatly extending the application scope of fiber sensors. As a proof of principle, we demonstrate cryogenic temperature sensing.
The insensitivity of photons to magnetic fields calls for symmetry-based approaches in the design of photonic topological insulators. We experimentally demonstrate the realization of pseudomagnetic fields in deformed photonic crystals, and investigate the resulting photonic Landau-levels and guided topological states. Akin to strained graphene sheets, deformations (i.e., designed strain) in photonic crystals generate synthetic gauge fields for photons, leading to localization at flat bands and new types of topologically protected edge states at the boundaries of strained photonic crystals. We reveal that tailoring these gauge fields via strain-engineering yields new control over light dispersion, localization, and allows broadband minimization of radiation losses.
Nano- and micromechanical oscillators act as great sensors of a wide variety of signals, but their sensitivity and bandwidth can be limited by quantum backaction imposed by optomechanical displacement measurement. We experimentally demonstrate a new paradigm for optomechanical measurement and control based on strong interactions with short light pulses. Using unique nanophotonic optomechanical cavities, we show that single pulsed measurements can achieve sub-quantum-limit resolution. Moreover, we demonstrate a new protocol to deterministically produce squeezed mechanical states, which can reduce single-quadrature fluctuations to arbitrarily small magnitudes. We discuss the application of the resulting squeezing and entanglement for mechanical quantum sensing.
In topological photonic crystals, suitable crystal symmetries bring the concepts of topological insulators to nanoscale photonic components. We experimentally probe topological states in nanophotonic cavities and waveguides, and investigate to what extent light confinement, transport, and light-matter interactions can benefit from robustness offered by topological design paradigms. We show how the signatures of topological band structures are imprinted on the complex near and far fields of photonic crystal states, and how these can be used to control near-to-far field coupling. Using near-field investigation, we quantify the scattering of topological edge states at sharp corners and controlled disorder. Finally, we show that spin-orbit coupling in the near field of topological photonic crystals can become disconnected from the edge state pseudospin, important to the design of chiral light-matter interfaces.
We investigate the possibility of probing the electrical status of a p-i-n junction via an optical fiber, without the need for electrical contacts. A photonic crystal with a resonance in the near-infrared is etched in a thin membrane of III-V semiconductor with an embedded p-i-n junction and placed on the cleaved facet of a fiber. We measure the effect of photoexcited carriers on the built-in voltage of the diode through the Pockels effect. This may enable the all-optical read-out of electrical signals from sensors in a distant or inaccessible environment.
Nano-optomechanical structures enable displacement and force sensing with ultimate precision, but their practical application is hindered by the complexity of the optical coupling. Here, we demonstrate that nano-optomechanical sensing can be realized on a fiber-tip without the use of any coupling optics. We use a photonic crystal optomechanical structure fabricated using standard wafer-scale processing. Afterwards the sensors are transferred on a fiber-tip using a simple pickup method. We demonstrate thermomechanical noise measurements with a high signal to noise ratio, using simple reflectance read-out via the fiber.
We study the emergence of nonreciprocal and topologically nontrivial phonon transport in nano-optomechanical networks. We develop on-chip nanophotonic systems in which multiple mechanical modes are strongly coupled through radiation pressure. Through temporal modulation and retardation, suitable laser control fields can break time-reversal symmetry and introduce controlled gain and loss at will on any of the network link and nodes. We reveal the emergence of nanomechanical circulation, helical quantum Hall states, and chiral thermal transport. Exploiting optomechanical gain, we study the combination of broken time-reversal symmetry and non-Hermiticity. This leads to rich phenomenology including magnetic-field tuning of exceptional points and unidirectional phononic amplification. It promises to serve as building blocks for new bosonic topological phases in the domain of nanomechanics, which is rich in applications in sensing and signal processing.
Topologically tailored photonic crystals offer robust transport of optical states in quantum and classical systems. However, quantifying the robustness of edge states in topologically protected PhCs has remained elusive. In our recent work, we report a rigorous quantitative evaluation of topological photonic edge eigenstates, emulating the quantum valley Hall effect (VPC), and analyze their transport properties in the telecom wavelength range using a phase-resolved near-field optical microscope. Our results demonstrate that the backscattering energy ratio for the VPC is two orders of magnitude smaller compared to that in a conventional W1 waveguide. Such an evaluation opens a pathway for creating quantum photonic networks that can achieve secure and robust communications.
Bringing topological physics from condensed matter to the optical domain offers unprecedented prospects in the control of light. Recently, the photonic analogue of the quantum spin Hall effect (QSHE) was proposed in 2D photonic crystal (PhC) structures featuring an interface between two topological distinct domains. Photonic spin-orbit coupling, mediated by the specific lattice symmetries, results in the emergence of helical edge states, guided along the interface in a protected manner. We fabricate and study topological PhC cavities emulating the QSHE that are coupled to the radiation continuum and perform imaging and Fourier spectroscopy in the far field to characterize their properties. We examine the robustness of cavity spectra and intrinsic loss against varying cavity size and shape, and demonstrate pseudo-spin conserved coupling between topological waveguides and cavities. The reliance on only passive media render such components promising building blocks for on-chip devices.
Obtaining nanoscale spatial information without real-space imaging, but from diffraction patterns, is already a valuable tool in metrology We investigate the potential of metasurfaces for nano-optical sensing and metrology with subwavelength resolution. We aim to exploit complex light scattering from a metasurface, programmable illumination, and retrieval of spatial information of a sample from far-field scattering images. We demonstrate an inversion technique based on singular value decomposition whereby we can retrieve the spatial position of a pointlike light source in a plasmon antenna with lambda/50 resolution just on basis of measured far field radiation patterns. Also, we argue that wavefront-shaping applied to plasmon oligomers enables selective generation of sub-diffractive field patterns that could form an optimal and complete basis for spatially-resolved sensing at the nanoscale. Our endeavours use stochastic optimization to choose wavefronts and plasmonic structures.
Topological states of light can be induced in nanophotonic systems by encoding spin or valley degrees of freedom in the electromagnetic vector field. We study topological light propagation and storage in waveguides and cavities in two-dimensional photonic crystals at telecom wavelengths, directly imaging their propagation and band structure in experiment. Through phase- and polarization-resolved measurement of the states' electromagnetic fields, we reveal their origin in photonic spin-orbit coupling. Our quantitative measurement techniques allow us to test the level of topological protection in these systems, which rely on spatial symmetries to achieve topological robustnes. We study topological protection of backreflection at sharp corners and defects and discuss the merits of these principles in realistic nanophotonic devices.
Leveraging topological protection in the photonic domain could lead to new ways to transport information on-chip, potentially increasing its robustness to scattering at disorder. We realize a photonic analogue of topological insulators based on the quantum spin Hall effect in symmetry-broken photonic crystals. We directly observe the propagation of topological edge states at telecom wavelengths in a silicon-on-insulator platform. Analyzing their properties through their far-field radiation allows characterizing their inherent spin, dispersion, and propagation. We reveal that the radiation of the topological states carries a signature of their origin in photonic spin-orbit coupling, linking the unidirectional propagation of two states with opposite pseudospin to circular far-field polarization. Polarimetric Fourier spectroscopy allows mapping the edge state dispersion and characterize their quality factors. The positive and negative group velocity modes can be selectively excited with circular polarization of opposite handedness. Moreover, we detect a small gap at the edge state crossing that is related to spin-spin scattering, inherent to the symmetry breaking at the edge, and a defining difference between photonic and electronic topological insulators. We image edge state propagation in real-space microscopy, and show how they can be routed at sharp waveguide junctions, attesting to their topologically protected nature. Thus, we observe the unique nature of topologically protected light transport in photonic crystals, through a technique that holds great promise for developing novel topological systems for various applications, including integrated photonic components, quantum optical interfaces, and nanoscale lasing.
Strong interaction between light and a single quantum emitter is pivotal to many applications, including single photon sources and quantum information processing. Typically, plasmonic antennas or optical cavities are used to boost this interaction. The former can focus light in a deeply subwavelength region, whereas the latter can store light for up to billions of oscillations.
In our work, we combine these two opposite elements into a single coupled system. First, we show theoretically [1] that hybrid cavity-antenna systems can achieve Purcell enhancements far exceeding those of the bare cavity and antenna, and can do so at any desired bandwidth. This requires a delicate balance between spoiling the cavity with the antenna on the one hand, and cooperative and interference effects on the other.
We then present our experimental results on hybrid systems using a whispering-gallery mode cavity and an aluminum plasmonic antenna. Using taper-coupled excitation of the hybrid mode, we study quality factors and radiation patterns, demonstrating that we can control the antenna-cavity coupling strength by varying their respective frequency detuning. We show that we can achieve modes that retain quality factors around 10^4, while creating a strongly localized field around the antenna. As such, we can exploit the benefits of plasmonic confinement without suffering from the usual losses. Finally, we present first studies of fluorescent emitters coupled to the hybrid modes.
[1] Doeleman, H. M., Verhagen, E., & Koenderink, A. F., "Antenna–Cavity Hybrids: Matching Polar Opposites for Purcell Enhancements at Any Linewidth." ACS Photonics 3.10 (2016): 1943-1951.
A prototypical experiment in cavity quantum electrodynamics involves controlling the light-matter interaction by tuning the frequency of a cavity mode in- and out-of resonance with the frequency of a quantum emitter,1-3 while the field amplitude is generally unaltered. The opposite situation, where one perturbs the spatial pattern of a cavity mode without changing its frequency, has been considered only recently in a few works.4, 5 Changing the amplitude of the field at the emitter's position has important applications, at it allows a real-time control of the light-matter coupling rate, and therefore a direct control of processes such as spontaneous emission and Rabi oscillations. In view of this large potential, in this paper we discuss general design principles that allow obtaining large variations of the electromagnetic field, without change of the frequency, upon an external perturbation of the cavity. We showcase the application of these rules to two photonic structures, a single Fabry-Perot cavity and a coupled three-cavity system. As showed by our analysis and by the examples provided, a small frequency spacing between the modes of the unperturbed cavity is an important requirement to obtain large field variations upon small perturbations. In this regard, a coupled-cavity system, where the frequency spacing is controlled by the interaction rates between the single cavities, constitutes the most promising system to achieve large modulations of the field amplitude.
Hybrid nanophotonic structures are structures that integrate different nanoscale platforms to harness light-matter interaction. We propose that combinations of plasmonic antennas inside modest-Q dielectric cavities can lead to very high Purcell factors, yielding plasmonic mode volumes at essentially cavity quality factors. The underlying physics is subtle: for instance, how plasmon antennas with large cross sections spoil or improve cavities and vice
versa, contains physics beyond perturbation theory, depending on interplays of back-action, and interferences. This is evident from the fact that the local density of states of hybrid systems shows the rich physics of Fano interferences. I will discuss recent scattering experiments performed on toroidal microcavities coupled to plasmon particle arrays that probe both cavity resonance shifts and particle polarizability changes illustrating these insights. Furthermore I will present our efforts to probe single plasmon antennas coupled to emitters and complex environments using scatterometry. An integral part of this approach is the recently developed measurement method of `k-space polarimetry’, a microscopy technique to completely classify the intensity and polarization state of light radiated by a single nano-object into any emission direction that is based on back focal plane imaging and Stokes polarimetry. I show benchmarks of this technique for the cases of scattering, fluorescence, and cathodoluminescence applied to directional surface plasmon polariton antennas.
Andrea Fiore, Zarko Zobenica, Rob van der Heijden, Maurangelo Petruzzella, Francesco Pagliano, Rick Leijssen, Ewold Verhagen, Tian Xia, Leonardo Midolo, YongJin Cho, Frank van Otten
Nanophotonic structures with narrow optical resonances, such as high-quality factor photonic crystal cavities, in principle enable spectral sensing with high resolution. This can also result in high-sensitivity displacement and/or acceleration sensing if a part of the cavity is compliant. However, the control of the resonance and its optical read-out are complex and usually not integrated with the sensing part. In this talk we will introduce a novel nano-opto-electromechanical system (NOEMS), where actuation, sensing and read-out are integrated in the same device. It consists of a double-membrane photonic crystal cavity, where the resonant wavelength is tuned by electrostatically controlling the separation between the membranes. The output current signal provides direct information about either the wavelength of the incident light or the cavity resonance. This nanophotonic sensing system can be employed to measure the spectrum of incident light, to determine the wavelength of a laser line with pm-range resolution, or equivalently to measure tiny displacements.
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