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      Vol. 0 ， Issue 0 ， 2020

        Reviews

      • Chuan Fei Guo,Tianyi Sun,Feng Cao,Qian Liu,Zhifeng Ren
        Vol. 3, Issue 2, (2014) DOI: 10.1038/lsa.2014.42
        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.  
        Keywords：conversion efficiency;light trapping;photovoltaics;plasmonics;solar absorber   
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        Published：2024-09-30
      • Martin J Booth
        Vol. 3, Issue 2, (2014) DOI: 10.1038/lsa.2014.46
        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.  
        Keywords：aberrations;adaptive optics;microscopy   
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        Published：2024-09-30
      • Min Gu,Xiangping Li,Yaoyu Cao
        Vol. 3, Issue 2, (2014) DOI: 10.1038/lsa.2014.58
        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.  
        Keywords：big data centers;optical data storage;optical storage arrays;super-resolution   
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        Published：2024-09-30
      • Koji Sugioka,Ya Cheng
        Vol. 3, Issue 2, (2014) DOI: 10.1038/lsa.2014.30
        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.  
        Keywords：3D fabrication;industrial application;micromachining;nanofabrication;ultrafast laser   
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        Published：2024-09-30

        Original Articles

      • Stefan Witte,Vasco T Tenner,Daniel WE Noom,Kjeld SE Eikema
        Vol. 3, Issue 2, (2014) DOI: 10.1038/lsa.2014.44
        Abstract：Lensless imaging is an approach to microscopy in which a high-resolution image of an object is reconstructed from one or more measured diffraction patterns, providing a solution in situations where the use of imaging optics is not possible. However, current lensless imaging methods are typically limited by the need for a light source with a narrow, stable and accurately known spectrum. We have developed a general approach to lensless imaging without spectral bandwidth limitations or sample requirements. We use two time-delayed coherent light pulses and show that scanning the pulse-to-pulse time delay allows the reconstruction of diffraction-limited images for all the spectral components in the pulse. In addition, we introduce an iterative phase retrieval algorithm that uses these spectrally resolved Fresnel diffraction patterns to obtain high-resolution images of complex extended objects. We demonstrate this two-pulse imaging method with octave-spanning visible light sources, in both transmission and reflection geometries, and with broadband extreme-ultraviolet radiation from a high-harmonic generation source. Our approach enables effective use of low-flux ultra-broadband sources, such as table-top high-harmonic generation systems, for high-resolution imaging.  
        Keywords：coherent diffractive imaging;extreme ultraviolet;Fourier transform spectroscopy;high-harmonic generation;lensless imaging   
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        Published：2024-09-30
      • Ebrahim Karimi,Sebastian A Schulz,Israel De Leon,Hammam Qassim,Jeremy Upham,Robert W Boyd
        Vol. 3, Issue 2, (2014) DOI: 10.1038/lsa.2014.48
        Abstract：Light beams with a helical phase-front possess orbital angular momentum along their direction of propagation in addition to the spin angular momentum that describes their polarisation. Until recently, it was thought that these two 'rotational' motions of light were largely independent and could not be coupled during light–matter interactions. However, it is now known that interactions with carefully designed complex media can result in spin-to-orbit coupling, where a change of the spin angular momentum will modify the orbital angular momentum and vice versa. In this work, we propose and demonstrate that the birefringence of plasmonic nanostructures can be wielded to transform circularly polarised light into light carrying orbital angular momentum. A device operating at visible wavelengths is designed from a space-variant array of subwavelength plasmonic nano-antennas. Experiment confirms that circularly polarised light transmitted through the device is imbued with orbital angular momentum of ±2ħ (with conversion efficiency of at least 1%). This technology paves the way towards ultrathin orbital angular momentum generators that could be integrated into applications for spectroscopy, nanoscale sensing and classical or quantum communications using integrated photonic devices.  
        Keywords：light orbital angular momentum;light spin angular momentum;plasmonic metasurface   
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        Published：2024-09-30
      • Robert Kammel,Roland Ackermann,Jens Thomas,Jörg Götte,Stefan Skupin,Andreas Tünnermann,Stefan Nolte
        Vol. 3, Issue 2, (2014) DOI: 10.1038/lsa.2014.50
        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.  
        Keywords：filamentation;fs-laser surgery;laser-induced optical breakdown;materials processing;plasma shadowgraphy   
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        Published：2024-09-30
      • Fusheng Ma,Yu-Sheng Lin,Xinhai Zhang,Chengkuo Lee
        Vol. 3, Issue 2, (2014) DOI: 10.1038/lsa.2014.52
        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.  
        Keywords：MEMS;reconfigurable metamaterials;split-ring resonator;terahertz;tunability   
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        Published：2024-09-30
      • Luca Alloatti,Robert Palmer,Sebastian Diebold,Kai Philipp Pahl,Baoquan Chen,Raluca Dinu,Maryse Fournier,Jean-Marc Fedeli,Thomas Zwick,Wolfgang Freude,Christian Koos,Juerg Leuthold
        Vol. 3, Issue 2, (2014) DOI: 10.1038/lsa.2014.54
        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.  
        Keywords：100GHz;high-speed silicon modulator;nanophotonics;silicon–organic hybrid   
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        Published：2024-09-30
      • Kitty Kumar,Kenneth KC Lee,Jianzhao Li,Jun Nogami,Nazir P Kherani,Peter R Herman
        Vol. 3, Issue 2, (2014) DOI: 10.1038/lsa.2014.38
        Abstract：The confinement of laser interactions inside transparent materials assisted by tight optical focusing and short-pulsed nonlinear interactions has driven many high-resolution patterning and probing applications in science and technology. In thin transparent films, laser interactions confined to the film/substrate interface have underpinned blistering and ejection processes for nanofluidic channel fabrication, film patterning and cell catapulting. Here, we harness femtosecond lasers to drive nonlinear interactions within Fabry–Perot interference fringes to define narrow nanolength scale zones for highly resolved internal structuring of a film of refractive index, nfilm, at fringe maxima separated by λ/2nfilm. This novel interaction internally cleaves the film to open subwavelength internal cavities and form thin membranes at single or multiple depths from which follow significant opportunities for writing multilevel nanofluidic channels inside the film, as well as ejecting nanodisks at quantized film depths for coloring and three-dimensional surface patterning that promise new compact types of lab-in-film devices.  
        Keywords：blistering;dielectrics;laser processing;quantized etching;subwavelength processing;thin films   
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        Published：2024-09-30
      • Pinhas Girshovitz,Natan T Shaked
        Vol. 3, Issue 2, (2014) DOI: 10.1038/lsa.2014.32
        Abstract：We present a new interferometric and holographic approach, named interferometry with doubled imaging area (IDIA), with which it is possible to double the camera field of view while performing off-axis interferometric imaging, without changing the imaging parameters, such as the magnification and the resolution. This technique enables quantitative amplitude and phase imaging of wider samples without reducing the acquisition frame rate due to scanning. The method is implemented using a compact interferometric module that connects to a regular digital camera, and is useful in a wide range of applications in which neither the field of view nor the acquisition rate can be compromised. Specifically, the IDIA principle allows doubling the off-axis interferometric field of view, which might be narrower than the camera field of view due to low-coherence illumination. We demonstrate the proposed technique for scan-free quantitative optical thickness imaging of microscopic biological samples, including live neurons and a human sperm cell in rapid motion under high magnification. In addition, we used the IDIA principle to perform non-destructive profilometry during a rapid lithography process of transparent structures.  
        Keywords：cell imaging;holography;interferometry;lithography;quantitative optical microscopy   
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        Published：2024-09-30
      • Alexander M Rubenchik,Michail P Fedoruk,Sergei K Turitsyn
        Vol. 3, Issue 2, (2014) DOI: 10.1038/lsa.2014.40
        Abstract：A ground-based laser system for space-debris cleaning will use powerful laser pulses that can self-focus while propagating through the atmosphere. We demonstrate that for the relevant laser parameters, this self-focusing can noticeably decrease the laser intensity on the target. We show that the detrimental effect can be, to a great extent, compensated for by applying the optimal initial beam defocusing. The effect of laser elevation on the system performance is discussed.  
        Keywords：laser;self-focusing;space debris   
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      • Lixiang Chen,Jijin Lei,Jacquiline Romero
        Vol. 3, Issue 2, (2014) DOI: 10.1038/lsa.2014.34
        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.  
        Keywords：dimensionality;Klyshko picture;optical vortices;orbital angular momentum;quantum imaging   
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        Published：2024-09-30
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