Incident Laser Field Research Articles

Enhancement of second-order vortex harmonics in epsilon-near-zero materials

When a spatially-inhomogeneous vortex laser field irradiating an epsilon-near-zero (ENZ) material is theoretically investigated, second-order vortex harmonics occur due to the symmetry breaking from the field spatial-inhomogeneity. Their signal intensity is sensitive to three factors: the ENZ field enhancing ratio, the ENZ material thickness, and the field inhomogeneity degree. Their competition supports a high conversion efficiency of around 10−6 for second-order harmonics from the incident fundamental laser field under an optimum thickness of ENZ materials.

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.

Study of Bragg scattering in the presence of dipole-dipole interaction in plasmonic nanohybrids

We have developed a theory of the Bragg scattering for metallic nanohybrid made of an ensemble of metallic nanorods doped in a substrate. The substrate can gas, liquid or solid. An external laser field is applied to study the Bragg scattered light. The photons from the incident laser interact with the surface plasmons od nanorods and produce surface plasmon polaritons (SPPs). The incident laser field also induced dipoles in the ensemble of nanorods and they interact with each other via the dipole-dipole interaction (DDI). We have developed a theory for Bragg scattering for metallic nanohybrids using the coupled-mode formulism based on Maxwell’s equation in the presence of SPP and DDI fields. It is found that the theory of Bragg scattered depends on the susceptibility induced by the SPP and DDI fields. We used the quantum mechanical density matrix method to calculate the susceptibility. An analytical expression of the Bragg scattered light intensity is obtained. These expressions can be useful for experimental scientists and engineers who can used them to compare their experiments and make new types of plasmonic devices. Next, we have compared our theory with the experiment data for a nanohybrid made of ensemble of Au-nanoris doped in water. We found a good agreement between theory and experiments. We have also performed the numerical simulations to study the effect of SPP and DDI fields on the Bragg intensity. We have predicted an enhancement the Brag intensity due to the SPP and DDI couplings. The enhancement is due to the two extra scattering mechanisms of the SPP and DDI polaritons with acoustic phonons. We have also found that the one peak in the Bragg intensity can be split int many peaks due the SPP coupling, DDI coupling and phase factor. The splitting is due the Bragg factor appearing in the theory, and it includes the coupling of the incident laser, SPP and DDI electric fields with of acoustic phonons. The enhancement effect can be used to fabricate new types of nanosensors. Similarity, splitting phenomenon can be used to fabricate new types nanoswitches where one peak can be considered as the OFF position and many peaks can be considered as the ON position.

A Thermodynamic Comparison of Nanotip and Nanoblade Geometries for Ultrafast Laser Field Emission via the Finite Element Method

Strong laser field emission from metals is a growing area of study, owing to its applications in high-brightness cathodes and potentially as a high harmonic generation source. Nanopatterned plasmonic cathodes localize and enhance incident laser fields, reducing the spot size and increasing the current density. Experiments have demonstrated that the nanoblade structure outperforms nanotips in the peak fields achieved before damage is inflicted. With more intense surface fields come brighter emissions, and thus investigating the thermomechanical properties of these structures is crucial in their characterization. We study, using the finite element method, the electron and lattice temperatures for varying geometries, as well as the opening angles, peak surface fields, and apex radii of curvature. While we underestimate the energy deposited into the lattice here, a comparison of the geometries is still helpful for understanding why one structure performs better than the other. We find that the opening angle—not the structure dimensionality—is what primarily determines the thermal performance of these structures.

Variable laser profiles and polarization states in the wave-mixing scheme for the generation of THz radiation in collisional-magnetized clustered-plasma

The four-wave mixing (FWM) mechanism for the generation of Terahertz (THz) radiation in magnetized-collisional clustered-plasma is studied. Clusters can trap the incident laser fields, leading to efficient energy transfer to plasma electrons and the creation of a strong nonlinear current. The size and density of clusters can be tuned to control the emission of THz radiation. The smaller clusters can bring about higher intensity THz radiation, while larger clusters can contribute to the broader THz frequency range. The THz radiation angular distribution patterns in the forward direction have been assessed, and the impact of plasma interaction length, cluster radius, and plasma electrons collision frequency on the generated THz wave patterns has been investigated. The FWM analysis highlights the importance of plasma clusters as well as magnetic fields in the efficient generation of THz emission. The model also takes into consideration how an induced electron current density initiated by DC external magnetic field can boost radiation power without changing the directivity diagram.

Determining carrier-envelope phase of an ultrashort laser pulse in quantum well via tuning inhomogeneity

When a few-cycle Inhomogeneous laser pulse propagates in parabolic quantum wells, a soliton pulse can occur after a certain propagation distance. The duration and peak intensity of such a soliton pulse are both sensitively dependent on the carrier-envelope phase (CEP) of the incident few-cycle pulse. This kind of CEP-dependence follows a sinusoidal modulation pattern with the Inhomogeneous degree of the incident laser field, which provides a route to determine the CEP by just shaping the field's inhomogeneity analogous to that by using polar media.

Quantum well-shape dependence of optical harmonic radiation

We theoretically investigate the spatial-non-homogeneous laser pulse propagation through the quantum well. It is shown that the odd-order and even-order harmonic generations can be found due to breaking the inversion symmetry of the system. Importantly, the spectral intensity of high-order harmonics can be enhanced by changing the propagation distances and spatial-non-homogeneity of incident laser field. Moreover, the quantum well’s shape has an important influence on the peak intensities of high-order harmonic generations, which may provide a new way to improve the spectral intensity of high-order harmonic generations.

Nonlinearity in surface plasmon polaritons at interface between triple quantum dot and nanocomposite medium

This paper focuses on investigating the generation and manipulation of surface plasmon polaritons (SPPs) at the interfaces between quantum dot (QD) molecules and nanocomposite media. We utilize three different gate voltage values and incident laser field strengths to control the propagation of SPPs. We find a significant improvement in the SPPs' propagation length at the interfaces between the QD molecules and nanocomposite media. Additionally, under the Kerr effect, the gate voltage variation provides opportunities to control the SPPs' propagation. Through modifying the intensity and frequency of the incident probe field, we can manipulate various properties of SPPs at the suggested model's interfaces. Our results indicate a large enhancement in the SPPs' propagation length at the interfaces. Notably, we estimate the skin depth and wavelength of SPPs for three distinct values of the incident probe field and gate voltage applied in the QD molecules.

Electric-field enhancement caused by subwavelength-sized particles located on the surface of multilayer dielectric mirrors.

A laser pulse impinging on the surface of an optical component can interact with particles, such as contamination debris, to produce a scattered electric field, which, either by itself or combined with the incident laser field, coherently can significantly increase the local field intensity. This effect can be of critical importance as it can reduce the laser-induced-damage threshold of the affected component. In this work, we use a field-propagation code to improve understanding regarding the factors that determine the magnitude and location of the electric-field enhancement for the case of subwavelength-sized particles located on the surface of multilayer dielectric mirrors.

Optical current generation in graphene: CEP control vs. ω + 2ω control

Abstract The injection of directional currents in solids with strong optical fields has attracted tremendous attention as a route to realize ultrafast electronics based on the quantum-mechanical nature of electrons at femto- to attosecond timescales. Such currents are usually the result of an asymmetric population distribution imprinted by the temporal symmetry of the driving field. Here we compare two experimental schemes that allow control over the amplitude and direction of light-field-driven currents excited in graphene. Both schemes rely on shaping the incident laser field with one parameter only: either the carrier-envelope phase (CEP) of a single laser pulse or the relative phase between pulses oscillating at angular frequencies ω and 2ω, both for comparable laser parameters. We observe that the efficiency in generating a current via two-color-control exceeds that of CEP control by more than two orders of magnitude (7 nA vs. 18 pA), as the ω + 2ω field exhibits significantly more asymmetry in its temporal shape. We support this finding with numerical simulations that clearly show that two-color current control in graphene is superior, even down to single-cycle pulse durations. We expect our results to be relevant to experimentally access fundamental properties of any solid at ultrafast timescales, as well as for the emerging field of petahertz electronics.

Direct Imprinting of Laser Field on Halide Perovskite Single Crystal for Advanced Photonic Applications

AbstractSingle crystal halide perovskites with microscale dimensions are an emerging class of objects for various advanced photonic and optoelectronic applications. Particularly, defect tolerance and broadband tunability of luminescence make them one of the most prospective candidates to develop microlasers for visible range. However, their post‐processing by standard nanolithography methods face a number of problems related to worsening of their properties, thus making gentle laser processing one of best solutions for perovskite patterning. Here, it is shown that femtosecond laser irradiation of single‐crystal halide perovskite CsPbBr3 allows for its precise and ultraclean ablation fully controlled at subwavelength scale by the intensity and polarization distribution of the complex laser field applied. Indeed, the extremely low thermal conductivity (over 300 times lower than that of silicon) and ultrafast thermalization rate makes it possible to reduce heat‐affected zone and avoid melting layer contribution, while the high refractive index (larger than 2) provides high spatial resolution in case of irradiation of pre‐patterned focusing perovskite nanostructures. These features allow for direct imprinting of the incident laser field at wavelength λ = 515 nm, creating micro‐lens and various light‐emitting metasurfaces with deeply subwavelength spatial resolution (down to λ/7).

Photon-induced atom recoil in collectively interacting planar arrays

The recoil of atoms in arrays due to the emission or absorption of photons is studied for sub-wavelength interatomic spacing. The atoms in the array interact with each other through collective dipole-dipole interactions and with the incident laser field in the low intensity limit. Shining uniform light on the array gives rise to patterns of excitation and recoil in the array. These arise due to the interference of different eigenmodes of excitation. The relation between the recoil and the decay dynamics is studied when the array is in its excitation eigenstates. The recoil experienced by a subradiant collective decay is substantially larger than from independent atom decay. A method to calculate the rate of recoil when steady state has been achieved with a constant influx of photons is also described.

Measurement and control of optical nonlinearities in dispersive dielectric multilayers.

Dispersive dielectric multilayer mirrors, high-dispersion chirped mirrors in particular, are widely used in modern ultrafast optics to manipulate spectral chirps of ultrashort laser pulses. Dispersive mirrors are routinely designed for dispersion compensation in ultrafast lasers and are assumed to be linear optical components. In this work, we report the experimental characterization of an unexpectedly strong nonlinear response in these chirped mirrors. At modest peak intensities <2 TW/cm2-well below the known laser-induced damage threshold of these dielectric structures-we observed a strong reflectivity decrease, local heating, transient spectral modifications, and time-dependent absorption of the incident pulse. Through computational analysis, we found that the incident laser field can be enhanced by an order of magnitude in the dielectric layers of the structure. The field enhancement leads to a wavelength-dependent nonlinear absorption, that shows no signs of cumulative damage before catastrophic failure. The nonlinear absorption is not a simply two-photon process but instead is likely mediated by defects that facilitate two-photon absorption. To mitigate this issue, we designed and fabricated a dispersive multilayer design that strategically suppresses the field enhancement in the high-index layers, shifting the high-field regions to the larger-bandgap, low-index layers. This strategy significantly increases the maximum peak intensity that the mirror can sustain. However, our finding of an onset of nonlinear absorption even at 'modest' fluence and peak intensity has significant implications for numerous past published experimental works employing dispersive mirrors. Additionally, our results will guide future ultrafast experimental work and ultrafast laser design.

Three Dimensional Alternating-Phase Focusing for Dielectric-Laser Electron Accelerators.

The concept of dielectric-laser acceleration (DLA) provides the highest gradients among breakdown-limited (nonplasma) particle accelerators and thus the potential of miniaturization. The implementation of a fully scalable electron accelerator on a microchip by two-dimensional alternating phase focusing (APF), which relies on homogeneous laser fields and external magnetic focusing in the third direction, was recently proposed. In this Letter, we generalize the APF for DLA scheme to 3D, such that stable beam transport and acceleration is attained without any external equipment, while the structures can still be fabricated by entirely two-dimensional lithographic techniques. In the new scheme, we obtain significantly higher accelerating gradients at given incident laser field by additionally exploiting the new horizontal edge. This enables ultralow injection energies of about 2.5keV (β=0.1) and bulky high voltage equipment as used in previous DLA experiments can be omitted. DLAs have applications in ultrafast time-resolved electron microscopy and diffraction. Our findings are crucial for the miniaturization of the entire setup and pave the way towards integration of DLAs in optical fiber driven endoscopes, e.g., for medical purposes.

Control of electronic and optical properties of a laser dressed double quantum dot molecule by lateral electric field

We study the confined states of a molecule formed by two laterally coupled quantum dots. The static electric field and incident plane wave laser field are considered with different polarization. The respective Shrödinger equation is obtained in the framework of the high-frequency limit of Floquet theory. It is shown that the electric field has a qualitative impact on the spatial symmetry of electronic states. An in-depth study of electric and laser field combined effects show a strong correlation between them. Additionally, we show that the linear, nonlinear and total optical absorption coefficients and relative refractive index of the considered structure can be effectively tuned by changing electric field strength and intense laser field parameter. Our results indicate the possibility of controlling the optical response of double quantum dots molecule for future applications in nanostructure-based quantum information science.

Driven Imposters: Controlling Expectations in Many-Body Systems.

We present a framework for controlling the observables of a general correlated electron system driven by an incident laser field. The approach provides a prescription for the driving required to generate an arbitrary predetermined evolution for the expectation value of a chosen observable, together with a constraint on the maximum size of this expectation. To demonstrate this, we determine the laser fields required to exactly control the current in a Fermi-Hubbard system under a range of model parameters, fully controlling the nonlinear high-harmonic generation and optically observed electron dynamics in the system. This is achieved for both the uncorrelated metalliclike state and deep in the strongly correlated Mott insulating regime, flipping the optical responses of the two systems so as to mimic the other, creating "driven imposters." We also present a general framework for the control of other dynamical variables, opening a new route for the design of driven materials with customized properties.

Impact of Nonlinear Kerr Effect on the Focusing Performance of Optical Lens with High-Intensity Laser Incidence

The rapid development of high-energy and high-power laser technology provides an important experimental means for the research of extreme physical state in the laboratory and for the design of large laser facilities for realizing inertial confinement fusion. However, when the incident laser field is very strong, the Kerr effect of materials affects the nominal performance of optical elements. In this work, the impact of Kerr effect on the focusing performance of an optical lens is studied by calculating and comparing the filed patterns of focal spots for three different incident laser beams together with three different levels of light intensities. The traditional transfer function of an optical lens is firstly modified according to the theory of nonlinear Kerr effect. We use the two-dimensional fast Fourier transform algorithm and angular spectrum algorithm to numerically calculate the field distributions of focal spots in the nominal focal plane of lens and its adjacent planes based on the Fresnel diffraction integral formula. The obtained results show that the Kerr effect affects the focusing characteristics of lens, especially for the incidence of high-order Gaussian beams, such as Hermite-Gaussian beams and Laguerre-Gaussian beams. At the same time, the focal length and refractive index of lens also change the field patterns of focal spots. The presented methodology is of great value in engineering applications where the practical problem with beam size up to 100 mm can be calculated using a common laptop computer. The work provides an efficient numerical technique for high-intensity incident laser beams focused by lens that takes Kerr effect into consideration, which has potential applications in high energy density physics and large laser facilities for inertial confinement fusion.

Stimulated Brillouin scattering in optical fibers with end reflections excited by broadband pump waves

The transient theory of stimulated Brillouin scattering (SBS) in optical fibers is used to investigate the effects of weak feedback of incident broadband laser fields exhibiting different spectral shapes. The proper boundary conditions are given for the forward- and backward-propagating electric fields inside the fiber with a finite amount of reflectivity at the fiber ends. When the linewidth of the laser approaches the Brillouin frequency shift, a weak reflection of the laser field overlaps with the SBS gain and can be amplified in the fiber, resulting in an apparent reduction in SBS threshold. It is shown that the spectral shape of the incident field and its overlap with the SBS gain spectrum affects the SBS signal. Numerical examples are presented for incident laser fields that (1) simulate a pulsed, multilongitudinal-mode laser and (2) are a constant amplitude with a sinusoidal phase modulation. For the pulsed field with mode spacing much less than the Brillouin linewidth, calculations show that an end reflection of 0.01% (−40 dB) of the laser field with a linewidth equal to 0.75 of the Brillouin frequency shift can reduce the effective SBS threshold by a factor of 1.5. When the incident laser field is constant amplitude and phase modulated with a sinusoid, calculations predict sharp and distinct enhancement of the reflected power when the ratio of the Brillouin frequency shift to the modulation frequency is an integer for end reflections as low as 0.001% (−50 dB). Experiments performed with a sinusoidal phase modulated signal confirm the theoretical predictions. These results have implications on the design of high-power fiber laser systems that utilize spectral broadening to suppress SBS.

In situ and ex-situ physical scenario of the femtosecond laser-induced periodic surface structures.

Laser-induced periodic surface structures (LIPSS) are a universal phenomenon that can allow tailoring nanoelectronics and nanophotonics devices. However, there is an issue about the formation mechanism of LIPSS, and the current research mainly focuses on the formation process of the individual structures, such as the low spatial frequency LIPSS (LSFL), sub-wavelength structures, and laser-induced periodic annular surface structures (LIPASS). A whole process formation picture of the series of these periodic structures is still missing. In this study, a pump-probing setup is applied to ensure the real-time and in situ monitoring of surface modification under different pulse numbers. LSFL firstly appears on the surface after two laser shots, and then, laser-induced orthogonal periodic structures (LIOPS) become the dominant morphology after five laser shots, which result from the local field enhancement of the surface ripples. As the laser shots increase, the LSFL split leads to the formation of nanopillars, and the formation of the nanopillars under the surface LSFL (after ten laser shots) is due to the transition between the LSFL and HSFL with an orientation parallel to the laser polarization. A dip surrounded by annular periodic fringes after 50 laser shots is observed, which is due to the interference of the incident laser field and the reflected laser field on the crater surface. Finally, a direct writing technique for fabrication of nano-gratings is also reported.

Coherent power amplification of third-order harmonic femtosecond pulses at thin-film up-conversion nanoparticles

Third harmonic generation (THG) is a nonlinear optical process attractive in high-resolution interfacial studies, sub-wavelength light manipulation, and bio-molecular detection due to its capability of converting low-energy quanta into a quantum of a higher energy. One of the limitations in utilizing THG is its low power conversion efficiency; thus, various THG enhancement methods have been researched by involving plasmonic coupling effects or utilizing electric band gap resonances at quantum dots or two-dimensional materials. Meanwhile, lanthanide ion-doped up-conversion nanoparticles (UCNPs) can be excited by a multi-photon process similar to THG, but its interaction or resonance with THG has not been studied to date. In this Communication, we demonstrate the first coherent amplification of third-order harmonic femtosecond pulses at multi-layered UCNP thin-film with an amplification factor of 7.8. This amplification is made by the resonance interaction of incident femtosecond laser field, generated third-order harmonics, and the electric band gaps of UCNPs. The power contribution of the third-order harmonic and the up-conversion luminescence (UCL) is strongly dependent on the sample geometry due to the reabsorption effect. For in-depth understanding of the emission characteristics, spectral-domain, time-domain, radio-frequency (RF) domain, and polarization-dependence analysis were addressed. This coherent amplification of third harmonic (TH) at UCNP thin-films enables us to attain higher power, shorter wavelength, and ultra-short femtosecond pulses generated from a simple thin-film structure near to the target samples, which will pave a way to an ultrafast short-wavelength laser platform for material characterization, sub-wavelength photonics, and biomolecular detection.