Showing papers in "Optics Letters in 2012"
TL;DR: It is found that the nonlinear refractive index decreases with increasing excitation flux but slower than the absorption, suggesting that graphene may be a very promising nonlinear medium, paving the way for graphene-based nonlinear photonics.
Abstract: Under strong laser illumination, few-layer graphene exhibits both a transmittance increase due to saturable absorption and a nonlinear phase shift. Here, we unambiguously distinguish these two nonlinear optical effects and identify both real and imaginary parts of the complex nonlinear refractive index of graphene. We show that graphene possesses a giant nonlinear refractive index n2≃10−7 cm2 W−1, almost 9 orders of magnitude larger than bulk dielectrics. We find that the nonlinear refractive index decreases with increasing excitation flux but slower than the absorption. This suggests that graphene may be a very promising nonlinear medium, paving the way for graphene-based nonlinear photonics.
578 citations
TL;DR: Two algorithms, PROOF and FROG-CRAB, were employed to retrieve the pulse from the experimental spectrogram, yielding nearly identical results.
Abstract: A single isolated attosecond pulse of 67 as was composed from an extreme UV supercontinuum covering 55–130 eV generated by the double optical gating technique. Phase mismatch was used to exclude the single-atom cutoff of the spectrum that possesses unfavorable attochirp, allowing the positive attochirp of the remaining spectrum to be compensated by the negative dispersion of a zirconium foil. Two algorithms, PROOF and FROG-CRAB, were employed to retrieve the pulse from the experimental spectrogram, yielding nearly identical results.
510 citations
TL;DR: A plasmonic nanosensor based on Fano resonance in the strong-confinement metal-dielectric-metal waveguide side-coupled with a pair of nanoresonators yields a sensitivity of ~900 nm/RIU and a figure of merit of ~500, remarkable values compared with those of plAsmonic sensors supported by perfect absorbers.
Abstract: We propose a plasmonic nanosensor based on Fano resonance in the strong-confinement metal–dielectric–metal waveguide side-coupled with a pair of nanoresonators. Due to the coherent interference of the splitting discrete and quasi-continuum modes, the reflection spectrum possesses a sharp asymmetric Fano resonance dip, which is dependent on the cavity–cavity phase and the refractive index change of the dielectric. The physical features contribute to a highly efficient plasmonic sensor for refractive index sensing. The nanosensor yields a sensitivity of ∼900 nm/RIU and a figure of merit of ∼500, remarkable values compared with those of plasmonic sensors supported by perfect absorbers.
349 citations
TL;DR: These high-Q metamaterials can be used in ultrasensitive label-free terahertz sensing, dense photonic integration, and narrowband filtering and are experimentally demonstrated with an ultrahigh quality (Q) factor of 227.5.
Abstract: We experimentally demonstrate a planar terahertz Fano metamaterial with an ultrahigh quality (Q) factor of 227. This is achieved by the excitation of the nonradiative dark modes by introducing a tiny asymmetry in the metamaterial structure. The extremely sharp quadrupole and Fano resonances are excited at normal incidence for orthogonal polarizations of the electric field. In order to capture the narrow linewidth of the dark resonance modes, we perform high resolution terahertz time–domain measurements with a scan length of 200 picoseconds and frequency resolution of 5 GHz. These high-Q metamaterials can be used in ultrasensitive label-free terahertz sensing, dense photonic integration, and narrowband filtering.
261 citations
TL;DR: In this article, the design, numerical simulations and experimental measurements of terahertz metamaterial absorbers with a broad and flat absorption top over a wide incidence angle range for either transverse electric or transverse magnetic polarization depending on the incident direction are presented.
Abstract: We present the design, numerical simulations and experimental measurements of terahertz metamaterial absorbers with a broad and flat absorption top over a wide incidence angle range for either transverse electric or transverse magnetic polarization depending on the incident direction. The metamaterial absorber unit cell consists of two sets of structures resonating at different but close frequencies. The overall absorption spectrum is the superposition of individual components and becomes flat at the top over a significant bandwidth. The experimental results are in excellent agreement with numerical simulations.
260 citations
TL;DR: A flat metasurface with a parabolic reflection-phase distribution can focus an impinging plane wave to a point image in reflection geometry and is in good agreement with full wave simulations, model calculations, and theoretical analyses.
Abstract: We show that a flat metasurface with a parabolic reflection-phase distribution can focus an impinging plane wave to a point image in reflection geometry. Our system is much thinner than conventional geometric-optics devices and does not suffer the energy-loss issues encountered by many metamaterial devices working in transmission geometry. We designed realistic microwave samples and performed near-field scanning experiments to verify the focusing effect. Experimental results are in good agreement with full wave simulations, model calculations, and theoretical analyses.
259 citations
TL;DR: A miniature fiber-tip pressure sensor was built by using an extremely thin graphene film as the diaphragm, which acts as a light reflector and forms a low finesse Fabry-Perot interferometer.
Abstract: A miniature fiber-tip pressure sensor was built by using an extremely thin graphene film as the diaphragm. The graphene also acts as a light reflector, which, in conjunction with the reflection at the fiber end-air interface, forms a low finesse Fabry-Perot interferometer. The graphene based sensor demonstrated pressure sensitivity over 39.4 nm/kPa with a diaphragm diameter of 25 μm. The use of graphene as diaphragm material would allow highly sensitive and compact fiber-tip sensors.
252 citations
TL;DR: It is demonstrated experimentally that a patchwork of four metal-insulator-metal patches leads to an unpolarized wideband omnidirectional infrared absorption that paves the way to the design of wideband efficient plasmonic absorbers in the infrared spectrum.
Abstract: In this Letter, we demonstrate experimentally that a patchwork of four metal-insulator-metal patches leads to an unpolarized wideband omnidirectional infrared absorption. Our structure absorbs 70% of the incident light on a 2.5 μm bandwidth at 8.5 μm. It paves the way to the design of wideband efficient plasmonic absorbers in the infrared spectrum.
247 citations
TL;DR: A broadband infrared absorber is proposed by engineering the frequency dispersion of metamaterial surface (metasurface) to mimic an ideal absorbing sheet by demonstrating a polarization-independent absorber with absorption larger than 97% and the concept of dispersion engineering may provide helpful guidance for the design of a broadband absorber.
Abstract: We propose a broadband infrared absorber by engineering the frequency dispersion of metamaterial surface (metasurface) to mimic an ideal absorbing sheet. With a thin layer of structured nichrome, a polarization-independent absorber with absorption larger than 97% is numerically demonstrated over a larger than one octave bandwidth. It is shown that the bandwidth enhancement is related with the transformation of the Drude model of free electron gas in metal film to the Lorentz oscillator model of a bound electron in the structured metallic surface. We believe that the concept of dispersion engineering may provide helpful guidance for the design of a broadband absorber.
245 citations
TL;DR: A demonstration of quantum key distribution (QKD) over a standard telecom fiber exceeding 50 dB in loss and 250 km in length is reported, with careful optimization of the 1 bit delayed Faraday-Michelson interferometer and the use of the superconducting single photon detector (SSPD).
Abstract: We report a demonstration of quantum key distribution (QKD) over a standard telecom fiber exceeding 50 dB in loss and 250 km in length. The differential phase shift QKD protocol was chosen and implemented with a 2 GHz system clock rate. By careful optimization of the 1 bit delayed Faraday–Michelson interferometer and the use of the superconducting single photon detector (SSPD), we achieved a quantum bit error rate below 2% when the fiber length was no more than 205 km, and of 3.45% for a 260 km fiber with 52.9 dB loss. We also improved the quantum efficiency of SSPD to obtain a high key rate for 50 km length.
238 citations
TL;DR: These results verify the advantage of longer pump pulses and support the expectation that mJ-level THz pulses will be available by cooling the crystal and using large pumped area.
Abstract: Recent theoretical calculations predicted an order-of-magnitude increase in the efficiency of terahertz pulse generation by optical rectification in lithium niobate when 500 fs long pump pulses are used, rather than the commonly used ∼100 fs pulses. Even by using longer than optimal pump pulses of 1.3 ps duration, 2.5× higher THz pulse energy (125 μJ) was measured with 2.5× higher pump-to-THz energy conversion efficiency (0.25%) than reported previously with shorter pulses. These results verify the advantage of longer pump pulses and support the expectation that mJ-level THz pulses will be available by cooling the crystal and using large pumped area.
TL;DR: Planar, scalar, optical Schell-model, and quasi-homogeneous sources with correlations that are Fourier transforms of multi-Gaussian functions are introduced and it is demonstrated that far fields produced by these families of sources carry interesting characteristics.
Abstract: Planar, scalar, optical Schell-model, and quasi-homogeneous sources with correlations that are Fourier transforms of multi-Gaussian functions are introduced. It is demonstrated that far fields produced by these families of sources carry interesting characteristics, being flatlike with adjustable steepness of the edge. Beam conditions for such sources are also derived.
TL;DR: A new (to the authors' knowledge) method to determine the modal content of the fiber is proposed and high purity of the desired vortex state is demonstrated (97% after 20 m, even after bends and twists).
Abstract: We present a fiber-based method for generating vortex beams with a tunable value of orbital angular momentum from −1ℏ to +1ℏ per photon. We propose a new (to our knowledge) method to determine the modal content of the fiber and demonstrate high purity of the desired vortex state (97% after 20 m, even after bends and twists). This method has immediate utility for the multitude of applications in science and technology that exploit vortex light states.
TL;DR: The results offer potential for a robust, integrated, and low-cost supercontinuum source for applications including frequency metrology, optical coherence tomography, confocal microscopy, and optical communications.
Abstract: We demonstrate supercontinuum generation spanning 1.6 octaves in silicon nitride waveguides. Using a 4.3 cm-long waveguide, with an effective nonlinearity of γ=1.2 W−1 m−1, we generate a spectrum extending from 665 nm to 2025 nm (at −30 dB) with 160 pJ pulses. Our results offer potential for a robust, integrated, and low-cost supercontinuum source for applications including frequency metrology, optical coherence tomography, confocal microscopy, and optical communications.
TL;DR: The experimentally demonstrate a wideband near-perfect light absorber in the midwave IR region using a multiplexed plasmonic metal structure that absorbs more than 98% of the incident light over a much wider spectral band than regular nonmultiplexed structure perfect light absorbers in the early IR region.
Abstract: We experimentally demonstrate a wideband near-perfect light absorber in the midwave IR region using a multiplexed plasmonic metal structure. The wideband near-perfect light absorber is made of two different size gold metal squares multiplexed on a thin dielectric spacing layer on top of a thick metal layer in each unit cell. We also fabricate regular nonmultiplexed structure perfect light absorbers. The multiplexed structure IR absorber absorbs more than 98% of the incident light over a much wider spectral band than regular nonmultiplexed structure perfect light absorbers in the midwave IR region.
TL;DR: A fiber-optic extrinsic Fabry-Perot interferometer pressure sensor based on a nanothick silver diaphragm that exhibits a relatively linear response within the pressure variation range of 0-50 kPa, with a high pressure sensitivity of 70.5 nm/kPa.
Abstract: We present a fiber-optic extrinsic Fabry–Perot interferometer pressure sensor based on a nanothick silver diaphragm. The sensing diaphragm, with a thickness measured in a few hundreds of nanometers, is fabricated by the electroless plating method, which provides a simple fabrication process involving a high-quality diaphragm at a low cost. The sensor exhibits a relatively linear response within the pressure variation range of 0–50 kPa, with a high pressure sensitivity of 70.5 nm/kPa. This sensor is expected to have potential applications in the field of highly sensitive pressure sensors.
TL;DR: This work experimentally studied the degradation of mode purity for light beams carrying orbital angular momentum (OAM) propagating through simulated atmospheric turbulence, showing that turbulence uniformly degrades the purity of all the modes within this range, irrespective of mode number.
Abstract: We have experimentally studied the degradation of mode purity for light beams carrying orbital angular momentum (OAM) propagating through simulated atmospheric turbulence. The turbulence is modeled as a randomly varying phase aberration, which obeys statistics postulated by Kolmogorov turbulence theory. We introduce this simulated turbulence through the use of a phase-only spatial light modulator. Once the turbulence is introduced, the degradation in mode quality results in crosstalk between OAM modes. We study this crosstalk in OAM for 11 modes, showing that turbulence uniformly degrades the purity of all the modes within this range, irrespective of mode number.
TL;DR: In this paper, a formal equivalence between generalized refraction and blazed diffraction gratings was established, and the relative merits of the two approaches were discussed, as well as the relative importance of different approaches.
Abstract: When an electromagnetic wave is obliquely incident on the interface between two homogeneous media with different refractive indices, the requirement of phase continuity across the interface generally leads to a shift in the trajectory of the wave. When a linearly position-dependent phase shift is imposed at the interface, the resulting refraction may be described using a generalized version of Snell's law. In this Letter, we establish a formal equivalence between generalized refraction and blazed diffraction gratings, further discussing the relative merits of the two approaches.
TL;DR: This Letter presents a novel absolute phase recovery technique with phase coding that uses phase instead of intensity to determine codewords, and it could achieve a faster measurement speed, since three additional images can represent more than 8(2(3) unique codeword for phase unwrapping.
Abstract: This Letter presents a novel absolute phase recovery technique with phase coding. Unlike the conventional gray-coding method, the codeword is embedded into the phase and then used to determine the fringe order for absolute phase retrieval. This technique is robust because it uses phase instead of intensity to determine codewords, and it could achieve a faster measurement speed, since three additional images can represent more than 8(23) unique codewords for phase unwrapping. Experimental results will be presented to verify the performance of the proposed technique.
TL;DR: The τ interferometer is a portable and inexpensive device for obtaining spatial interferograms of microscopic biological samples without the strict stability and the highly coherent illumination that are usually required for interferometric microscopy setups.
Abstract: This Letter presents the τ interferometer, a portable and inexpensive device for obtaining spatial interferograms of microscopic biological samples without the strict stability and the highly coherent illumination that are usually required for interferometric microscopy setups The device is built using off-the-shelf optical elements and can easily operate with low-coherence illumination, while being positioned in the output of a conventional inverted microscope The interferograms are processed into the quantitative amplitude and phase profiles of the sample Based on the phase profile, the optical-path-delay profile is obtained with temporal stability of 018 nm and spatial stability of 042 nm Further experimental demonstration of using the τ interferometer for imaging the quantitative thickness profile of a live red blood cell is provided
TL;DR: The design and fabrication of an array of thin microwaveguides, which terminates at a three-dimensionally distributed set of points, appropriate for delivering light to targets distributed in a 3D pattern throughout the brain.
Abstract: To deliver light to the brain for neuroscientific and neuroengineering applications like optogenetics, in which light is used to activate or silence neurons expressing specific photosensitive proteins, optical fibers are commonly used. However, an optical fiber is limited to delivering light to a single target within the 3D structure of the brain. Here, we describe the design and fabrication of an array of thin microwaveguides, which terminates at a three-dimensionally distributed set of points, appropriate for delivering light to targets distributed in a 3D pattern throughout the brain.
TL;DR: This work has shown that a variety of dispersive media, including a disordered photonic crystal lattice, an array of Bragg fibers, and even a random medium, can be used in spectrometers despite high insertion loss, limiting the device performance.
Abstract: We propose and demonstrate that a conventional multimode fiber can function as a high-resolution, low-loss spectrometer. The proposed spectrometer consists only of the fiber and a camera that images the speckle pattern generated by interference among the fiber modes. Although this speckle pattern is detrimental to many applications, it encodes information about the spectral content of the input signal, which can be recovered using calibration data. We achieve a spectral resolution of 0.15 nm over 25 nm bandwidth using 1 m long fiber, and 0.03 nm resolution over 5 nm bandwidth with a 5 m fiber. The insertion loss is less than 10%, and the signal-to-noise ratio in the reconstructed spectra is more than 1000.
TL;DR: In this paper, a generalized mode volume that can be easily evaluated based on the mode calculation methods typically applied in the literature is introduced, allowing one to compute the Purcell effect and other interesting optical phenomena in a rigorous and unambiguous way.
Abstract: We show explicitly how the commonly adopted prescription for calculating effective mode volumes is wrong and leads to uncontrolled errors. Instead, we introduce a generalized mode volume that can be easily evaluated based on the mode calculation methods typically applied in the literature, and which allows one to compute the Purcell effect and other interesting optical phenomena in a rigorous and unambiguous way.
TL;DR: A simple and robust refractive index (RI) sensor based on a Mach-Zehnder interferometer has been demonstrated and a tunable optical delay line (ODL) is inserted into the other arm to compensate for the variation of the optical length difference.
Abstract: A simple and robust refractive index (RI) sensor based on a Mach–Zehnder interferometer has been demonstrated. A section of optical microfiber drawn from silica fiber is employed as the sensing arm. Because of the evanescent field, a slight change of the ambient RI will lead to the variation of the microfiber propagation constant, which will further change the optical length. In order to compensate the variation of the optical length difference, a tunable optical delay line (ODL) is inserted into the other arm. By measuring the delay of the ODL, the ambient RI can be simply demodulated. A high RI sensitivity of about 7159 μm/refractive index unit is achieved at microfiber diameter of 2.0 μm.
TL;DR: Two-dimensional electronic spectroscopy allows fundamentally new insights into the structure and dynamics of multi-chromophore systems and the fact that it automatically measures absorptive spectra.
Abstract: We introduce the translating wedge-based identical pulses encoding system, a novel device for the generation of collinear, interferometrically locked ultrashort pulse pairs. By means of birefringent wedges, we are able to control the pulse delay with attosecond precision and stability better that λ/360, without affecting the pulse duration and in a spectral range that spans from UV to mid-IR. This device is expected to dramatically simplify two-dimensional spectroscopy experiments.
TL;DR: A decomposition of guided modes propagating in optical fibers is implemented and it is shown that the observed field can be reconstructed with very high fidelity.
Abstract: A procedure for the real-time analysis of laser modes using a phase-only spatial light modulator is outlined. The procedure involves encoding into digital holograms by complex amplitude modulation a set of orthonormal basis functions into which the initial field is decomposed. This approach allows any function to be encoded and refreshed in real time (60 Hz). We implement a decomposition of guided modes propagating in optical fibers and show that we can successfully reconstruct the observed field with very high fidelity.
TL;DR: Enhanced thermal quenching property of PiG compared to phosphor with conventional silicone resin suggests its prominent feasibility for high-power/high-brightness white LEDs.
Abstract: Phosphor-in-glass (PiG) typed robust color converters were fabricated using Pb-free silicate glasses for high-power white LED applications. SiO2–B2O3–RO(R=Ba,Zn) glass powder showed good sintering behavior and high visible transparency under the sintering condition of 750 °C for 30 min without noticeable interaction with phosphors. By simply changing the thickness of the PiG plate or mixing ratio of glass to Y3Al5O12:Ce3+ phosphor, CIE chromaticity coordinates of the LED can be easily controlled. Enhanced thermal quenching property of PiG compared to phosphor with conventional silicone resin suggests its prominent feasibility for high-power/high-brightness white LEDs.
TL;DR: A germanium strip waveguide designed for single mode transmission of light in transverse magnetic polarization generated from quantum cascade lasers at a wavelength of 5.8 μm is demonstrated.
Abstract: Mid-infrared photonics in silicon needs low-loss integrated waveguides. While monocrystalline germanium waveguides on silicon have been proposed, experimental realization has not been reported. Here we demonstrate a germanium strip waveguide on a silicon substrate. It is designed for single mode transmission of light in transverse magnetic (TM) polarization generated from quantum cascade lasers at a wavelength of 5.8 μm. The propagation losses were measured with the Fabry–Perot resonance method. The lowest achieved propagation loss is 2.5 dB/cm, while the bending loss is measured to be 0.12 dB for a 90° bend with a radius of 115 μm.
TL;DR: It is experimentally demonstrated a passively mode-locked femtosecond laser by using a graphene-based saturable absorber mirror (graphene SAM) in the spectral region of 2 μm.
Abstract: We experimentally demonstrated a passively mode-locked femtosecond laser by using a graphene-based saturable absorber mirror (graphene SAM) in the spectral region of 2 μm. The graphene SAM was fabricated by transferring chemical-vapor-deposited, high-quality, and large-area graphene on a highly reflective plane mirror. Stable mode-locked laser pulses as short as 729 fs were obtained with a repetition rate of 98.7 MHz and an average output power of 60.2 mW at 2018 nm.
TL;DR: This Letter presents an efficient, fast, and straightforward two-step demodulating method based on a Gram-Schmidt (GS) orthonormalization approach based on determining an Orthonormalized interferogram basis from the two supplied interferograms using the GS method.
Abstract: This Letter presents an efficient, fast, and straightforward two-step demodulating method based on a Gram-Schmidt (GS) orthonormalization approach. The phase-shift value has not to be known and can take any value inside the range (0,2π), excluding the singular case, where it corresponds to π. The proposed method is based on determining an orthonormalized interferogram basis from the two supplied interferograms using the GS method. We have applied the proposed method to simulated and experimental interferograms, obtaining satisfactory results. A complete MATLAB software package is provided at http://goo.gl/IZKF3.