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Funding Information: Jenny ja Antti Wihurin Rahasto (00210075); Academy of Finland (308394); Aalto Science Institute AScI; Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN). Publisher Copyright: © 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement Light sheets are optical beam-like fields with one-dimensional intensity localization. Ideally, the field intensity should be independent of the longitudinal and one of the transverse coordinates, which is difficult to achieve even for truncated light sheets. In this work, we present a general theoretical framework for intensity-interferometric continuous wave (cw) light sheets formed by overlapping the interference fringe patterns of mutually uncorrelated frequency components of the field. We show that the key parameters of the light sheets can be calculated using simple analytical expressions. We propose a practical way to generate such light sheets with the help of prisms and demonstrate numerically the abilities of the method. Both bright and dark light sheets with an exceptionally small thickness and long divergence-free propagation distance are possible to generate. We also show that the transverse profile of the generated light sheets can be shaped by modifying the spectrum of the light. We believe ourfindings advance the beam-engineering technology and its applications.

Spatial coherence is a fundamental property of light with many applications. However, if a light source is spatially incoherent and structured, its near-field coherence is difficult to evaluate directly because of the complexity of the emitted random light. In this work, we propose a general approach to calculate the near-field mutual intensity and transverse coherence length of propagating radiation from a periodically structured incoherent source using optical reciprocity. If the structure period is subwavelength and much smaller than the source, the mutual intensity reduces to a Fourier transform of the angular intensity spectrum of the radiation. Tailoring the near-field spatial coherence of incoherent sources by structuring can be used, e.g., for applications in optical sensing and imaging.

Some imaging techniques reduce the effect of optical aberrations either by detecting and actively compensating for them or by utilizing interferometry. A microscope based on a Mach-Zehnder interferometer has been recently introduced to allow obtaining sharp images of light-transmitting objects in the presence of strong aberrations. However, the method is not capable of imaging microstructures on opaque substrates. In this work, we use a Michelson interferometer to demonstrate imaging of reflecting and back-scattering objects on any substrate with micrometer-scale resolution. The system is remarkably insensitive to both deterministic and random aberrations that can completely destroy the object's intensity image.

In an optically absorbing or amplifying linear medium, the energy flow density of interfering optical waves is in general periodically modulated in space. This makes the wave transmission through a material boundary, as described by the Fresnel transmission coefficients, nonreciprocal and apparently violating the energy conservation law. The modulation has been previously described in connection to ordinary homogeneous nonmagnetic materials. In this work, we extend the description to nanomaterials with designed structural units that can be magnetic at optical frequencies. We find that in such a “metamaterial” the modulation in energy flow can be used to enhance optical far-field emission in spite of the fact that the material is highly absorbing. We also demonstrate a nanomaterial design that absorbs light, but simultaneously eliminates the power flow modulation and returns the reciprocity, which is impossible to achieve with a nonmagnetic material. We anticipate that these unusual optical effects can be usedto increase the efficiency of nanostructured light emitters and absorbers, such as light-emitting diodes and solar cells.

Wave retarders, including quarter- and half-wave plates, are used in many optical systems for polarization conversion. They are usually realized with anisotropic crystalline materials. However, much thinner and possibly also less expensive wave plates can be made of micro- and nanostructures. We present a new way to create thin-film optical retarders based on a highly birefringent metamaterial. The wave plate is capable of low-loss, broadband operation, which we verify both numerically and experimentally. Owing to the remarkable simplicity of our design, the wave plates operating on the proposed principle can meet the requirements of large-scale production and find widespread application in optics and photonics.

The ongoing rapid development of digital camera technology together with increasing computing power enabling fast image analysis has opened up possibilities to further advance the performance of full-field interferometry. We present a software-based stabilization method for stroboscopic homodyne full-field interferometry that enables phase sensitive absolute-amplitude measurements of surface vibrations in microelectromechanical devices. The reference signal for the stabilization is obtained from a freely selectable region in the acquired interference images, resulting in a compact interferometer design without the need for additional optical components for monitoring the operation point. The proposed stabilization method is implemented in an LED-based stroboscopic Michelson-type full-field interferometer. To demonstrate the performance of the setup, an out-of-plane vibration field at 12 MHz in a square-plate silicon resonator is characterized. The data analysis reveals that a minimum detectable amplitude of less than 30 pm is achieved in the measurement. These first results already demonstrate the potential of the software-stabilization concept and serve to advance the detection of low-amplitude surface vibrations with full-field interferometry. [2014-0355]

Spatial dispersion is an intriguing property of essentially all nanostructured optical media. In particular, it makes optical waves with equal frequencies and polarizations have different wavelengths, if they propagate in different directions. This can offer new approaches to control light radiation and propagation. Spatially dispersive nanomaterials, such as metamaterials, are often treated in terms of wave parameters, such as refractive index and impedance retrieved from reflection and transmission coefficients of the material at each incidence angle. Usually, however, the waves are approximated as transverse, which simplifies the description, but yields wrong results, if spatial dispersion or optical anisotropy is significant. In this work, we present a method to calculate the wave parameters of a general spatially dispersive and optically anisotropic medium without applying such an approximation. The method allows one to evaluate the true impedances and field vectors of the effective waves, obtaining thusthe true light intensity and energy propagation direction in the medium. The equations are applied to several examples of spatially dispersive and anisotropic materials. The method introduces new insights into optics of nanostructured media and extends the design of such media towards optical phenomena involving significant spatial dispersion.

Spatial dispersion makes optical properties of materials depend on the direction of light propagation. The effect can be applied to control optical emission by sources embedded in such media. We propose a method to determine the radiation pattern of essentially any emitter located in a general spatially dispersive and optically anisotropic medium. The method is based on a decomposition of the source into waves of electric current, each creating optical plane waves whose properties are determined by the wave parameters, the refractive index, and impedance. The method is computationally fast and very accurate even in strongly spatially dispersive plasmonic metamaterials. In particular, we observe large modification of dipole emission in a bifacial and a diffraction-compensating metamaterial. The method is applicable to a large variety of nanostructured materials and, therefore, we believe that it can find numerous applications in nano-optics.

The field of plasmonics allows for confinement and control of light on the nanoscale. Due to potentially strong resonant interactions that light can have with metal nanoscale structures, metals are a good candidate to tailor interactions with light, e.g., periodic arrays of subwavelength metal structures can support extremely narrow resonances and show enhanced transmission. The field of plasmonics has evolved from using simple geometries to the desire to create complex nanostructures for improved control. The availability of fabrication techniques that provide for complex structures, however, is paired with the seemingly inevitable increase in complexity of fabrication techniques themselves. We present a facile and scalable method for the fabrication of periodic arrays of unique three-dimensional subwavelength-sized structures such as tapered holes and pyramidically shaped subwavelength-sized particles. The procedure consists of holographic inscription of a two-dimensional surface-relief grating in an azobenzene-containing polymer film, evaporative gold deposition and broad-beam ion milling of the relief structure. The method allows the fabrication of highly uniform arrays with tunable lattice parameters and dimensions over large sample areas. The optical response of the fabricated structures is determined experimentally and through simulation, which confirm the unique plasmonic response of the structures. While the proposed fabrication method has clear benefits for plasmonics, it could easily be applied also in other fields, for example by using other coating materials.

We consider a highly anisotropic metamaterial structure, composed of parallel metal nanostripes, and show that a thin layer of the material can be used as a tunable partial polarizer. The transmittance of the structure for TE-polarized waves depends strongly on the incidence angle, while for TM-polarized waves, it stays high and essentially constant. In particular, using the structure, the degree of polarization of a partially polarized or unpolarized light can be tuned by changing the incidence angle. The TE-wave transmittance drops from, c.a., 1 to 0 when the incidence angle increases by 5 deg only, owing to the presence of an unusual higher-order odd-symmetric TM mode that we have revealed in the structure. The tuning can be made smoother by introducing another layer of a similar metal-nanostripe structure on top of the first one. The new design allows the TE-wave transmittance to decrease gradually towards 0 with the incidence angle increasing from 0 to about 30 deg. Our structures serve as an essential optical component for studies involving partially polarized light.

While nanoscale local enhancement of fluorescence is a well-studied phenomenon, enhancement of far-field radiation of large-area samples using nanostructures has been explored much less. In this work, we demonstrate planar nanostructures with exceptionally high far-field intensity enhancement factors and in addition show that the enhancement can be made controllable via the polarization state of the pump light. We also find that, counterintuitively, the Purcell effect plays only a small part in the observed far-field enhancement. The results of these studies can be applied, for example, in thin-film optical emitters, fluorescence sensors, and wide-field fluorescence microscopy.

We introduce a fast and cost-effective technique to fabricate large-area periodically nanopatterned metal samples and apply this technique to create reflective nano-grid wave plates for optical wavelengths. The technique makes use of azo-polymer-based interference lithography and a special imprint method that enables creating large-area metal nanopatterns with high vertical walls. We fabricate and experimentally test a gold nano-grid wave plate that operates as reflective λ/4 -plate for λ = 604 nm and λ/2 -plate for λ = 997 nm.

Cancellation of optical diffraction is an intriguing phenomenon enabling optical fields to preserve their transverse intensity profiles upon propagation. In this work, we introduce a metamaterial design that exhibits this phenomenon for three-dimensional optical beams. As an advantage over other diffraction-compensating materials, our metamaterial is impedance-matched to glass, which suppresses optical reflection at the glass-metamaterial interface. The material is designed for beams formed by TM-polarized plane-wave components. We show, however, that unpolarized optical images with arbitrary shapes can be transferred over remarkable distances in the material without distortion. We foresee multiple applications of our results in integrated optics and optical imaging. (C) 2016 Optical Society of America

Planar optical waveguides made of designable spatially dispersive nanomaterials can offer new capabilities for nanophotonic components. As an example, a thin slab waveguide can be designed to compensate for optical diffraction and provide divergence-free propagation for strongly focused optical beams. Optical signals in such waveguides can be transferred in narrow channels formed by the light itself. We introduce here a theoretical method for characterization and design of nanostructured waveguides taking into account their inherent spatial dispersion and anisotropy. Using the method, we design a diffraction-compensating slab waveguide that contains only a single layer of silver nanorods. The waveguide shows low propagation loss and broadband diffraction compensation, potentially allowing transfer of optical information at a THz rate.

The pulse-buildup in miniature passively Q-switched lasers occurs in a similar time-scale as the phonon-assisted thermalization of the laser manifolds. We study how the time-dynamics of the laser is affected by the thermalization and relaxation processes using a dimensionless geometrical rate equation model for a homogeneously broadened four-level gain medium. The pulse length is found to depend strongly on the thermalization rate when the ratio of the ground-state absorption cross section of the saturable absorber to the spectroscopic emission cross section of the laser transition is small (lsim 10). Numerically calculated design curves are shown for the 1064 nm transition in Nd:YAG, and guidelines are given for applying the model to other transitions. Experimental results are used for estimating the thermalization time constant of Nd:YAG for the 1 mum transition group and the ground-state absorption cross section of the Cr4+:YAG saturable absorber crystal.

We design, fabricate, and characterize a metasurface, whose multipole optical response depends significantly on the illumination direction. The metasurface is composed of gold-nanodisc dimers embedded in glass. In spite of their nanoscale size, the dimers exhibit a dominating electric-current-quadrupole response in a wide range of wavelengths around 700 nm when illuminated from one side, and a primarily electric-dipole response when illuminated from the opposite side. This leads to two consequences. First, the reflection coefficient of the metasurface considerably differs for the two sides of illumination. Second, quadrupole excitation results in a significant local enhancement of both electric and magnetic fields around the dimers. Our experimental spectroscopic data are in good agreement with simulations obtained using a multipole expansion model.

We present phase-sensitive absolute amplitude measurements of surface acoustic wave fields obtained using a stroboscopic white-light interferometer. The data analysis makes use of the high resolution available in the measured interferometric phase data, enabling the characterization of the out-of-plane surface vibration fields in electrically excited microstructures with better than 100 pm amplitude resolution. The setup uses a supercontinuum light source with tailored spectral properties for obtaining the high amplitude resolution. The duration of the light pulses is less than 300 ps to allow the detection of high frequencies. These capabilities enabled a detailed measurement of the focusing of surface acoustic waves by an annular interdigital transducer structure operating at 74 MHz, featuring a maximum vibration amplitude of 3 nm.

Acoustic wave fields in a thin-film bulk acoustic wave resonator are studied using a heterodyne laser interferometer. The measurement area is extended outside the active electrode region of the resonator, so that wave fields in both the active and surrounding regions can be characterized. At frequencies at which the region surrounding the resonator does not support laterally propagating acoustic waves, the analysis of the measurement data shows exponentially decaying amplitude fields outside the active resonator area, as suggested by theory. The magnitude of the imaginary wave vectors is determined by fitting an exponential function to the measured amplitude data, and thereby the experimentally determined dispersion diagram is extended into the region of imaginary wave numbers.

Functional optical metamaterials usually consist of absorbing, anisotropic and often non-centrosymmetric structures of a size that is only a few times smaller than the wavelength of visible light. If the structures would be substantially smaller, excitation of higher-order electromagnetic multipoles in them, including magnetic dipoles, would be inefficient. As a result, the material would act as an ordinary electric-dipole material. The required non-negligible size of metamolecules, however, makes the material spatially dispersive, so that its optical characteristics depend on light propagation direction. This phenomenon significantly complicates the description of metamaterials in terms of conventional electric permittivity and magnetic permeability tensors. In this work, we present a simple semianalytical method to describe such spatially dispersive metamaterials, which are also allowed to be optically anisotropic and non-centrosymmetric. Applying the method, we show that a strong spatial dispersion, combined with absorption and optical anisotropy, can be used to efficiently control propagational characteristics of optical beams.

We present a picosecond supercontinuum light source designed for stroboscopic white-light interferometry. This source offers a potential for high-resolution characterization of vibrational fields in electromechanical components with frequencies up to the GHz range. The light source concept combines a gain-switched laser diode, the output of which is amplified in a two-stage fiber amplifier, with supercontinuum generation in a microstructured optical fiber. Implemented in our white-light interferometer setup, optical pulses with optimized spectral properties and below 310 ps duration are used for stroboscopic illumination at freely adjustable repetition rates. The performance of the source is demonstrated by characterizing the surface vibration field of a square-plate silicon MEMS resonator at 3.37 MHz. A minimum detectable vibration amplitude of less than 100 pm is reached.

We introduce and theoretically describe internally twisted optical metamaterials, in which the metamolecules can have an arbitrary, but common, orientation in the unit cells. The molecules, and consequently the material, are not chiral, but they are allowed to be noncentrosymmetric. While such internally twisted crystalline structures are difficult to find in natural materials, metamaterials of this type can be designed and fabricated at will. Here, we present a theoretical method that enables a detailed analysis of such metamaterials. The method establishes a connection between the optical properties of a metamaterial and the plane-wave optical response of a single two-dimensional array of metamolecules. In the theory, the effective wave parameters, such as the refractive index and wave impedance, are retrieved. Using the model, we show that these parameters can dramatically depend on the wave propagation direction and metamolecular orientation, which can be used, along with optical anisotropy, to efficiently adjust and control the plane-wave content of optical beams.

In optical metamaterials, the scatterers are usually aligned symmetrically with respect to the unit cells of the material. In this work, we consider metamaterials in which the "metamolecules" can be non-centrosymmetric and have an arbitrary, but common, orientation in the unit cells. Such internally twisted crystalline structures are difficult to find in natural materials, but metamaterials of this type can be designed and fabricated at will. Here we present a theoretical method that enables a detailed analysis of internally twisted non-centrosymmetric metamaterials. The method establishes a connection between the optical properties of a metamaterial and the plane-wave optical response of a single two-dimensional array of metamolecules. In this theory, the effective wave parameters, such as the refractive index and wave impedance, are retrieved. Using the model, we show that these parameters can dramatically depend on the wave propagation direction and metamolecular orientation in a metamaterial. This dependence provides a possibility to adjust and control the plane-wave content of optical beams propagating in the material.

Effects of external optical feedback from a reflection grating on the linewidth of a GaAlAs semiconductor laser operating at 780 nm are investigated. Accurate linewidth measurements as a function of the laser frequency tuning have been performed by applying the self-homodyne technique with a short delay line. A realistic coupled-cavity model, which incorporates the frequency-dependent reflection from the grating, is used to explain the measured data. The agreement between the theoretical and experimental results was found to be good.

In near-field optics the resolution and sensitivity of measurements depend on the abundance of evanescent waves in relation to propagating waves. The electromagnetic field propagator is related to the scalar spherical wave, for which the Weyl expansion is a half-space representation containing both evanescent and homogeneous plane waves. Making use of these results, we decompose the dyadic free-space Green function into its evanescent and homogeneous parts and show that some approaches put forward in the literature are inconsistent with this formulation. We express the results in a form that is suitable for numerical computation and illustrate the field decomposition for a point dipole in some typical cases.

We present experiments on optical writing and erasing of microscopic magnetic domain patterns on an epitaxial ferrite-garnet film. The film is initially uniformly magnetized normally to the surface. A weak constant magnetic field pointing in the same direction as the magnetization is applied, and magnetic domain patterns are written and erased by locally heating the film with a pulsed focused laser beam. For switching the magnetization, in both directions, only the beam diameter and the peak power of the laser pulse need to be controlled; the direction and strength of the external magnetic field are kept unchanged. The switching is essentially all-optical, which widens the area of potential applications of the method. We also show that domain patterns can be optically recorded in multilayered ferrite-garnet samples. Large Faraday-rotation angles in our films enable both spatial and temporal observations of the magnetization reversal.

We describe a LED-based stroboscopic white-light interferometer and a data analysis method that allow mapping out-of-plane surface vibration fields in electrically excited microstructures with sub-nm amplitude resolution for vibration frequencies ranging up to tens of MHz. The data analysis, which is performed entirely in the frequency domain, makes use of the high resolution available in the measured interferometric phase data. For demonstration, we image the surface vibration fields in a square-plate silicon MEMS resonator for three vibration modes ranging in frequency between 3 and 14 MHz. The minimum detectable vibration amplitude in this case was less than 100 pm.

In this work we study the interaction of visible light with plasmonic nanodimers that, under particular illumination conditions, do not exhibit any electric dipole excitations. It has previously been found out that the dipole suppression phenomenon disappears when the illumination direction is reversed. As a consequence, a homogeneous nanomaterial consisting of such nanodimers can be expected to be spatially dispersive, such that the conventional electric polarization vanishes for certain directions of light propagation. In order to reveal the complete picture of the light-nanodimer interaction, we analyze the multipole excitations in a dimer at various illumination angles. In particular, we introduce an analytical model for the multipole polarizability tensors of the dimer that, in contrast to conventional polarizability tensors, depend on the light propagation direction. The model is verified by rigorous numerical calculations. It can be used to gain insight into the properties of optical nanomaterials, such as metamaterials, in which higher-order multipoles can be efficiently excited.

We present an approach for numerically solving the multimode generalized nonlinear Schrödinger equation (MM-GNLSE). We propose to transform the MM-GNLSE to a system of first-order ordinary differential equations (ODEs) that can then be solved using readily available ODE solvers, thus making modeling of pulse propagation in multimode fibers easier. The solver is verified for the simplest multimode case in which only the two orthogonal polarization states in a non-birefringent microstructured optical fiber are involved. Also, the nonlinear dynamics of the degree and state of spectral polarization are presented for this case.

We introduce a simple theoretical model that describes the interaction of light with optical metamaterials in terms of interfering optical plane waves. In this model, a metamaterial is considered to consist of planar arrays of densely packed nanoparticles. In the analysis, each such array reduces to an infinitely thin homogeneous sheet. The transmission and reflection coefficients of this sheet are found to be equal to those of an isolated nanoparticle array and, therefore, they are easy to evaluate numerically for arbitrary shapes and arrangements of the particles. The presented theory enables fast calculation of electromagnetic fields interacting with a metamaterial slab of an arbitrary size, which, for example, can be used to retrieve the effective refractive index and wave impedance in the material. The model is also shown to accurately describe optically anisotropic metamaterials that in addition exhibit strong spatial dispersion, such as bifacial metamaterials.

We introduce a formalism that describes the interaction of light with bifacial optical nanomaterials. They are artificial noncentrosymmetric materials in which counter-propagating waves behave differently. We derive electromagnetic material parameters for uniaxial crystalline media in terms of the complex transmission and reflection coefficients of a single layer of the constituent nanoscatterers, which makes the numerical evaluation of these parameters very efficient. In addition, we present generalized Fresnel coefficients for such bifacial nanomaterials and investigate the fundamental role of higher-order electromagnetic multipoles on the bifaciality. We find that two counter-propagating waves in the material must experience the same refractive index, but they can have dramatically different wave impedances. The use of our model in practice is demonstrated with a particular example of a bifacial nanomaterial that exhibits a directional impedance matching to the surrounding medium.

In subwavelength-sized particles, light-induced multipole moments of orders higher than the electric dipole are usually negligibly small, which allows for the light-matter interaction to be accurately treated within the electric dipole approximation. In this work we show that in a specially designed meta-atom, a disc metadimer, the electric quadrupole and magnetic dipole can be the only excitable multipoles. This condition is achieved in a narrow but tunable spectral range of visible light both for individual metadimers and for a periodic array of such particles. The electromagnetic fields scattered by the metadimers fundamentally differ from those created by electric dipoles. A metamaterial composed of such metadimers will therefore exhibit unusual optical properties.

Optical properties of natural or designed materials are determined by the electromagnetic multipole moments that light can excite in the constituent particles. In this paper, we present an approach to calculating the multipole excitations in arbitrary arrays of nanoscatterers in a dielectric host medium. We introduce a simple and illustrative multipole decomposition of the electric currents excited in the scatterers and connect this decomposition to the classical multipole expansion of the scattered field. In particular, we find that completely different multipoles can produce identical scattered fields. The presented multipole theory can be used as a basis for the design and characterization of optical nanomaterials.

We present an experimental study of the nonlinear behavior of a square-extensional (SE) mode microelectromechanicalresonator, actuated with a piezoelectricAlNthin film. The acoustic vibration fields of the device are characterized using laser probing. A nonlinear vibration behavior of the SE mode is observed above a drive power level of −10 dBm such that the vibration amplitude of the SE mode saturates and a rotational in-plane vibration mode is excited at a significantly lower frequency (0.725 MHz) than the SE mode (16.670 MHz). Interestingly, the measured∼10 nm saturation amplitude of the SE mode is more than a decade below the amplitude value at which mechanical or electromechanical nonlinearities are estimated to become significant.

International audience; For more than a century, physicists have searched for a unique and general form for the force density that an electromagnetic field imposes on a medium. The existing equations for this quantity, obtained, e.g., by Minkowski, Einstein and Laub, Abraham, and Helmholtz, are manifestly different and, as such, give different predictions in some particular physical situations. The cases of real media with dispersion, dissipation, and nonlinearities as well as materials with optical gain have not been much addressed. Here we present an unambiguous general equation for the force density expressed in terms of a new energy-momentum tensor of the field. The equation can be applied to almost any natural or designed material. We show how the results obtained by Minkowski, Helmholtz, Einstein and Laub, and Abraham appear as special cases of this general treatment. Surprisingly, our result turns out to be physically very close to the almost forgotten result of Einstein and Laub. Electromagnetic force density and energy-momentum tensor in an arbitrary continuous medium2

International audience; We consider launching a monochromatic surface acoustic wave packet on a large set of random scatterers. The interference of the multiple scattered waves creates a random pattern of ripples on the crystal surface that is recorded by optical interferometry. The Fourier transform of the amplitude and phase data of the measured wave field unveils the complete slowness curve, i.e., the wave-vector as a function of the propagation angle. A simple acoustic speckle model is proposed to explain this observation.

Adding holes in a periodic arrangement to metallic thin films greatly affects the optical properties of the metal film. The compound structure can exert a large influence on electromagnetic fields that interact with the hole array. Many parameters affect the actual response of the hole arrays to electromagnetic fields, such as the periodicity, the size of the holes and their shape. Here, we will show by calculation that the angular emission and lifetime of emitters, embedded in hole arrays comprised of rectangular holes with varying aspect ratio, depend strongly on the hole aspect ratio. Specifically, changing the aspect ratio of the holes leads to a large variety in far-field emission patterns and a more than 10-fold changes in decay rate of a single emitter placed in the central hole of such an array.

We investigate into a new type of liquid crystal (LC) heterogeneous system that contains hydrogen-bonded polymer-azo-dye complexes at a low concentration. The suspension shows a unique self-orienting property and a spontaneous anchoring transition from planar to homeotropic alignment on a rubbed polyimide surface. We suggest a simple polymer adsorption model to explain the universal homeotropic orienting property. The complex-doped LC suspension can be used as a new liquid crystalline material that does not require any additional alignment processing or treatment.

The potentials of polarization spectroscopy in combustion studies are reported. This well-established technique of nonlinear laser spectroscopy is applied to the detection of OH molecules in a premixed acetylene-oxygen and a propane-air flame. The polarization spectrum is recorded in theA2Σ−X2π(0, 0) band. The spatial distribution of OH is measured in the flames. The saturation of the signal as a function of the laser pulse intensity is also investigated.

Acoustic wave fields both within the active electrode area of a solidly mounted 1.8 GHz bulk acoustic waveresonator, and around it in the surrounding region, are measured using a heterodyne laser interferometer. Plate-wave dispersion diagrams for both regions are extracted from the measurement data. The experimental dispersion data reveal the cutoff frequencies of the acoustic vibration modes in the region surrounding the resonator, and, therefore, the energy trapping range of the resonator can readily be determined. The measureddispersionproperties of the surrounding region, together with the abruptly diminishing amplitude of the dispersion curves in the resonator, signal the onset of acoustic leakage from the resonator. This information is important for verifying and further developing the simulation tools used for the design of the resonators. Experimental wave field images, dispersion diagrams for both regions, and the threshold for energy leakage are discussed.

A heterodyne laser interferometer is used to study acoustic wave fields excited in a 1.8 GHz AlN thin film bulk acoustic waveresonator. The electrical response of the resonator exhibits a strong thickness resonance onto which spurious modes, caused by lateral standing plate waves, are superposed. Optical interferometermeasurements are used to extract dispersion curves of the laterally propagating waves responsible for the spurious responses. A discrete eigenmode spectrum due to the finite lateral dimensions of the resonator is observed. An equivalent circuit model for a multimode resonator is fitted to the mechanical resonator response extracted along a single curve in the dispersion diagram, and is used to determine properties, such as Q-values, of the individual lateral eigenmodes.Measuredwave field images, extracted dispersion curves, and the eigenmode spectrum with the model fitting results are presented.

We perform a systematic study of the resonant transmission of visible and near-infrared (NIR) light through a single subwavelength slit in a gold film when the parameters defining the structure are varied. We further examine the optical properties of a related nanostructure, a cross with subwavelength sized features. Focused ion beam (FIB) milling was used to fabricate nanoslits and crosses with linewidths ranging from 26 nm to 85 nm. The dimensions of the structure are found to affect strongly the transmittance spectrum. For example, as the slit becomes narrower the resonance is observed to both sharpen and shift significantly. Our observations are in good agreement with our earlier numerical calculations on the optical properties of nanoslits.

We study experimentally the spectral evolution of supercontinua in two different microstructured fibers that are pumped with nanosecond pulses from dual-wavelength sources of either 1064/532 nm or 946/473 nm output. The experimental findings are compared with simulations based on numerically solving the nonlinear Schrodinger equation. The role of cascaded cross-phase modulation processes and the group-delay properties of the fiber are emphasized and demonstrated to determine the extent of the broadening of the continua to the visible wavelengths.

We analyze the spectral properties of resonant transmission of light through a sub-wavelength slit in a metal film. We show that the enhanced transmission can be understood in terms of interfering surface-wave-like modes propagating in the slit. We characterize the effect of geometrical and material properties of the slit on the transmission spectrum. Furthermore, we show that the wavelength of the transmission resonance strongly depends on the surrounding medium. This effect may be utilized in sensors, imaging, and the detection of, e.g. biomolecules.

We report on a simple technique to measure the anomalous dispersion of small-core microstructured fibers using short optical pulses. The method relies on the spectral modulation resulting from the evolution of the input pulse into a propagating soliton wave. The technique allows for a direct measurement of the dispersion at the desired wavelength from a single pulse. The measurement error is estimated to be less than 10%.

Two stationary, partially polarized electromagnetic beams with equal degrees of polarization may exhibit completely different time evolutions of the instantaneous polarization state. In this work, we derive a statistical quantity that describes the rate at which the field intensity in the beam, on average, is redistributed between the beam's polarization state at any time and the state orthogonal to it. This method allows one to treat the dynamical properties of the polarization fluctuations both theoretically and experimentally. We demonstrate the method by applying it to important special cases, such as fields obeying Gaussian statistics, black-body radiation pencils and depolarized laser beams. We also prove that a geometric approach introduced earlier is closely connected with the present model.

Conoscopic interferometry is applied for determining the crystal orientation of lithium niobate and other commonly employed substrate wafers for integrated-optic and surface-acoustic-wave devices. The method is particularly applicable for detecting the orientation of the optic axes of the strongly birefringent niobate but is less sensitive for lithium tantalate or quartz. Conoscopic interference is a low-cost and easy-to-use method that is especially suitable for laboratory usage.

We investigate the dynamics of the instantaneous polarization state of stationary, partially polarized random electromagnetic beamlike fields. An intensity-normalized correlation function of the instantaneous Poincaré vector is introduced for the characterization of the time evolution of the polarization state. This polarization correlation function enables us to define a polarization time and a polarization length over which the polarization state remains substantially unchanged. In the case of Gaussian statistics, the polarization correlation function is shown to assume a simple form in terms of the parameters employed to characterize partial coherence and partial polarization of electromagnetic fields. The formalism is demonstrated for a partially polarized, temporally Gaussian-correlated beam, and black-body radiation. The results are expected to find a range of applications in investigations of phenomena where polarization fluctuations of light play an important role.

International audience; Surface acoustic wave propagation within a two-dimensional phononic band gap structure has been studied using a heterodyne laser interferometer. Acoustic waves are launched by interdigital transducers towards a square lattice of holes etched in a piezoelectric medium. Interferometer measurements performed at frequencies lying below, within, and above the expected band gap frequency range provide direct information of the wave interaction with the phononic crystal, revealing anisotropic scattering into higher diffraction orders depending on the apparent grating pitch at the boundary between the phononic crystal and free surface. Furthermore, the measurements also confirm the existence of an elastic band gap, in accordance with previous electrical measurements and theoretical predictions.