Incident Femtosecond Pulses Research Articles

High‐Order Nonlinear Frequency Conversion in Transparent Conducting Oxide Thin Films

AbstractThe study of conductive oxides has gained momentum within the photonics community due to their unique linear and nonlinear optical properties. Despite recent experiments reporting on high harmonic generation from thin films, the optical/electronic behavior of these compounds at the nanoscale is still not fully understood due to the lack of a suitable theoretical model. In the present work, aluminum zinc oxide is excited near its epsilon‐near‐zero crossing point using incident femtosecond pulses having peak power densities in the 1 TW cm−2 range. A relatively efficient frequency up‐conversion including even and odd harmonics up to the seventh order is observed. A hydrodynamic‐Maxwell theoretical approach is adopted, capable of simultaneously taking into account linear and nonlinear dispersions, nonlocal effects, surface, magnetic, and bulk nonlinearities in a spectral region that spans over two and a half octaves from the UV to the NIR region. The study enables a deeper understanding of the fundamental material parameters regulating optical nonlinearities, providing important insights to engineer this class of materials for applications in sensing, ultra‐fast physics, and spectroscopy.

Self-trapping and switching of solitonic pulses in mismatched dual-core highly nonlinear fibers

We investigate experimentally and theoretically effects of the inter-core propagation mismatch on nonlinear switching in dual-core high-index-contrast soft-glass optical fibers. Incident femtosecond pulses of various energy are fed into a single (“straight”) core, to identify transitions between different dynamical regimes, viz., inter-core oscillations, self-trapping in the cross core, and retaining the pulse in the straight core. The transfer between channels, which has solitonic character, is controlled by the pulse’s energy. A model based on the system of coupled nonlinear Schrödinger equations reveals the effect of the mismatch parameter and pulse duration on the diagram of the various energy dependent dynamical regimes. Optimal values of the mismatch and pulse width, which ensure stable performance of the nonlinear switching, are identified. The theoretical predictions are in agreement with experimental findings.

Generation of adjustable femtosecond double pulses by diffraction of a chirped femtosecond pulse from volume Bragg gratings

Based on Kogelnik’s coupled wave theory, femtosecond double pulses with adjustable pulse shape and interval are generated by diffraction of a chirped femtosecond pulse from transmitted volume Bragg gratings (VBGs). It is found that duration of the diffracted double pulses depends on the chirped parameter of the incident femtosecond pulse. When the chirp parameter is larger than zero, the double pulses are compressed. However, when it is less than zero, the double pulses are broadened. The degree of compression and broadening of the double pulses are different from each other. Moreover, the pulse interval between the double pulses is determined mainly by thickness and refractive index modulation of the VBG. This paper provides a theoretical foundation to design proper VBGs for generating femtosecond double pulses with different waveform.

Femtosecond Laser Fabrication of Hybrid Metal-Dielectric Structures with Nonlinear Photoluminescence

Fabrication of hybrid micro- and nanostructures with a strong nonlinear response is challenging and represents a great interest due to a wide range of photonic applications. Usually, such structures are produced by quite complicated and time-consuming techniques. This work demonstrates laser-induced hybrid metal-dielectric structures with strong nonlinear properties obtained by a single-step fabrication process. We determine the influence of several incident femtosecond pulses on the Au/Si bi-layer film on produced structure morphology. The created hybrid systems represent isolated nanoparticles with a height of 250–500 nm exceeding the total thickness of the Au-Si bi-layer. It is shown that fabricated hybrid nanostructures demonstrate enhancement of the SHG signal (up to two orders of magnitude) compared to the initial planar sample and a broadband photoluminescence signal (more than 200 nm in width) in the visible spectral region. We establish the correlation between nonlinear signal and phase composition provided by Raman scattering measurements. Such laser-induced structures have significant potential in optical sensing applications and can be used as components for different nanophotonic devices.

Femtosecond supercontinuum generation in scattering media

We report our results on the effect of random multiple scattering on supercontinuum (SC) generation by intense light pulses. Spectral measurements are performed on the SC generated when femtosecond laser pulses propagate in water containing randomly suspended microspheres. We observed that the SC emission in the presence of scatterers is significantly suppressed in the blue side of the spectrum in comparison to that observed in neat water. We attribute this to the dissipation of incident energy, strong scattering of the shorter wavelengths, and stretching of the pulse temporally. This reasoning is supported by our experiments in neat water where we alter the shape of the incident femtosecond pulse by means of an acousto-optic programmable dispersive filter, and find similar suppression in the blue side with the lengthening of the input pulse. We believe our study will help in understanding the physics governing applications that involve transmission of high-intensity ultrashort pulses through long distances in scattering media like the atmosphere and sea water.

Experimental research of four-wave mixing and soliton effects in a photonic crystal fiber pumped femtosecond pulses at the wavelength located normal dispersion regime away from the zero dispersion point

In this paper, we show experimentally that a pair of signal wave and idler wave bands is generated by four-wave mixing, and a pair of optical solitons and dispersion wave bands is induced by intrapulse Raman scattering and non-soliton radiation, and we also observe the power saturation phenomenon of the photonic crystal fiber (PCF) by using the incident femtosecond pulses in normal dispersion region away from the zero-dispersion wavelength of the PCF. The dispersion and nonlinear characteristics of the PCF are studied by the finite element method. The possible positions of the signal wave band and the idler wave band that is generated in the fiber satisfying the phase-matching condition are simulated by four-wave mixing phase matching, the results are in good agreement with the experimental results and show that even the pump laser pulse in the normal dispersion region can also produce the PCF four-wave mixing and optical solitons effects. In the present study it also revealed that the four-wave mixing experiment is generated by the fourth-order dispersion parameter β4, and the origins of optical solitons and dispersion wave bands generation are further explained.

Energy scaling of a nonlinear compression setup using passive coherent combining

Passive spatial and temporal coherent combining schemes are implemented to scale the output energy of a nonlinear temporal compression setup. By generating 32 replicas of the incident femtosecond pulses, the output of a high-energy fiber chirped-pulse amplifier can be compressed using self-phase modulation in a large-mode-area rod-type fiber at peak-power levels well beyond the self-focusing power. We demonstrate the generation of 71 fs 7.5 μJ pulses at 100 kHz repetition rate, corresponding to a peak power of 86 MW.

Formation of a Single Attosecond Pulse via Interaction of Resonant Radiation with a Strongly Perturbed Atomic Transition

We propose a technique to form a single few-cycle attosecond pulse from vacuum ultraviolet or extreme ultraviolet radiation via resonant interaction with hydrogenlike atoms, irradiated by a high-intensity far-off-resonant laser field. The laser field strongly perturbs excited atomic energy levels via the Stark effect and ionizes atoms from the excited states. We show that an isolated attosecond pulse can be formed using either a short incident femtosecond pulse of the resonant radiation or a steep front edge of the laser field. We propose an experimental realization of a single subfemtosecond pulse formation at 121.6 nm in atomic hydrogen and a single sub-100 as pulse formation at 13.5 nm in Li(2+) plasma.

Polarization effects in diffraction-induced laser pulse splitting in one-dimensional photonic crystals

The polarization effects in the diffraction-induced pulse splitting (DIPS) observed under the dynamical Bragg diffraction in the Laue geometry in linear one-dimensional photonic crystals (PCs) are studied theoretically and experimentally. It is demonstrated that the characteristic length of the laser pulse path in a PC, or splitting length, used to describe the temporal pulse splitting, as well as the number of the outgoing femtosecond pulses, are influenced significantly by the polarization of the incident laser pulse. We have observed that the characteristic splitting time in porous quartz PCs for the s-polarized probe pulse is approximately 1.5 times smaller as compared with that measured for the p-polarized radiation. These results are supported by the theoretical description and ensure that the polarization sensitivity of the DIPS effect is due to a large lattice-induced dispersion of the PC. It is also shown that the number of output pulses can be varied from two up to four in both transmission and diffraction directions depending on the polarization of incident femtosecond pulses.

Third harmonic generation in graphene and few-layer graphite films

We observe optical third harmonic generation from graphene and few-layer graphite flakes produced by exfoliation. The emission scales with the cube of the intensity of the incident near-infrared femtosecond pulses and has a wavelength that is one-third of the incident wavelength, both consistent with third harmonic generation. We extract an effective third-order susceptibility for graphene that is on the order of 10${}^{\ensuremath{-}16}$ m${}^{2}$/V${}^{2}$, which is comparable to that for materials that are resonantly excited, but larger than for materials that are transparent at the fundamental and third harmonic wavelengths.

Femtosecond pulse shaping using plasmonic snowflake nanoantennas

We have theoretically demonstrated femtosecond pulse manipulation at the nanoscale using the plasmonic snowflake antenna's ability to localize light over a broad spectrum. To analyze the interaction of the incident femtosecond pulse with the plasmonic nanoantenna, we first decompose the diffraction limited incident femtosecond pulse into its spectral components. The interaction of each spectral component with the nanoantenna is analyzed using finite element technique. The time domain response of the plasmonic antenna is obtained using inverse Fourier transformation. It is shown that the rich spectral characteristics of the plasmonic snowflake nanoantenna allow manipulation of the femtosecond pulses over a wide spectrum. Light localization around the gap region of the nanoantenna is shown for femtosecond pulses. As the alignment of incident light polarization is varied, different antenna elements oscillate, which in turn creates a different spectrum and a distinct femtosecond response.

Light field characteristics of reflective thin-films irradiated by femtosecond laser pulses

The formation of interference field in optical thin-films is a process of unsteady state when irradiated by femtosecond pulses. Based on the methods of plane-wave decomposition and the using of thin-film characteristic matrix, the model is presented focusing on the issues of mirror reflectance and internal light field in femtosecond timescale. The numerical results reveal that with the decrease of the pulse width a reduction of reflection performance occurs. In addition, the traditional approach that simplifies the incident femtosecond pulse to monochromatic continuous wave cannot get the exact value of the light field magnitude and the temporal profile of ultrashort laser pulse should be taken into account to reach more accurate results.

Ultrashort pulse temporal contrast enhancement based on noncollinear optical-parametric amplification

A pulse clean technique in time domain combining noncollinear optical-parametric amplification and second-harmonic generation is demonstrated. The measurement-limited >10(10) temporal contrast ratio of the 0.5 mJ/40 fs pulse near 800 nm is obtained over a large temporal range extending from <1 ps before the main pulse. The contrast ratio enhancement as fourth power of the initial contrast is demonstrated. The total efficiency is >6% for the 8.2 mJ/40 fs initial incident femtosecond pulse.

Maxwell-Drude-Bloch dissipative few-cycle optical solitons

We study the propagation of few-cycle pulses in two-component medium consisting of nonlinear amplifying and absorbing two-level centers embedded into a linear and conductive host material. First we present a linear theory of propagation of short pulses in a purely conductive material, and demonstrate the diffusive behavior for the evolution of the low-frequency components of the magnetic field in the case of relatively strong conductivity. Then, numerical simulations carried out in the frame of the full nonlinear theory involving the Maxwell-Drude-Bloch model reveal the stable creation and propagation of few-cycle dissipative solitons under excitation by incident femtosecond optical pulses of relatively high energies. The broadband losses that are introduced by the medium conductivity represent the main stabilization mechanism for the dissipative few-cycle solitons.

Near-Degenerate Four-Wave-Mixing Microscopy

Fluorescence microscopy has been widely used to explore the nanoscale world because of its superb sensitivity, but it is limited to fluorescent samples. Hence, various spectroscopic contrasts have been explored for imaging nonfluorescent species. Here we report a multiphoton microscopy based on single-beam near-degenerate four wave mixing (ND-FWM), by detecting a coherent signal generated by the sample at frequencies close to the "edge" of the spectrally "truncated" incident femtosecond pulses. ND-FWM microscopy allows label-free biomedical imaging with high sensitivity and spatial resolution. In particular, by achieving a nearly perfect phase matching condition, ND-FWM generates almost the highest nonlinear coherent signal in a bulk medium and provides a contrast mechanism different from other nonlinear imaging techniques. More importantly, we developed an electronic resonant version of ND-FWM for absorbing but nonfluorescent molecules. Ultrasensitive chromophore detection (approximately 50 molecules) and hemoglobin imaging are demonstrated, by harnessing a fully (triply) resonant enhancement of the nonlinear polarization and using optical heterodyne detection.

Microscopic spallation mechanisms induced by a pulse laser at the solid-state interface

This paper presents a study of the transient behavior of structural dynamics and the associated innovatory microscopic spallation mechanism at the solid-state interface, induced by an incident femtosecond pulse laser. By detailed structural dynamic analysis, using the technique of molecular dynamics simulation, the spallation mechanism at the solid–solid interface is observed. The occurrence of structural spallation is mainly characterized by extraordinary expansion dynamics and tensile stress that induces interior structural void defect coalescence, eventually leading to cracking. The microscopic phenomenon of moderate ductile fracturing at the solid–solid interface is identified. A high strain rate in the order of 109 s-1 is observed. Both aforementioned phenomena are analogous to the experimental results of metal-film spallation excited by a pulse laser. Moreover, it is also shown that the critical value of the stain rate is one of the dominant factors that influences the occurrence and mechanism of structural spallation. The results of simulations reveal that the thin-film structure is safe if the strain rate is below certain critical values. The critical damage threshold is evaluated and technical suggestions to avoid interfacial fracture are also presented.

Saturation of ablation channels micro-machined in fused silica with many femtosecond laser pulses

We investigate the effect of saturation in the propagation of ablation channels performed in fused silica with many incident femtosecond pulses and laser fluence slightly above the ultrafast ablation threshold. A 110 fs Ti:Sapphire laser system is used in the experiments and the results are compared with theoretical predictions performed with a numerical model developed by the authors. Diffraction of the incoming pulses at the entrance of the channel as well as reflections at the walls of the channel play a crucial role in the progress of the crater as it is shown by means of the numerical results. The effect of the pulse duration in the shape of the ablation channel is also investigated.

Hysteresis of the spectral components of a femtosecond pulse propagating in a nonlinear medium

Numerical simulation based on the one-dimensional nonlinear Maxwell equations is used to study the frequency that provides the maximum spectral intensity in a selected time interval versus the amplitude of an incident femtosecond pulse. The hysteretic variation of this frequency with amplitude is shown to be a possibility. Analysis is performed in the approximation of an optically thick layer for a medium with a saturating restoring force.

The Bragg reflection characteristics of the fibre grating formed by chirped ultr ashort laser pulses

Under the illumination of 800 nm femtosecond laser pulse, two-photon absorption phenomenon can occur in the photosensitive glass As2S3. When a chirped femtosecond laser pulse meets with another femtosecond laser pulse in an As2S3 fibre, the interference pattern can be recorded permanently as a chirped grating. In this paper, the relation between the structure of the chirped fibre grating and the parameters of the incident femtosecond pulse is described. Numerical calculations show that the chirped fibre grating has a broad Bragg reflection spectrum.