Measuring an ultrashort, ultraviolet pulse in a slowly responding, absorbing medium.

Frequency-resolved optical gating (FROG) is a common technique for measuring ultrashort laser pulses using an instantaneous, nonlinear-optical interaction as a fast time-gate to measure the pulse intensity and phase. But at high frequencies, materials are often absorbing and it is not always possible to find a medium with a fast nonlinear-optical response. Here we show that an ultrashort, ultraviolet (UV) pulse can be measured in a strongly absorbing medium, using the absorption as the nonlinear-optical time-gate. To do this, we build on our recent implementation of FROG, known as induced-grating cross-correlation FROG (IG XFROG), where an unknown, higher-frequency pulse creates a transient grating that is probed with a lower-frequency, more easily detectable reference pulse. We demonstrate this with an 800 nm reference pulse to characterize 400 nm or 267 nm pulses using ZnS as the nonlinear-optical medium, which is absorptive at and below 400 nm. By scanning the delay between the two UV pulses which create the transient grating, we show that the phase-sensitive instantaneous four-wave-mixing contribution to the nonlinear signal field can be detected and separated from the slower, incoherent part of the response. Measuring a spectrally-resolved cross-correlation in this way and then applying a simple model for the response of the medium, we show that a modified generalized projections (GP) phase-retrieval algorithm can be used to extract the pulse amplitude and phase. We test this approach by measuring chirped UV pulses centered at 400 nm and 267 nm. Since interband absorption (or even photoionization) is not strongly wavelength-dependent, we expect IG XFROG to be applicable deeper into the UV.

Encoding the complete electric field of an ultraviolet ultrashort laser pulse in a near-infrared nonlinear-optical signal.

We introduce a variation on the cross-correlation frequency-resolved optical gating (XFROG) technique that uses a near-infrared (NIR) nonlinear-optical signal to characterize pulses in the ultraviolet (UV). Using a transient-grating XFROG beam geometry, we create a grating using two copies of the unknown UV pulse and diffract a NIR reference pulse from it. We show that, by varying the delay between the UV pulses creating the grating, the UV pulse intensity-and-phase information can be encoded into a NIR signal. We also implemented a modified generalized-projections phase-retrieval algorithm for retrieving the UV pulses from these spectrograms. We performed proof-of-principle measurements of chirped pulses and double pulses, all at 400 nm. This approach should be extendable deeper into the UV and potentially even into the extreme UV or x-ray range.

Effects of residual coherence on the scintillation of a partially coherent beam.

Recent results show that partially coherent beams (PCB) can be conveniently generated in a multimode fiber and modulated with data at gigabit per second rates, which makes them attractive for free-space optical communication through turbulent atmosphere. An important feature of these realistic beams in contrast to model ones is the presence of residual coherence between pairs of points spatially separated by more than a few coherence radii on the beam aperture. In the present work we experimentally study the influence of this residual coherence on the scintillation of a partially coherent beam in a laboratory turbulence. It is shown that the total scintillation can be considered as a combination of scintillations of the coherent and incoherent parts of the full beam. When residual coherence is large the scintillation is mostly due to speckle motion on the detector. In the opposite case, the scintillation index settles at a low value pertaining to "ideal" homogeneous PCB.

Different measures of speckle and coherence at the output of a multimode optical fiber.

Excitation of a multimode fiber with a focused spatially coherent light of finite bandwidth results in a partially coherent light at the output of the fiber. Here we study the properties of speckle and classical coherence of such light with analytical theory, numerical modeling, and experimentally. Of particular interest is the relationship between measures of coherence and speckle and their dependence on input source bandwidth and fiber length. Speckle contrast is easy to measure experimentally and there exist at least two different methods to generate ensembles of random speckles. We show that speckle contrast evaluated over the ensemble of external diffusers is related to the number of effective modes-one of the characteristics of beam global coherence. The other speckle contrast measure evaluated over the ensemble of random bends and twists of the fiber is related to residual coherence, which is the pedestal on the average modulus of the complex degree of the coherence function on the output endface of the fiber.

Coherence and speckle contrast at the output of a stationary multimode optical fiber.

We relate classic coherence properties of light at the output of a multimode optical fiber excited by a spatially coherent broadband source to speckle contrast measured by two different methods. Speckle contrast measured with an external diffuser is related to the effective number of modes, while that measured over the ensemble of random bends and twists of the fiber is related to the residual coherence defined as a spatial average of the modulus of the classic complex degree of coherence between pairs of widely separated points at the fiber output.

Electrical Control of near-Field Energy Transfer between Quantum Dots and Two-Dimensional Semiconductors.

We investigate near-field energy transfer between chemically synthesized quantum dots (QDs) and two-dimensional semiconductors. We fabricate devices in which electrostatically gated semiconducting monolayer molybdenum disulfide (MoS2) is placed atop a homogeneous self-assembled layer of core-shell CdSSe QDs. We demonstrate efficient nonradiative Förster resonant energy transfer (FRET) from QDs into MoS2 and prove that modest gate-induced variation in the excitonic absorption of MoS2 leads to large (∼500%) changes in the FRET rate. This in turn allows for up to ∼75% electrical modulation of QD photoluminescence intensity. The hybrid QD/MoS2 devices operate within a small voltage range, allow for continuous modification of the QD photoluminescence intensity, and can be used for selective tuning of QDs emitting in the visible-IR range.

Intuitive model for the scintillations of a partially coherent beam.

An intuitive model for the scintillation index of a partially coherent beam is developed in which essentially the only critical parameter is the properly defined Fresnel number equal to the ratio of the "working" aperture area to the area of the Fresnel zone. The model transpired from and is supported by numerical simulations using Rytov method for weak fluctuations regime and Tatarskii turbulence spectrum with inner scale. The ratio of the scintillation index of a partially coherent beam to that of a plane wave displays a characteristic minimum, the magnitude of which and its distance from the transmitter are easily explained using the intuitive model. A theoretical asymptotic is found for the scintillation index of a source with decreasing coherence at this minimum.

Spatial coherence at the output of multimode optical fibers.

The modulus of the complex degree of coherence is directly measured at the output of a step-index multimode optical fiber using lateral-sheering, delay-dithering Mach-Zehnder interferometer. Pumping the multimode fiber with monochromatic light always results in spatially-coherent output, whereas for the broadband pumping the modal dispersion of the fiber leads to a partially coherent output. While the coherence radius is a function of the numerical aperture only, the residual coherence outside the main peak is an interesting function of two dimensionless parameters: the number of non-degenerate modes and the ratio of the modal dispersion to the coherence time of the source. We develop a simple model describing this residual coherence and verify its predictions experimentally.

Lateral-shearing, delay-dithering Mach-Zehnder interferometer for spatial coherence measurement.

An image-shearing interferometer of Mach-Zehnder type with corner cubes is introduced for the purpose of measuring spatial coherence at the output of inhomogeneous optical sources, such as multimode fibers (MMFs). One arm of the interferometer is modulated in optical delay to produce dynamic interference fringes. Fringe visibility and the two individual intensities are measured nearly simultaneously to allow direct calculation of the modulus of the complex degree of coherence as a function of the lateral shear between the two interferometer arms. Spatial degree of coherence is measured for a step-index MMF pumped with monochromatic and broadband optical sources.

Diabolical point and conical-like diffraction in periodic plasmonic nanostructures.

We present the formation of a singular (diabolical) point in k-space from a periodic metal-dielectric waveguide array. The singularity originates from the balance between alternating normal and anomalous coupling. We numerically demonstrate a strong diffraction anomaly (conical-like diffraction) near the singular point. We also show the evolution of the diffraction pattern with band deformation. The resultant peculiar propagation dynamics of surface plasmon polaritons could provide a new toolset for manipulating light on the nano-scale.

Dirac dynamics in one-dimensional graphene-like plasmonic crystals: pseudo-spin, chirality, and diffraction anomaly.

We introduce a new class of plasmonic crystals possessing graphene-like internal symmetries and Dirac-type spectrum in k-space. We study dynamics of surface plasmon polaritons supported in the plasmonic crystals by employing the formalism of Dirac dynamics for relativistic quantum particles. Through an analogy with graphene, we introduce a concept of pseudo-spin and chirality to indicate built-in symmetry of the plasmonic crystals near Dirac point. The surface plasmon polaritons with different pseudo-spin states are shown to split in the crystals into two beams, analogous to spin Hall effect.

Coherent mid-infrared broadband continuum generation in non-uniform ZBLAN fiber taper.

A simple method is described for efficient, asymmetric and coherent continuum generation in the mid-infrared region based on the dynamics of a stabilized soliton in the vicinity of a second dispersion zero of a nonlinear fiber. The mechanism involves nonlinear soliton compression, Raman self-frequency shift and resonant emission of a dispersive (Cherenkov) wave in a non-uniformly tapered ZBLAN fluoride fiber pumped by a low-power compact femtosecond laser at 1.55 microm. The fiber taper features a continuous shift of the second zero dispersion wavelength, which facilitates the progressive shift in the wavelength of the dispersive wave generated by the stabilized soliton. Numerical solution of the generalized nonlinear Schrödinger equation, which accounts for the exact wave-length dependence of dispersion and nonlinear coefficients, shows robust generation of near-octave continuum spanning 1.5-3 microm wavelength range.

Subwavelength hybrid terahertz waveguides.

We introduce and present general properties of hybrid terahertz waveguides. Weakly confined Zenneck waves on a metal-dielectric interface at terahertz frequencies can be transformed to a strongly confined yet low-loss subwavelength mode through coupling with a photonic mode of a nearby high-index dielectric strip. We analyze confinement, attenuation, and dispersion properties of this mode. The proposed design is suitable for planar integration and allows easy fabrication on chip scale. The superior waveguiding properties at terahertz frequencies could enable the hybrid terahertz waveguides as building blocks for terahertz integrated circuits.

Emission properties of quantum dots in polymer optical fibres.

CdSe/ZnS core-shell quantum dots have been embedded within microstructured polymer optical fibres. The emission properties of quantum dots within fibres have been explored to show that variation in concentration, sample length and pumping regimes effects the emission characteristics of these quantum dots.

Supercontinuum generation and soliton timing jitter in SF6 soft glass photonic crystal fibers.

We use broadband frequency-resolved optical gating (FROG) and cross-correlation FROG (XFROG) techniques to study the details of the supercontinuum generated in extruded soft glass SF6 photonic crystal fibers pumped with envelope-modulated 100 fs pulses at telecom wavelengths. Strong temporal jitter of solitons is observed with highly non-Gaussian statistics, which is related to the statistics of the pump pulse envelope shape fluctuations. The ripples present on the input pulse seed the modulation instability at high pump powers, affecting soliton fission. Numerical modeling confirms strong sensitivity of the soliton fission process to the presence of ripples on the pump pulse envelope.

Ultra-thin metasurface microwave flat lens for broadband applications.

We demonstrate a metasurface-based ultrathin flat lens operating at microwave frequencies. A series of subwavelength metallic split-ring resonators, which are sandwiched between two cross-polarized metallic gratings, are defined to obtain a radially symmetric parabolic phase distribution, covering relative phase differences ranging from 0 to 2.5π radians to create a lens. The tri-layer lens exhibits focusing/collimating of broadband microwaves from 7.0 to 10.0 GHz, with a gain enhancement of 17 dBi at a central wavelength 9.0 GHz while fed by a rectangular horn antenna. The measured focal length agrees reasonably well with design, achieving a 3 dB directionality <4.5° and confirming high-quality beam collimation along the propagation direction. The demonstrated metasurface flat lens enables light-weight, low-cost, and easily deployable flat transceivers for microwave communication, detection, and imaging applications.

Soliton dynamics in optical fibers using the generalized traveling-wave method.

The generalized traveling wave method (GTWM) is applied to the nonlinear Schrödinger (NLS) equation with general perturbations in order to obtain the equations of motion for an ansatz with six collective coordinates, namely the soliton position, the amplitude, the inverse of the soliton width, the velocity, the chirp, and the phase. The advantage of the new ansatz is that it yields three pairs of canonically conjugated coordinates and momenta that all are well-behaved. The new ansatz is applied to model the dynamics of a soliton in a dispersion-shifted optical fiber described by the generalized NLS, including dissipation, higher-order dispersion, Raman scattering, and self-steepening perturbations. It is shown that the GTWM is equivalent to the modified method of moments, which considers the time variation of the norm, the first and the second moment of the norm, the momentum, the first moment of the momentum, and the energy for the perturbed NLS equation.

Polarization dependent harmonic generation in microstructured fibers.

We report on the control of visible harmonic generation in microstructured fiber through the polarization state of the fundamental radiation. By coupling ë=1.55 ìm femtosecond pulses that have the same peak power into a short length (Z=20 cm) of high- microstructured fiber, we observe the generation of distinct visible spectral components in the visible at the output of the fiber in dependence of the input pulse's polarization state.

Nonlinear generation of very high-order UV modes in microstructured fibers.

Cobweb microstructured optical fibers are often strongly multimode in the visible and near infrared regions. This may lead to a number of intermodally phase-matched nonlinear processes. Here we describe a process of nonlinear generation of very high-order UV modes by pumping such fibers with 100 fs Ti:sapphire pulses. Wavelengths as short as 260 nm are generated through a mechanism distinct from supercontinuum generation.

Phase-matched third harmonic generation in microstructured fibers.

Strong guiding provided by the high-delta microstructured fibers allows for efficient intermodally phase-matched harmonic generation with femtosecond pumping at telecom wavelengths. Visible harmonics are generated in a number of distinct transverse modes of the structure. We present a detailed experimental and theoretical study of the third harmonic generation in such fibers including phase-matching wavelengths, far-field intensity distributions and polarization dependence. Good agreement between the theory and experiment is achieved.