Diagnostic techniques for photonic materials based on Raman and Brillouin spectroscopies

The elastic and vibrational properties of a material,bulk or planar waveguide,are studied by Brillouin and Raman spectroscopy to follow the process of nanocrystals growth in glass-ceramics. The nanoparticles cause the appearance,in the low fre-quency Raman spectrum,of characteristic peaks,whose position depends on the size of the crystals. At the same time,sharp crystal peaks,due to optical phonons,appear in the Raman spectra,allowing the determination of the nucleated phase,and a frequency shift of the Brillouin peaks is observed.

Controlling the Cassie-to-Wenzel Transition: an Easy Route towards the Realization of Tridimensional Arrays of Biological Objects

In this paper we provide evidence that the Cassie-to-Wenzel transition, despite its detrimental effects on the wetting properties of superhydrophobic surfaces, can be exploited as an effective micro-fabrication strategy to obtain highly ordered arrays of biological objects. To this purpose we fabricated a patterned surface wetted in the Cassie state, where we deposited a droplet containing genomic DNA. We observed that, when the droplet wets the surface in the Cassie state, an array of DNA filaments pinned on the top edges between pillars is formed. Conversely, when the Cassie-to-Wenzel transition occurs, DNA can be pinned at different height between pillars. These results open the way to the realization of tridimensional arrays of biological objects.

Er^(3+)-activated silica inverse opals synthesized by the solgel method

We present the details of the sol-gel processing used to realize inverse silica opal,where the silica was activated with 0.3 mol% of Er3+ ions. The template(direct opal) was obtained assembling polystyrene spheres of the dimensions of 260 nm by means of a vertical deposition technique. The Er3+-activated silica inverse opal was obtained infiltrating,into the void of the template,the silica sol doped with Er3+ ions and subsequently removing the polystyrene spheres by means of calcinations. Scanning electron microscope showed that the inverse opals possess a fcc structure with a air hollows of about 210 nm and a photonic band gap,in the visible range,was observed from reflectance measurements. Spectroscopic properties of Er3+-activated silica inverse opal were investigated by luminescence spectroscopy,where,upon excitation at 514.5 nm,an emission of 4I13/2 → 4I15/2 of Er3+ ions transition with a 21 nm bandwidth was observed. Moreover the 4I13/2 level decay curve presents a single-exponential profile,with a measured lifetime of 18 ms,corresponding a high quantum efficiency of the system.

Effect of Yb^(3+) concentration on Er^(3+) luminescence properties of sol-gelderived silica-alumina xerogels

Er3+/Yb3+ co-doped silica-alumina monolithic xerogels were prepared with the same concentration on 2 000 Er/Si ppm and 6 000 Al/Si ppm.The Yb/Si content was varied from 0 to 4 000 ppm.Densification of the samples was achieved by thermal treatment in air at 950 ℃ for 120 hours with a heating rate 0.1 ℃/min.Photoluminescence spectroscopy was used to obtain information about the effective excitation efficiency of Er3+ ions by co-doping with Yb 3+ ions.The infrared-to-visible up-conversion luminescence has been investigated upon continuous wave excitation at 980 nm.The samples exhibit red,green and blue up-conversion emission.It is found that the intensity of up-conversion luminescence increases with increasing Yb3+ doping concentration.An opposite behavior is observed for the 4I13/2 luminescence of the Er3+ ions.

Time reversal of a discrete system coupled to a continuum based on non-Hermitian flip

Time reversal in quantum or classical systems described by an Hermitian Hamiltonian is a physically allowed process, which requires in principle inverting the sign of the Hamiltonian. Here we consider the problem of time reversal of a subsystem of discrete states coupled to an external environment characterized by a continuum of states, into which they generally decay. It is shown that, by flipping the discrete-continuum coupling from an Hermitian to a non-Hermitian interaction, thus resulting in a non unitary dynamics, time reversal of the subsystem of discrete states can be achieved, while the continuum of states is not reversed. Exact time reversal requires frequency degeneracy of the discrete states,or large frequency mismatch among the discrete states as compared to the strength of indirect coupling mediated by the continuum. Interestingly, periodic and frequent switch of the discrete-continuum coupling results in a frozen dynamics of the subsystem of discrete states.

Particle focusing by 3D inertial microfluidics

Three-dimensional(3D)particle focusing in microfluidics is a fundamental capability with a wide range of applications,such as on-chip flow cytometry,where high-throughput analysis at the single-cell level is performed.Currently,3D focusing is achieved mainly in devices with complex layouts,additional sheath fluids,and complex pumping systems.In this work,we present a compact microfluidic device capable of 3D particle focusing at high flow rates and with a small footprint,without the requirement of external fields or lateral sheath flows,but using only a single-inlet,single-outlet microfluidic sequence of straight channels and tightly curving vertical loops.This device exploits inertial fluidic effects that occur in a laminar regime at sufficiently high flow rates,manipulating the particle positions by the combination of inertial lift forces and Dean drag forces.The device is fabricated by femtosecond laser irradiation followed by chemical etching,which is a simple two-step process enabling the creation of 3D microfluidic networks in fused silica glass substrates.The use of tightly curving three-dimensional microfluidic loops produces strong Dean drag forces along the whole loop but also induces an asymmetric Dean flow decay in the subsequent straight channel,thus producing rapid cross-sectional mixing flows that assist with 3D particle focusing.The use of out-of-plane loops favors a compact parallelization of multiple focusing channels,allowing one to process large amounts of samples.In addition,the low fluidic resistance of the channel network is compatible with vacuum driven flows.The resulting device is quite interesting for high-throughput on-chip flow cytometry.

Unidirectional lasing in semiconductor microring lasers at an exceptional point [Invited]

Recent experiments demonstrated that chiral symmetry breaking at an exceptional point(EP) is a viable route to achieve unidirectional laser emission in microring lasers. By a detailed semiconductor laser rate equation model,we show here that unidirectional laser emission at an EP is a robust regime. Slight deviations from the EP condition can break preferential unidirectional lasing near threshold via a Hopf instability. However, abovea "second" laser threshold, unidirectional emission is restored.

Optimal photonic indistinguishability tests in multimode networks

Particle indistinguishability is at the heart of quantum statistics that regulates fundamental phenomena such as the electronic band structure of solids, Bose-Einstein condensation and superconductivity.Moreover, it is necessary in practical applications such as linear optical quantum computation and simulation, in particular for Boson Sampling devices.It is thus crucial to develop tools to certify genuine multiphoton interference between multiple sources.Our approach employs the total variation distance to find those transformations that minimize the error probability in discriminating the behaviors of distinguishable and indistinguishable photons.In particular, we show that so-called Sylvester interferometers are near-optimal for this task.By using Bayesian tests and inference, we numerically show that Sylvester transformations largely outperform most Haar-random unitaries in terms of sample size required.Furthermore, we experimentally demonstrate the efficacy of the transformation using an efficient 3 D integrated circuits in the single-and multiple-source cases.We then discuss the extension of this approach to a larger number of photons and modes.These results open the way to the application of Sylvester interferometers for optimal assessment of multiphoton interference experiments.

Deep reinforcement learning for quantum multiparameter estimation

Estimation of physical quantities is at the core of most scientific research,and the use of quantum devices promises to enhance its performances.In real scenarios,it is fundamental to consider that resources are limited,and Bayesian adaptive estimation represents a powerful approach to efficiently allocate,during the estimation process,all the available resources.However,this framework relies on the precise knowledge of the system model,retrieved with a fine calibration,with results that are often computationally and experimentally demanding.We introduce a model-free and deep-learning-based approach to efficiently implement realistic Bayesian quantum metrology tasks accomplishing all the relevant challenges,without relying on any a priori knowledge of the system.To overcome this need,a neural network is trained directly on experimental data to learn the multiparameter Bayesian update.Then the system is set at its optimal working point through feedback provided by a reinforcement learning algorithm trained to reconstruct and enhance experiment heuristics of the investigated quantum sensor.Notably,we prove experimentally the achievement of higher estimation performances than standard methods,demonstrating the strength of the combination of these two black-box algorithms on an integrated photonic circuit.Our work represents an important step toward fully artificial intelligence-based quantum metrology.

Photonic implementation of boson sampling:a review

Boson sampling is a computational problem that has recently been proposed as a candidate to obtain an unequivocal quantum computational advantage.The problem consists in sampling from the output distribution of indistinguishable bosons in a linear interferometer.There is strong evidence that such an experiment is hard to classically simulate,but it is naturally solved by dedicated photonic quantum hardware,comprising single photons,linear evolution,and photodetection.This prospect has stimulated much effort resulting in the experimental implementation of progressively larger devices.We review recent advances in photonic boson sampling,describing both the technological improvements achieved and the future challenges.We also discuss recent proposals and implementations of variants of the original problem,theoretical issues occurring when imperfections are considered,and advances in the development of suitable techniques for validation of boson sampling experiments.We conclude by discussing the future application of photonic boson sampling devices beyond the original theoretical scope.

Passively Q-switched femtosecond-laser-written thulium waveguide laser based on evanescent field interaction with carbon nanotubes

Surface channel waveguides(WGs) were fabricated in a monoclinic Tm^(3+):KLu(WO_4)_2 crystal by femtosecond direct laser writing(fs-DLW). The WGs consisted of a half-ring cladding with diameters of 50 and 60 μm located just beneath the crystal surface. They were characterized by confocal laser microscopy and μ-Raman spectroscopy,indicating a reduced crystallinity and stress-induced birefringence of the WG cladding. In continuous-wave(CW)mode, under Ti:sapphire laser pumping at 802 nm, the maximum output power reached 171.1 mW at 1847.4 nm,corresponding to a slope efficiency η of 37.8% for the 60 μm diameter WG. The WG propagation loss was0.7 0.3 d B∕cm. The top surface of the WGs was spin-coated by a polymethyl methacrylate film containing randomly oriented(spaghetti-like) arc-discharge single-walled carbon nanotubes serving as a saturable absorber based on evanescent field coupling. Stable passively Q-switched(PQS) operation was achieved. The PQS60 μm diameter WG laser generated a record output power of 150 m W at 1846.8 nm with η = 34.6%. The conversion efficiency with respect to the CW mode was 87.6%. The best pulse characteristics(energy/duration)were 105.6 nJ/98 ns at a repetition rate of 1.42 MHz.

Enhanced generation of nondegenerate photon pairs in nonlinear metasurfaces

We predict theoretically a regime of photon-pair generation driven by the interplay of multiple bound states in the continuum resonances in nonlinear metasurfaces.This nondegenerate photon-pair generation is derived from the hyperbolic topology of the transverse phase matching and can enable orders-of-magnitude enhancement of the photon rate and spectral brightness,as compared to the degenerate regime.We show through comprehensive simulations that the entanglement of the photon pairs can be tuned by varying the pump polarization,which can underpin future advances and applications of ultracompact quantum light sources.

Ultrafast control of fractional orbital angular momentum of microlaser emissions

On-chip integrated laser sources of structured light carrying fractional orbital angular momentum(FOAM)are highly desirable for the forefront development of optical communication and quantum information-processing technologies.While integrated vortex beam generators have been previously demonstrated in different optical settings,ultrafast control and sweep of FOAM light with low-power control,suitable for high-speed optical communication and computing,remains challenging.Here we demonstrate fast control of the FOAM from a vortex semiconductor microlaser based on fast transient mixing of integer laser vorticities induced by a control pulse.A continuous FOAM sweep between charge 0 and charge+2 is demonstrated in a 100 ps time window,with the ultimate speed limit being established by the carrier recombination time in the gain medium.Our results provide a new route to generating vortex microlasers carrying FOAM that are switchable at GHz frequencies by an ultrafast control pulse.

Planarized THz quantum cascade lasers for broadband coherent photonics

Recently,there has been a growing interest in integrated THz photonics for various applications in communications,spectroscopy and sensing.We present a new integrated photonic platform based on active and passive elements integrated in a double-metal,high-confinement waveguide layout planarized with a low-loss polymer.An extended top metallization keeps waveguide losses low while improving dispersion,thermal and RF properties,as it enables to decouple the design of THz and microwave cavities.Free-running on-chip quantum cascade laser combs spanning 800 GHz,harmonic states with over 1.1 THz bandwidth and RF-injected broadband incoherent states spanning over nearly 1.6 THz are observed using a homogeneous quantum-cascade active core.With a strong external RF drive,actively mode-locked pulses as short as 4.4 ps can be produced,as measured by SWIFTS.We demonstrate as well passive waveguides with low insertion loss,enabling the tuning of the laser cavity boundary conditions and the cointegration of active and passive elements on the same THz photonic chip.