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Picosecond mIR-spectroscopy reveals polaron stabilization processes in perovskite quantum dots on a few ps time-scale, considerably faster than the bulk analogue.

Confinement of electromagnetic fields at the subwavelength scale via metamaterial paradigms is an established method to engineer light-matter interaction in most common material systems, from insulators to semiconductors, from metals to superconductors. In recent years, this approach has been extended to the realm of topological materials, providing a new avenue to access nontrivial features of their electronic band structure. In this review, we survey various topological material classes from a photonics standpoint, including crystal growth and lithographic structuring methods. We discuss how exotic electronic features such as spin-selective Dirac plasmon polaritons in topological insulators or hyperbolic plasmon polaritons in Weyl semimetals may give rise to unconventional magneto-optic, non-linear and circular photogalvanic effects in metamaterials across the visible to infrared spectrum. Finally, we dwell on how these effects may be dynamically controlled by applying external perturbations in the form of electric and magnetic fields or ultrafast optical pulses. Through these examples and future perspectives, we argue that topological insulator, semimetal and superconductor metamaterials are unique systems to bridge the missing links between nanophotonic, electronic and spintronic technologies., Comment: 56 pages, 20 figures

Emerging immersive visual communication technologies require light sources with complex functionality for dynamic control of polarization, directivity, wavefront, spectrum, and intensity of light. Currently, this is mostly achieved by free space bulk optic elements, limiting the adoption of these technologies. Flat optics based on artificially structured metasurfaces that operate at the sub-wavelength scale are a viable solution, however their integration into electrically driven devices remains challenging. Here we demonstrate a radically new approach of monolithic integration of a dielectric metasurface into a perovskite light-emitting transistor. We show that nanogratings directly structured on top of the transistor channel yield an 8-fold increase of electroluminescence intensity and dynamic tunability of polarization. This new light-emitting metatransistor device concept opens unlimited opportunities for light management strategies based on metasurface design and integration. This article is protected by copyright. All rights reserved. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Submitted/Accepted version This research was supported by the Agency for Science, Technology and Research A*STAR-AME programmatic grant on Nanoantenna Spatial Light Modulators for Next-Gen Display Technologies (Grant no. A18A7b0058) and the Singapore Ministry of Education MOE Tier 3 (Grant no. MOE2016-T3-1-006).

Organic semiconductors have shown great potential as efficient bioelectronic materials. Specifically, photovoltaic polymers such as the workhorse poly(thiophene) derivatives, when stimulated with visible light, can depolarize neurons and generate action potentials, an effect that has been also employed for rescuing vision in blind rats. In this context, however, the coupling of such materials with optically resonant structures to enhance those photodriven biological effects is still in its infancy. Here, we employ the optical coupling between a nanostructured metasurface and poly(3-hexylthiophene) (P3HT) to improve the bioelectronic effects occurring upon photostimulation at the abiotic-biotic interface. In particular, we designed a spectrally tuned aluminum metasurface that can resonate with P3HT, hence augmenting the effective field experienced by the polymer. In turn, this leads to an 8-fold increase in invoked inward current in cells. This enhanced activation strategy could be useful to increase the effectiveness of P3HT-based prosthetic implants for degenerative retinal disorders. Ministry of Education (MOE) Published version This work was supported by the Singapore Ministry of Education Academic Research Fund Tier 1 (2018-T1-002- 040).

Advanced quantum information protocols rely on the operation of multinodal quantum networks where entanglement is distributed across the nodes. Existing protocols of entanglement generation are probabilistic, with the efficiency dropping exponentially with the size of the system. We formulate an approach for the deterministic generation of entangled states of a multinodal quantum network of arbitrary size by coupling a single photon standing wave with the nodes of the network. We show experimentally how this can be implemented in a simple binodal system. Since this approach relies on collective excitation of the network—not on local interaction with individual nodes—it allows generation of entanglement with unitary efficiency, independent of the size and the nature of the network.

Hybrid metal halide perovskites exhibit well-defined semiconducting properties and efficient optoelectronic performance considering their soft crystal structure and low-energy lattice motions. The response of such a crystal lattice to light-induced charges is a fundamental question, for which experimental insight into ultrafast time scales is still sought. Here, we use infrared-activated vibrations (IRAV) of the organic components within the hybrid perovskite lattice as a sensitive probe for local structural reorganizations after photoexcitation, with femtosecond resolution. We find that the IRAV signal response shows a delayed rise of about 3 ps and subsequent decay of pronounced monomolecular character, distinguishing it from absorption associated with free carriers. We interpret our results as a two-step carrier localization process. Initially, carriers localize transiently in local energy minima formed by lattice fluctuations. A subpopulation of these can then fall into deeper trapped states over picoseconds, likely due to local reorganization of the organic molecules surrounding the carriers.

Hybrid halide perovskites have achieved excellent efficiencies and remarkable performances, not just in photovoltaic cells, but also in light-emitting devices, like Light Emitting Diodes, Light Emi...

Despite the rapid development of perovskite light-emitting diodes (PeLEDs) in recent years, blue PeLEDs’ efficiencies are still inferior to those of their red and green counterparts. The poor performance is associated with, among other factors, halide segregation in bromide-chloride materials and energy funneling to lowest bandgaps in multilayered Ruddlesden–Popper (RP) systems. This study reports that compositional engineering through prudent selection of the A-site cation in a pure bromide RP system can result in a narrow distribution of layered domains. With a narrow distribution centered around the desired RP domain, efficient energy cascade to yield blue emission is ensured. Coupled with rapid nucleation induced by an antisolvent deposition technique, record efficiencies of 2.34 and 5.08%, corresponding to color-stable deep blue (∼465 nm) and cyan (∼493 nm), respectively, were attained. This composition and process engineering to design favorable structural landscape is transferrable to other material systems, which paves the way for high-performance PeLEDs. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

We demonstrate a new methodology of modelling phenomena of quantum photonics using the analogy in the evolution of quantum states of a single photon and a transmon, superconducting charge device of the IBM computer.

Halide perovskite metasurfaces are attracting increasing interest for applications in light-emitting and display technologies. To access the wide range of colors required for these applications, the main mechanism exploited thus far has been chemical engineering of the perovskite compounds – this constitutes a significant limitation for the dynamic switching of optical response desirable in actual devices. Here we demonstrate polarization-dependent, dynamic control of structural color and emission wavelength in an all-dielectric phase-change halide perovskite nanograting metasurface, by temperature tuning. This is underpinned by the significant change in the perovskite optical constants which accompanies its phase-transition around room temperature. The functionalities demonstrated in our work bearing potential for applications in light-emitting devices, displays and spatial-light-modulators. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Published version This research was supported by the A*STAR-AME programmatic fund on Nanoantenna Spatial Light Modulators for Next-Gen Display Technologies (grant A18A7b0058), and the Singapore Ministry of Education (Tier 3 grant MOE2016-T3-1-006).

Common methods to achieve photon number resolution rely on fast on-off single-photon detectors in conjunction with temporal or spatial mode multiplexing. Yet, these methods suffer from an inherent trade-off between the efficiency of photon number discrimination and photon detection rate. Here, we introduce a method of photon number resolving detection that overcomes these limitations by replacing mode multiplexing with coherent absorption of a single optical mode in a distributed detector array. Distributed coherent absorption ensures complete and uniform absorption of light among the constituent detectors, enabling fast and efficient photon number resolution. As a proof-of-concept, we consider the case of a distributed array of superconducting nanowire single-photon detectors with realistic parameters and show that deterministic absorption and arbitrarily high photon number discrimination efficiency can be achieved by increasing the number of detectors in the array. Photon number resolution without optical mode multiplication provides a simple yet effective method to discriminate an arbitrary number of photons in large arrays of on-off detectors or in smaller arrays of mode multiplexed detectors., Comment: 17 pages, 8 figures

The Rashba effect, i.e., the splitting of electronic spin-polarized bands in the momentum space of a crystal with broken inversion symmetry, has enabled the realization of spin-orbitronic devices, in which spins are manipulated by spin-orbit coupling. In optics, where the helicity of light polarization represents the spin degree of freedom for spin-momentum coupling, the optical Rashba effect is manifested by the splitting of optical states with opposite chirality in the momentum space. Previous realizations of the optical Rashba effect relied on passive devices determining the surface plasmon or light propagation inside nanostructures, or the directional emission of chiral luminescence when hybridized with light-emitting media. An active device underpinned by the optical Rashba effect is demonstrated here, in which a monolithic halide perovskite metasurface emits highly directional chiral photoluminescence. An all-dielectric metasurface design with broken in-plane inversion symmetry is directly embossed into the high-refractive-index, light-emitting perovskite film, yielding a degree of circular polarization of photoluminescence of 60% at room temperature. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) This research was supported by the A*STAR-AME programmatic fund on Nanoantenna Spatial Light Modulators for Next-Gen Display Technologies (grant A18A7b0058), and the Singapore Ministry of Education (Tier 3 grant MOE2016-T3-1-006).

Quantum photonics technologies, like wavelength division multiplexing (WDM), for high-rate quantum key distribution require narrowband, spectrally tunable single photon emitters. Physical methods that rely on the application of large mechanical strain to epitaxial quantum dots or electric and magnetic fields to color centers in 2D metal dichalcogenides provide limited spectral tunability. Here we adopt a chemical approach to synthesize a family of colloidal mixed-cation perovskite quantum dots (Cs1-xFAxPbBr3) that show highly photostable, compositionally tunable single photon emission at room temperature, spanning more than 30 nm in the visible wavelength spectral range. We find that tailoring the stoichiometry of the organic formamidinium (FA) cation in all-inorganic cesium lead bromide (CsPbBr3) perovskite quantum dots detunes the electronic band structure while preserving their excellent single photon emission characteristics. We argue that the mixed-cation perovskite quantum dots studied in this work offer a new platform for the realization of color-tunable single photon emitters that could be readily integrated in a diversity of quantum photonic devices. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Submitted/Accepted version Research in NTU was supported by the Agency for Science, Technology and Research A*STAR-AME Programmatic Grant on Nanoantenna Spatial Light Modulators for Next-Gen Display Technologies (Grant No. A18A7b0058) and the Singapore Ministry of Education MOE Tier 3 (Grant No. MOE2016-T3-1-006). Research in LKB was supported by the ANR Project IPER-Nano2 (ANR-18CE30-0023) and by the European Union’s Horizon 2020 Research and Innovation Program Nanobright (No. 828972). Q.G. and A.B. are members of the Institut Universitaire de France (IUF).

Nanophotonics has emerged in recent years as the branch of science capable of connecting conventional optics with nanoscale phenomena. The key feature enabling this convergence is the ability of nanosized photonic objects to confine light within volumes comparable to or smaller than their physical sizes, far beyond the wavelength of the optical excitation. Proper arrangements of a large number of nanophotonic building blocks on two-dimensional landscapes, known as metamaterials and metasurfaces, allow for considerable enhancement of the response of the individual constituent parts and give access to optical functionalities, which would otherwise be unattainable.

Metasurfaces supporting optical bound states in the continuum (BICs) are emerging as simple and compact optical cavities to realize polarization-vortex lasers. The winding of the polarization around the singularity defines topological charges which are generally set by the cavity design and cannot be altered without changing geometrical parameters. Here, a subwavelength-thin phase-change halide perovskite BIC metasurface functioning as a tunable polarization vortex microlaser is demonstrated. Upon the perovskite structural phase transitions, both its refractive index and gain vary substantially, inducing reversible and bistable switching between distinct polarization vortexes underpinned by opposite topological charges. Dynamic tuning and switching of the resulting vector beams may find use in microscopy imaging, particle trapping and manipulation, and optical data storage. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version The research was supported by the Agency for Science, Technology and Research A*STAR-AME programmatic grant on Nanoantenna Spatial Light Modulators for Next-Gen Display Technologies (Grant no. A18A7b0058), the Singapore Ministry of Education (Grant no. MOE2016-T3-1-006), National Research Foundation of Singapore (Grant no. NRF-NRFI2017-01), and IET A F Harvey Engineering Research Prize 2016.

Modeling of photonic devices traditionally involves solving the equations of light–matter interaction and light propagation. Here, we demonstrate an alternative modeling methodology by reproducing the optical device functionality using a quantum computer. As an illustration, we simulate the quantum interference of light on a thin absorbing film. Such interference can lead to either perfect absorption or total transmission of light through the film, the phenomena attracting attention for data processing applications in classical and quantum information networks. We map the behavior of the photon in the interference experiment to the evolution of a quantum state of transmon, a superconducting charge qubit of the IBM quantum computer. Details of the real optical experiment are flawlessly reproduced on the quantum computer. We argue that the superiority of this methodology shall be apparent in modeling complex multi-photon optical phenomena and devices.

The Rashba effect, i.e., the splitting of electronic spin-polarized bands in the momentum space of a crystal with broken inversion symmetry, has enabled the realization of spin-orbitronic devices, in which spins are manipulated by spin-orbit coupling. In optics, where the helicity of light polarization represents the spin degree of freedom for spin-momentum coupling, the optical Rashba effect is manifested by the splitting of optical states with opposite chirality in the momentum space. Previous realizations of the optical Rashba effect relied on passive devices determining either the propagation direction of surface plasmons or circularly polarized light into nanostructures, or the directional emission of polarized luminescence from metamaterials hybridized with light-emitting media. Here we demonstrate an active device underpinned by the optical Rashba effect, in which a monolithic halide perovskite metasurface emits highly directional chiral photoluminescence. An all-dielectric metasurface design with broken in-plane inversion symmetry is directly embossed into the high refractive index, light-emitting perovskite film, yielding a degree of circular polarization of photoluminescence of 40% at room temperature - more than one order of magnitude greater than in state of art chiral perovskites.

We report a single photon detector based on NbTiN microbridges, suitable for operation within telecommunication wavelengths. We observed an excellent signal-to-noise ratio of the readout signal while the corresponding jitter contributed by electrical noise was measured to be less than 10 ps. Routing the current through a parallel electrical connection to set the microbridge back to the superconducting state after photon absorption enabled us to overcome the hysteresis of the state transition. Our approach combines facile fabrication of fast microscale detectors with efficient current redistribution mechanism, enabling prospective applications in quantum photonics which requires accurate estimation of photon arrival events.

We provide proof of concept demonstrations of halide perovskite light emitting metadevices, like a phase-change tunable vortex laser with optical bistability and a chiral light emitting metasurface with high degree of circular polarization.

We show theoretically and demonstrate experimentally that coherent absorption of a single photon in a quantum network allows deterministic generation of entanglement between the network’s nodes. Quantum networks, consisting of multiple nodes connected by quantum channels, are in the basis of quantum technology allowing protocols of quantum computation and quantum communication. To implement these protocols, quantum entanglement should be distributed across the network. A conventional approach of entanglement generation is realized by a sequential splitting of a single photon on a series of beamsplitters and independent coupling of each ‘part’ of the photon into the corresponding node of the network [1] , Fig. 1(a) . Efficient entanglement generation in a large scale quantum network is a challenging task as a highly efficient light-matter interaction in each node of the network is required: even in a network of 100 nodes the probability of successful entanglement generation would be around 10 −5 .

We report on-demand and site-specific creation of near-infrared color centers in GaN films by Ga+ implantation where the luminescence is attributed to optical transitions of neutral gallium atoms originating from implanted Ga+ ions.

Under circularly polarized light illumination, Dirac surface electrons in topological insulators show a natural chiral response due to spin-momentum locking, which can be interrogated by probing the dependence of photocurrent on the polarization of incident light. Here we report, for the first time, that patterning topological insulator BSTS (Bi 1.5 Sb 0.5 Te 1.8 Se 1.2 ) crystals with mirror-symmetric, planar-chiral metamaterial designs generates giant enhancement and reversal of photogalvanic currents.

Surface modified semiconductor quantum dots (QDs) with a tunable photoluminescence (PL) are particularly desirable for many photo-responsive devices. In a step forward towards this goal using 3-mercaptopropionic acid (MPA) the CdTe QDs of different sizes are synthesized. The MPA not only controls the size of the QDs but its dense shell around the surface also passivates the surface traps. Consequently, the CdTe QDs exhibit narrow emission with full-width at half-maximum (FWHM) of ∼0.3 eV. These QDs are attached to (E)-4-((3-formyl-4-hydroxyphenyl) diazenyl) benzoic acid (FHDBA) to fabricate a photochrome-fluorophore assembly. Ultraviolet (UV) irradiation induces trans-cis isomerization in FHDBA. Upon storing FHDBA in dark for ∼30 min, it completely reverts to trans-isomer. After photoisomerization the absorption band (n – π* transition) of cis-isomer of FHDBA overlaps with the emission band of CdTe QDs. Following UV excitation photoinduced electron transfer (ET) from conduction band (CB) of CdTe QDs to the LUMO of the cis-isomer of FHDBA quench the fluorescence of QDs by ∼16 times. Forster resonance energy transfer (FRET) may also quench the fluorescence but its contribution is minor. The photoinduced reversible trans-cis interconversion of FHDBA followed by ET in QDs-FHDBA assembly and the dual function of MPA as coupling strategy may open the avenues to design QDs luminescent photoswitchable probes for biomedical and energy storage applications.

Hybrid metal halide perovskites exhibit well-defined semiconducting properties and efficient optoelectronic performance considering their soft crystal structure and low-energy lattice motions. The response of such a crystal lattice to light-induced charges is a fundamental question, for which experimental insight into ultrafast time scales is still sought. Here, we use infrared-activated vibrations (IRAV) of the organic components within the hybrid perovskite lattice as a sensitive probe for local structural reorganizations after photoexcitation, with femtosecond resolution. We find that the IRAV signal response shows a delayed rise of about 3 ps and subsequent decay of pronounced monomolecular character, distinguishing it from absorption associated with free carriers. We interpret our results as a two-step carrier localization process. Initially, carriers localize transiently in local energy minima formed by lattice fluctuations. A subpopulation of these can then fall into deeper trapped states over picoseconds, likely due to local reorganization of the organic molecules surrounding the carriers.

One of the most striking manifestations of electronic properties of topological insulators is the dependence of the photocurrent direction on the helicity of circularly polarized optical excitation. The helicity dependent photocurrents, underpinned by spin-momentum locking of surface Dirac electrons, are weak and easily overshadowed by bulk contributions. Here we show that the chiral response can be enhanced by nanostructuring. The tight confinement of electromagnetic fields in the resonant nanostructure enhances the photoexcitation of spin-polarized surface states of topological insulator Bi1.5Sb0.5Te1.8Se1.2, leading to an 11-fold increase of the circular photogalvanic effect and an unprecedented photocurrent dichroism (\r{ho}circ=0.87) at room temperature. The control of spin-transport in topological materials by structural design is a previously unrecognised ability of metamaterials that bridges the gap between nanophotonics and spin-electronics, providing new opportunities for developing polarization sensitive photodetectors., Comment: 15 pages, 4 figures, 1 table

A four-step route for the synthesis of 5-azatetraceene (benzo[b]acridine) has been developed, employing a base-catalysed Friedlander condensation reaction between 3-amino-2-napthaldehyde and cyclohexanone as the key step followed by dehydrogenation of the intermediate. The optical and electrochemical properties of the 5-azatetracene were investigated by UV-vis and photoluminescence spectroscopy, and by cyclic voltammetry and compared with those of tetracene. It is found that 5-azatetracene shows broader absorption in the visible region than tetracene, exhibits a higher luminescence quantum efficiency, and possesses a lower-lying LUMO level and smaller HOMO-LUMO band gap. Time-resolved PL spectroscopy was used to elucidate the reasons for the more efficient luminescence of 5-azatetracene. Field-effect transistor measurements revealed the ambipolar nature of charge transport in 5-azatetracene. Ministry of Education (MOE) Accepted version We acknowledge funding from the Singapore Ministry of Education through the Academic Research Fund Tier 1 grant RG117/15 and Tier 2 grant MOE2019-T2-1-085

We demonstrate that coherent absorption offers a robust and efficient way to generate quantum entanglement in multi-nodal quantum networks. Proof-of principle experiment in a bi-nodal network is reported.

We report a long awaited experimental observation of resonant enhancement of luminescence from an ensemble of quantum dots embedded in a photonic waveguide operating in the epsilon-near-zero cutoff regime.

We report a resonant enhancement of luminescence intensity from an ensemble of CdS/ZnS quantum dots embedded in a nanoscale rectangular photonic waveguide operating at the epsilon-near-zero regime., 6 pages, 2 figures

Dissipation was traditionally considered as a destructive effect for quantum phenomena such as quantum light interference. In spite of this, correctly designed dissipation may provide an additional degree of freedom for quantum light control. Here we investigate, both theoretically and experimentally, different aspects of coherent quantum light interaction with lossy beamsplitters. Applications of dissipative interference for quantum technology are discussed.

Despite being very weak, the chiral optical response of natural media plays a fundamental role in several areas of photonics, medicine, pharmacology, and quantum optics. Topological insulators are a class of materials in which spin-momentum locking induces a preferential response of surface electrons to circularly polarized light. However, the resulting spin-polarized photocurrents are often hindered by the strong contribution of bulk electrons. Here we show that the intrinsic circular photogalvanic effect in topological insulator BSTS (Bi1.5Sb0.5Te1.8Se1.2), probed by helicity dependent photocurrent, can be enhanced by one order of magnitude when a non-chiral metamaterial design is patterned on the crystal surface. This method can be adopted to control the polarization properties of Dirac materials beyond topological insulators by metamaterial design, opening up new opportunities for the detection of quantum light, molecular sensing, and the realization of opto-spintronic devices.

Multimode fibers (MMFs) have the potential to carry complex images for endoscopy and related applications, but decoding the complex speckle patterns produced by mode-mixing and modal dispersion in MMFs is a serious challenge. Several groups have recently shown that convolutional neural networks (CNNs) can be trained to perform high-fidelity MMF image reconstruction. We find that a considerably simpler neural network architecture, the single hidden layer dense neural network, performs at least as well as previously-used CNNs in terms of image reconstruction fidelity, and is superior in terms of training time and computing resources required. The trained networks can accurately reconstruct MMF images collected over a week after the cessation of the training set, with the dense network performing as well as the CNN over the entire period., 17 pages, 10 figures