Incident Laser Frequency Research Articles

Vortex harmonic generation in indium tin oxide thin film irradiated by a two-color field.

When a two-color Laguerre-Gaussian laser beam propagates through an indium tin oxide (ITO) material, the spatial distributions of odd- and even-order vortex harmonics carrying orbital angular momentum (OAM) are studied. The origin of vortex harmonics can be directly clarified by investigating their dependence on the incident laser field amplitude and frequency. In addition, it is shown that the spectral intensities of vortex harmonics are sensitive to the epsilon-near-zero nonlinear enhancing effects and the thickness of ITO materials. Thus the vortex harmonics can be conveniently tunable, which provides a wider potential application in optical communications based on high-order OAM coherent vortex beams.

Computational Design of Molecular Probes for Electronic Preresonance Raman Scattering Microscopy.

Recently developed electronic preresonance stimulated Raman scattering (epr-SRS) microscopy, in which the Raman signal of a dye is significantly boosted by setting the incident laser frequency near the electronic excitation energy, has pushed the sensitivity of SRS microscopy close to that offered by confocal fluorescence microscopy. Prominently, the maintained narrow line-width of epr-SRS also offers high multiplexity that breaks the "color barrier" in optical microscopy. However, detailed understanding of the fundamental mechanism in these epr-SRS dyes still remains elusive. Here, we combine experiments with theoretical modeling to investigate the structure-function relationship, aiming to facilitate the design of new probes and expanding epr-SRS palettes. Our ab initio approach employing the displaced harmonic oscillator (DHO) model provides a consistent agreement between simulated and experimental SRS intensities of various triple-bond bearing epr-SRS probes with distinct scaffolds. We further review two popular approximate expressions for epr-SRS, namely the short-time and Albrecht A-term equations, and compare them to the DHO model. Overall, the theory allows us to illustrate how the observed intensity differences between molecular scaffolds stem from the coupling strength between the electronic excitation and the targeted vibrational mode, leading to a general design strategy for highly sensitive next-generation vibrational imaging probes.

Broadband electromagnetic emission via mode conversion mediated by stimulated Raman scattering in inhomogeneous plasma

Electromagnetic emission via linear mode conversion from electron plasma waves (EPWs) excited by stimulated Raman scattering (SRS) of an incident laser pulse in inhomogeneous plasma is investigated theoretically and numerically. It is found that the mode conversion can occur naturally in underdense plasma region below the quarter critical density provided that EPWs are generated due to the development of backward SRS when the laser pulse is incident at certain angle with the plasma density gradient. The produced radiation may cover a broad frequency range up to half of the incident laser frequency. The dependence of the radiation conversion efficiency on the laser intensity, incident angle, laser pulse duration, plasma density scale length, and initial electron temperature is analyzed based on one-dimensional particle-in-cell simulation. In two-dimensional geometry, due to the development of sideward SRS, it is found that the mode conversion to occur even at normal incidence of the laser pulse. The radiation frequency, bandwidth, duration, and amplitude can be well controlled by the laser and plasma parameters, suggesting that it may provide a new source of tunable broadband radiation as well as a diagnosis of the development of SRS.

Gain without inversion and enhancement of refractive index via intervalley quantum coherence transfer in hybrid WS2 -metallic nanoantenna systems

We theoretically study formation of gain without inversion and resonant enhancement of refractive index with suppressed absorption in a system consisting of a monolayer of transition metal dichalcogenide (${\mathrm{WS}}_{2}$) and a metallic nanoantenna. We show that these processes happen when the hybrid system interacts with a right-circularly polarized laser beam, allowing quantum coherence transfer to happen from the K to ${\mathrm{K}}^{\ensuremath{'}}$ valley. Our results show that the quantum coherence transfer can convert the ${\mathrm{K}}^{\ensuremath{'}}$ valley into a conjugated system wherein the real and imaginary parts of its polarization become similar to the imaginary and real parts of the K valley polarization, respectively. This allows the maximum refractive index in the ${\mathrm{K}}^{\ensuremath{'}}$ valley to occur as the absorption becomes zero. We characterize the conjugated valley system by inspecting the evolution of its Bloch vector as the incident laser frequency varies. The theoretical approach includes density matrix formalism while considering a pure dephasing rate that represents the fast decay of exciton valley polarization in transition metal dichalcogenide monolayers.

Second harmonic generation of a self-focused Hermite-Gaussian laser in plasma

Second harmonic generation (SHG) using intense Hermite-Gaussian laser beam (HGLB) propagating through the plasma for mode-indices m = 0 and m = 1 is reported in the present work. Ponderomotive force induced density perturbation beats with the oscillatory velocity of electrons at incident laser frequency, generate the second harmonic nonlinear current that give rise to SHG. Using paraxial approximations, we have derived the coupled equations for the beam width parameter of HGLB and second harmonic's normalized amplitude. Resonance condition is obtained by considering wiggler magnetic field which providing an extra momentum to the second harmonic photon and this result a significant increase in the amplitude of SHG. Our analysis shows the prominent rise in normalized amplitude of second harmonic on increasing the value of the intensity of fundamental laser pulse, normalized wiggler magnetic field and normalized density of plasma. It is notified that the gain of SHG is more prominent for m = 1. Dependency of laser and plasma parameters on SHG is also reported in the current work.

Absorption of Electromagnetic Waves in Non-Dispersive Plasma due to Effect of Ponderomotive Force

The absorption coefficient of electromagnetic wave propagating through overdense absorbing plasma is calculated by using Fresnel’s formula. Also, the force acting per unit volume of the electron fluid is calculated along z direction. The Fresnel’s formula is reduced in terms of conductivity and angular frequency so that nature of absorption coefficient with respect to conductivity of plasma and angular frequency of wave is studied, with the assumption that conductivity and angular frequency are two independent terms. By keeping conductivity of plasma constant, it is seen that the absorption coefficient is increased with increase in angular frequency and also absorption coefficient has the lowest value for the highest value of conductivity. At constant angular frequency of incidence, the absorption coefficient is decreased with increase in conductivity and also absorption coefficient has the highest value of angular frequency of incidence. Also the nature of force on the plasma is studied by considering that the conductivity of plasma is very large with compare to angular frequency of incident laser. The expression of absorption coefficient is separated into real and imaginary parts, and then plots are studied with respect to these parts. It is found that in both cases, the force on plasma is existed for short in time and distance, and then damps exponentially. Also, the variation of potential with distance is cleared that the potential is shielded within multiple distance of Debye length.

Formation of relativistic electromagnetic solitons in over-dense plasmas

We report the formation of electromagnetic solitons in over-dense plasmas in the relativistic transparency regime. By using one-dimensional and two-dimensional particle-in-cell simulations, the formation and basic properties of these long-lived relativistic electromagnetic solitons are studied. The predicted mechanism of soliton formation is different from the existing investigations. The latter ones are found to exist in the wake of the high-intensity laser pulse during the interaction with a low density plasma, and such solitons are made of low-frequency, spatially localized electromagnetic fields. While for the former ones, frequency of solitons formed in the relativistic transparency regime is comparable to incident laser frequency. Moreover, a threshold of plasma density under which stable solitons can be formed is analyzed. These newly predicted solitons are expected to be observed in the present-day laser-plasma experiments.

Photoinduced Nonequilibrium Topological States in Strained Black Phosphorus.

Black phosphorus (BP), an elemental semiconductor, has attracted tremendous interest because it exhibits a wealth of interesting electronic and optoelectronic properties in equilibrium condition. The nonequilibrium electronic structures of bulk BP under a periodic field of laser remain unexplored, but can lead to intriguing topological optoelectronic properties. Here we show that, under the irradiation of circularly polarized light (CPL), BP exhibits a photon-dressed Floquet-Dirac semimetal state, which can be continuously tuned by changing the direction, intensity, and frequency of the incident laser. The topological phase transition from type-I to type-II Floquet-Dirac fermions manifests a new form of type-III phase, which exists in a wide range of intensities and frequencies of the incident laser. Furthermore, topological surface states exhibit nonequilibrium electron transport in a direction locked by the helicity of CPL. Our findings not only deepen our understanding of fundamental properties of BP in relation to topology but also extend optoelectronic device applications of BP to the nonequilibrium regime.

Localized surface plasmon resonance enhanced magneto-optical Kerr effect in Ni80Fe20 thin films coated with Au nanorods

The influence of the localized surface plasmon resonance on the magneto-optical Kerr effect (MOKE) was investigated in Ni80Fe20 thin films coated with gold nanorods. The nanorods are dimensionally tuned to support localized surface plasmon resonance near the incident laser frequency. A significant enhancement of magneto-optical response is observed, in which the MOKE signal is increased by over 40% compared to the reference Ni80Fe20 films. The spectral dependence of MOKE signals exhibits the maximum magneto-optical response centered around the longitudinal surface plasmon resonance wavelength of gold nanorods. Finite-difference time-domain modeling confirms that the excitation of the localized surface plasmons leads to the enhancement of magneto-optical responses.

Self-aligning concave relativistic plasma mirror with adjustable focus

We report an experimental-computational study of the optical properties of plasma mirrors (PMs) at the incident laser frequency when irradiated directly at relativistic intensity (1018<I0<1019 W/cm2) by near-normally incident (4°), high-contrast, 30 fs, 800 nm laser pulses. We find that such relativistic PMs are highly reflective (0.6–0.8) and focus a significant fraction of reflected light to intensity as large as ∼10I0 at distance f as small as ∼25 μm from the PM, provided that pre-pulses do not exceed 1014 W/cm2 prior to ∼20 ps before arrival of the main pulse peak. Particle-in-cell simulations show that focusing results from denting of the reflecting surface by light pressure combined with relativistic transparency and that reflectivity and f can be adjusted by controlling pre-plasma length L over the range 0.5 ≲ L ≲ 3 μm. Pump-probe reflectivity measurements show that the PM's focusing properties evolve on a ps time scale.

Vortex dynamics in superfluids governed by an interacting gauge theory

We study the dynamics of a vortex in a quasi two-dimensional Bose gas consisting of light-matter coupled atoms forming two-component pseudo spins. The gas is subject to a density dependent gauge potential, hence governed by an interacting gauge theory, which stems from a collisionally induced detuning between the incident laser frequency and the atomic energy levels. This provides a back-action between the synthetic gauge potential and the matter field. A Lagrangian approach is used to derive an expression for the force acting on a vortex in such a gas. We discuss the similarities between this force and the one predicted by Iordanskii, Lifshitz and Pitaevskii when scattering between a superfluid vortex and the thermal component is taken into account.

Imaging of Anomalous Internal Reflections of Hyperbolic Phonon-Polaritons in Hexagonal Boron Nitride.

We use scanning near-field optical microscopy to study the response of hexagonal boron nitride nanocones at infrared frequencies, where this material behaves as a hyperbolic medium. The obtained images are dominated by a series of "hot" rings that occur on the sloped sidewalls of the nanocones. The ring positions depend on the incident laser frequency and the nanocone shape. Both dependences are consistent with directional propagation of hyperbolic phonon-polariton rays that are launched at the edges and zigzag through the interior of the nanocones, sustaining multiple internal reflections off the sidewalls. Additionally, we observe a strong overall enhancement of the near-field signal at discrete resonance frequencies. These resonances attest to low dielectric losses that permit coherent standing waves of the subdiffractional polaritons to form. We comment on potential applications of such shape-dependent resonances and the field concentration at the hot rings.

Stimulated compton scattering of surface plasma wave excited over metallic surface by a laser

AbstractA high-frequency surface plasma wave (SPW) excited over metallic surface irradiated by a laser beam, can undergo stimulated Compton scattering if phase velocity of daughter plasma wave is equal to the Fermi velocity for metal. The pump SPW${\rm (}{{\rm \omega} _0},{\vec k_{0{\rm z}}})$parametrically excites a quasi-electrostatic plasma wave${\rm (\omega}, {\vec k_{\rm z}})$and a backscattered sideband SPW${\rm (}{{\rm \omega} _1},{\vec k_{1{\rm z}}})$at resonance ω0= ω − ω1and${\vec k_{0{\rm z}}} = {\vec k_{\rm z}} - {\vec k_{1{\rm z}}}$. The growth rate of Compton process increases with the frequency of incident laser and turns out to be 5.425 × 1010rad/s at laser frequency ω0= 0.7595 × 1015rad/s for incident laser amplitudeA0L= 11 × 1011V/m, laser spot size b = 1.38 × 10−5m, and free electron density of metaln0= 5.85 × 1028/m3. The excitation of highly damped quasi-electrostatic plasma wave in this parametric process provide a better nonlinear option for surface heating as compared with direct laser heating. The process can also be used for diagnostics purposes.

Investigation on the influence of atomic potentials on the above threshold ionization

We systematically investigate the influence of atomic potentials on the above-threshold ionization (ATI) spectra in one-dimensional (1D) cases and compare them with the three-dimensional (3D) case by numerically solving the time-dependent Schrödinger equation. It is found that the direct ionization plateau and the rescattering plateau of the ATI spectrum in the 3D case can be well reproduced by the 1D ATI spectra calculated from the supersolid-core potential and the soft-core potential, respectively. By analyzing the factors that affect the yield of the ATI spectrum, we propose a modified-potential with which we can reproduce the overall 3D ATI spectrum. In addition, the influence of the incident laser intensities and frequencies on the ATI spectra calculated from the proposed modified potential is studied.

Measurement of a Heavy-Hole Hyperfine Interaction in InGaAs Quantum Dots Using Resonance Fluorescence

We measure the strength and the sign of hyperfine interaction of a heavy hole with nuclear spins in single self-assembled quantum dots. Our experiments utilize the locking of a quantum dot resonance to an incident laser frequency to generate nuclear spin polarization. By monitoring the resulting Overhauser shift of optical transitions that are split either by electron or exciton Zeeman energy with respect to the locked transition using resonance fluorescence, we find that the ratio of the heavy-hole and electron hyperfine interactions is -0.09 ± 0.02 in three quantum dots. Since hyperfine interactions constitute the principal decoherence source for spin qubits, we expect our results to be important for efforts aimed at using heavy-hole spins in quantum information processing.

Confluence of resonant laser excitation and bidirectional quantum-dot nuclear-spin polarization

Resonant laser scattering along with photon correlation measurements established the atom-like character of quantum dots. Here, we show that for a wide range of experimental parameters it is impossible to isolate elementary quantum-dot excitations from a strong influence of nuclear spins; the absorption lineshapes at magnetic fields exceeding 1 T indicate that the nuclear spins get polarized by an amount that ensures locking of the quantum-dot resonance to the incident laser frequency. In stark contrast to earlier experiments, this nuclear-spin polarization is bidirectional, allowing the combined electron–nuclear-spin system to track the changes in laser frequency dynamically on both sides of the resonance. This unexpected feature stems from a competition between two spin-pumping processes that attempt to polarize nuclear spins in opposite directions. We find that the confluence of laser excitation and nuclear-spin polarization suppresses the fluctuations in resonant absorption. A master-equation analysis suggests narrowing of the nuclear-spin distribution, pointing to applications in quantum information processing.

Observation of Normal and Anomalous Dispersions in a Microsphere Taper Fiber System

A dielectric microsphere may be considered as an artificial photonic atom. The electromagnetic modes in the microsphere are completely assigned with indexes similar to those used to characterize simple atomic systems. Microsphere modes, as well as microring or microdisk, trap light in circular orbits near the sphere surface and strongly modify the phase of the interacting light, resulting in very steep dispersions. Pulse propagation experiments were performed in a system consisting of a fiber taper coupled with a silica glass microsphere, and it was shown that the light velocity in the system can be largely changed through the control of the coupling strength. Slow light was also demonstrated in a system in which two ultrahigh-Q silica microspheres of different diameters were coupled in tandem to a fiber taper demonstrating classical analog of electromagnetically induced transparency in atomic system. Here, we employed interferometric techniques based on the Mach-Zehnder interferometer for examining extremely steep dispersions induced by whispering gallery modes with a quality factor on the order of Q 1⁄4 10. Such a narrow resonance in the dielectric microsphere is comparable to that in cooled atomic systems. We successfully observed both normal and anomalous dispersions at the resonant frequency, depending on the coupling strength between the sphere and the fiber. Figure 1 shows our experimental system based on an interferometric setup. We used the second harmonics of a single mode ring cavity Nd3þ yttrium aluminum garnet (Nd3þ:YAG) laser with a line width of 1 kHz at 532 nm. The laser frequency was tuned using thermal control of the ring cavity length, over a mode-hop-free range of greater than 10GHz. The reference arm was empty, while the sample arm contained the microsphere-fiber system. Silica microspheres were fabricated from standard telecommunication optical fiber. The end of the etched fiber was fused using a CO2 laser and the microsphere formed due to the surface tension. The experiments were performed with a microsphere 60 mm in diameter. The fiber taper was also fabricated from the same telecommunication fiber. The typical fiber waist diameter at the coupling region was 0.5 mm. A large number of spectrally overlapping resonance modes exist within a microsphere. The fiber taper method is essential for coupling the incident light to a specific whispering gallery mode selectively and efficiently. The microsphere was attached to a translation stage controlled by a piezo actuator. The relative position between the sphere and the fiber were carefully adjusted so that we could observe spectrally isolated whispering gallery modes with proper coupling strength. The optical path length and dispersion were equalized in each arm as completely as possible. The fringe pattern was captured using a charge-coupled device (CCD) camera for each frequency increment. The observed interference patterns appeared as concentric circles due to the interference between the plain reference wave and the spherical wave from the sample arm. A least square fit for each fringe pattern was performed to extract the phase information in the transmitted light. Figures 2(a) and 2(b) show examples of transmission spectra through the microsphere fiber taper coupled system. These spectra were taken in two different whispering gallery modes at different laser frequencies in the mode-hop-free laser tuning range. The transmission dips appear when the incident laser frequencies are resonant at specific whispering gallery modes in the microsphere. The two whispering gallery modes shown in Figs. 2(a) and 2(b) are denoted as WG1 and WG2, respectively. The transmission minimum was Tmin 1⁄4 0:12, and the resonance width was 1⁄4 BS

Robust Ultrafast Currents in Molecular Wires through Stark Shifts

A novel way to induce ultrafast currents in molecular wires using two incident laser frequencies, omega and 2omega, is demonstrated. The mechanism relies on Stark shifts, instead of near-resonance photon absorption, to transfer population to the excited states and exploits the temporal profile of the field to generate phase-controllable transport. Calculations in a trans- polyacetylene oligomer coupled to metallic leads indicate that the mechanism is highly efficient and robust to ultrafast electronic dephasing processes induced by vibronic couplings.

Photon-Assisted Conductance Structure for a Quantum Dot

We investigate theoretically the electron transport of a two-levelquantum dot irradiated under a weak laser field at low temperatures inthe rotating wave approximation. Using the method of the Keldyshequation of motion for nonequilibrium Green function, we examine theconductance for the system with photon polarization perpendicular tothe tunnelling current direction. It is demonstrated that by analyticanalysing and numerical examples, a feature of conductance peaksplitting appears, and the dependence of conductance on the incidentlaser frequency and self-energy are discussed.

Efficient stimulated Raman pumping for quantum state resolved surface reactivity measurements

We describe the use of stimulated Raman pumping in a molecular beam to perform quantum state resolved gas-surface reactivity measurements for molecules prepared in totally symmetric vibrational states. Vibrational states of homonuclear diatomics as well as totally symmetric vibrations of polyatomic molecules cannot be prepared by direct infrared excitation but are accessible through stimulated Raman pumping by two laser fields when the difference between the incident laser frequencies matches the vibration. We generate a suitable resonant pair of high-energy pump and Stokes laser beams in an injection seeded Raman amplifier filled with the sample gas and equipped with internal gas recirculation. The ability to partially saturate the Raman pumping process in the molecular beam is used to quantify the fraction of vibrationally excited molecules in the irradiated volume, which is needed for quantitative reactivity measurements. We illustrate the method with state resolved reactivity measurements for CH4, prepared in its symmetric C–H stretch vibration on a Ni(100) single crystal surface.