Concept: Ultrashort pulse
We demonstrate experimentally ultrafast all-optical switching in subwavelength nonlinear dielectric nanostructures exhibiting localized magnetic Mie resonances. We employ amorphous silicon nanodisks to achieve strong self-modulation of femtosecond pulses with a depth of 60\% at picojoule-per-disk pump energies. In the pump-probe measurements, we reveal that switching in the nanodisks can be governed by pulse-limited 65-fs-long two-photon absorption being enhanced by a factor of 80 with respect to the unstructured silicon film. We also show that undesirable free-carrier effects can be suppressed by a proper spectral positioning of the magnetic resonance, making such a structure the fastest all-optical switch operating at the nanoscale.
An important challenge in the field of three-dimensional ultrafast laser processing is to achieve permanent modifications in the bulk of silicon and narrow-gap materials. Recent attempts by increasing the energy of infrared ultrashort pulses have simply failed. Here, we establish that it is because focusing with a maximum numerical aperture of about 1.5 with conventional schemes does not allow overcoming strong nonlinear and plasma effects in the pre-focal region. We circumvent this limitation by exploiting solid-immersion focusing, in analogy to techniques applied in advanced microscopy and lithography. By creating the conditions for an interaction with an extreme numerical aperture near 3 in a perfect spherical sample, repeatable femtosecond optical breakdown and controllable refractive index modifications are achieved inside silicon. This opens the door to the direct writing of three-dimensional monolithic devices for silicon photonics. It also provides perspectives for new strong-field physics and warm-dense-matter plasma experiments.Ultrafast laser processing is a versatile three-dimensional photonic structuring method but it has been limited to wide band gap materials like glasses. Here, Chanal et al. demonstrate direct refractive-index modification in the bulk of silicon by extreme localization of the energy deposition.
Carbon nanomaterials exhibit novel characteristics including enhanced thermal, electrical, mechanical, and biological properties. Nanodiamonds; first discovered in meteorites are found to be biocompatible, non-toxic and have distinct optical properties. Here we show that nanodiamonds with the size of <5 nm are formed directly from ethanol via 1025 nm femtosecond laser irradiation. The absorption of laser energy by ethanol increased non-linearly above 100 μJ accompanied by a white light continuum arises from fs laser filamentation. At laser energy higher than 300 μJ, emission spectra of C, O and H in the plasma were detected, indicating the dissociation of C2H5OH. Nucleation of the carbon species in the confined plasma within the laser filaments leads to the formation of nanodiamonds. The energy dependence and the roles of the nonlinear phenomenon to the formation of homogeneous nanodiamonds are discussed. This work brings new possibility for bottom-up nanomaterials synthesis based on nano and ultrafast laser physics.
An open challenge in the important field of femtosecond laser material processing is the controlled internal structuring of dielectric materials. Although the availability of high energy high repetition rate femtosecond lasers has led to many advances in this field, writing structures within transparent dielectrics at intensities exceeding 10(13) W/cm(2) has remained difficult as it is associated with significant nonlinear spatial distortion. This letter reports the existence of a new propagation regime for femtosecond pulses at high power that overcomes this challenge, associated with the generation of a hollow uniform and intense light tube that remains propagation invariant even at intensities associated with dense plasma formation. This regime is seeded from higher order nondiffracting Bessel beams, which carry an optical vortex charge. Numerical simulations are quantitatively confirmed by experiments where a novel experimental approach allows direct imaging of the 3D fluence distribution within transparent solids. We also analyze the transitions to other propagation regimes in near and far fields. We demonstrate how the generation of plasma in this tubular geometry can lead to applications in ultrafast laser material processing in terms of single shot index writing, and discuss how it opens important perspectives for material compression and filamentation guiding in atmosphere.
The critical outcome of the interactions between light and nanostructured ferromagnetic film has been demonstrated that the ultrashort laser pulse can directly switch the magentic domains. It leads to the dawn of a new era for breaking the ultimate physical limit for the speed of magnetic switching and manipulation, which is relevant to current and future information storage. However, the fundamental understanding for the interactions between spins, electrons and lattices far from equilibrium induced by laser pulse is still lacking in the case of magnetic heterostructures with nanoscale domain configurations. Here we carried out the time resolved magneto-optical Kerr (TRMOKE) experiments and atomistic simulations to investigate the dominant mechanism of ultrafast magnetization loss after femtosecond laser pulse excitation in [Co/Pt]20 multilayers with nanoscale magnetic domains. It is found that the ultrafast demagnetization takes place at almost the same rate under various magnetic configurations, indicating that there is no acceleration effect induced by neighboring domains. In addition, both in experiment and atomistic simulations, we find a dependence of the behavior of ultrafast demagnetization time on the laser fluence, which is in contrast to the observations of spin transport within magnetic domains.The remarkable agreement between experiment and atomistic simulations indicates that the local dissipation of spin angular momentum is the dominant demagnetization mechanism in this system. More interestingly, the comparison between atomistic spin dynamic simulation and the longitudinal spin flip model highlights that the transversal spin fluctuations mechanism is responsible for the ultrafast demagnetization in the case of inhomogeneous magnetic structures. This is a significant progress in clarifying the microscopic mechanism underlying the ultrafast demagnetization in the inhomogeneous magnetic structures.
We demonstrate theoretically that ultrafast heating of metal nanoparticles by the laser pulse should lead to the generation of coherent terahertz (THz) radiation during the heat redistribution process. It is shown that after the femtosecond laser pulse action, the time-dependent gradient of the electronic temperature induces low-frequency particle polarization with the characteristic timescale of about fractions of a picosecond. In the case of the directed metallic pattern, the THz pulse waveform can be controlled by changing the geometry of the particle. The proposed THz generation mechanism can be the basis for interpretation of recent experiments with metallic nanoparticles and nanostructures.
This paper reports the ultrafast imaging on the formation of periodic surface ripples induced by a single 800 nm, 50 fs laser pulse. The evolution process is observed on a Si surface with a prefabricated nanogroove. The ripples emerge very quickly, only 3 ps after the laser pulse with a fluence of 0.18 J/cm2irradiating on the surface, and last for several hundreds of picoseconds. The ultrafast dynamics of laser-matter interaction, such as free carrier excitation, carrier and lattice heating, surface plasmon polariton (SPP) excitation, etc, are studied theoretically. The theoretical and experimental results support that the periodic ripples are caused by the periodic energy deposition due to SPP excitation. The emerge time could identify the surface melting causing the formation of periodic ripples, and exclude the other thermal effects, for example, hydrodynamics.
Few-layer black phosphorus (BP), which exhibits excellent optical and electronic properties, has great potential application in nanodevices. However, BP inevitably suffers from the rapid degradation in ambient air due to the high reactivity of P atoms with oxygen and water, which greatly hinders its wide applications. Herein, we demonstrate the electrostatic functionalization as an effective way to simultaneously enhance the stability and dispersity of aqueous phase exfoliated few-layer BP. The poly dimethyldiallyl ammonium chloride (PDDA) is selected to spontaneously and uniformly adsorb on the surface of few-layer BP via electrostatic interaction. The positive charge-center of N atom of PDDA, which passivates the lone-pair electrons of P, plays a critical role in stabilizing the BP. Meanwhile, the PDDA could serve as hydrophilic ligands to improve the dispersity of exfoliated BP in water. The thinner PDDA-BP nanosheets can stabilize in both air and water even after 15 days exposure. Finally, the uniform PDDA-BP-polymer film was used as saturable absorber to realize passive mode-locking operations in a fiber laser, delivering a train of ultrafast pulses with the duration of 1.2 ps at 1557.8 nm. This work provides a new way to obtain highly stable few-layer BP, which shows great promise in ultrafast optics application.
We demonstrate experimentally a generic method for the synthesis of optical femtosecond pulses based on Gaussian, Airy and Hermite-Gauss functions, which are transformed to exhibit fringes with tunable width. The width of the fringes is set in some cases to be much narrower than the inverse of the spectral bandwidth. Such pulses might be useful for ultrafast spectroscopy, coherent control and nonlinear optics.
We report the broadband nonlinear optical (NLO) properties of CsPbBr3 perovskite films achieved from colloidal nanocrystals prepared following a room temperature and open atmosphere anti-solvent precipitation method. The NLO studies were performed on the films of nanocubes (NCs) and nanorods (NRs) using the Z-scan technique with 1 kHz femtosecond pulses at 600, 700, and 800 nm. Large two-photon absorption cross sections (∼105 GM) were retrieved by fitting the open-aperture Z-scan data. Strong third-order NLO susceptibility (∼10-10 esu) was observed in these films. At higher peak intensities a switching of sign (in both NCs and NRs) in the real and imaginary parts of the NLO susceptibility was observed from the studies on these CsPbBr3 nanocrystals. The obtained NLO coefficients clearly suggest that these materials are promising for ultrafast photonic applications.