Three-Dimensional Reconfigurable Optical Singularities in Bilayer Photonic Crystals.

Metasurfaces and photonic crystals have revolutionized classical and quantum manipulation of light and opened the door to studying various optical singularities related to phases and polarization states. However, traditional nanophotonic devices lack reconfigurability, hindering the dynamic switching and optimization of optical singularities. This paper delves into the underexplored concept of tunable bilayer photonic crystals (BPhCs), which offer rich interlayer coupling effects. Utilizing silicon nitride-based BPhCs, we demonstrate tunable bidirectional and unidirectional polarization singularities, along with spatiotemporal phase singularities. Leveraging these tunable singularities, we achieve dynamic modulation of bound-state-in-continuum states, unidirectional guided resonances, and both longitudinal and transverse orbital angular momentum. Our work paves the way for multidimensional control over polarization and phase, inspiring new directions in ultrafast optics, optoelectronics, and quantum optics.

Experimental probe of twist angle-dependent band structure of on-chip optical bilayer photonic crystal.

Recently, twisted bilayer photonic materials have been extensively used for creating and studying photonic tunability through interlayer couplings. While twisted bilayer photonic materials have been experimentally demonstrated in microwave regimes, a robust platform for experimentally measuring optical frequencies has been elusive. Here, we demonstrate the first on-chip optical twisted bilayer photonic crystal with twist angle-tunable dispersion and great simulation-experiment agreement. Our results reveal a highly tunable band structure of twisted bilayer photonic crystals due to moiré scattering. This work opens the door to realizing unconventional twisted bilayer properties and novel applications in optical frequency regimes.

Photonic flatband resonances for free-electron radiation.

Flatbands have become a cornerstone of contemporary condensed-matter physics and photonics. In electronics, flatbands entail comparable energy bandwidth and Coulomb interaction, leading to correlated phenomena such as the fractional quantum Hall effect and recently those in magic-angle systems. In photonics, they enable properties including slow light1 and lasing2. Notably, flatbands support supercollimation-diffractionless wavepacket propagation-in both systems3,4. Despite these intense parallel efforts, flatbands have never been shown to affect the core interaction between free electrons and photons. Their interaction, pivotal for free-electron lasers5, microscopy and spectroscopy6,7, and particle accelerators8,9, is, in fact, limited by a dimensionality mismatch between localized electrons and extended photons. Here we reveal theoretically that photonic flatbands can overcome this mismatch and thus remarkably boost their interaction. We design flatband resonances in a silicon-on-insulator photonic crystal slab to control and enhance the associated free-electron radiation by tuning their trajectory and velocity. We observe signatures of flatband enhancement, recording a two-order increase from the conventional diffraction-enabled Smith-Purcell radiation. The enhancement enables polarization shaping of free-electron radiation and characterization of photonic bands through electron-beam measurements. Our results support the use of flatbands as test beds for strong light-electron interaction, particularly relevant for efficient and compact free-electron light sources and accelerators.

Relaxed Phase-Matching Constraints in Zero-Index Waveguides.

The utility of all parametric nonlinear optical processes is hampered by phase-matching requirements. Quasi-phase-matching, birefringent phase matching, and higher-order-mode phase matching have all been developed to address this constraint, but the methods demonstrated to date suffer from the inconvenience of only being phase matched for a single, specific arrangement of beams, typically copropagating, resulting in cumbersome experimental configurations and large footprints for integrated devices. Here, we experimentally demonstrate that these phase-matching requirements may be satisfied in a parametric nonlinear optical process for multiple, if not all, configurations of input and output beams when using low-index media. Our measurement constitutes the first experimental observation of direction-independent phase matching for a medium sufficiently long for phase matching to be relevant. We demonstrate four-wave mixing from spectrally distinct co- and counterpropagating pump and probe beams, the backward generation of a nonlinear signal, and excitation by an out-of-plane probe beam. These results explicitly show that the unique properties of low-index media relax traditional phase-matching constraints, which can be exploited to facilitate nonlinear interactions and miniaturize nonlinear devices, thus adding to the established exceptional properties of low-index materials.

Momentum considerations inside near-zero index materials.

Near-zero index (NZI) materials, i.e., materials having a phase refractive index close to zero, are known to enhance or inhibit light-matter interactions. Most theoretical derivations of fundamental radiative processes rely on energetic considerations and detailed balance equations, but not on momentum considerations. Because momentum exchange should also be incorporated into theoretical models, we investigate momentum inside the three categories of NZI materials, i.e., inside epsilon-and-mu-near-zero (EMNZ), epsilon-near-zero (ENZ) and mu-near-zero (MNZ) materials. In the context of Abraham-Minkowski debate in dispersive materials, we show that Minkowski-canonical momentum of light is zero inside all categories of NZI materials while Abraham-kinetic momentum of light is zero in ENZ and MNZ materials but nonzero inside EMNZ materials. We theoretically demonstrate that momentum recoil, transfer momentum from the field to the atom and Doppler shift are inhibited in NZI materials. Fundamental radiative processes inhibition is also explained due to those momentum considerations inside three-dimensional NZI materials. Absence of diffraction pattern in slits experiments is seen as a consequence of zero Minkowski momentum. Lastly, consequence on Heisenberg inequality, microscopy applications and on the canonical momentum as generator of translations are discussed. Those findings are appealing for a better understanding of fundamental light-matter interactions at the nanoscale as well as for lasing applications.

Genetic-algorithm-aided ultra-broadband perfect absorbers using plasmonic metamaterials.

Complete absorption of electromagnetic waves is paramount in today's applications, ranging from photovoltaics to cross-talk prevention into sensitive devices. In this context, we use a genetic algorithm (GA) strategy to optimize absorption properties of periodic arrays of truncated square-based pyramids made of alternating stacks of metal/dielectric layers. We target ultra-broadband quasi-perfect absorption of normally incident electromagnetic radiations in the visible and near-infrared ranges (wavelength comprised between 420 and 1600 nm). We compare the results one can obtain by considering one, two or three stacks of either Ni, Ti, Al, Cr, Ag, Cu, Au or W for the metal, and poly(methyl methacrylate) (PMMA) for the dielectric. More than 1017 configurations of geometrical parameters are explored and reduced to a few optimal ones. This extensive study shows that Ni/PMMA, Ti/PMMA, Cr/PMMA and W/PMMA provide high-quality solutions with an integrated absorptance higher than 99% over the considered wavelength range, when considering realistic implementation of these ultra-broadband perfect electromagnetic absorbers. Robustness of optimal solutions with respect to geometrical parameters is investigated and local absorption maps are provided. Moreover, we confirm that these optimal solutions maintain quasi-perfect broadband absorption properties over a broad angular range when changing the inclination of the incident radiation. The study also reveals that noble metals (Au, Ag, Cu) do not provide the highest performance for the present application.

Intracellular Cargo Delivery Induced by Irradiating Polymer Substrates with Nanosecond-Pulsed Lasers.

There is a great need in the biomedical field to efficiently, and cost-effectively, deliver membrane-impermeable molecules into the cellular cytoplasm. However, the cell membrane is a selectively permeable barrier, and large molecules often cannot pass through the phospholipid bilayer. We show that nanosecond laser-activated polymer surfaces of commercial polyvinyl tape and black polystyrene Petri dishes can transiently permeabilize cells for high-throughput, diverse cargo delivery of sizes of up to 150 kDa. The polymer surfaces are biocompatible and support normal cell growth of adherent cells. We determine the optimal irradiation conditions for poration, influx of fluorescent molecules into the cell, and post-treatment viability of the cells. The simple and low-cost substrates we use have no thin-metal structures, do not require cleanroom fabrication, and provide spatial selectivity and scalability for biomedical applications.

Dirac-like cone-based electromagnetic zero-index metamaterials.

Metamaterials with a Dirac-like cone dispersion at the center of the Brillouin zone behave like an isotropic and impedance-matched zero refractive index material at the Dirac-point frequency. Such metamaterials can be realized in the form of either bulk metamaterials with efficient coupling to free-space light or on-chip metamaterials that are efficiently coupled to integrated photonic circuits. These materials enable the interactions of a spatially uniform electromagnetic mode with matter over a large area in arbitrary shapes. This unique optical property paves the way for many applications, including arbitrarily shaped high-transmission waveguides, nonlinear enhancement, and phase mismatch-free nonlinear signal generation, and collective emission of many emitters. This review summarizes the Dirac-like cone-based zero-index metamaterials' fundamental physics, design, experimental realizations, and potential applications.

Modeling the optical properties of twisted bilayer photonic crystals.

We demonstrate a photonic analog of twisted bilayer graphene that has ultra-flat photonic bands and exhibits extreme slow-light behavior. Our twisted bilayer photonic device, which has an operating wavelength in the C-band of the telecom window, uses two crystalline silicon photonic crystal slabs separated by a methyl methacrylate tunneling layer. We numerically determine the magic angle using a finite-element method and the corresponding photonic band structure, which exhibits a flat band over the entire Brillouin zone. This flat band causes the group velocity to approach zero and introduces light localization, which enhances the electromagnetic field at the expense of bandwidth. Using our original plane-wave continuum model, we find that the photonic system has a larger band asymmetry. The band structure can easily be engineered by adjusting the device geometry, giving significant freedom in the design of devices. Our work provides a fundamental understanding of the photonic properties of twisted bilayer photonic crystals and opens the door to the nanoscale-based enhancement of nonlinear effects.

Ultra-low-loss on-chip zero-index materials.

Light travels in a zero-index medium without accumulating a spatial phase, resulting in perfect spatial coherence. Such coherence brings several potential applications, including arbitrarily shaped waveguides, phase-mismatch-free nonlinear propagation, large-area single-mode lasers, and extended superradiance. A promising platform to achieve these applications is an integrated Dirac-cone material that features an impedance-matched zero index. Although an integrated Dirac-cone material eliminates ohmic losses via its purely dielectric structure, it still entails out-of-plane radiation loss, limiting its applications to a small scale. We design an ultra-low-loss integrated Dirac cone material by achieving destructive interference above and below the material. The material consists of a square array of low-aspect-ratio silicon pillars embedded in silicon dioxide, featuring easy fabrication using a standard planar process. This design paves the way for leveraging the perfect spatial coherence of large-area zero-index materials in linear, nonlinear, and quantum optics.

Low-Loss Zero-Index Materials.

Materials with a zero refractive index support electromagnetic modes that exhibit stationary phase profiles. While such materials have been realized across the visible and near-infrared spectral range, radiative and dissipative optical losses have hindered their development. We reduce losses in zero-index, on-chip photonic crystals by introducing high-Q resonances via resonance-trapped and symmetry-protected states. Using these approaches, we experimentally obtain quality factors of 2.6 × 103 and 7.8 × 103 at near-infrared wavelengths, corresponding to an order-of-magnitude reduction in propagation loss over previous designs. Our work presents a viable approach to fabricate zero-index on-chip nanophotonic devices with low-loss.

Optically Induced Molecular Logic Operations.

Molecular electronics is a promising route for down-sizing electronic devices. Tip-enhanced Raman spectroscopy provides us a setup to probe current-driven molecular junctions that are considered as prototypes of molecular electronic devices. In this setup, the plasmonic tip concentrates optical fields to a degree that allows observing optical response of single molecules. Simultaneously, the tip can also induce a localized optical angular momentum, which has been seldomly considered in previous studies. Here, we propose that the induced optical angular momentum can interact with the probed molecule and strongly modify the response signal. Specifically, we demonstrate the ability to control the vibrational resonance of current-driven molecular junctions with the optical angular momentum. This precise control of light-matter interactions at the nanoscale allows us to demonstrate multiple logic operations. These results provide a fundamental understanding of future molecular electronics applications.

Detecting Laser-Volatilized Salts with a Miniature 100-GHz Spectrometer.

Rotational transitions are unique identifiers of molecular species, including isotopologues. This article describes the rotational detections of two laser-volatilized salts, NaCl and KCl, made with a miniature Fourier transform millimeter-wave (FTmmW) cavity spectrometer that could one day be used to measure solid composition in the field or in space. The two salts are relevant targets for icy moons in the outer solar system, and in principle, other molecular solids could be analyzed with the FTmmW instrument. By coupling the spectrometer to a collisionally cooling laser ablation source, (a) we demonstrate that the FTmmW instrument is sensitive enough to detect ablation products, and (b) we use the small size of the FTmmW cavity to measure ablation product signal along the carrier gas beam. We find that for 532 nm nanosecond pulses, ablated molecules are widely dispersed in the carrier-gas jet. In addition to the miniature spectrometer results, we present several complementary measurements intended to characterize the laser ablation process. For pulse energies between 10 and 30 mJ, the ablation product yield increases linearly, reaching approximately 1012 salt molecules per 30 mJ pulse. Using mass spectrometry, we observe Li+, Na+, and K+ in the plumes of ablated NaCl, KCl, and LiCl, which implies dissociation of the volatilized material. We do not observe salt ions (e.g., NaCl+). However, with 800 nm femtosecond laser pulses, the triatomic ion clusters Li2Cl+, Na2Cl+, and K2Cl+ are produced. Finally, we observe incomplete volatilization with the nanosecond pulses: some of the ejecta are liquid droplets. The insights about ablation plume physics gleaned from these experiments should guide future implementations of the laser-volatilization technique.

Peer Instruction-Based Feedback Sessions Improve the Retention of Knowledge in Medical Students / Feedback Baseado em "Peer Instruction" Melhora a Retenção de Conhecimento em Estudantes de Medicina

ABSTRACT Peer Instruction (PI) is an interactive teaching-learning process between colleagues and has been applied in various universities throughout the world. This active teaching methodology improves students' performance and their capacity to resolve problems when they perform activities with their study colleagues. There are no systematic studies about the use of PI in assessment feedback. The aim of our study is to identify whether the use of PI on assessment feedback improves the retention of basic concepts in medical programs. For this study 226 undergraduate students (Y2 = 115, Y3 = 111) enrolled in a Brazilian medical school were invited to participate. After taking the regular exam (RE), the students of the control group (125) could individually receive feedback (review of the exam) from the professor according to the course routine, and the students in the study group (101) were invited to participate in an immediate intervention after the RE with a feedback developed session using the peer instruction teaching method. At the conclusion of the feedback session, the students again answered the post-feedback exam (PFE) so that we could identify any changes in the answers compared with the regular exam taken before feedback and 6 months later, we applied a diagnostic exam (DE) of identify whether the students retained the concepts covered in the previous exams. The control and study groups are statistically significantly different in the RE (p = 0.0014) and DE (p < 0.000). The study group demonstrated better performance in both exams than the control group. When we gave feedback, using PI immediately after the exam, retention of basic science knowledge jumped to 39%, increasing by 15%. The students that had assessment feedback had the opportunity to discuss their misconceptions. These students had the highest number of correct answers with assimilate knowledge and fewer assimilation of wrong answers, therefore, students who received immediate feedback had less tendency to make the same conceptual errors. PI in the feedback was effective in improving retention of basic science knowledge.

RESUMO Instrução de pares (PI) é um processo interativo de ensino-aprendizagem entre os estudantes e tem sido aplicado em várias universidades em todo o mundo. Essa metodologia de ensino ativa melhora o desempenho dos alunos e sua capacidade de resolver problemas quando realizam atividades com seus colegas. Não há estudos sistemáticos sobre o uso de PI no feedback da avaliação. O objetivo do nosso estudo é identificar se o uso do PI no feedback da avaliação melhora a retenção de conceitos básicos em educação médica. Foram convidados a participar deste estudo 226 estudantes de graduação (segundo ano = 115, terceiro ano = 111) matriculados em uma escola de Medicina no Brasil. Após o exame regular (ER), os alunos do grupo controle (125) poderiam solicitar a revisão de prova de acordo com a rotina do curso, e os alunos do grupo de estudo (101) foram convidados a participar de uma intervenção imediata após o ER com uma sessão de feedback usando-se o método de ensino por instrução de pares. No final do feedback, os alunos responderam novamente ao exame pós-feedback (PFE) para que pudéssemos identificar quaisquer alterações nas respostas em comparação ao exame regular feito antes do feedback. Após seis meses, aplicamos um exame de diagnóstico (DE) para identificar se os alunos mantiveram os conceitos abordados nos exames anteriores. O desempenho dos estudantes dos grupos controle e estudo são estatisticamente diferentes no RE (p = 0,0014) e no DE (p < 0,000). O grupo de estudo demonstrou melhor desempenho em ambos os exames do que o grupo controle. Com a sessão de feedback, usando-se PI imediatamente após o exame, a retenção do conhecimento básico foi de 39%, aumentando em 15%. Os alunos que tiveram feedback de avaliação tiveram a oportunidade de discutir suas dificuldades. Esses alunos apresentaram o maior número de respostas corretas assimiladas e menor assimilação de respostas erradas. Portanto, os alunos que receberam feedback imediato apresentaram menor tendência a cometer os mesmos erros conceituais da primeira avaliação. PI no feedback foi eficaz em melhorar a retenção de conhecimentos básicos em estudantes de Medicina.

Manipulating the flow of light using Dirac-cone zero-index metamaterials.

Metamaterials with a refractive index of zero exhibit properties that are important for integrated optics. Possessing an infinite effective wavelength and zero spatial phase change, zero-index metamaterials may be especially useful for routing on-chip photonic processes and reducing the footprint of nonlinear interactions. Zero-index has only been achieved recently in an integrated platform through a Dirac-cone dispersion, enabling some of these more exciting applications in an integrated platform. This paper presents an overview of Dirac-cone zero-index metamaterials, including the fundamental physics, history and demonstration in the optical regime, as well as current challenges and future directions.

A comparison of inverted and upright laser-activated titanium nitride micropyramids for intracellular delivery.

The delivery of biomolecules into cells relies on porating the plasma membrane to allow exterior molecules to enter the cell via diffusion. Various established delivery methods, including electroporation and viral techniques, come with drawbacks such as low viability or immunotoxicity, respectively. An optics-based delivery method that uses laser pulses to excite plasmonic titanium nitride (TiN) micropyramids presents an opportunity to overcome these shortcomings. This laser excitation generates localized nano-scale heating effects and bubbles, which produce transient pores in the cell membrane for payload entry. TiN is a promising plasmonic material due to its high hardness and thermal stability. In this study, two designs of TiN micropyramid arrays are constructed and tested. These designs include inverted and upright pyramid structures, each coated with a 50-nm layer of TiN. Simulation software shows that the inverted and upright designs reach temperatures of 875 °C and 307 °C, respectively, upon laser irradiation. Collectively, experimental results show that these reusable designs achieve maximum cell poration efficiency greater than 80% and viability greater than 90% when delivering calcein dye to target cells. Overall, we demonstrate that TiN microstructures are strong candidates for future use in biomedical devices for intracellular delivery and regenerative medicine.

Laser-Activated Self-Assembled Thermoplasmonic Nanocavity Substrates for Intracellular Delivery.

Intracellular delivery is crucial for cellular engineering and the development of therapeutics. Laser-activated thermoplasmonic nanostructured surfaces are a promising platform for high-efficiency, high-viability, high-throughput intracellular delivery. Their fabrication, however, typically involves complicated nanofabrication techniques, limiting the approach's applicability. Here, colloidal self-assembly and templating are used to fabricate large arrays of thermoplasmonic nanocavities simply and cost-effectively. These laser-activated substrates are used to deliver membrane-impermeable dye into cells at an efficiency of 78% and throughput of 30 000 cells min-1 while maintaining 87% cell viability. Proof-of-concept data show delivery of large cargoes ranging from 0.6 to 2000 kDa to cells without compromising viability.

Light Spread Manipulation in Scintillators Using Laser Induced Optical Barriers.

We are using the Laser Induced Optical Barriers (LIOB) technique to fabricate scintillator detectors with combined performance characteristics of the two standard detector types, mechanically pixelated arrays and monolithic crystals. This is done by incorporation of so-called optical barriers that have a refractive index lower than that of the crystal bulk. Such barriers can redirect the scintillation light and allow for control of the light spread in the detector. Previous work has shown that the LIOB technique has the potential to achieve detectors with high transversal and depth of interaction (DOI) resolution simultaneously in a single-side readout configuration, suitable for high resolution PET imaging. However, all designs studied thus far present edge effect issues similarly as in the standard detector categories. In this work we take advantage of the inherent flexibility of the LIOB technique and investigate alternative barrier patterns with the aim to address this problem. Light transport simulations of barrier patterns in LYSO:Ce, with deeper barrier walls moving towards the detector edge show great promise in reducing the edge effect, however there is a trade-off in terms of achievable DOI information. Furthermore, fabrication and characterization of a 20 mm thick LYSO:Ce detector with optical barriers forming a pattern of 1 × 1 × 20mm3 pixel like structures show that light channeling in laser-processed detectors in agreement with optical barriers with refractive index between 1.2 and 1.4 is achievable.

Analysis of poration-induced changes in cells from laser-activated plasmonic substrates.

Laser-exposed plasmonic substrates permeabilize the plasma membrane of cells when in close contact to deliver cell-impermeable cargo. While studies have determined the cargo delivery efficiency and viability of laser-exposed plasmonic substrates, morphological changes in a cell have not been quantified. We porated myoblast C2C12 cells on a plasmonic pyramid array using a 532-nm laser with 850-ps pulse length and time-lapse fluorescence imaging to quantify cellular changes. We obtain a poration efficiency of 80%, viability of 90%, and a pore radius of 20 nm. We quantified area changes in the plasma membrane attached to the substrate (10% decrease), nucleus (5 - 10% decrease), and cytoplasm (5 - 10% decrease) over 1 h after laser treatment. Cytoskeleton fibers show a change of 50% in the alignment, or coherency, of fibers, which stabilizes after 10 mins. We investigate structural and morphological changes due to the poration process to enable the safe development of this technique for therapeutic applications.

Monolithic CMOS-compatible zero-index metamaterials.

Zero-index materials exhibit exotic optical properties that can be utilized for integrated-optics applications. However, practical implementation requires compatibility with complementary metallic-oxide-semiconductor (CMOS) technologies. We demonstrate a CMOS-compatible zero-index metamaterial consisting of a square array of air holes in a 220-nm-thick silicon-on-insulator (SOI) wafer. This design supports zero-index modes with Dirac-cone dispersion. The metamaterial is entirely composed of silicon and offers compatibility through low-aspect-ratio structures that can be simply fabricated in a standard device layer. This platform enables mass adoption and exploration of zero-index-based photonic devices at low cost and high fidelity.