Strong Light Field Research Articles

Dual-band enhanced optical absorptance of graphene with resonant tunneling in total internal reflection regime

We propose a stratified structure, where the monolayer graphene is sandwiched between two thin film stacks with optimized period number. A band-pass filter based on the stratified structure is realized. The dual-band enhanced optical absorptance of graphene with resonant tunneling in total internal reflection (TIR) regime is investigated theoretically. Both absorptance peaks are enhanced up to almost 100% in the pass-band filter based on resonant tunneling through a graphene sheet over the communication wavelength range due to the strong light field confinement. With the increasing of the bottom mirror layers, the values of the dual-band absorptance peaks improve accordingly while the peak locations are unchanged. The variation of the angle and wavelength can be exploited to realize tunable filters with good rejection.

The anomalous interaction of electrons in strong pulsed light fields

The force of effective interaction between two nonrelativistic electrons inside the strong pulsed laser field of two mutually perpendicular light waves is studied theoretically. The magnetic interaction of effective electron currents makes the main contribution to this force. The magnitude and direction of the electron currents depend on the intensity of the waves. The force of magnetic interaction of effective electron currents may exceed the Coulomb force at long distances. As a result, the effective force of an electron pair’s interaction in the laser fields can be attractive as well as repulsive. Thus, the effect of attraction allows increasing the confinement time of electrons in comparison with the corresponding value without any external fields. The anomalous effect of electron repulsion was predicted. This effect allows electrons (after interaction) to scatter over distances exceeding by an order of magnitude corresponding distances without external fields. The appearance of the effective force of attraction may be experimentally verified in the framework of modern research projects where sources of pulsed laser radiation are used (SLAC, FAIR, XFEL, ELI, XCELS).

8th Symposium on Frequency Standards and Metrology 2015

8th Symposium on Frequency Standards and Metrology 2015

X-ray fluorescence spectrum of highly charged Fe ions driven by strong free-electron-laser fields

The influence of nonlinear dynamical effects is analyzed on the observed spectra of controversial 3C and 3D astrophysically relevant x-ray lines in neonlike Fe and the A, B, and C lines in natriumlike Fe ions. First, a large-scale configuration-interaction calculation of oscillator strengths is performed with the inclusion of higher-order electron-correlation effects. Also, quantum-electrodynamic corrections to the transition energies are calculated. Further considered dynamical effects provide a possible resolution of the discrepancy between theory and experiment found by recent x-ray free-electron-laser measurements of these controversial lines. We find that, for strong x-ray sources, the modeling of the spectral lines by a peak with an area proportional to the oscillator strength is not sufficient and nonlinear dynamical effects have to be taken into account. Thus, we advocate the use of light–matter-interaction models also valid for strong light fields in the analysis and interpretation of the associated astrophysical and laboratory spectra. We investigate line-strength ratios distinguishing between the coherent and incoherent parts of the emission spectrum. In addition, the spectrum of Fe, an autoionizing ion which was also present in the recent laboratory experiment, is analyzed as well.

Singular perturbations approach to localized surface-plasmon resonance: Nearly touching metal nanospheres

Metallic nano-structures characterised by multiple geometric length scales support low-frequency surface-plasmon modes, which enable strong light localization and field enhancement. We suggest studying such configurations using singular perturbation methods, and demonstrate the efficacy of this approach by considering, in the quasi-static limit, a pair of nearly touching metallic nano-spheres subjected to an incident electromagnetic wave polarized with the electric field along the line of sphere centers. Rather than attempting an exact analytical solution, we construct the pertinent (longitudinal) eigen-modes by matching relatively simple asymptotic expansions valid in overlapping spatial domains. We thereby arrive at an effective boundary eigenvalue problem in a half-space representing the metal region in the vicinity of the gap. Coupling with the gap field gives rise to a mixed-type boundary condition with varying coefficients, whereas coupling with the particle-scale field enters through an integral eigenvalue selection rule involving the electrostatic capacitance of the configuration. By solving the reduced problem we obtain accurate closed-form expressions for the resonance values of the metal dielectric function. Furthermore, together with an energy-like integral relation, the latter eigen-solutions yield also closed-form approximations for the induced-dipole moment and gap-field enhancement under resonance. We demonstrate agreement between the asymptotic formulas and a semi-numerical computation. The analysis, underpinned by asymptotic scaling arguments, elucidates how metal polarization together with geometrical confinement enables a strong plasmon-frequency redshift and amplified near-field at resonance.

Metal semishell-substrate coupled structures with enlargened near-field enhancement area

Metal antennas-substrate coupled structures have unique features of strong nanoscale light confinement and field enhancement at the gap, which make them attractive and promising in a variety of applications such as sensing, lasing, and solar cell. Here, we built a coupled system consisting of a gold spherical semishell antenna and gold substrate. Through numerical simulation, it is found that due to the unique geometric shape of the antenna, the near-field enhancement area of the localized resonance is enlarged dramatically in the designed plasmonic structure, which is beneficial to increasing the active region, allowing more molecular adsorbed and also replaced easily. Moreover, the far-field extinction spectra of the localized dipole mode in the designed structure has higher quality factor and is more robust to the gap separation than its solid counterpart, which may relax the fabrication tolerances.

Generation of current sheets and giant quasistatic magnetic fields at the ionization of vacuum in extremely strong light fields

The self-consistent dynamics of an electron–positron plasma, which is formed during the generation of quantum-electrodynamic cascades, in a superstrong field of counterpropagating linearly polarized waves is examined. It is shown that the formation of thin (on a wavelength scale) current sheets which generate quasistatic magnetic fields comparable to the corresponding fields of incident waves plays an important role in the dynamics of a cascade for fields above a certain threshold. The fraction of the laser energy transformed into the energy of quasistatic magnetic fields can exceed 20%.

Astrophysical line diagnosis requires nonlinear dynamical atomic modeling.

Line intensities and oscillator strengths for the controversial 3C and 3D astrophysically relevant lines in neonlike Fe(16+) ions are calculated. A large-scale configuration-interaction calculation of oscillator strengths is performed with the inclusion of higher-order electron-correlation effects, suggesting that these contributions cannot explain existing discrepancies between theory and experiment. Then, we investigate nonlinear dynamical effects, showing that, for strong x-ray sources, the modeling of the spectral lines by a peak with an area proportional to the oscillator strength is not sufficient. The dynamical effects give a possible resolution of discrepancies of theory and experiment found by recent measurements, which motivates the use of light-matter interaction models also valid for strong light fields in the analysis and interpretation of astrophysical and laboratory spectra.

Large and well-defined Rabi splitting in a semiconductor nanogap cavity.

We propose a nanogap structure composed of semiconductor nanoparticles forming an optical cavity. The resonant excitation of excitons in the nanoparticles can generate a localized strong light field in the gap region, also called "hot spot". The spectral width of the hot spot is significantly narrow because of the small exciton damping and the dephasing at low temperature, so the semiconductor nanogap structure acts as a high-Q cavity. In addition, the interaction between light and matter at the nanogap is significantly larger than that in a conventional microcavity, because the former has a small cavity-mode volume beyond the diffraction limit. We theoretically demonstrate the large and well-defined vacuum-Rabi splitting of a two-level emitter placed inside the semiconductor nanogap cavity: the Rabi splitting energy of 1.7 meV for the transition dipole moment of the emitter (25 Debye) is about 6.3 times larger than the spectral width. An optical cavity providing such a large and well-defined Rabi splitting is highly suited for studying characteristic features of the cavity quantum electrodynamics and for the development of new applications.

Nonlinear optics of graphene in the presence of Rabi oscillation

When graphene is subjected to a strong light field, its nonlinear optical response allows only odd harmonic emissions because of a centrosymmetric structure. We demonstrate that this limitation no longer holds when the generation of an electric current density is considered within a quantum theory because of the involvement of the Rabi oscillation in the nonlinear optical response. By transforming the time-dependent Dirac equation into the length gauge, the Rabi frequency in graphene can be defined, allowing the identification of the Rabi coupling effect on the current-induced harmonic spectra. In particular, a peak at the spectral position of the second harmonic of the incident light can be generated in graphene when the intensity of the incident light causes the Rabi frequency to become comparable with the frequency of the incident light, which was demonstrated in ZnO with a measurement of carrier-envelope-offset frequency [Phys. Rev. Lett.90, 217404 (2003)10.1103/PhysRevLett.90.217404PRLTAO0031-9007]. Moreover, as the light intensity increases, we show that a broadband harmonic spectrum is generated and that a compact electron distribution caused by multiphoton absorptions is produced because of highly nonlinear effects associated with the Rabi oscillation.

Resources of polarimetric sensitivity in spin noise spectroscopy

We attract attention to the fact that the ultimate (shot-noise-limited) polarimetric sensitivity can be enhanced by orders of magnitude leaving the photon flux incident onto the photodetector on the same low level. This opportunity is of crucial importance for present-day spin noise spectroscopy, where a direct increase of sensitivity by increasing the probe beam power is strongly restricted by the admissible input power of the broadband photodetectors. The gain in sensitivity is achieved by replacing the 45-deg polarization geometry commonly used in conventional schemes with balanced detectors by geometries with stronger polarization extinction. The efficiency of these high-extinction polarization geometries with enhancement of the detected signal by more than an order of magnitude is demonstrated by measurements of the spin noise spectra of bulk n:GaAs in the spectral range 835-918 nm. It is shown that the inevitable growth of the probe beam power with the sensitivity gain makes spin noise spectroscopy much more perturbative, but, at the same time, opens up fresh opportunities for studying nonlinear interactions of strong light fields with spin ensembles.

Anti-Stokes-enhanced tunneling ionization of molecules

We consider the influence of vibrational motion of nuclei on the tunneling ionization of a molecule in a strong light field. Unlike the molecular Ammosov-Delone-Krainov (MO-ADK) model [X. M. Tong et al., Phys. Rev. A 66, 033402 (2002)], the Franck-Condon factors that take into consideration the quantization of nuclear vibrations are introduced in the theory. Modification of the vibrational motion of nuclei by the external field leads to the time-dependent constant in the asymptotic form of the electronic wave function. This in turn leads to a more significant modification of the Keldysh theory for the tunnel effect in molecules as compared to the MO-ADK model. We take into account the possibility of vibrational quantum numbers changing in a neutral molecule and in a molecular ion in the process of the tunneling ionization, which in principle enables one to get molecular ions of a certain isotopic composition if the vibrational states of the neutral molecules with this isotopic composition are pumped before ionization. We present numerical results illustrating this possibility for the isotopes of a hydrogen molecule.

Ionization and Coulomb explosion of small group 10 transition metal oxide clusters in strong light fields

The ionization properties of small group 10 metal oxide clusters are explored using ultrafast pulses centered at 624 nm. Maximum atomic charge states resulting from Coulomb explosion were observed to be Ni(3+), Pd(3+), Pt(5+), and O(2+) species with similar ionization potentials ~30-35 eV. Ion signal as a function of laser intensity of each charge state of Ni, Pd, Pt, and O resulting from Coulomb explosion was mapped and compared to that predicted from semi-classical tunneling theory using sequential ionization potentials to quantify observed enhancements in ionization. The saturation intensity (I(sat)) of each charge state is measured and compared to previous studies on group 5 transition metal oxides. The atomic charge states of nickel showed a large enhancement in ionization compared to palladium and platinum, reflective of the differing bonding properties of each metal with oxygen. Results indicate that nickel oxide clusters undergo a greater extent of ionization enhancement as a result of multiple ionization mechanisms. The ionization enhancement behavior of each metal oxide species is explored herein.

Interaction of classical nonrelativistic identically charged particles in a strong pulsed light field

In this review we consider the effective interaction force of identically charged nonrelativistic particles (electrons, light and heavy atoms nuclei) in a strong pulsed laser field. It is shown that under certain conditions, the effective interaction force can become a force of attraction. This effect significantly changes the kinematics of the interaction between the particles and can lead to bound states.

Applications of Oxide Nanomaterials in Nonlinear Optics

ABSTRACTNonlinear optical effects are revealed when strong light fields interact with matter. It has been shown that nanomaterials exhibit properties which are very different from the bulk, and in many cases, the nonlinear optical (NLO) efficiency of nanomaterials is found to be higher in comparison. Recently there has been substantial interest in developing novel NLO media for various applications. Even though several organic as well as inorganic materials have been studied in this connection, only a limited number of NLO reports exist for oxide nanomaterials. Therefore, in this paper we present results of NLO measurements recently conducted in our laboratory in three different oxide nanosystems. It is found that oxide nanomaterials are generally robust, and exhibit good NLO efficiencies, which make them potential candidates for photonic and optoelectronic applications.

Laser control of the low-temperature e −+O 2 + reaction

The dissociative recombination theory of slow electrons with molecular ions in a strong monochromatic light field is developed. Classifications of all possible transitions and reaction mechanisms are presented. The potential energy curves of the oxygen molecule O2** dissociative states and partial cross sections are calculated.

Efficient frequency downconversion at the single photon level from the red spectral range to the telecommunications C-band

We report on single photon frequency downconversion from the red part of the spectrum (738 nm) to the telecommunications C-band. By mixing attenuated laser pulses with an average photon number per pulse < 1 with a strong continuous light field at 1403 nm in a periodically poled Zn:LiNbO3 ridge waveguide an internal conversion efficiency of ∼ 73% is achieved. We further investigate the noise properties of the process by measuring the output spectrum. Our results indicate that by narrow spectral filtering a quantum interface should be feasible which bridges the wavelength gap between quantum emitters like color centers in diamond emitting in the red part of the spectrum and low-loss fiber-optic telecommunications wavelengths.

Waveform control of orientation-dependent ionization of DCl in few-cycle laser fields

Strong few-cycle light fields with stable electric field waveforms allow controlling electrons on time scales down to the attosecond domain. We have studied the dissociative ionization of randomly oriented DCl in 5 fs light fields at 720 nm in the tunneling regime. Momentum distributions of D(+) and Cl(+) fragments were recorded via velocity-map imaging. A waveform-dependent anti-correlated directional emission of D(+) and Cl(+) fragments is observed. Comparison of our results with calculations indicates that tailoring of the light field via the carrier envelope phase permits the control over the orientation of DCl(+) and in turn the directional emission of charged fragments upon the breakup of the molecular ion.

Observation of the Rabi Oscillation of Light Driven by an Atomic Spin Wave

Coherent conversion between a Raman pump field and its Stokes field is observed in a Raman process with a strong atomic spin wave initially prepared by another Raman process operated in the stimulated emission regime. The oscillatory behavior resembles the Rabi oscillation in atomic population in a two-level atomic system driven by a strong light field. The Rabi-like oscillation frequency is found to be related to the strength of the prebuilt atomic spin wave. High conversion efficiency of 40% from the Raman pump field to the Stokes field is recorded and it is independent of the input Raman pump field. This process can act as a photon frequency multiplexer and may find wide applications in quantum information science.

Tailoring spectral position and width of field enhancement by focused ion‐beam patterning of plasmonic nanoparticles

AbstractWe propose to use ion‐beam patterning of plasmonic nanoparticles made by standard electron beam lithography, plasmonic metal deposition, and lift‐off sequence. With this approach new three‐dimensional (3D) tailoring of nanoparticles becomes possible: cut through the particle and into the substrate, cut‐through at an angle, trim or bore nanoparticles or substrate. Here, we numerically simulate the expected optical properties of such 3D patterned nanoparticles. We show a particular case when spectrally broad extinction band can be created with a strong light field enhancement on a Si substrate.magnified imageNanoparticles with a 10–20 nm nano‐gap cut by ion‐beam at the chosen location and to the required depth into the substrate. Scanning electron microscopy (SEM) image of the fabricated Au‐nanoparticles.(© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)