Incident Light Field Research Articles

The electric field modulation by three-dimensional crack on fused silica subsurface

The main factor of laser-induced damage is the modulation to electromagnetic field of laser by the crack on the subsurface. In this paper, a three-dimensional crack model on the exit surface is presented. Three-dimensional finite-difference time-domain (FDTD) method is employed to simulate the electric field intensity distribution in the vicinity of crack on fused silica subsurface. The roles of the crack width, depth, length and the gradient angle in the modulation to the incident light field are analyzed in detail. Results show that the crack size plays an important role in the electric modulation. With the increasing depth and width, the peak value of maximal electric field intensity appears in fused silica. However, the maximal electric field intensity tends to be a constant when the crack length reaches 1 μm. Besides, the enhancement of light intensity becomes obvious when total internal reflection occurs in fused silica. Our calculated results provide an advisable theoretical criterion to the corresponding experiment.

Enhancement of 2-Photon Absorption of a Dye in a Polymer Microsphere Based on an Optical Cavity Effect

We examined optical cavity effects on the 2-photon absorption efficiency of a dye (Rohdamine B; RhB) doped in polymer microspheres (ion-exchanging resin and poly(methyl methacrylate) (PMMA)). When 1064 nm pulsed-laser light was irradiated on single polymer microspheres doped with RhB under an optical microscope, we clearly observed dye fluorescence from individual microspheres, although the dye was transparent at 1064 nm. We confirmed that the fluorescence intensity was proportional to the square of the laser power for excitation. In contrast, such fluorescence was not observed from RhB doped in a PMMA film, in which the enhancement of a light electric field by an optical cavity effect was never expected. Theoretical calculations indicated that the microsphere possessed several peak values of the quality factor (Q = 10(2)-10(5)) at around certain particle diameters under 1064 nm irradiation. This means that the electromagnetic field of incident light is enhanced through light confinement in a microsphere. Based on these results, we conclude that the 2-photon absorption probability of RhB would be considerably enhanced by the optical cavity effects of the microsphere.

Quantitatively characterizing fluctuations of dielectric susceptibility of tissue with Fourier domain optical coherence tomography

A new model of Fourier domain optical coherence tomography (FDOCT) is proposed, valid within the first Born approximation, which takes the fluctuations of the dielectric susceptibility of tissue into account. It is shown that the spectral electrical power at the detector in the FDOCT system is proportional to the Fourier component of the spatial correlation function of the dielectric susceptibility of the tissue, proportional to the squares of the spectrum of the incident light field and the amplitude reflectance of the reference mirror. One possible application of the obtained result is to use the measured spectral data of the spatial correlation function of the dielectric susceptibility to quantitatively characterize properties of tissue.

Dynamic multiple-beam counter-propagating optical traps using optical phase-conjugation

Counter-propagating optical traps are widely used where long working distances, axially symmetric trapping potentials, or standing light waves are required. We demonstrate that optical phase-conjugation can automatically provide a counter-propagating replica of a wide range of incident light fields in an optical trapping configuration. The resulting counter-propagating traps are self-adjusting and adapt dynamically to changes of the input light field. It is shown that not only single counter-propagating traps can be implemented by phase-conjugation, but also structured light fields can be used. This step towards more complex traps enables advanced state-of-the-art applications where multiple traps or other elaborated trapping scenarios are required. The resulting traps cannot only be used statically, but they can be rearranged in real-time and allow for interactive dynamic manipulation.

Effect of ellipticity on Hanle electromagnetically induced absorption and transparency resonances with longitudinal and transverse magnetic fields

The effect of incident light field ellipticity on the electromagnetically induced absorption (EIA) and electromagnetically induced transparency (EIT) resonances has been studied experimentally and computationally in Hanle configuration with longitudinal and transverse magnetic fields. We identify the Zeeman coherences that influence the resonance profile and study the role of coherence-transfer from excited to ground state via spontaneous emission as a function of ellipticity for the F{sub g}=2{yields}F{sub e}=3 transition of {sup 87}Rb. The EIT resonance observed with the light field locked on the F{sub g}=1{yields}F{sub e}=2 transition of {sup 87}Rb is an influence of the nearby F{sub g}=1{yields}F{sub e}=0 closed and F{sub g}=1{yields}F{sub e}=1 open transitions. With increase in ellipticity the observed EIA and EIT resonances diminish in amplitude for a longitudinal magnetic field and are enhanced for a transverse magnetic field. We computationally account for these observations and discuss the factors that influence the EIA and EIT resonance amplitudes as a function of ellipticity and show that for a transverse field scan the ellipticity dependence of the EIA resonance amplitude can be accounted for without invoking the Doppler effect unlike for a longitudinal field scan. We also show that the maximum in the EIA resonance amplitude obtained for nonzeromore » ellipticities with a longitudinal magnetic field depends on the closedness of the atomic system.« less

Formation of periodic surface nanostructures on Ti 3+:Al 2O 3 crystals using femtosecond laser pulses

Periodic surface nanostructures are observed on Ti 3+:Al 2O 3 single crystals that have been irradiated by a single focused beam from a femtosecond pulsed laser (wavelength: 800 nm; pulse duration: 130 and 152 fs). Atomic force microscopy images of single-ablated zones and modified structures created by fixing and translating samples through the focal region of a linearly polarized laser beam reveal self-organized periodic surface nanostructures (ripples) with a subwavelength spacing, which are oriented perpendicular to the electric-field vector of the laser beam. The period of the subwavelength ripples obtained by linearly polarized laser irradiation varies from ∼ λ/5 to 2 λ/5 ( λ: incident laser wavelength) depending on the laser pulse energy. This phenomenon can be explained by assuming that the incident light field interferes with the electric field of electron plasma waves propagating inside the material; this interference periodically modulates the electron plasma density and modifies the surface ablation. In addition, for the first time, we observe screw-shaped nanostructures in the focal spot of circularly polarized beam irradiation. The morphology of these nanostructures appears to reflect the circular polarization of the laser light.

Nanolithography method by using localized surface plasmon mask generated with polydimethylsiloxane soft mold on thin metal film

We propose a photolithographic method to fabricate nanostructures by employing a localized surface plasmon (LSP) mask generated by a soft mold on a thin metal film. The soft mold can be formed by transparent materials, such as polydimethylsiloxane, contacting firmly to the metal film. The pattern edges of the mold, serving as the fine tapers, can be used to excite LSPs and accumulate a large amount of localized energy from the incident light field, providing a modulated optical field in the resist with nanometer feature size. Nanolithographic results with a minimum feature size of 30 nm are demonstrated.

Accurate optical wavefront reconstruction based on reciprocity of an optical path using low resolution spatial light modulators

A method for high precision optical wavefront reconstruction using low resolution spatial light modulators (SLMs) was proposed. It utilizes an adiabatic waveguide taper consisting of a plurality of single-mode waveguides to decompose an incident light field into simple fundamental modes of the single-mode waveguides. By digital generation of the conjugate fields of those simple fundamental modes a field proportional to the original incident light field might be reconstructed accurately based on reciprocity. Devices based on the method using transparent and reflective SLMs possess no aberration like that of a conventional optic lens and are able to achieve diffraction limited resolution. Specifically on the surface of the narrow end of a taper a resolution much higher than half of the wavelength is attainable. The device may work in linear mode and possesses unlimited theoretical 3D space-bandwidth product (SBP). The SBP of a real device is limited by the accuracy of SLMs. A pair of 8-bit SLMs with 1000 × 1000 = 10 6 pixels could provide a SBP of about 5 × 10 4. The SBP may expand by 16 times if 10-bit SLMs with the same number of pixels are employed or 16 successive frames are used to display one scene. The device might be used as high precision optical tweezers, or employed for continuous or discrete real-time 3D display, 3D measurement, machine vision, etc.

Classical Signature of Ponderomotive Squeezing in a Suspended Mirror Resonator

The radiation pressure coupling between a low-mass moving mirror and an incident light field has been experimentally studied in a high-finesse Fabry-Perot cavity. Using classical intensity noise in order to mimic radiation pressure quantum fluctuations, the physics of ponderomotive squeezing comes into play as a result of the opto-mechanical correlations between the field quadratures. The same scheme can be used to probe ponderomotive squeezing at the quantum level, thus opening new routes in quantum optics and high sensitivity measurement experiments.

Nanogap-Assisted Surface Plasmon Nanolithography

Nanoparticles of noble metals exhibit localized surface plasmons (LSP) associated with enhancement of the electromagnetic field due to its localization in nanometric domains at the surface of nanoparticles. Especially, nanogap metallic structures show strong enhancement of the electromagnetic field at the nanogap position. This can be achieved up to 105 times larger than the incident light field. We have explored the possibility to use the localized field for nanogap-assisted surface plasmon nanolithography. For this purpose, monodisperse arrays consisting of pairs of rectangular gold nanoblocks separated by nanogaps were fabricated on glass substrates using electron beam lithography and lift-off techniques. Nanoblock structures were used as photomasks for contact exposure of thin films of positive photoresist on glass substrates. Thus, ordered clusters of nanoparticles separated by nanogaps can provide a stable and versatile platform for further development of optical subwavelength nanolithography. To th...

Surface plasmon-enhanced emission from Ag-coated Ce doped Y3Al5O12 thin films phosphor capped with a dielectric layer of SiO2

The effect of a surface plasmon (SP) on the yellow emission of Ce doped Y3Al5O12 (YAG:Ce) phosphor thin films was studied. The coating of YAG:Ce thin films with silver nanoislands on a 3-nm-thick SiO2 spacer significantly enhanced their photoluminescence (PL). The PL enhancement is caused by the resonant coupling of photoemission in YAG:Ce into SP and the electric field of incident light at the metal interfaces. Varying the mass thickness and the annealing condition of the Ag layer indicated that the PL intensity was highest at an Ag mass thickness of 20 nm. Therefore, yellow emission was enhanced by tuning the matching conditions of thin film phosphor-SP coupling by controlling the structure of silver nanoislands. The PL intensity can be further and remarkably enhanced by capping Ag islands with a 10-nm-thick SiO2 layer as a dielectric medium.

Numerical simulation of light intensity distribution in the vicinity of three-dimensional Hertzian conical scratch on fused silica subsurface

The main factor of laser induced damage is the modulation caused by the scratch in the subsurface of the fused silica. The three-dimensional Hertzian conical scratch (HCS) model on the exit surface is establised in this article. Three-dimensional finite-difference time-domain method is used to calculate and simulate the electric field intensity distribution in the vicinity of HCS. The effects of depth, radius and gradient angle of the HCS on incident light field modulation are analyzed, respectively. The results show that the electric field enhancement in the center of HCS is the largest. This area is apt to be damaged. The maximal electric field intensity in the fused silica will be enhanced as the depth of the HCS changes from λ to 19λ/2. The fused silica is liable to be damaged while the radius of HCS is below 15λ. When the radius is larger than 175λ, the maximal electric fields in the fused silica tend to be 2.5 V/m. And the maximal electric fields are independent on the radius. The effect of light intensity enhancement is more obviously when total internal reflection occurs between internal interface of HCS and the rear surface of fused silica.

Bianisotropic Photonic Metamaterials

In metamaterials, electric (magnetic) dipoles can be excited by the electric (magnetic) component of the incident light field. Moreover, in the description of bianisotropic metamaterials, cross terms occur, i.e., magnetic dipoles can also be excited by the electric-field component of the incident light and vice versa. For the cross terms, in the general bianisotropic case, the exciting field and dipole vectors include an arbitrary angle. For the special case of chirality, the angle is zero. In the spirit of a brief tutorial, a very simple electric-circuit description of the split-ring resonator is used to give a basic introduction to the cross terms. Mathematical details of the effective parameter retrieval are presented. Furthermore, we briefly review recent experiments on bianisotropic metamaterials operating at optical frequencies.

Surface Plasmon-Enhanced Emission from Metal-Island-Coated YAG:Ce Thin-Film Phosphor

This work experimentally verifies that surface plasmon (SP) coupling can enhance the yellow emission of Ce-doped (YAG:Ce) thin-film phosphors. The effect of SP on yellow emission from YAG:Ce thin-film phosphor coated with a metal layer is examined. Among Ag, Al, and Au materials, the emission intensity of YAG:Ce thin-film phosphor by coating a silver layer with a thickness of nanometers was significantly enhanced by 10-fold. The enormous enhancement on photoluminescence (PL) is attributable to the resonant coupling of emission in YAG:Ce with an SP and the electric field of incident light at the metal interfaces. The mass thickness and annealing conditions of the deposited layer can be varied to transform the Ag layer into an islandlike structure; the PL intensity is highest at a Ag thickness of 20 nm.

Recording and controlling the 4D light field in a microscope using microlens arrays

By inserting a microlens array at the intermediate image plane of an optical microscope, one can record four-dimensional light fields of biological specimens in a single snapshot. Unlike a conventional photograph, light fields permit manipulation of viewpoint and focus after the snapshot has been taken, subject to the resolution of the camera and the diffraction limit of the optical system. By inserting a second microlens array and video projector into the microscope's illumination path, one can control the incident light field falling on the specimen in a similar way. In this paper, we describe a prototype system we have built that implements these ideas, and we demonstrate two applications for it: simulating exotic microscope illumination modalities and correcting for optical aberrations digitally.

The quantum acousto-optic effect in Bose–Einstein condensate

We investigate the interaction between a single mode light field and an elongated cigar shaped Bose–Einstein condensate (BEC), subject to a temporal modulation of the trap frequency in the tight confinement direction. Under appropriate conditions, the longitudinal sound like waves (Faraday waves) in the direction of weak confinement acts as a dynamic diffraction grating for the incident light field analogous to the acousto-optic effect in classical optics. The change in the refractive index due to the periodic modulation of the BEC density is responsible for the acousto-optic effect. The dynamics is characterised by Bragg scattering of light from the matter wave Faraday grating and simultaneous Bragg scattering of the condensate atoms from the optical grating formed due to the interference between the incident light and the diffracted light fields. Varying the intensity of the incident laser beam we observe the transition from the acousto-optic effect regime to the atomic Bragg scattering regime, where Rabi oscillations between two momentum levels of the atoms are observed. We show that the acousto-optic effect is reduced as the atomic interaction is increased.

Structural Characterization and Photoelectrochemical Properties of Gold Nanoparticle Multistructures Prepared by Layer-by-Layer Deposition

Surface plasmon, which is induced by the oscillation of free electrons at the surface of a metal, generates remarkable enhancement of localized electric field owing to its interaction with the incident light field, which is called localized surface plasmon resonance (LSPR). In this study, we have fabricated densely packed multistructures of gold nanoparticle films by a layer-by-layer approach using polyions and films of LSPR-active gold nanoparticles prepared at the liquid-liquid interface. The structure and the optical properties of the multistructure were evaluated by transmission absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and quartz crystal microbalance (QCM) measurements. In addition, the photoelectrochemical investigations of these films verified the anodic photocurrent generation due to photoexcitation of the gold nanoparticles.

Collapses and revivals of stored orbital angular momentum of light in a cold-atom ensemble

We report on the storage of orbital angular momentum of light in a cold ensemble of cesium atoms. We employ Bragg diffraction to retrieve the stored optical information impressed into the atomic coherence by the incident light fields. The stored information can be manipulated by an applied magnetic field and we were able to observe collapses and revivals due to the rotation of the stored atomic Zeeman coherence for times longer than $15\text{ }\ensuremath{\mu}\text{s}$.

Photodetection using Bose-Einstein-condensed atoms in a microtrap

A model of photodetection using a Bose-Einstein condensate in an atom-chip-based microtrap is analyzed. Atoms absorb photons from the incident light field, receive part of the photon momentum, and leave the trap potential. Upon counting escaped atoms within predetermined time intervals, the photon statistics of the incident light is mapped onto the atom-count statistics. Whereas traditional photodetection theory treats the emission centers of photoelectrons as distinguishable, here the centers of escaping atoms are condensed and thus indistinguishable atoms. From this, an enhancement of the photon-number resolution as compared to the commonly known counting formula is derived for the case of low saturation.

Dynamics of Bragg diffraction in a stored light grating in cold atoms

We report on a detailed investigation of the dynamics and the saturation of a light grating stored in a sample of cold cesium atoms. We employ Bragg diffraction to retrieve the stored optical information impressed into the atomic coherence by the incident light fields. The diffracted efficiency is studied as a function of the intensities of both writing and reading laser beams. A theoretical model is developed to predict the temporal pulse shape of the retrieved signal and compares reasonably well with the observed results.