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Vol. 2 ， Issue 1-2 ， 2013
- Abstract：Resonant light pressure effects can open new degrees of freedom in optical manipulation with microparticles, but they have been traditionally considered as relatively subtle effects. Using a simplified two-dimensional model of surface electromagnetic waves evanescently coupled to whispering gallery modes (WGMs) in transparent circular cavities, we show that under resonant conditions the peaks of the optical forces can approach theoretical limits imposed by the momentum conservation law on totally absorbing particles. Experimentally, we proved the existence of strong peaks of the optical forces by studying the optical propulsion of dielectric microspheres along tapered microfibers. We observed giant optical propelling velocities ~0.45 mm s−1 for some of the 15-20 µm polystyrene microspheres in water for guided powers limited at ~43 mW. Such velocities exceed previous observations by more than an order of magnitude, thereby providing evidence for the strongly enhanced resonant optical forces. We analyzed the statistical properties of the velocity distribution function measured for slightly disordered (~1% size variations) ensembles of microspheres with mean diameters varying from 3 to 20 µm. These results demonstrate a principal possibility of optical sorting of microspheres with the positions of WGM resonances overlapped at the wavelength of the laser source. They can be used as building blocks of the lossless coupled resonator optical waveguides and various integrated optoelectronics devices.Keywords：optical propulsion;optical forces;whispering gallery modes;microspheres;evanescent coupling
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Published：2024-09-30 - Abstract：Light sources based on reliable and energy-efficient light-emitting diodes (LEDs) are instrumental in the development of solid-state lighting (SSL). Most research efforts in SSL have focused on improving both the intrinsic quantum efficiency (QE) and the stability of light emitters. For this reason, it is broadly accepted that with the advent of highly efficient (QE close to 1) and stable emitters, the fundamental research phase of SSL is coming to an end. In this study, we demonstrate a very large improvement in SSL emission (above 70-fold directional enhancement for p-polarized emission and 60-fold enhancement for unpolarized emission) using nanophotonic structures. This is attained by coupling emitters with very high QE to collective plasmonic resonances in periodic arrays of aluminum nanoantennas. Our results open a new path for fundamental and applied research in SSL in which plasmonic nanostructures are able to mold the spectral and angular distribution of the emission with unprecedented precision.Keywords：diffraction;fluorescence;LED;nanophotonics;plasmonics
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Published：2024-09-30 - Abstract：Mesenchymal stem cells (MSCs) have been cited as contributors to heart repair through cardiogenic differentiation and multiple cellular interactions, including the paracrine effect, cell fusion, and mechanical and electrical couplings. Due to heart–muscle complexity, progress in the development of knowledge concerning the role of MSCs in cardiac repair is heavily based on MSC–cardiomyocyte coculture. In conventional coculture systems, however, the in vivo cardiac muscle structure, in which rod-shaped cells are connected end-to-end, is not sustained; instead, irregularly shaped cells spread randomly, resulting in randomly distributed cell junctions. Consequently, contact-mediated cell–cell interactions (e.g., the electrical triggering signal and the mechanical contraction wave that propagate through MSC–cardiomyocyte junctions) occur randomly. Thus, the data generated on the beneficial effects of MSCs may be irrelevant to in vivo biological processes. In this study, we explored whether cardiomyocyte alignment, the most important phenotype, is relevant to stem cell cardiogenic differentiation. Here, we report (ⅰ) the construction of a laser-patterned, biochip-based, stem cell–cardiomyocyte coculture model with controlled cell alignment; and (ⅱ) single-cell-level data on stem cell cardiogenic differentiation under in vivo-like cardiomyocyte alignment conditions.Keywords：cardiogenic differentiation;mesenchymal stem cells;microenvironment;optical force;single-cell analysis
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Published：2024-09-30 - Abstract：We experimentally demonstrate the focusing of visible light with ultra-thin, planar metasurfaces made of concentrically perforated, 30-nm-thick gold films. The perforated nano-voids—Babinet-inverted (complementary) nano-antennas—create discrete phase shifts and form a desired wavefront of cross-polarized, scattered light. The signal-to-noise ratio in our complementary nano-antenna design is at least one order of magnitude higher than in previous metallic nano-antenna designs. We first study our proof-of-concept 'metalens' with extremely strong focusing ability: focusing at a distance of only 2.5 μm is achieved experimentally with a 4-μm-diameter lens for light at a wavelength of 676 nm. We then extend our work with one of these 'metalenses' and achieve a wavelength-controllable focal length. Optical characterization of the lens confirms that switching the incident wavelength from 676 to 476 nm changes the focal length from 7 to 10 µm, which opens up new opportunities for tuning and spatially separating light at different wavelengths within small, micrometer-scale areas. All the proposed designs can be embedded on-chip or at the end of an optical fiber. The designs also all work for two orthogonal, linear polarizations of incident light.Keywords：metalens;nano-antennas;plasmonics;wavefront shaping
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Published：2024-09-30 - Abstract：Surface plasmon polaritons (SPPs) have been widely exploited in various scientific communities, ranging from physics, chemistry to biology, due to the strong confinement of light to the metal surface. For many applications, it is important that the free space photon can be coupled to SPPs in a controllable manner. In this Letter, we apply the concept of interfacial phase discontinuity for circularly polarizations on a metasurface to the design of a novel type of polarization-dependent SPP unidirectional excitation at normal incidence. Selective unidirectional excitation of SPPs along opposite directions is experimentally demonstrated at optical frequencies by simply switching the helicity of the incident light. This approach, in conjunction with dynamic polarization modulation techniques, opens gateway towards integrated plasmonic circuits with electrically reconfigurable functionalities.Keywords：metamaterials;metasurface;surface plasmon
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Published：2024-09-30 - Abstract：Colloidal semiconductor quantum dots (QDs) constitute a perfect material for ink-jet printable large area displays, photovoltaics, light-emitting diode, bio-imaging luminescent markers and many other applications. For this purpose, efficient light emission/absorption and spectral tunability are necessary conditions. These are currently fulfilled by the direct bandgap materials. Si-QDs could offer the solution to major hurdles posed by these materials, namely, toxicity (e.g., Cd-, Pb- or As-based QDs), scarcity (e.g., QD with In, Se, Te) and/or instability. Here we show that by combining quantum confinement with dedicated surface engineering, the biggest drawback of Si—the indirect bandgap nature—can be overcome, and a 'direct bandgap' variety of Si-QDs is created. We demonstrate this transformation on chemically synthesized Si-QDs using state-of-the-art optical spectroscopy and theoretical modelling. The carbon surface termination gives rise to drastic modification in electron and hole wavefunctions and radiative transitions between the lowest excited states of electron and hole attain 'direct bandgap-like' (phonon-less) character. This results in efficient fast emission, tunable within the visible spectral range by QD size. These findings are fully justified within a tight-binding theoretical model. When the C surface termination is replaced by oxygen, the emission is converted into the well-known red luminescence, with microsecond decay and limited spectral tunability. In that way, the 'direct bandgap' Si-QDs convert into the 'traditional' indirect bandgap form, thoroughly investigated in the past.Keywords：direct bandgap;optical spectroscopy;optoelectronics;organic capping;silicon quantum dots;tight binding model
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Published：2024-09-30 - Abstract：In a conventional flat plate solar cell under direct sunlight, light is received from the solar disk, but is re-emitted isotropically. This isotropic emission corresponds to a significant entropy increase in the solar cell, with a corresponding drop in efficiency. Here, using a detailed balance model, we show that limiting the emission angle of a high-quality GaAs solar cell is a feasible route to achieving power conversion efficiencies above 38% with a single junction. The highest efficiencies are predicted for a thin, light trapping cell with an ideal back reflector, though the scheme is robust to a non-ideal back reflector. Comparison with a conventional planar cell geometry illustrates that limiting emission angle in a light trapping geometry not only allows for much thinner cells, but also for significantly higher overall efficiencies with an excellent rear reflector. Finally, we present ray-tracing and detailed balance analysis of two angular coupler designs, show that significant efficiency improvements are possible with these couplers, and demonstrate initial fabrication of one coupler design.Keywords：detailed balance;GaAs solar cell;light trapping;photovoltaics;two photon lithography
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Published：2024-09-30 - Abstract：Phosphor-converted white light-emitting diodes for indoor illumination need to be warm-white (i.e., correlated color temperature <4000 K) with good color rendition (i.e., color rendering index >80). However, no single-phosphor, single-emitting-center-converted white light-emitting diodes can simultaneously satisfy the color temperature and rendition requirements due to the lack of sufficient red spectral component in the phosphors' emission spectrum. Here, we report a new yellow Ba0.93Eu0.07Al2O4 phosphor that has a new orthorhombic lattice structure and exhibits a broad yellow photoluminescence band with sufficient red spectral component. Warm-white emissions with correlated color temperature <4000 K and color rendering index >80 were readily achieved when combining the Ba0.93Eu0.07Al2O4 phosphor with a blue light-emitting diode (440–470 nm). This study demonstrates that warm-white light-emitting diodes with high color rendition (i.e., color rendering index >80) can be achieved based on single-phosphor, single-emitting-center conversion.Keywords：luminescence;single-phosphor-conversion;warm-white light-emitting diode;yellow phosphor
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Published：2024-09-30 - Abstract：Splitting white light into its constituent spectral components has been of interest ever since Newton first discovered the phenomenon of color separation. Many devices have since been conceived to achieve efficient wavelength separation, yet a large number of applications, e.g., in display technology, still use simple color absorption or rejection filters that absorb or reflect unwanted wavelengths, thus wasting luminous energy. Here, we demonstrate a novel microsized device concept that enables efficient color routing. The device operation is based on differential material dispersion in a waveguide array, which causes different wavelength signals to couple selectively into appropriate waveguides. A theoretical power delivery of greater than 50% for a tricolor wavelength router is obtained, compared to 33% expected from geometry alone. The principle of operation is demonstrated experimentally for a dual-color light field, where we achieve a higher than 70% routing efficiency (compared to 50% from geometry), thus highlighting the feasibility of this novel and promising approach.Keywords：color filters;dispersion;propagation;route;waveguides
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Published：2024-09-30 - Abstract：A systematic study has been conducted on microcavity organic light emitting diodes (OLEDs) based on green, red and blue phosphorescent emitters to elucidate the microcavity effects for different color emitters. We found that the luminance output is determined by the reflectivity of the semitransparent electrode and the photopic response of the green, red and blue emitters. While the luminance enhancements of blue and red phosphorescent microcavity devices are small, a current efficiency as high as 224 cd A−1 is obtained in the green phosphorescent microcavity OLEDs.Keywords：efficiency enhancements;microcavity;organic light emitting devices;phosphorescence;photopic luminosity
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Published：2024-09-30 - Abstract：Thermomechanical damage of nodules in dielectric multilayer coatings that are irradiated by nanosecond laser pulses has been interpreted with respect to mechanical properties and electric-field enhancement. However, the effect of electric-field enhancement in nodular damage, especially the influence of electric-field distributions, has never been directly demonstrated through experimental results, which prevents the achievement of a clear understanding of the damage process of nodular defects. Here, a systematic and comparative study was designed to reveal how electric-field distributions affect the damage behavior of nodules. To obtain reliable results, two series of artificial nodules with different geometries and film absorption characteristics were prepared from monodisperse silica microspheres. After establishing simplified geometrical models of the nodules, the electric-field enhancement was simulated using a three-dimensional finite-difference time-domain code. Then, the damage morphologies of the artificial nodules were directly compared with the simulated electric-field intensity profiles. For both series of nodules, the damage morphologies reproduced our simulated electric-field intensity distributions very well. These results indicated that the electric-field distribution was actually a bridge that connected the nodular mechanical properties to the final thermomechanical damage. Understanding of the damage mechanism of nodules was deepened by obtaining data on the influence of electric-field distributions on the damage behavior of nodules.Keywords：electric field;nanosecond-pulse-laser damage;nodular defects;thin films
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Published：2024-09-30 - Abstract：Optically coupled microcavities have emerged as photonic structures with promising properties for investigation of fundamental science as well as for applications. We report on the fabrication and spatially resolved spectroscopy of on-chip photonic molecule (PM) lasers consisting of two coupled, dye-doped polymeric microdisks on a silicon substrate. We investigate the fundamental lasing properties with focus on the spatial distribution of modes, the coupling dependent suppression of lasing modes, and in particular the application-oriented operation of these devices in aqueous environments. By depositing an additional polymer layer onto the lithographically structured cavities made of dye-doped poly(methyl methacrylate), coupling-gap widths below 150 nm with aspect ratios of the micro-/nanostructure exceeding 9∶1 are achieved. This enables strong optical coupling at visible wavelengths despite relatively small resonator radii of 25 µm. The lasing properties of dye-doped PMs are investigated using spatially resolved micro-photoluminescence (μ-PL) spectroscopy. This technique allows for the direct imaging of whispering-gallery modes (WGMs) in the photonics molecules. For subwavelength coupling gaps, we observe lasing from delocalized eigenstates of the PMs (termed in the following as super-modes). Using size-mismatched cavities, the lasing mode suppression for different coupling-gap widths is investigated. We further demonstrate single-mode lasing operation in aqueous environments with PMs, which are realized on a low-cost, polymer-on-silicon platform.Keywords：microcavity lasers;photonic molecules;visible lasers;whispering-gallery modes
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Published：2024-09-30 - Abstract：The typically tiny effect of radiation damping on a moving body can be amplified to a favorable extent by exploiting the sharp reflectivity slope at one edge of an optically induced stop-band in atoms loaded into an optical lattice. In this paper, this phenomenon is demonstrated for the periodically trapped and coherently driven cold 87Rb atoms, where radiation damping might be much larger than that anticipated in previous proposals and become comparable with radiation pressure. Such an enhancement could be observed even at speeds of only a few meters per second with less than 1.0% absorption, making radiation damping experimentally accessible.Keywords：optically induced stop-band;ordered atomic structure;radiation damping;radiation pressure
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Published：2024-09-30 - Abstract：Photonic devices increasingly require three-dimensional control of refractive index, but existing fabrication methods such as femtosecond micromachining, multilayer lithography and bulk diffusion can only address a select scale range, are often limited in complexity or thickness and have low throughput. We introduce a new fabrication method and polymeric material that can efficiently create mm3 optical devices with programmable, gradient index of refraction with arbitrary feature size. Index contrast of 0.1 is demonstrated, which is 100 times larger than femtosecond micromachining, and 20 times larger than commercial holographic photopolymers. This is achieved by repetitive microfluidic layering of a self-developing photopolymer structured by projection lithography. The process has the unusual property that total fabrication time for a fixed thickness decreases with the number of layers, enabling fabrication 105 faster than femtosecond micromachining. We demonstrate the process by sequentially writing 100 layers to fabricate a mm thick waveguide array.Keywords：holographic photopolymers;lithography;photonic device fabrication;single photon polymerization;three-dimensional structuring
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Published：2024-09-30 - Abstract：We report on two novel routes to generating a train of attosecond pulses from a broad discrete spectrum in the near-infrared–visible–ultraviolet range. One extends an integer-temporal-Talbot (ITT) concept to include high-order spectral dispersions and generates a pulse train that completely satisfies the transform-limited condition. The other numerically explores the optimum conditions under which we can obtain an attosecond pulse train that approximately satisfies the transform-limited condition. The second method is more practical than the first. Either of these methods is extremely simple and robust; we need only to place a few thin dispersive materials in the optical path and to adjust their thicknesses without spatially dispersing the frequency components. We numerically demonstrate the generation of a train of attosecond pulses with a transform-limited pulse duration of 728 as and a repetition period of 8.03 fs in gaseous parahydrogen.Keywords：adiabatic excitation of Raman transition;attosecond pulse generation;integer-temporal-Talbot (ITT) method;molecular modulation
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Published：2024-09-30 - Abstract：The resonant frequencies of the excited surface waves on a metal hole array with respect to the incident angle were studied in the terahertz region. The experimental and theoretical results demonstrate that the resonant peak of surface wave excitation splits into two when transmitted through a metal hole array off-normally. The high-order mode with resonant frequency above the cutoff frequency fc (plasma frequency effect) has a shorter attenuation length than that of the low-order mode whose resonant frequency is below fc. The reason is that the high-order mode is a coupled mode consisting of surface wave and hole modes, while the low-order mode is just an excited surface wave (which can be considered as the spoof surface plasmons). Our investigation may open a door to distinguish the spoof surface plasmons and the coupled modes of surface waves and hole modes.Keywords：angular dependence;cutoff frequency;metal hole arrays;surface waves;terahertz
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Published：2024-09-30 - Abstract：Modern miniaturization and the digitalization of characterization instruments greatly facilitate the diffusion of technological advances in new fields and generate innovative applications. The concept of a portable, inexpensive and semi-automated biosensing platform, or lab-on-a-chip, is a vision shared by many researchers and venture industries. Under this scope, we present a semiconductor monolithic integration approach to conduct surface plasmon resonance studies. This technology is already commonly used for biochemical characterization in pharmaceutical industries, but we have reduced the technological platform to a few nanometers in scale on a semiconductor chip. We evaluate the signal quality of this nanophotonic device using hyperspectral-imaging technology, and we compare its performance with that of a standard prism-based commercial system. Two standard biochemical agents are employed for this characterization study: bovine serum albumin and inactivated influenza A virus. Time resolutions of data acquisition varying between 360 and 2.2 s are presented, yielding 2.7×10−5–1.5×10−6 RIU resolutions, respectively.Keywords：GaAs/AlGaAs quantum well device;hyperspectral imaging technology;influenza A virus;integrated surface plasmon resonance biosensor
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Published：2024-09-30
Original Articles
Editorial
- Abstract：The strong light–matter interaction in ZnO-embedded microcavities has received great attention in recent years, due to its ability to generate the robust bosonic quasiparticles, exciton-polaritons, at or above room temperature. This review introduces the strong coupling effect in ZnO-based microcavities and describes the recent progress in this field. In addition, the report contains a systematic analysis of the room-temperature strong-coupling effects from relaxation to polariton lasing. The stable room temperature operation of polaritonic effects in a ZnO microcavity promises a wide range of practical applications in the future, such as ultra-low power consumption coherent light emitters in the ultraviolet region, polaritonic transport, and other fundamental of quantum optics in solid-state systems.Keywords：microcavity;polariton;strong coupling;ZnO
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Published：2024-09-30
Review
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