Showing papers in "Light-Science & Applications in 2014"

Coding metamaterials, digital metamaterials and programmable metamaterials

Abstract: Smart materials offering great freedom in manipulating electromagnetic radiation have been developed. This exciting new concept was realized by Tie Jun Cui and co-workers at the Southeast University, China, who developed digital metamaterials consisting of two kinds of unit cells whose different phase responses allow them to act as ‘0’ and ‘1’ bits. These cells can be judiciously arranged in sequences to enable controlled manipulation of electromagnetic waves. This is one-bit coding; higher-bit coding is possible by employing more kinds of unit cells. The researchers developed a metamaterial cell whose binary response can be controlled by a biased diode. By using a field-programmable gate array, they demonstrated that this digital metamaterial can be programmed. Such metamaterials are attractive for controlling radiation beams in antennas and for realizing other ‘smart’ metamaterials.

Ultrafast lasers—reliable tools for advanced materials processing

Abstract: The unique characteristics of ultrafast lasers, such as picosecond and femtosecond lasers, have opened up new avenues in materials processing that employ ultrashort pulse widths and extremely high peak intensities. Thus, ultrafast lasers are currently used widely for both fundamental research and practical applications. This review describes the characteristics of ultrafast laser processing and the recent advancements and applications of both surface and volume processing. Surface processing includes micromachining, micro- and nanostructuring, and nanoablation, while volume processing includes two-photon polymerization and three-dimensional (3D) processing within transparent materials. Commercial and industrial applications of ultrafast laser processing are also introduced, and a summary of the technology with future outlooks are also given. Scientists in Asia have reviewed the role of ultrafast lasers in materials processing. Koji Sugioka from RIKEN in Japan and Ya Cheng from the Shanghai Institute of Optics and Fine Mechanics in China describe how femtosecond and picosecond lasers can be used to perform useful tasks in both surface and volume processing. Such lasers can cut, drill and ablate a variety of materials with high precision, including metals, semiconductors, ceramics and glasses. They can also polymerize organic materials that contain a suitable photosensitizer and can three-dimensionally process inside transparent materials such as glass, and are already being used to fabricate medical stents, repair photomasks, drill ink-jet nozzles and pattern solar cells. The researchers also explain the characteristics of such lasers and the interaction of ultrashort, intense pulses of light with matter.

Generating optical orbital angular momentum at visible wavelengths using a plasmonic metasurface

Abstract: Visible, circularly polarised light can be transformed into light-carrying orbital angular momentum by a plasmonic metasurface. That is the finding of Ebrahim Karimi and co-workers at the University of Ottawa in Canada and the University of Rochester in the United States. Light with orbital angular momentum (owing to a twisted phase front) is traditionally generated using specially designed optical elements such as spatial light modulator, cylindrical lens mode converters and q-plate. The researchers have now shown that a plasmonic metasurface comprising an array of nano-antennas can couple spin-to-orbital angular momentum at thickness much smaller than the wavelength of the light with an efficiency of around 3%. The conversion takes place due to the birefringence present in the nanostructure array. This approach could yield ultrathin generators of visible light with orbital angular momentum, for potential applications in spectroscopy, imaging, sensing and quantum information.

Adaptive optical microscopy: the ongoing quest for a perfect image

Abstract: Adaptive optics is becoming a valuable tool for high resolution microscopy, providing correction for aberrations introduced by the refractive index structure of specimens. This is proving particularly promising for applications that require images from deep within biological tissue specimens. We review recent developments in adaptive microscopy, including methods and applications. A range of advances in different microscope modalities is covered and prospects for the future are discussed. Adaptive optics is used to improve image quality across a wide range of microscopy techniques. Martin Booth from the University of Oxford in the UK reviews how technologies such as deformable mirrors and spatial light modulators, which compensate for aberrations by locally controlling the wavefront of a light wave, are now improving the performance of multiphoton, confocal, widefield and super-resolution microscopes. The benefits of such improvements are especially appealing for images captured from within biological tissue (focal distances of tens to hundreds of micrometres), where low-order aberrations associated with smooth phase variations occur. One future challenge is the development of efficient measurement and correction schemes for higher-order phase variations.

Metallic nanostructures for light trapping in energy-harvesting devices

Abstract: Solar energy is abundant and environmentally friendly. Light trapping in solar-energy-harvesting devices or structures is of critical importance. This article reviews light trapping with metallic nanostructures for thin film solar cells and selective solar absorbers. The metallic nanostructures can either be used in reducing material thickness and device cost or in improving light absorbance and thereby improving conversion efficiency. The metallic nanostructures can contribute to light trapping by scattering and increasing the path length of light, by generating strong electromagnetic field in the active layer, or by multiple reflections/absorptions. We have also discussed the adverse effect of metallic nanostructures and how to solve these problems and take full advantage of the light-trapping effect. In recent years, researchers have demonstrated a number of new schemes for enhancing the absorption of light in solar cells. Chuan Fei Guo and colleagues from the University of Houston in the USA and National Center for Nanoscience and Technology of China in Beijing have now reviewed the use of metallic nanostructures for trapping light in photovoltaic devices. In particular, the presence of metallic nanoparticles in a solar cell or a solar absorber can aid light absorption by inducing strong, local field-enhancement effects or coupling to resonant plasmon modes. Such particles can also promote scattering and thus increase path lengths for light within the device. Solar cells that utilize this approach are either more efficient or substantially thinner than those that do not, thus reducing material costs and creating the opportunity for ultrathin, flexible devices.

Light scattering and surface plasmons on small spherical particles

Abstract: Light scattering by small particles has a long and interesting history in physics. Nonetheless, it continues to surprise with new insights and applications. This includes new discoveries, such as novel plasmonic effects, as well as exciting theoretical and experimental developments such as optical trapping, anomalous light scattering, optical tweezers, nanospasers, and novel aspects and realizations of Fano resonances. These have led to important new applications, including several ones in the biomedical area and in sensing techniques at the single-molecule level. There are additionally many potential future applications in optical devices and solar energy technologies. Here we review the fundamental aspects of light scattering by small spherical particles, emphasizing the phenomenological treatments and new developments in this field. The interaction of light with small spherical particles has long been a topic of interest to researchers. Indeed, understanding many natural phenomena, including rainbows and the solar corona, requires knowledge of how light behaves in such circumstances. Xiaofeng Fan and co-workers from Jilin University in China and Oak Ridge National Laboratory in the USA have now reviewed the physics and applications that arise during the interaction of light with small spherical particles. The researchers describe how Mie theory can be used to describe optical scattering by small dielectric particles, and, in the case of metallic particles, how light excites surface plasmons to generate an optical response featuring asymmetric Fano resonances. In the special case when metallic particles are surrounded by an optical gain medium, plasmons can be amplified; the resulting device is known as a ‘spaser’.

Fundamentals of phase-only liquid crystal on silicon (LCOS) devices

Abstract: This paper describes the fundamentals of phase-only liquid crystal on silicon (LCOS) technology, which have not been previously discussed in detail. This technology is widely utilized in high efficiency applications for real-time holography and diffractive optics. The paper begins with a brief introduction on the developmental trajectory of phase-only LCOS technology, followed by the correct selection of liquid crystal (LC) materials and corresponding electro-optic effects in such devices. Attention is focused on the essential requirements of the physical aspects of the LC layer as well as the indispensable parameters for the response time of the device. Furthermore, the basic functionalities embedded in the complementary metal oxide semiconductor (CMOS) silicon backplane for phase-only LCOS devices are illustrated, including two typical addressing schemes. Finally, the application of phase-only LCOS devices in real-time holography will be introduced in association with the use of cutting-edge computer-generated holograms. The capabilities and applications of phase-only liquid crystal on silicon (LCOS) technology are reviewed by scientists in China and the United Kingdom. Zichen Zhang and co-workers from Tsinghua University in Beijing and the University of Cambridge describe how an electronically controlled liquid-crystal pixel array on a silicon backplane can be useful for manipulating the phase of coherent light. Such LCOS phase modulators have many potential applications, including real-time holography, head-up displays, wavelength-selective switches and reconfigurable optical add-drop multiplexers. Commercial devices currently offer array sizes of up to about 1-inch diagonal with pixel pitches in the range 6–20 μm. However, the technology is still immature; the liquid-crystal materials and silicon control backplane need to be optimized in order to realize the potential of such devices.

Healthy, natural, efficient and tunable lighting: four-package white LEDs for optimizing the circadian effect, color quality and vision performance

Abstract: Researchers in Korea have discovered that carefully designed white LEDs can help optimize an individual's circadian rhythm. Ji Hye Oh and co-workers from Kookmin University studied four-package white LEDs, which consist of phosphor-converted red, amber and green LEDs, a blue LED and a long-wavelength dichroic filter. They compared these devices with a variety of traditional forms of lighting, and found that such four-package LED designs yield a high luminous efficacy, excellent color rendering and a tunable circadian effect for regulating the secretion of melatonin. The figures of merit suggest that carefully designed white LEDs could play an important role in creating smart lighting systems that help maintain good health, bring energy savings and provide optimal visualization of colors.

Optical storage arrays: a perspective for future big data storage

Abstract: The advance of nanophotonics has provided a variety of avenues for light–matter interaction at the nanometer scale through the enriched mechanisms for physical and chemical reactions induced by nanometer-confined optical probes in nanocomposite materials. These emerging nanophotonic devices and materials have enabled researchers to develop disruptive methods of tremendously increasing the storage capacity of current optical memory. In this paper, we present a review of the recent advancements in nanophotonics-enabled optical storage techniques. Particularly, we offer our perspective of using them as optical storage arrays for next-generation exabyte data centers. The science and technology of nanophotonics can help dramatically increase the capacity of optical discs. After reviewing research into next-generation optical data storage, Min Gu, Xiangping Li and Yaoyu Cao from the Swinburne University of Technology in Australia have offered their perspective of the creation of exabyte-scale optical data centers. They report that developments in ’super-resolution recording‚, which allow a light-sensitive material to be exposed to a focal spot that is smaller than the diffraction limit of light, will allow the size of recorded bits to shrink to just a few nanometres in size. This would ultimately allow a single disk to store petabytes of data and thus constitute a key component in optical storage arrays for ultrahigh-capacity optical data centers.

Handheld high-throughput plasmonic biosensor using computational on-chip imaging

Abstract: We demonstrate a handheld on-chip biosensing technology that employs plasmonic microarrays coupled with a lens-free computational imaging system towards multiplexed and high-throughput screening of biomolecular interactions for point-of-care applications and resource-limited settings. This lightweight and field-portable biosensing device, weighing 60 g and 7.5 cm tall, utilizes a compact optoelectronic sensor array to record the diffraction patterns of plasmonic nanostructures under uniform illumination by a single-light emitting diode tuned to the plasmonic mode of the nanoapertures. Employing a sensitive plasmonic array design that is combined with lens-free computational imaging, we demonstrate label-free and quantitative detection of biomolecules with a protein layer thickness down to 3 nm. Integrating large-scale plasmonic microarrays, our on-chip imaging platform enables simultaneous detection of protein mono- and bilayers on the same platform over a wide range of biomolecule concentrations. In this handheld device, we also employ an iterative phase retrieval-based image reconstruction method, which offers the ability to digitally image a highly multiplexed array of sensors on the same plasmonic chip, making this approach especially suitable for high-throughput diagnostic applications in field settings. Light: Science & Applications (2014) 3, e122; doi:10.1038/lsa.2014.3; published online 3 January 2014

100 GHz silicon-organic hybrid modulator

Abstract: Electro-optic modulation at frequencies of 100 GHz and beyond is important for photonic-electronic signal processing at the highest speeds. To date, however, only a small number of devices exist that can operate up to this frequency. In this study, we demonstrate that this frequency range can be addressed by nanophotonic, silicon-based modulators. We exploit the ultrafast Pockels effect by using the silicon–organic hybrid (SOH) platform, which combines highly nonlinear organic molecules with silicon waveguides. Until now, the bandwidth of these devices was limited by the losses of the radiofrequency (RF) signal and the RC (resistor-capacitor) time constant of the silicon structure. The RF losses are overcome by using a device as short as 500 µm, and the RC time constant is decreased by using a highly conductive electron accumulation layer and an improved gate insulator. Using this method, we demonstrate for the first time an integrated silicon modulator with a 3dB bandwidth at an operating frequency beyond 100 GHz. Our results clearly indicate that the RC time constant is not a fundamental speed limitation of SOH devices at these frequencies. Our device has a voltage–length product of only VπL=11 V mm, which compares favorably with the best silicon-photonic modulators available today. Using cladding materials with stronger nonlinearities, the voltage–length product is expected to improve by more than an order of magnitude. Researchers have developed an integrated silicon–organic device capable of modulating light at frequencies of up to 100 GHz. The ultrafast modulator, fabricated by Luca Alloatti and co-workers, relies on the electro-optic Pockels effect that occurs in a polymer cladding covering a silicon slot waveguide. Application of an electric field to the polymer causes its refractive index to change, which in turn modifies the phase of light passing through it. The 500-µm-long device has a half-wave voltage of 22 V, meaning that this voltage is required in order to achieve a π phase shift for the output light. However, the researchers are confident that using a cladding material with a stronger nonlinearity could improve this figure of merit by a factor of ten.

Design and fabrication of broadband ultralow reflectivity black Si surfaces by laser micro/nanoprocessing

Abstract: Light collection efficiency is an important factor that affects the performance of many optical and optoelectronic devices. In these devices, the high reflectivity of interfaces can hinder efficient light collection. To minimize unwanted reflection, anti-reflection surfaces can be fabricated by micro/nanopatterning. In this paper, we investigate the fabrication of broadband anti-reflection Si surfaces by laser micro/nanoprocessing. Laser direct writing is applied to create microstructures on Si surfaces that reduce light reflection by light trapping. In addition, laser interference lithography and metal assisted chemical etching are adopted to fabricate the Si nanowire arrays. The anti-reflection performance is greatly improved by the high aspect ratio subwavelength structures, which create gradients of refractive index from the ambient air to the substrate. Furthermore, by decoration of the Si nanowires with metallic nanoparticles, surface plasmon resonance can be used to further control the broadband reflections, reducing the reflection to below 1.0% across from 300 to 1200 nm. An average reflection of 0.8% is achieved.

Efficient unidirectional polarization-controlled excitation of surface plasmon polaritons

Abstract: Devices capable of converting light of any polarization into surface plasmon polaritons could aid the development of on-chip optical circuits. Anders Pors and co-workers from the University of Southern Denmark and the University of Burgundy in France have now fabricated such a ‘coupler’ from gradient metasurfaces — arrays of miniature gold patches on a thin dielectric layer covering a metal-coated glass substrate. Their device is compatible with wavelengths of light in the telecommunications window (around 1,500 nm) and does not require a specific polarization in order to excite plasmons. Furthermore, it allows the direction of plasmon propagation to be controlled by changing the polarization of the incident beam. Calculations indicate that the coupling efficiency could be as high as 40%, with a directivity of around 50:1.

A visible light-driven plasmonic photocatalyst

Abstract: Researchers in Japan have developed a visible-light-driven photocatalyst by exploiting the plasmonic properties of gold nanoparticles. Francesca Pincella and co-workers fabricated the photocatalyst by depositing a two-dimensional array of gold nanoparticles on top of a transparent conductive substrate of indium-tin-oxide-coated quartz. They then covered the gold nanoparticles with a monolayer of trimethoxyoctylsilane, which acts as an anchoring agent for the final layer of titania nanocrystals. Experiments involving the photocatalytic breakdown of methylene blue under visible (700 nm) and ultraviolet (250–380 nm) light suggest that operation is considerably superior to that of conventional titania photocatalysts. The performance in the visible region is attributed to two-photon absorption, which is boosted by the plasmon resonance and near-field enhancement of the gold nanoparticles.

Nanowire-supported plasmonic waveguide for remote excitation of surface-enhanced Raman scattering

Abstract: Due to its amazing ability to manipulate light at the nanoscale, plasmonics has become one of the most interesting topics in the field of light-matter interaction. As a promising application of plasmonics, surface-enhanced Raman scattering (SERS) has been widely used in scientific investigations and material analysis. The large enhanced Raman signals are mainly caused by the extremely enhanced electromagnetic field that results from localized surface plasmon polaritons. Recently, a novel SERS technology called remote SERS has been reported, combining both localized surface plasmon polaritons and propagating surface plasmon polaritons (PSPPs, or called plasmonic waveguide), which may be found in prominent applications in special circumstances compared to traditional local SERS. In this article, we review the mechanism of remote SERS and its development since it was first reported in 2009. Various remote metal systems based on plasmonic waveguides, such as nanoparticle-nanowire systems, single nanowire systems, crossed nanowire systems and nanowire dimer systems, are introduced, and recent novel applications, such as sensors, plasmon-driven surface-catalyzed reactions and Raman optical activity, are also presented. Furthermore, studies of remote SERS in dielectric and organic systems based on dielectric waveguides remind us that this useful technology has additional, tremendous application prospects that have not been realized in metal systems.

Quantum digital spiral imaging

Abstract: We demonstrate that the combination of digital spiral imaging with high-dimensional orbital angular momentum (OAM) entanglement can be used for efficiently probing and identifying pure phase objects, where the probing light does not necessarily touch the object, via the experimental, non-local decomposition of non-integer pure phase vortices in OAM-entangled photon pairs. The entangled photons are generated by parametric downconversion and then measured with spatial light modulators and single-mode fibers. The fractional phase vortices are defined in the idler photons, while their corresponding spiral spectra are obtained non-locally by scanning the measured OAM states in the signal photons. We conceptually illustrate our results with the biphoton Klyshko picture and the effective dimensionality to demonstrate the high-dimensional nature of the associated quantum OAM channels. Our result is a proof of concept that quantum imaging techniques exploiting high-dimensional entanglement can potentially be used for remote sensing.

Adding fifth dimension to optoacoustic imaging: volumetric time-resolved spectrally enriched tomography

Abstract: Xose Luis Dean-Ben and Daniel Razansky at the Technical University of Munich and the Helmholtz Centre Munich have demonstrated three-dimensional multispectral optoacoustic bioimaging in real time. Optoacoustic imaging is a technique wherein a laser beam absorbed by biological tissue induces the creation of a characteristic acoustic signal, which is then detected in the same way as an ultrasound. The technique plays an important role in imaging biological processes in the body due to its excellent optical contrast and high spatial resolution in deep tissues. To detect various substances such as contrast agents in living tissue, scans at multiple wavelengths are required. By employing an ultrafast millisecond timescale laser wavelength tuning along with instantaneous acquisition of volumetric image data, the researchers demonstrated a fully five-dimensional imaging system, thus providing unprecedented flexibility in the mapping of biological processes in-vivo.

Multi-ion cooperative processes in Yb3+ clusters

Abstract: Cooperative luminescence (CL) occurs in spectral regions in which single ions do not have energy levels. It was first observed more than 40 years ago, and all results reported so far are from a pair of ions. In this work, upconverted CL of three Yb3+ ions was observed in the ultraviolet (UV) region under near-infrared (NIR) excitation. The UV CL intensity showed a cubic dependence on the NIR pump power, whereas the luminescence lifetime was nearly one-third the luminescence lifetime of single Yb3+ ions. The triplet CL (TCL) has a clear spectral structure, in which most emission peaks are consistent with the self-convoluted spectra from single Yb3+ ions. Blue shifts were observed for certain peaks, indicating complex interactions among the excited Yb3+ ions. The probability of the TCL process versus the average distances among three Yb3+ ions was derived via the first- and second-order corrections to the wave functions of lanthanide ions, indicating that the formation of Yb3+ clusters containing closely spaced ions favors the occurrence of the multi-ion interaction processes. Furthermore, the cooperative sensitization of one Gd3+ ion by four excited Yb3+ ions (Yb3+-tetramer) was demonstrated experimentally, which exhibited a novel upconversion mechanism—cluster sensitization. Our results are intriguing for further exploring quantum transitions that simultaneously involve multiple ions. Scientists in China have observed ultraviolet light emission from a cluster of rare-earth Yb3+ ions in a CaF2 matrix. Zhen-Yu Liu and co-workers from Jilin University excited the polycrystalline powders with 978 nm near-infrared laser light. The resulting up-converted emission in the ultraviolet is believed to be due to a phenomenon called co-operative luminescence, whereby multiple Yb3+ ions emit a single shorter-wavelength photon by simultaneous depopulation from their excited states. In this particular case, three Yb3+ ions are thought to be involved — a hypothesis confirmed by low-temperature laser spectroscopy of the sample. This process is interesting because it could lead to light emission at wavelengths that lie outside the absorption and emission bands of single ions.

Emission properties of nanolasers during the transition to lasing

Abstract: This review addresses ongoing discussions involving nanolaser experiments, particularly those related to thresholdless lasing or few-emitter devices. A quantum-optical (quantum-mechanical active medium and radiation field) theory is used to examine the emission properties of nanolasers under different experimental configurations. The active medium is treated as inhomogeneously broadened semiconductor quantum dots embedded in a quantum well, where carriers are introduced via current injection. Comparisons are made between a conventional laser and a nanolaser with a spontaneous emission factor of unity, as well as a laser with only a few quantum dots providing the gain. It is found that the combined exploration of intensity, coherence time, photon autocorrelation function and carrier spectral hole burning can provide a unique and consistent picture of nanolasers in the new regimes of laser operation during the transition from thermal to coherent emission. Furthermore, by reducing the number of quantum dots in the optical cavity, a clear indication of non-classical photon statistics is observed before the single-quantum-dot limit is reached. Nanolasers operate in a regime distinctly different to that of conventional lasers, and a consistent model of nanolasing physics is needed. Whereas conventional lasers are characterized by a marked intensity jump at the onset of lasing, micro- and nanocavity lasers with well-developed three-dimensional optical mode confinement can have a vanishing small intensity jump approaching thresholdless behavior. Weng Chow from Sandia National Laboratories in the United States, with colleagues Frank Jahnke and Christopher Gies from the University of Bremen in Germany, has reviewed recent nanolaser experiments and developed a model based on quantum-optical theory to examine photon statistics in the different operational regime of nanolasers compared with conventional lasers. The results dispute the notation of thresholdless lasing and suggests the emergence of non-classical photon statistics for few-atom or few-quantum-dot active regions.

Tunable multiband terahertz metamaterials using a reconfigurable electric split-ring resonator array

Abstract: We demonstrate micromachined reconfigurable metamaterials working at multiple frequencies simultaneously in the terahertz range. The proposed metamaterial structures can be structurally reconfigured by employing flexible microelectromechanical system-based cantilevers in the resonators, which are designed to deform out of plane under an external stimulus. The proposed metamaterial structures provide not only multiband resonance frequency operation but also polarization-dependent tunability. Three kinds of metamaterials are investigated as electric split-ring resonator (eSRR) arrays with different positions of the split. By moving the position of the split away from the resonator's center, the eSRR exhibits anisotropy, with the dipole resonance splitting into two resonances. The dipole–dipole coupling strength can be continuously adjusted, which enables the electromagnetic response to be tailored by adjusting the direct current (DC) voltage between the released cantilevers and the silicon substrate. The observed tunability of the eSRRs is found to be dependent on the polarization of the incident terahertz wave. This polarization-dependent tunability is demonstrated by both experimental measurements and electromagnetic simulations. Researchers in Singapore have developed metamaterials whose shape and optical properties can be easily reconfigured. Metamaterials are artificial structures much smaller than the wavelength of light that can achieve functionalities not possible with conventional optics. Chengkuo Lee and colleagues at the National University of Singapore achieved reconfigurable control by fabricating metamaterials on micrometer-scale cantilevers. Applying an electrical voltage bends the cantilevers vertically and thus changes the shape of each component in the metamaterial. As the active elements of the device move away from each other with increasing cantilever height, the optical resonance of the device shifts. The researchers performed their investigation in the terahertz regime, although in principle the concept can be applied to the entire electromagnetic spectrum. This approach could lead to novel applications in which optical functionalities can be freely tuned by an electrical voltage.

Manipulating Bloch surface waves in 2D: a platform concept-based flat lens

Abstract: Controlling Bloch surface waves in a multilayer dielectric yields a new platform for creating two-dimensional photonic circuitry. Libo Yu and co-workers from the Swiss Federal Institute of Technology in Lausanne used an ultrathin (λ/15) polymer coating to modify the local effective refractive index of a dielectric multilayer stack that guides Bloch surface waves. Careful design of the shape of a polymer layer makes it possible to deflect, diffract and focus the waves within a planar geometry. The researchers used this approach to construct a flat lens capable of operating at a wavelength of 1.5 µm, and used a multiheterodyne scanning near-field optical microscope to observe the near-field behavior of the platform. They say that many other forms of photonic component should be possible.

Hybrid 2D-3D optical devices for integrated optics by direct laser writing

Abstract: Integrated optical chips have already been established for application in optical communication. They also offer interesting future perspectives for integrated quantum optics on a chip. At present, however, they are mostly fabricated using essentially planar fabrication approaches like electron-beam lithography or UV optical lithography. Many further design options would arise if one had complete fabrication freedom in regard to the third dimension normal to the chip without having to give up the virtues and the know-how of existing planar fabrication technologies. As a step in this direction, we here use three-dimensional dip-in direct-laser-writing optical lithography to fabricate three-dimensional polymeric functional devices on pre-fabricated planar optical chips containing Si3N4 waveguides as well as grating couplers made by standard electron-beam lithography. The first example is a polymeric dielectric rectangular-shaped waveguide which is connected to Si3N4 waveguides and that is adiabatically twisted along its axis to achieve geometrical rotation of linear polarization on the chip. The rotator’s broadband performance at around 1550 nm wavelength is verified by polarization-dependent grating couplers. Such polarization rotation on the optical chip cannot easily be achieved by other means. The second example is a whispering-gallery-mode optical resonator connected to Si3N4 waveguides on the chip via polymeric waveguides. By mechanically connecting the latter to the disk, we can control the coupling to the resonator and, at the same time, guarantee mechanical stability of the three-dimensional architecture on the chip. Direct laser writing is a popular scheme for constructing three-dimensional integrated optical structures. Martin Schumann and co-workers from the Karlsruhe Institute of Technology and the Institute of Nanotechnology in Germany used two-photon polymerization to create three-dimensional polymer objects such as bridge waveguides, a twisted-waveguide polarization rotator and free-standing disk resonators. The structures, which would be difficult or impossible to construct using planar lithography, were successfully integrated with silicon optical chips featuring silicon nitride waveguides that guide light in the 1,550 nm telecommunications wavelength window. The researchers say that their approach could also be used to provide convenient access to three-dimensional photonic crystals. An advanced form of this approach that exploits higher resolutions would allow the construction of structures that are compatible with visible wavelengths.

Protein-based soft micro-optics fabricated by femtosecond laser direct writing

Abstract: In this work, we report a novel soft diffractive micro-optics, called ‘microscale kinoform phase-type lens (micro-KPL)’, which is fabricated by femtosecond laser direct writing (FsLDW) using bovine serum albumin (BSA) as building blocks and flexible polydimethylsiloxane (PDMS) slices as substrates. By carefully optimizing various process parameters of FsLDW (e.g., average laser power density, scanning step, exposure time on a single point and protein concentration), the as-formed protein micro-KPLs exhibit excellent surface quality, well-defined three-dimensional (3D) geometry and distinctive optical properties, even in relatively harsh operation environments (for instance, in strong acid or base). Laser shaping, imaging and other optical performances can be easily achieved. More importantly, micro-KPLs also have unique flexible and stretchable properties as well as good biocompatibility and biodegradability. Therefore, such protein hydrogel-based micro-optics may have great potential applications, such as in flexible and stretchable photonics and optics, soft integrated optical microsystems and bioimplantable devices. Scientists have used direct laser writing to fabricate flexible and biocompatible optical elements from a protein hydrogel. Yun-Lu Sun and co-workers from Jilin University in China and Pohang University of Science and Technology in Korea say that the miniature optical components produced using this approach could be useful for use in photonic implants or as stretchable optical devices. They fabricated soft, phase-type diffractive lenses with diameters of 50–100 µm by focusing femtosecond pulses from a Ti:sapphire laser into an aqueous ‘protein ink’ comprising a mixture of bovine serum albumin and the photosensitizer methylene blue. Irradiated regions underwent two-photon polymerization to form a soft protein hydrogel. By moving both the laser beam and the sample, the researchers successfully fabricated a three-dimensional optic featuring the desired series of concentric rings needed to act as a phase-type lens.

Tweezing and manipulating micro- and nanoparticles by optical nonlinear endoscopy

Abstract: The precise control and manipulation of micro- and nanoparticles using an optical endoscope are potentially important in biomedical studies, bedside diagnosis and treatment in an aquatic internal organ environment, but they have not yet been achieved. Here, for the first time, we demonstrate optical nonlinear endoscopic tweezers (ONETs) for directly controlling and manipulating aquatic micro- and nanobeads as well as gold nanorods. It is found that two-photon absorption can enhance the trapping force on fluorescent nanobeads by up to four orders of magnitude compared with dielectric nanobeads of the same size. More importantly, two-photon excitation leads to a plasmon-mediated optothermal attracting force on nanorods, which can extend far beyond the focal spot. This new phenomenon facilitates a snowball effect that allows the fast uploading of nanorods to a targeted cell followed by thermal treatment within 1 min. As two-photon absorption allows an operation wavelength at the center of the transmission window of human tissue, our work demonstrates that ONET is potentially an unprecedented tool for precisely specifying the location and dosage of drug particles and for rapidly uploading metallic nanoparticles to individual cancer cells for treatment. Two-photon absorption can dramatically enhance the trapping force applied to fluorescent nanobeads and metallic nanoparticles. Min Gu and co-workers at the Swinburne University of Technology in Australia say that their fibre-based optical nonlinear endoscopic tweezers, which exploit two-photon absorption, can provide a trapping force that is three to four orders of magnitude stronger than usual. Their device could therefore be a potentially important tool for in vivo biomedical studies. In principle, the nonlinear tweezers allow large amounts of gold nanorods to be delivered rapidly to a desired location, for example to kill cancerous cells. This allows operation with a near-infrared source at the peak in transmission of biological tissue (800 nm), thus allowing greater penetration and reduced photodamage in the surrounding area.

Colored ultrathin hybrid photovoltaics with high quantum efficiency

Abstract: Most current solar panels are fabricated via complex processes using expensive semiconductor materials, and they are rigid and heavy with a dull, black appearance. As a result of their non-aesthetic appearance and weight, they are primarily installed on rooftops to minimize their negative impact on building appearance. The large surfaces and interiors of modern buildings are not efficiently utilized for potential electric power generation. Here, we introduce dual-function solar cells based on ultrathin dopant-free amorphous silicon embedded in an optical cavity that not only efficiently extract the photogenerated carriers but also display distinctive colors with the desired angle-insensitive appearances. Light-energy-harvesting colored signage is demonstrated. Furthermore, a cascaded photovoltaics scheme based on tunable spectrum splitting can be employed to increase power efficiency by absorbing a broader band of light energy. This study pioneers a new approach to architecturally compatible and decorative thin-film photovoltaics. Ultrathin solar cells that can also act as coloured signs and displays have been developed by scientists in the USA. Kyu-Tae Lee and co-workers from the University of Michigan fabricated them by embedding a very thin (10–30 nm) layer of amorphous silicon within a metal–semiconductor–metal optical cavity. The cavity acts as a Fabry–Perot resonator that reflects a particular colour; importantly, it is insensitive to the polarization state or angle (up to 60°) of the incident light. By combining cells of different thickness (and hence different colours), arbitrary patterns or images can be created. The design has yielded solar cells with power conversion efficiencies of around 3%, despite using an amorphous silicon layer that is ten times thinner than those usually found in solar cells.

Surface plasmon excitation in semitransparent inverted polymer photovoltaic devices and their applications as label-free optical sensors

Abstract: Herein, we report on surface plasmon (SP)-sensitive semitransparent inverted polymer photovoltaic (PV) devices that are based on multilayered material systems consisting of poly(3-hexylthiophene): fullerene-derivative bulk-heterojunction PV layers and thin gold or silver anodes. We demonstrate that these PV devices allow the simultaneous generation of both electrical power and SPs on their anodes for photoexcitation just above the optical absorption edge of the PV layers, resulting not only in attenuated total reflection, but also in attenuated photocurrent generation (APG) under the SP resonance (SPR) condition. Moreover, we also confirm that the biomolecular interaction of biotin–streptavidin on the PV devices can be precisely detected via apparent SPR angle shifts in the APG spectra, even without the need for complex attenuated total reflection configurations. We highlight our view that APG measurements made using these PV devices show great potential for the development of future generations of compact and highly sensitive SPR-based optical sensors. Researchers investigate surface plasmon excitation in photovoltaic devices and explore their application as highly sensitive optical sensors. Byoungchoo Park and co-workers at Kwangwoon University in South Korea have developed semitransparent inverted polymer photovoltaic devices with a planar multilayer structure that are capable of simultaneously generating electrical power and surface plasmons on irradiation. When surface plasmon resonance is induced in these devices, both total internal reflection and photocurrent generation are attenuated. The researchers demonstrated that this property can be exploited to sensitively detect biological compounds. Specifically, they used apparent shifts in the surface plasmon resonance angle in attenuated photocurrent generation to detect the biomolecular interaction of biotin-streptavidin. They conclude that these devices have great potential as next-generation optical sensors that are simple, inexpensive, compact and highly sensitive.

Doubling the field of view in off-axis low-coherence interferometric imaging

Abstract: Researchers in Israel have developed a method for doubling the field of view in single-exposure depth-resolved holographic imaging. Pinhas Girshovitz and Natan Shaked from Tel Aviv University based their technique on digital interferometric microscopy, which uses interference effects to extract quantitative information on the thickness or height profile of the imaged object. Such information is useful, for example, in imaging of biological samples without labeling, or for optically profiling elements for non-destructive testing with sub-nanometer accuracy. Unfortunately, to enable capturing the entire quantitative image in a single exposure, a reduction in the field of view occurs. Girshovitz and Shaked proposed a way to optically compress more information into the same camera frame without loss in the imaging details or the magnification. A compact and portable interferometric module, positioned only in the output of the imaging system, splits the image beam into two components that are then superimposed at different angles on the imaging camera. This multiplexing approach increases the information in the image and doubles the field of view over previous techniques. Therefore, they were able to image wider samples or acquire the samples in faster frame rates. The main experimental demonstrations include fast high-magnification quantitative imaging of a swimming sperm cell, and non-destructive profiling of a thin element during rapid lithography process.

Enhancing precision in fs-laser material processing by simultaneous spatial and temporal focusing

Abstract: In recent years, femtosecond (fs)-lasers have evolved into a versatile tool for high precision micromachining of transparent materials because nonlinear absorption in the focus can result in refractive index modifications or material disruptions. However, when high pulse energies or low numerical apertures are required, nonlinear side effects such as self-focusing, filamentation or white light generation can decrease the modification quality. In this paper, we apply simultaneous spatial and temporal focusing (SSTF) to overcome these limitations. The main advantage of SSTF is that the ultrashort pulse is only formed at the focal plane, thereby confining the intensity distribution strongly to the focal volume and suppressing detrimental nonlinear side effects. Thus, we investigate the optical breakdown within a water cell by pump-probe shadowgraphy, comparing conventional focusing and SSTF under equivalent focusing conditions. The plasma formation is well confined for low pulse energies <2 µJ, but higher pulse energies lead to the filamentation and break-up of the disruptions for conventional focusing, thereby decreasing the modification quality. In contrast, plasma induced by SSTF stays well confined to the focal plane, even for high pulse energies up to 8 µJ, preventing extended filaments, side branches or break-up of the disruptions. Furthermore, while conventional focusing leads to broadband supercontinuum generation, only marginal spectral broadening is observed using SSTF. These experimental findings are in excellent agreement with numerical simulations of the nonlinear pulse propagation and interaction processes. Therefore, SSTF appears to be a powerful tool to control the processing of transparent materials, e.g., for precise ophthalmic fs-surgery.

Experimental control of optical helicity in nanophotonics

Abstract: An analysis of light–matter interactions based on symmetries can provide valuable insight, particularly because it reveals which quantities are conserved and which ones can be transformed within a physical system. In this context, helicity can be a useful addition to more commonly considered observables such as angular momentum. The question arises how to treat helicity, the projection of the total angular momentum onto the linear momentum direction, in practical experiments. In this paper, we put forward a simple but versatile experimental treatment of helicity. We then apply the proposed method to the scattering of light by isolated cylindrical nanoapertures in a gold film. This allows us to study the helicity transformation taking place during the interaction of focused light with the nanoapertures. In particular, we observe from the transmitted light that the scaling of the helicity transformed component with the aperture size is very different to the direct helicity component.

Refractometric biosensing based on optical phase flips in sparse and short-range-ordered nanoplasmonic layers

Abstract: Noble metal nanoparticles support localized surface plasmon resonances (LSPRs) that are extremely sensitive to the local dielectric properties of the environment within distances up to 10-100 nm from the metal surface. The significant overlap between the sensing volume of the nanoparticles and the size of biological macromolecules has made LSPR biosensing a key field for the application of plasmonics. Recent advancements in evaluating plasmonic refractometric sensors have suggested that the phase detection of light can surpass the sensitivity of standard intensity-based detection techniques. Here, we experimentally confirm that the phase of light can be used to precisely track local refractive index changes induced by biomolecular reactions, even for dilute and layers of short-range-ordered plasmonic nanoparticles. In particular, we demonstrate that the sensitivity can be enhanced by tuning in to a zero reflection condition, in which an abrupt phase flip of the reflected light is achieved. Using a cost-effective interference fringe tracking technique, we demonstrate that phase measurements yield an approximately one order of magnitude larger relative shift compared with traditional LSPR measurements for the model system of NeutrAvidin binding to biotinylated nanodisks.