Showing papers in "Journal of Optics in 2012"
TL;DR: In this article, the authors focus on the class of nanostructured media with hyperbolic dispersion that have emerged as one of the most promising metamaterials with a multitude of practical applications from subwavelength imaging, nanoscale waveguiding, biosensing to nonlinear switching.
Abstract: Engineering optical properties using artificial nanostructured media known as metamaterials has led to breakthrough devices with capabilities from super-resolution imaging to invisibility. In this paper, we review metamaterials for quantum nanophotonic applications, a recent development in the field. This seeks to address many challenges in the field of quantum optics using advances in nanophotonics and nanofabrication. We focus on the class of nanostructured media with hyperbolic dispersion that have emerged as one of the most promising metamaterials with a multitude of practical applications from subwavelength imaging, nanoscale waveguiding, biosensing to nonlinear switching. We present the various design and characterization principles of hyperbolic metamaterials and explain the most important property of such media: a broadband enhancement in the electromagnetic density of states. We review several recent experiments that have explored this phenomenon using spontaneous emission from dye molecules and quantum dots. We finally point to future applications of hyperbolic metamaterials, using the broadband enhancement in the spontaneous emission to construct single-photon sources.
445 citations
TL;DR: In this paper, a back-contact light trapping surface for a-Si:H solar cells was designed and fabricated using a large-scale, relative inexpensive nano-imprint technique, which showed enhanced efficiency over standard randomly textured cells.
Abstract: Plasmonic nanostructures have been recently investigated as a possible way to improve absorption of light in solar cells. The strong interaction of small metal nanostructures with light allows control over the propagation of light at the nanoscale and thus the design of ultrathin solar cells in which light is trapped in the active layer and efficiently absorbed. In this paper we review some of our recent work in the field of plasmonics for improved solar cells. We have investigated two possible ways of integrating metal nanoparticles in a solar cell. First, a layer of Ag nanoparticles that improves the standard antireflection coating used for crystalline and amorphous silicon solar cells has been designed and fabricated. Second, regular and random arrays of metal nanostructures have been designed to couple light in waveguide modes of thin semiconductor layers. Using a large-scale, relative inexpensive nano-imprint technique, we have designed a back-contact light trapping surface for a-Si:H solar cells which show enhanced efficiency over standard randomly textured cells.
366 citations
TL;DR: In this article, a solar thermo-photovoltaic (STPV) collection system based on a large-area, nano-print-patterned film of plasmonic structures acting as an integrated solar absorber/narrow-band thermal emitter (SANTE) is presented.
Abstract: We present the concept of a solar thermo-photovoltaic (STPV) collection system based on a large-area, nanoimprint-patterned film of plasmonic structures acting as an integrated solar absorber/narrow-band thermal emitter (SANTE). The SANTE film concept is based on integrating broad-band solar radiation absorption with selective narrow-band thermal IR radiation which can be efficiently coupled to a photovoltaic (PV) cell for power generation. By employing a low reflectivity refractory metal (e.g., tungsten) as a plasmonic material, we demonstrate that the absorption spectrum of the SANTE film can be designed to be broad-band in the visible range and narrow-band in the infrared range. A detailed balance calculation demonstrates that the total STPV system efficiency exceeds the Shockley–Queisser limit for emitter temperatures above Te = 1200 K, and achieves an efficiency as high as 41% for Te = 2300 K. Emitter temperatures in this range are shown to be achievable under modest sun concentrations (less than 1000 suns) due to the thermal insulation provided by the SANTE film. An experimental demonstration of the wide-angle, frequency-selective absorptivity is presented.
355 citations
TL;DR: In this article, the feasibility of the realization of micromachined tunable metamaterials via structure reconfiguration and the current state of the art in the fabrication technologies of structurally reconfigurable metammaterial elements are reviewed.
Abstract: This paper reviews micromachined tunable metamaterials, whereby the tuning capabilities are based on the mechanical reconfiguration of the lattice and/or the metamaterial element geometry. The primary focus of this review is the feasibility of the realization of micromachined tunable metamaterials via structure reconfiguration and the current state of the art in the fabrication technologies of structurally reconfigurable metamaterial elements. The micromachined reconfigurable microstructures not only offer a new tuning method for metamaterials without being limited by the nonlinearity of constituent materials, but also enable a new paradigm of reconfigurable metamaterial-based devices with mechanical actuations. With recent development in nanomachining technology, it is possible to develop structurally reconfigurable metamaterials with faster tuning speed, higher density of integration and more flexible choice of the working frequencies.
284 citations
TL;DR: An overview of some mathematical concepts relevant to superresolution in linear optical systems and properties of bandlimited functions related to both instrumental and computational aspects of superresolution are presented.
Abstract: Optical imaging beyond the diffraction limit, i.e., optical superresolution, has been studied extensively in various contexts. This paper presents an overview of some mathematical concepts relevant to superresolution in linear optical systems. Properties of bandlimited functions are surveyed and are related to both instrumental and computational aspects of superresolution. The phenomenon of superoscillation and its relation to superresolution are discussed.
199 citations
TL;DR: The paper reviews some domains that appeared as emerging fields in the last years of the 20th century and have been developed later on in the 21st century, such as three-dimensional object recognition, biometric pattern matching, optical security and hybrid optical–digital processors.
Abstract: On the verge of the 50th anniversary of Vander Lugt’s formulation for pattern matching based on matched filtering and optical correlation, we acknowledge the very intense research activity developed in the field of correlation-based pattern recognition during this period of time. The paper reviews some domains that appeared as emerging fields in the last years of the 20th century and have been developed later on in the 21st century. Such is the case of three-dimensional (3D) object recognition, biometric pattern matching, optical security and hybrid optical–digital processors. 3D object recognition is a challenging case of multidimensional image recognition because of its implications in the recognition of real-world objects independent of their perspective. Biometric recognition is essentially pattern recognition for which the personal identification is based on the authentication of a specific physiological characteristic possessed by the subject (e.g. fingerprint, face, iris, retina, and multifactor combinations). Biometric recognition often appears combined with encryption–decryption processes to secure information. The optical implementations of correlation-based pattern recognition processes still rely on the 4f-correlator, the joint transform correlator, or some of their variants. But the many applications developed in the field have been pushing the systems for a continuous improvement of their architectures and algorithms, thus leading towards merged optical–digital solutions.
197 citations
TL;DR: In this article, the resolution limits of popular sub-diffraction and sub-wavelength imaging schemes are examined using a unified approach that allows rapid comparison of the relative merits and shortcomings of each technique.
Abstract: The resolution limits of popular sub-diffraction and sub-wavelength imaging schemes are examined using a unified approach that allows rapid comparison of the relative merits and shortcomings of each technique. This is intended to clarify the often confusing and constantly growing array of super-resolution techniques. Specific techniques examined include centroid-based techniques like PALM (photo-activated localization microscopy) and STORM (stochastic optical reconstruction microscopy), structured illumination techniques like SSIM (spatially structured illumination microscopy), STED (stimulated emission depletion), and GSD (ground state depletion), coherent techniques like MRI (magnetic resonance imaging), Rabi gradients, and light shift gradients, as well as quantum-inspired multi-photon techniques. It is found that the ultimate resolution for all these techniques can be described using a simple ratio of an oscillation frequency to an effective decay rate, which can be physically interpreted as the number of oscillations that can be observed before decay (i.e.?the quality factor Q of the imaging transition).
153 citations
TL;DR: In this paper, a general approach to solving the problems of inverse scattering in three-dimensional isotropic media with a spherically symmetric refractive index distribution is presented, based on equivalence of the central section of an inhomogeneous medium and corresponding geodesic lens.
Abstract: This paper presents a general approach to solving the problems of inverse scattering in three-dimensional isotropic media with a spherically symmetric refractive index distribution. It is based on equivalence of the central section of an inhomogeneous medium and corresponding geodesic lens, which is a non-Euclidean surface with constant refractive index. We use this approach for solving the Luneburg inverse problem and also for the derivation and design of absolute instruments that provide perfect imaging within the frame of geometrical optics. In addition, we solve the generalized Luneburg inverse problem, which leads to the discovery of a new class of magnifying lenses.
122 citations
TL;DR: In this paper, the use of a liquid-crystal spatial light modulator (SLM) device to convert a linearly polarized femtosecond laser beam into a radially or azimuthally polarized vortex beam is demonstrated.
Abstract: The use of a liquid-crystal spatial light modulator (SLM) device to convert a linearly polarized femtosecond laser beam into a radially or azimuthally polarized vortex beam is demonstrated. In order to verify the state of polarization at the focal plane, laser induced periodic surface structures (LIPSS) are produced on stainless steel, imprinting the complex vectorial polarization structures and confirming the efficacy of the SLM in producing the desired polarization modes. Stainless steel plates of various thicknesses are micromachined with the radially and azimuthally polarized vortex beams and the resulting cut-outs are analysed. The process efficiency and quality of each mode are compared with those of circular polarization. Radial polarization is confirmed to be the most efficient mode for machining high-aspect-ratio (depth/width > 3) channels thanks to its relatively higher absorptivity. Following our microprocessing tests, liquid-crystal SLMs emerged as a flexible off-the-shelf tool for producing radially and azimuthally polarized beams in existing ultrashort-pulse laser microprocessing systems.
106 citations
TL;DR: In this article, the authors use electromagnetic simulations to carry out a systematic study of broadband absorption in vertically-aligned semiconductor nanowire arrays for photovoltaic applications, and show that the ultimate efficiencies of optimized arrays exceed those of equal-height thin films for all six materials and over a wide range of heights.
Abstract: We use electromagnetic simulations to carry out a systematic study of broadband absorption in vertically-aligned semiconductor nanowire arrays for photovoltaic applications. We study six semiconductor materials that are commonly used for solar cells. We optimize the structural parameters of each nanowire array to maximize the ultimate efficiency. We plot the maximal ultimate efficiency as a function of height to determine how it approaches the perfect-absorption limit. We further show that the ultimate efficiencies of optimized nanowire arrays exceed those of equal-height thin films for all six materials and over a wide range of heights from 100?nm to 100??m.
102 citations
TL;DR: In this paper, the potential performance of thin silicon solar cells with either silicon (Si) or titanium dioxide (TiO2) gratings using numerical simulations was examined, and the results showed that submicron symmetric and skewed pyramids of Si or TiO2 are a highly effective way of achieving light trapping in thin film solar cells.
Abstract: Dielectric gratings are a promising method of achieving light trapping for thin crystalline silicon solar cells. In this paper, we systematically examine the potential performance of thin silicon solar cells with either silicon (Si) or titanium dioxide (TiO2) gratings using numerical simulations. The square pyramid structure with silicon nitride coating provides the best light trapping among all the symmetric structures investigated, with 89% of the expected short circuit current density of the Lambertian case. For structures where the grating is at the rear of the cell, we show that the light trapping provided by the square pyramid and the checkerboard structure is almost identical. Introducing asymmetry into the grating structures can further improve their light trapping properties. An optimized Si skewed pyramid grating on the front surface of the solar cell results in a maximum short circuit current density, Jsc, of 33.4 mA cm−2, which is 91% of the Jsc expected from an ideal Lambertian scatterer. An optimized Si skewed pyramid grating on the rear performs as well as a rear Lambertian scatterer and an optimized TiO2 grating on the rear results in 84% of the Jsc expected from an optimized Si grating. The results show that submicron symmetric and skewed pyramids of Si or TiO2 are a highly effective way of achieving light trapping in thin film solar cells. TiO2 structures would have the additional advantage of not increasing recombination within the cell.
TL;DR: In this paper, the results of experimental and theoretical investigations of diffraction ring patterns are reported, which are formed due to the self-phase modulation of a continuous wave laser beam propagating through a solution of dye in ethanol.
Abstract: In this work, the results of experimental and theoretical investigations of diffraction ring patterns are reported. Diffraction rings are formed due to the self-phase modulation of a continuous wave laser beam propagating through a solution of dye in ethanol. The self-phase modulation of the laser beam is the result of the changes in the refractive index due to the heating of the sample by a small absorbed fraction of the laser power. To find the thermal nonlinearity inside the liquid irradiated by a Gaussian laser beam, we have solved the heat transfer equation including conduction and convection effects in a liquid medium. The temporal dynamics and structural characteristics of the diffraction ring patterns are studied theoretically on the basis of a Fresnel–Kirchhoff diffraction integral in the approximation of an optically thin absorbing medium. The simulation results show that when a Gaussian laser beam is transmitted through a liquid medium the intensity distribution pattern in the far-field first forms a series of circular diffraction rings, but after a period of time the rings change to half circular symmetry due to convection.
TL;DR: In this article, the authors used a common self-assembly technique to fabricate a range of silver island films on transparent substrates and measure their reflectance and transmittance for visible and near infrared light.
Abstract: Silver nanoparticles can be used as light scattering elements for enhancing solar cell energy conversion efficiencies. The objective of our work is to gain more insight into the optical properties of silver nanoparticle films and their effect on the performance of solar cells. We use a common self-assembly technique to fabricate a range of silver island films on transparent substrates and measure their reflectance and transmittance for visible and near infrared light. We demonstrate that it is possible to represent silver island films by an effective medium with the same optical properties. The observed strong dependence on illumination side of the reflectance and absorptance, attributed to driving field effects, is reproduced very well. Thin-film silicon solar cells with embedded silver island films were fabricated and it was found that their performance is reduced due to parasitic absorption of light in the silver island film. Simulations of these solar cells, where the silver island film is represented as an effective medium layer, show a similar trend. This highlights the importance of minimizing parasitic absorption.
TL;DR: In this paper, a simple and compact method for implementing an in-fiber Mach-Zehnder interferometer, which is constructed with two optical paths, propagating through the core and the ring-shaped silica cladding modes in the double-cladding fibers, is proposed.
Abstract: We propose a simple and compact method for implementing an in-fiber Mach–Zehnder interferometer, which is constructed with two optical paths, propagating through the core and the ring-shaped silica cladding modes in the double-cladding fibers. Strong cladding-mode resonance across the thin inner cladding is used to excite the cladding modes. The measured spectra fringe presents high-contrast interference from cascading a pair of well-overlapped resonant spectra dips. In combination with the nonlinear polarization rotation (NPR) technique, switchable and tunable multi-channel laser outputs are experimentally demonstrated with a fluctuation of less than 0.1 dB.
TL;DR: This work shows that photon-limited encrypted distributions have sufficient information for successful decryption, authentication and signal retrieval of complex-valued encrypted data.
Abstract: The integration of photon-counting imaging techniques and optical encryption systems can improve information authentication robustness against intruder attacks. Photon-counting imaging generates distributions with far fewer photons than conventional imaging and provides substantial bandwidth reduction by generating sparse encrypted data. We show that photon-limited encrypted distributions have sufficient information for successful decryption, authentication and signal retrieval. Additional compression of the encrypted distribution is applied by limiting the number of phase values used to reproduce the phase information of the complex-valued encrypted data. The validity of this technique—with and without phase compression—is probed through simulated experiments for two types of input images: alphanumerical signs and dithered natural scenes.
TL;DR: In this paper, an infrared dual-band metamaterial absorber composed of simple periodically patterned structures was designed and demonstrated, and two distinct absorption peaks of 74% and 96% were obtained, which were in reasonable agreement with the simulations.
Abstract: We report the design, characterization, and experimental demonstration of an infrared dual-band metamaterial absorber composed of simple periodically patterned structures. Experimental results show that two distinct absorption peaks of 74% and 96% are obtained, which are in reasonable agreement with the simulations. We demonstrate two absorption resonances that are derived from the mixture of magnetic and electric plasmon resonances. The dual-band absorber is polarization insensitive and the absorption peaks remain high with large angles of incidence for both transverse electric and transverse magnetic polarizations, which provide more efficient absorptions for nonpolarized or oblique incident beams.
TL;DR: In this paper, a plasmonic nanocrescent structure was proposed to increase the efficiency of a single-junction solar cell with an upconverter-doped dielectric core and crescent-shaped metallic shell.
Abstract: Upconversion of sub-bandgap photons can increase the maximum efficiency of a single-junction solar cell from 30% to over 44%. However, upconverting materials often have small absorption cross-sections and poor radiative recombination efficiencies that limit their utility in solar applications. Here, we show that the efficiency of upconversion can be substantially enhanced with a suitably designed plasmonic nanostructure. The structure consists of a spherical nanocrescent composed of an upconverter-doped dielectric core and a crescent-shaped metallic shell. Using numerical techniques, we calculate a greater than 10-fold absorption enhancement for a broad range of sub-bandgap wavelengths throughout the entire upconverting core. Further, this nanocrescent enables a 100-fold increase in above-bandgap power emission toward the solar cell. Our results provide a framework for achieving low-power solar upconversion, potentially enabling a single-junction solar cell with an efficiency exceeding the Shockley–Queisser limit.
TL;DR: Using a generalized rigorous coupled wave analysis, the scattering spectra of arrays of varying wire radii, length, and lattice filling factors was calculated for a square array of 20 µm long wires with radii of 200 µm and a filling fraction of 30%.
Abstract: Reducing reflection and transmission losses in photovoltaic devices is essential for realizing highly efficient power conversion. Here, we theoretically investigate arrays of radial junction silicon wires to determine the optimal geometry for maximized light absorption. Using a generalized rigorous coupled wave analysis, we calculate the scattering spectra of arrays of varying wire radii, length, and lattice filling factors. Near unity absorption, far exceeding that of conventional thin film devices, is calculated for a square array of 20 µm long wires with radii of 200 nm and a filling fraction of 30%. These results suggest a potentially cost-effective route toward high efficiency solar cells.
TL;DR: In this paper, an optically implemented absorption modulation and redshift switch of metamaterial absorber at terahertz frequencies was demonstrated, where the active structure of hybrid metal-semiconductor split ring resonators can be tuned by applying an external pump power.
Abstract: We demonstrate an optically implemented absorption modulation and redshift switch of metamaterial absorber at terahertz frequencies. Hybrid metal–semiconductor split ring resonators (SRRs) form the active structure, which can be tuned by applying an external pump power. This enables effective controls of the absorption strength and absorption peak frequency. As a function of incident pump power, the conductivity of silicon pads filled in the gap of SRRs is tuned efficiently, resulting in the modulation of absorption magnitude with a modulation depth of 60.5%, and a broadband switch of absorption peak frequencies varying from 1.11 to 0.87 THz. Multiple-reflection interference theory is used to analyze the reflection spectrum quantitatively under various silicon conductivities, and the results are in good agreement with full-wave simulations. The optical-tuned absorber demonstrates the viability to incorporate metamaterials to mature semiconductor technologies and has potential applications as an active terahertz modulator and switch.
TL;DR: In this paper, the potential of 3D photonic crystals for photon management in solar cells and corresponding concepts are discussed, where the authors propose a three-dimensional (3D) photonic crystal for photonic management of solar cells.
Abstract: Photon management is a key element to optimize the optical and electro-optical performance of solar cells. The potential of 3D photonic crystals for photon management in solar cells and the corresponding concepts are discussed.
TL;DR: In this article, the existence of high Q-factor trapped mode resonances is revealed in a double-periodic subwavelength planar structure made up of paired dielectric bars.
Abstract: The results of research into resonant phenomena in double-periodic subwavelength planar structures made up of paired dielectric bars are presented. For the first time the existence of high Q-factor trapped mode resonances is revealed in these low-loss entirely dielectric structures. A large red shift of the trapped mode resonance of the structure is observed as compared with the resonant wavelength of the periodic structure with only one dielectric bar per unit cell. This shift of the resonant wavelength is caused by a strong coupling of the electromagnetic fields in the adjacent dielectric bar resonators.
TL;DR: In this paper, the nonlinear Schrodinger-Maxwell-Bloch (NLS-MB) equation with variable coefficients was considered and the Lax pair for the NLS MB system through AKNS formalism was derived.
Abstract: We consider the nonlinear Schrodinger–Maxwell–Bloch (NLS–MB) equation with variable coefficients. We construct the Lax pair for the NLS–MB system through AKNS formalism. We derived the two-soliton solution using the Darboux transformation. Finally we demonstrate the nonlinear tunneling of two solitons through dispersion and the nonlinear barrier or well. Our result shows that solitons can be compressed through the tunneling barrier or well with exponential decay for the special choice of variable coefficients.
TL;DR: In this paper, the authors present both theoretical and experimental cases for realizing optically switchable and tunable split-ring resonator (SRR) metamaterials operating in the THz regime.
Abstract: We present both theoretical and experimental cases for realizing optically switchable and tunable split-ring resonator (SRR) metamaterials operating in the THz regime. This is achieved by suitably placing photoconducting semiconductors in the various SRR designs. Exciting the semiconductor by an optical pump beam, the realization of single- and multi-band switching, blue-shift and red-shift tunability, and broad-band phase modulation are demonstrated.
TL;DR: In this article, the light management in silicon thin film solar cells, using photonic crystals (PhC) structures, is discussed by means of optical simulations performed on realistic thin-film solar cell stacks.
Abstract: In this paper, we discuss on light management in silicon thin film solar cells, using photonic crystals (PhC) structures. We particularly focus on photovoltaic devices including amorphous silicon absorbers patterned as 2D PhCs. Physical principles and design rules leading to the optimized configuration of the patterned cell are discussed by means of optical simulations performed on realistic thin film solar cell stacks. Theoretically, a maximum 40%rel increase of integrated absorption in the a-Si:H layer of the patterned cell is expected compared to the unpatterned case. Moreover, both simulation and optical characterization of the fabricated cells demonstrate the robustness of their optical properties with regards to the angle of incidence of the light and to the fabrication induced defects in the PhCs. Finally, the impact of the surface recombination due to the generation of new free surfaces with higher defect densities is addressed. We demonstrate that patterning still induces a substantial increase of the conversion efficiency, with a reasonable surface recombination velocity.
TL;DR: In this article, the plasmonic resonance modes and coupling effects of single silver nanobeads and dimers were investigated using the three-dimensional finite element method, and it was shown that only the bonding mode can be found for low-refractive index cores.
Abstract: We investigate the plasmonic resonance modes and coupling effects of single silver nanobeads and silver nanobead dimers. Numerical investigation using the three-dimensional finite element method indicates that silver nanobeads exhibit two plasmonic resonances corresponding to the bonding and anti-bonding modes, respectively. The boundary symmetry on the inner and outer surfaces of the silver nanobeads can be broken by increasing the refractive indices of the cores filling the dielectric holes. It is shown that only the bonding mode can be found for low-refractive index cores, whereas both bonding and anti-bonding modes can be found for high-refractive index cores.
TL;DR: In this paper, an acousto-optic tunable filter (AOTF) operating in the long-wave infrared (LWIR) region has been developed based on the wide-angle regime of light diffraction in the YZ plane of the birefringent crystal operating from 8.4 to 13.6 µm.
Abstract: The acoustic, optic and acousto-optic properties of tellurium crystals have been examined in order to develop an acousto-optic tunable filter (AOTF) operating in the long-wave infrared (LWIR) region. The AOTF design is based on the wide-angle regime of light diffraction in the YZ plane of the birefringent crystal operating from 8.4 to 13.6 µm. Device characteristics were obtained from both theoretical and experimental investigations. Experiments were carried out using both a 10.6 µm pulsed CO2 laser as well as a tunable CO2 laser operating in a continuous wave mode from 9.2 to 10.7 µm. The AOTF was tuned over the acoustic frequency range of 81.5–94.7 MHz. The filtering performance in the tellurium device was provided by a pure shear elastic wave propagating at a 95.8° angle with respect to the positive direction of the optic axis, while an ordinary polarized optical beam was incident at the Bragg angle of 6.0° relative to the acoustic wavefront. At 10.6 µm, the measured spectral bandwidth of the filter was 127 nm and the optical transmission coefficient was around 8.8% with 2.0 W drive power. This paper presents detailed results from both the theoretical as well as experimental device characterization including the spectral images obtained with a 256 × 256 mercury cadmium telluride camera cooled to 77 K.
TL;DR: In this article, a miniature fiber Fabry?Perot interferometer (FFPI) for temperature measurement is proposed and demonstrated, which consists of a section of singlemode fiber (SMF) tip coated with a thin film of polyvinyl alcohol (PVA) at the end of the fiber tip.
Abstract: A miniature fiber Fabry?Perot interferometer (FFPI) for temperature measurement is proposed and demonstrated. The sensor consists of a section of single-mode fiber (SMF) tip coated with a thin film of polyvinyl alcohol (PVA) at the end of the fiber tip. A well-defined interference pattern is obtained as the result of the FFPI based on Fresnel reflection. The sensing head is extremely sensitive to ambient temperature, and provides a stable temperature sensitivity with a maximum value up to 173.5?pm??C?1 above 80??C. This proposed sensor has advantages of low cost, ultra-compactness, a small degree of hysteresis and high stability.
TL;DR: In this article, the optical limiting properties of gold, silver and gold?silver alloy nanoparticles in methyl 2-methylprop-2-enoate for nanosecond laser pulses are presented.
Abstract: The near- and off-resonant optical limiting properties of gold, silver and gold?silver alloy nanoparticles in methyl 2-methylprop-2-enoate for nanosecond laser pulses are presented. The nanoparticles are generated by picosecond pulsed laser ablation in liquid having hydrodynamic diameters from 26 to 30?nm. We use a Q-switched Nd:YAG laser working at a wavelength of 1064 or 532?nm, with a pulse width of 3?ns to characterize their behaviour by laser energy and fluence dependent transmittance measurements. To elucidate the contribution of nonlinear scattering to the optical limiting properties the scattered light energy at an angle of 90??is measured. The experimental results show that these nanoparticles have a strong nonlinear attenuation which can be attributed to intraband, interband and free carrier absorption and a thermal-induced scattering only at high input energies. Our results indicate in addition that the surface plasmon resonance does not contribute to the nonlinear processes at high input energies.
TL;DR: The feasibility of implementing reconfigurable ultrafast all-optical NOR and NAND gates by employing a single Mach-Zehnder interferometer (MZI) with quantum-dot semiconductor optical amplifiers (QD-SOAs) is theoretically investigated in this paper.
Abstract: The feasibility of implementing reconfigurable ultrafast all-optical NOR and NAND gates by employing a single Mach–Zehnder interferometer (MZI) with quantum-dot semiconductor optical amplifiers (QD-SOAs) is theoretically investigated. The reconfiguration of the scheme that allows conversion from one gate to the other is achieved by simply turning on or off a clock signal, while the complement of one of the data signals is used too as input for both gates. By conducting a numerical simulation, the conditions under which the QD-SOA-based MZI must be adjusted to operate so as to simultaneously ensure an acceptable extinction ratio for the NOR and amplitude modulation for the NAND are specified. This procedure is more demanding than when each gate is considered separately. Nevertheless it is possible to extract technologically realistic and achievable guidelines for the data signals and QD-SOAs characteristics in order for these gates to be jointly designed without modifying the fundamental structure or data driving mode of the MZI switch. In this manner a permissible range and a proper selection of values for the critical performance parameters that is common for these universal logic gates is derived, which enables both of them to be realized with a logically correct and high quality outcome.
TL;DR: In this article, the absorption response of thin films on transparent substrates is estimated by subtracting the measured absorption spectrum of the bare substrate from that of the film on the substrate structure, in a non-straightforward way.
Abstract: Both a theoretical algorithm and an experimental procedure are discussed of a new route to determine the absorption/scattering properties of thin films deposited on transparent substrates. Notably, the non-measurable contribution of the film–substrate interface is inherently accounted for. While the experimental procedure exploits only measurable spectra combined according to a very simple algorithm, the theoretical derivation does not require numerical handling of the acquired spectra or any assumption on the film homogeneity and substrate thickness. The film absorption response is estimated by subtracting the measured absorption spectrum of the bare substrate from that of the film on the substrate structure but in a non-straightforward way. In fact, an assumption about the absorption profile of the overall structure is introduced and a corrective factor accounting for the relative film-to-substrate thickness. The method is tested on films of a well known material (ITO) as a function of the film structural quality and influence of the film–substrate interface, both deliberately changed by thickness tuning and doping. Results are found fully consistent with information obtained by standard optical analysis and band gap values reported in the literature. Additionally, comparison with a conventional method demonstrates that our route is generally more accurate even if particularly suited for very thin films.