Showing papers in "Optics Letters in 2007"
TL;DR: Focusing of coherent light through opaque scattering materials by control of the incident wavefront with a brightness up to a factor of 1000 higher than the brightness of the normal diffuse transmission is reported.
Abstract: We report focusing of coherent light through opaque scattering materials by control of the incident wavefront. The multiply scattered light forms a focus with a brightness that is up to a factor of 1000 higher than the brightness of the normal diffuse transmission.
1,364 citations
TL;DR: This work investigates the acceleration dynamics of quasi-diffraction-free Airy beams in both one- and two-dimensional configurations and shows that this class of finite energy waves can retain their intensity features over several diffraction lengths.
Abstract: We investigate the acceleration dynamics of quasi-diffraction-free Airy beams in both one- and two-dimensional configurations. We show that this class of finite energy waves can retain their intensity features over several diffraction lengths. The possibility of other physical realizations involving spatiotemporal Airy wave packets is also considered.
1,306 citations
TL;DR: Starting from Lagrangian principles, a formalism suitable for describing coupled optical parity-time symmetric systems is developed.
Abstract: Starting from Lagrangian principles we develop a formalism suitable for describing coupled optical parity-time symmetric systems.
1,005 citations
TL;DR: A recently established silver-based negative-index metamaterial design is miniaturized and a real part of the refractive index of -0.6 at a 780 nm wavelength is inferred--which is visible in the laboratory.
Abstract: We further miniaturize a recently established silver-based negative-index metamaterial design. By comparing transmittance, reflectance, and phase-sensitive time-of-flight experiments with theory, we infer a real part of the refractive index of −0.6 at a 780 nm wavelength—which is visible in the laboratory.
718 citations
TL;DR: A double-layer chiral planar metamaterial that exhibits pronounced circular dichroism at near-infrared wavelengths and the antisymmetric oscillation modes of the two coupled layers allow local magnetic-dipole moments and enhanced polarization effects compared with similar single-layer systems where only electric-d Dipole moments occur.
Abstract: We propose, fabricate, and study a double-layer chiral planar metamaterial that exhibits pronounced circular dichroism at near-infrared wavelengths. The antisymmetric oscillation modes of the two coupled layers allow local magnetic-dipole moments and enhanced polarization effects compared with similar single-layer systems where only electric-dipole moments occur. Experiment and rigorous theoretical calculations are in good agreement.
444 citations
TL;DR: Experimental results from Yb-doped fiber lasers agree with the trends of numerical simulations, and stable and self-starting pulses are generated with energies above 20 nJ, and these can be dechirped to <200 fs duration.
Abstract: We report a study of the scaling and limits to pulse energy in an all-normal-dispersion femtosecond fiber laser. Theoretical calculations show that operation at large normal cavity dispersion is possible in the presence of large nonlinear phase shifts, owing to strong pulse shaping by spectral filtering of the chirped pulse in the laser. Stable pulses are possible with energies of tens of nanojoules. Experimental results from Yb-doped fiber lasers agree with the trends of numerical simulations. Stable and self-starting pulses are generated with energies above 20 nJ, and these can be dechirped to <200 fs duration. Femtosecond pulses with peak powers near 100 kW are thus available from this simple and practical design.
402 citations
TL;DR: A computer-generated hologram is introduced onto SLM for performing the beam conversion and optical realization of a variety of polarization configurations confirms the reliability and flexibility of the method.
Abstract: We describe a convenient approach for generating arbitrary vector beams in a 4-f system with a spatial light modulator (SLM) and a common path interferometric arrangement. A computer-generated hologram is introduced onto SLM for performing the beam conversion. Optical realization of a variety of polarization configurations confirms the reliability and flexibility of our method.
381 citations
TL;DR: Multidirectional selective plane illumination microscopy (mSPIM) as mentioned in this paper reduces absorption and scattering artifacts and provides an evenly illuminated focal plane by pivoting the light sheet and illuminating the sample consecutively from opposing directions.
Abstract: Multidirectional selective plane illumination microscopy (mSPIM) reduces absorption and scattering artifacts and provides an evenly illuminated focal plane. mSPIM solves two common problems in light-sheet-based imaging techniques: The shadowing in the excitation path due to absorption in the specimen is eliminated by pivoting the light sheet; the spread of the light sheet by scattering in the sample is compensated by illuminating the sample consecutively from opposing directions. The resulting two images are computationally fused yielding a superior image. The effective light sheet is thinner, and the axial resolution is increased by square root 2 over single-directional SPIM. The multidirectional illumination proves essential in biological specimens such as millimeter-sized embryos. The performance of mSPIM is demonstrated by the imaging of live zebrafish embryos.
363 citations
TL;DR: An experimental demonstration of an integrated biochemical sensor based on a slot-waveguide microring resonator that detects a minimal refractive index variation of 2x10(-4) RIU.
Abstract: We report an experimental demonstration of an integrated biochemical sensor based on a slot-waveguide microring resonator. The microresonator is fabricated on a Si3N4-SiO2 platform and operates at a wavelength of 1.3 μm. The transmission spectrum of the sensor is measured with different ambient refractive indices ranging from n=1.33 to 1.42. A linear shift of the resonant wavelength with increasing ambient refractive index of 212 nm/refractive index units (RIU) is observed. The sensor detects a minimal refractive index variation of 2×10−4 RIU.
348 citations
TL;DR: In this article, phase and frequency stabilization of a diode laser at the thermal noise limit of a passive optical cavity was demonstrated. The system is compact and exploits a cavity design that reduces vibration sensitivity.
Abstract: We demonstrate phase and frequency stabilization of a diode laser at the thermal noise limit of a passive optical cavity. The system is compact and exploits a cavity design that reduces vibration sensitivity. The subhertz laser is characterized by comparison with a second independent system with similar fractional frequency stability (1x10(-15) at 1 s). The laser is further characterized by resolving a 2 Hz wide, ultranarrow optical clock transition in ultracold strontium.
330 citations
TL;DR: An analytical solution is provided in the case in which the density of generated carriers is relatively low; it is useful for estimating spectral bandwidth of pulses at low repetition rates.
Abstract: We study the effects of two-photon absorption on the self-phase modulation (SPM) process in silicon waveguides while including both free-carrier absorption and free-carrier dispersion An analytical solution is provided in the case in which the density of generated carriers is relatively low; it is useful for estimating spectral bandwidth of pulses at low repetition rates The free-carrier effects are studied numerically with emphasis on their role on the nonlinear phase shift and spectral broadening We also consider how the repetition rate of a pulse train affects the SPM process
TL;DR: A new method for recording digital holograms under incoherent illumination, which results in a complex-valued Fresnel hologram that is reconstructed in the computer when the 3D properties of the object are revealed.
Abstract: We present a new method for recording digital holograms under incoherent illumination. Light is reflected from a 3D object, propagates through a diffractive optical element (DOE), and is recorded by a digital camera. Three holograms are recorded sequentially, each for a different phase factor of the DOE. The three holograms are superposed in the computer, such that the result is a complex-valued Fresnel hologram. When this hologram is reconstructed in the computer, the 3D properties of the object are revealed.
TL;DR: A compact crossings for silicon-on-insulator photonic wires is presented using a 3 microm parabolic taper in each arm and locally applying a lower index contrast using a double-etch technique to reduce loss of confinement.
Abstract: We present compact crossings for silicon-on-insulator photonic wires. The waveguides are broadened using a 3μm parabolic taper in each arm. By locally applying a lower index contrast using a double-etch technique, loss of confinement is reduced and 97.5% transmission (−1.7dB) is achieved with only −40dB cross talk.
TL;DR: It is theoretically prove that a conventional Mach-Zehnder modulator can generate an optical frequency comb with excellent spectral flatness and predicts the conversion efficiency and bandwidth of the generated frequency comb.
Abstract: We theoretically prove that a conventional Mach-Zehnder modulator can generate an optical frequency comb with excellent spectral flatness. The modulator is asymmetrically dual driven by large amplitude sinusoidal signals with different amplitudes. The driving condition to obtain spectral flatness is analytically derived and optimized, yielding a simple formula. This formula also predicts the conversion efficiency and bandwidth of the generated frequency comb.
TL;DR: A sufficient condition for ensuring the satisfaction of the constraint of nonnegative definiteness on the choice of the mathematical form of spatial correlation functions for optical fields is discussed.
Abstract: The choice of the mathematical form of spatial correlation functions for optical fields is restricted by the constraint of nonnegative definiteness. We discuss a sufficient condition for ensuring the satisfaction of such a constraint.
TL;DR: It is shown through numerical simulations that silicon waveguides can be used to create a supercontinuum extending over 400 nm by launching femtosecond pulses as higher-order solitons.
Abstract: We show through numerical simulations that silicon waveguides can be used to create a supercontinuum extending over 400 nm by launching femtosecond pulses as higher-order solitons. The physical process behind continuum generation is related to soliton fission, self-phase modulation, and generation of Cherenkov radiation. In contrast with optical fibers, stimulated Raman scattering plays little role. As low-energy(≈1 pJ) pulses and short waveguides (<1 cm) are sufficient for continuum generation, the proposed scheme should prove useful for practical applications.
TL;DR: To the knowledge, this work presents the highest pulse energy ever extracted from fiber based femtosecond laser systems, and a nearly 2 orders of magnitude higher repetition rate than in previously published millijoule-level fiber CPA systems.
Abstract: We report on an ytterbium-doped fiber chirped-pulse amplification (CPA) system delivering millijoule level pulse energy at repetition rates above 100 kHz corresponding to an average power of more than 100 W. The compressed pulses are as short as 800 fs. As the main amplifier, an 80 μm core diameter short length photonic crystal fiber is employed, which allows the generation of pulse energies up to 1.45 mJ with a B-integral as low as 7 at a stretched pulse duration of 2 ns. A stretcher-compressor unit consisting of dielectric diffraction gratings is capable of handling the average power without beam and pulse quality distortions. To our knowledge, we present the highest pulse energy ever extracted from fiber based femtosecond laser systems, and a nearly 2 orders of magnitude higher repetition rate than in previously published millijoule-level fiber CPA systems.
TL;DR: An embedded fiber-taper saturable absorber is proposed and demonstrated based on a fiber taper embedded in a carbon nanotube/polymer composite and an all-fiber mode-locked ring laser is built.
Abstract: We propose and demonstrate a new saturable absorber based on a fiber taper embedded in a carbon nanotube/polymer composite. Greater than a 10% reduction in absorption (due to saturation) is directly measured for our saturable absorber. Using an embedded fiber-taper saturable absorber, we built an all-fiber mode-locked ring laser, which produces 594 fs/1.7 nJ pulses with a repetition rate of 13.3 MHz.
TL;DR: It is shown that, using structures manufacturable with today's nanotechnology, it is possible to increase the radiative decay rate by three orders of magnitude while keeping a quantum efficiency larger than 80% in the near-infrared regime.
Abstract: We apply two- and three-dimensional numerical calculations to study optical nanoantennae made of two coupled gold nanostructures, enclosing a single emitter in their gap. We show that, using structures manufacturable with today's nanotechnology, it is possible to increase the radiative decay rate by three orders of magnitude while keeping a quantum efficiency larger than 80% in the near-infrared regime. We examine the competition between the radiative and nonradiative processes in the presence of the antennae as a function of wavelength and antenna geometry. Our results hold great promise for improving the quantum efficiency of poor emitters such as silicon nanocrystals or carbon nanotubes.
TL;DR: In this paper, the authors demonstrate optical switching of electrically resonant terahertz planar metamaterials fabricated on ErAs/GaAs nanoisland superlattice substrates.
Abstract: We demonstrate optical switching of electrically resonant terahertz planar metamaterials fabricated on ErAs/GaAs nanoisland superlattice substrates. Photoexcited charge carriers in the superlattice shunt the capacitive regions of the constituent elements, thereby modulating the resonant response of the metamaterials. A switching recovery time of 20 ps results from fast carrier recombination in the ErAs/GaAs superlattice substrates.
TL;DR: It is measured that 6.3 dB of relative intensity squeezing is generated by stimulated, nondegenerate four-wave mixing in a hot rubidium vapor, which is of interest for experiments involving cold atoms or atomic ensembles.
Abstract: We have measured −3.5 dB (−8.1 dB corrected for losses) relative intensity squeezing between probe and conjugate beams generated by stimulated, nondegenerate four-wave mixing in hot rubidium vapor. Unlike early observations of squeezing in atomic vapors based on saturation of a two-level system, our scheme uses a resonant nonlinearity based on ground-state coherences in a three-level system. Since this scheme produces narrowband, squeezed light near an atomic resonance, it is of interest for experiments involving cold atoms or atomic ensembles.
TL;DR: A Fourier domain mode-locked (FDML) laser for ultra-high-speed optical coherence tomography (OCT) imaging of the human retina is demonstrated, demonstrating a speed improvement of approximately10x over typical high-speed OCT systems, paving the way for densely sampled volumetric data sets and new imaging protocols.
Abstract: A Fourier domain mode-locked (FDML) laser at 1050 nm for ultra-high-speed optical coherence tomography (OCT) imaging of the human retina is demonstrated. Achievable performance, physical limitations, design rules, and scaling principles for FDML operation and component choice in this wavelength range are discussed. The fiber-based FDML laser operates at a sweep rate of 236 kHz over a 63 nm tuning range, with 7 mW average output power. Ultra-high-speed retinal imaging is demonstrated at 236,000 axial scans per second. This represents a speed improvement of approximately10x over typical high-speed OCT systems, paving the way for densely sampled volumetric data sets and new imaging protocols.
TL;DR: A finite-element analysis of a diffraction problem involving a coated cylinder enabling the electromagnetic cloaking of a lossy object with sharp wedges located within its core finds that the electromagnetic field radiated by such a source located a fraction of a wavelength from the cloak is perturbed by less than 1%.
Abstract: We present a finite-element analysis of a diffraction problem involving a coated cylinder enabling the electromagnetic cloaking of a lossy object with sharp wedges located within its core. The coating consists of a heterogeneous anisotropic material deduced from a geometrical transformation as first proposed by Pendry [Science 312, 1780 (2006)]. We analyze the electromagnetic response of the cloak in the presence of an electric line source in p polarization and a loop of magnetic current in s polarization. We find that the electromagnetic field radiated by such a source located a fraction of a wavelength from the cloak is perturbed by less than 1%. When the source lies in the coating, it seems to radiate from a shifted location.
TL;DR: The transmission spectrum of fiber Bragg gratings with gratings planes tilted at a small angle relative to the fiber axis shows a large number of narrowband cladding mode resonances within a 100 nm wide spectrum.
Abstract: The transmission spectrum of fiber Bragg gratings with gratings planes tilted at a small angle (2°-10°) relative to the fiber axis shows a large number of narrowband cladding mode resonances within a 100 nm wide spectrum. When a gold coating with a thickness between 10 and 30 nm is deposited on the fiber, the transmission spectrum shows anomalous features for values of the outside medium refractive index between 1.4211 and 1.4499. These features are shown to correspond to the excitation of surface plasmon resonances at the external surface of the gold film.
TL;DR: In this article, a spatially variable retardation plate was used to transform a linear polarization distribution to radial/azimuthal distribution, and a radially and azimuthally polarized (0, 1) Laguerre-Gaussian beam was obtained.
Abstract: We demonstrate an efficient transformation of a linearly polarized Gaussian beam to a radially or an azimuthally polarized doughnut (0,1)* Laguerre-Gaussian beam of high purity. We use a spatially variable retardation plate, composed of eight sectors of a lambda/2 retardation plate, to transform a linear polarization distribution to radial/azimuthal distribution. We transformed an Nd:YAG Gaussian beam with M(2)=1.3 to a radially and azimuthally polarized (0,1)* Laguerre-Gaussian beams with M(2)=2.5 and degree of radial/azimuthal polarization of 96-98%.
TL;DR: A novel family of paraxial laser beams forming an overcomplete yet nonorthogonal set of modes that have a singular phase profile and are eigenfunctions of the photon orbital angular momentum are studied.
Abstract: We studied a novel family of paraxial laser beams forming an overcomplete yet nonorthogonal set of modes. These modes have a singular phase profile and are eigenfunctions of the photon orbital angular momentum. The intensity profile is characterized by a single brilliant ring with the singularity at its center, where the field amplitude vanishes. The complex amplitude is proportional to the degenerate (confluent) hypergeometric function, and therefore we term such beams hypergeometric-Gaussian (HyGG) modes. Unlike the recently introduced hypergeometric modes [Opt. Lett. 32, 742 (2007)], the HyGG modes carry a finite power and have been generated in this work with a liquid-crystal spatial light modulator. We briefly consider some subfamilies of the HyGG modes as the modified Bessel Gaussian modes, the modified exponential Gaussian modes, and the modified Laguerre-Gaussian modes.
TL;DR: A novel passive mode-locking scheme for pulsed lasers enhanced by the interaction of carbon nanotubes (CNTs) with the evanescent field of propagating light in a D-shaped optical fiber that ensures the preservation of the original morphology of the individual CNTs.
Abstract: We demonstrate a novel passive mode-locking scheme for pulsed lasers enhanced by the interaction of carbon nanotubes (CNTs) with the evanescent field of propagating light in a D-shaped optical fiber. The scheme features all-fiber operation as well as a long lateral interaction length, which guarantees a strong nonlinear effect from the nanotubes. Mode locking is achieved with less than 30% of the CNTs compared with the amount of nanotubes used for conventional schemes. Our method also ensures the preservation of the original morphology of the individual CNTs. The demonstrated pulsed laser with our CNT mode locker has a repetition rate of 5.88 MHz and a temporal pulse width of 470 fs.
TL;DR: It is shown that the depth-resolved distribution of fluorochromes in small animals can be imaged with 25 fmol sensitivity and 150 microm spatial resolution by means of multispectral photoacoustic imaging.
Abstract: Fluorochromes have become essential reporter molecules in biological research. We show that the depth-resolved distribution of fluorochromes in small animals can be imaged with 25 fmol sensitivity and 150 μm spatial resolution by means of multispectral photoacoustic imaging. The major advantage of the multispectral approach is the sensitive differentiation of chromophores and fluorochromes of interest based on self-reference measurements, as evidenced in this study by resolving a commonly used fluorochrome (Alexa Fluor 750) in mouse. The suggested method is well suited for enhancing visualization of functional and molecular information in vivo and longitudinally.
TL;DR: Values indicate that the tunable near-infrared input component is downconverted with a quantum efficiency that exceeds 30%.
Abstract: We report single-pass difference-frequency generation of mid-infrared femtosecond pulses tunable in the 3.2-4.8 μm range from a two-branch mode-locked erbium-doped fiber source. Average power levels of up to 1.1 mW at a repetition rate of 82 MHz are obtained in the mid infrared. This is achieved via nonlinear mixing of 170 mW, 65 fs pump pulses at a fixed wavelength of 1.58 μm, with 11.5 mW, 40 fs pulses tunable in the near-infrared range between 1.05 and 1.18 μm. These values indicate that the tunable near-infrared input component is downconverted with a quantum efficiency that exceeds 30%.
TL;DR: Buffered Fourier domain mode-locked (FDML) lasers are demonstrated for dynamic phase-sensitive optical coherence tomography (OCT) and 3D OCT phase microscopy and provide excellent displacement sensitivities at extremely high sweep speeds.
Abstract: Buffered Fourier domain mode-locked (FDML) lasers are demonstrated for dynamic phase-sensitive optical coherence tomography (OCT) and 3D OCT phase microscopy. Systems are operated at sweep speeds of 42, 117, and 370 kHz, and displacement sensitivities of 39, 52, and 102 pm are achieved, respectively. Sensitivities are comparable to spectrometer-based OCT phase microscopy systems, but much faster acquisition speeds are possible. An additional factor of sqrt 2 improvement in noise performance is observed for differential phase measurements, which is important for Doppler OCT. Dynamic measurements of piezoelectric transducer motion and static 3D OCT phase microscopy are demonstrated. Buffered FDML lasers provide excellent displacement sensitivities at extremely high sweep speeds.