Femtosecond Laser Excitation Research Articles

Energy splitting of CdSe quantum dots induced by intense femtosecond laser excitation

Microscopic photoluminescence (PL) spectra of self-assembled CdSe quantum dots (QDs) grown by molecular beam epitaxy were investigated under excitation of intense femtosecond laser. Two samples with different QD sizes were fabricated. One had a single layer of larger CdSe QDs while the other had three layers of smaller QDs. The second harmonic radiation at 420 nm obtained from a mode-locked tunable Ti-Sapphire laser was used as the excitation source. The laser power density was in the order of kW cm−2 and the peak power density was in the order of GW cm−2 for the 150 fs laser pulse with a repetition rate of 78 MHz. The intense femtosecond laser pulses generated strong surface acoustic waves and modulated energy bands of electrons and holes of CdSe QDs. Increasing of the laser power resulted in the PL peak of the CdSe QDs splitting into four peaks for both QD samples: two peaks shifted to a lower energy side and the other two shifted to a higher energy side. The strong strain fields led to the mixing of heavy-hole state and light-hole state in the quantum dots. The strain fields further modulated the energy bands of electrons and holes and produced splitting of both electron–heavy hole (e-hh) transition and electron–light hole (e-lh) transition. For the sample with a single layer of smaller QDs, the energy splitting for both e-hh and e-lh transitions reached 23.5 meV at a peak power density of 0.32 GW cm−2. For the sample with three layers of larger QDs, the energy splitting was 19.9 meV for e-hh transition and 17.9 meV for e-lh transition at a peak power of 1.1 GW cm−2.

Hard X-ray generation and plasma filament formation under interaction of femtosecond laser pulses with large molecular clusters

We have demonstrated that the efficiency of hard X-ray generation (with energy 2 − 4 keV) from the femtosecond laser excitation (intensity I ∼ 1016 W/cm2, pulse energy E ∼ 5 mJ) of large molecular clusters (SF6, CF3I) can reach ∼ 10−6 (with yield ∼ 109 photons/J). It was shown that at maximal X-ray yield, the plasma filament is essentially non-uniform and has a multifocal structure. It was observed that the amplitude of the central and trailing spectral components of the chirped laser pulse decreases after its interaction with the cluster target as a result of efficient laser energy absorption under self-focusing conditions.

Coercivity dynamics and origin of time-delayed magneto-optical hysteresis loops in pump-probe Kerr spectroscopy

Time-resolved magneto-optical Kerr effect (TR-MOKE) is used to measure hysteresis loops of a magnetic film at a different delayed time after femtosecond laser excitations. Coercivity dynamics of the magnetic film is obtained from TR-MOKE loops as usual. However, we point out that the dynamics of coercivity does not reflect the evolution of initial coercivity shown by the initial hysteresis loop with no laser excitation, but that of the final coercivity under laser excitation, where the final coercivity means one given by the hysteresis loop measured at a negative pump-probe delay time under pump excitation. TR-MOKE loops usually cannot track the evolution of coercivity of a magnetic system upon strong laser shot, and can do that only when the magnetic system is reversible under a low excitation. The oscillatory recovery behavior of the final coercivity is observed, and is ascribed to metastability of zero magnetization state.

Third order nonlinear optical property of Bi_2Se_3

The third order nonlinear optical property of Bi₂Se₃, a kind of topological insulator (TI), has been investigated under femto-second laser excitation. The open and closed aperture Z-scan measurements were used to unambiguously distinguish the real and imaginary part of the third order optical nonlinearity of the TI. When excited at 800 nm, the TI exhibits saturable absorption with a saturation intensity of 10.12 GW/cm² and a modulation depth of 61.2%, and a giant nonlinear refractive index of 10⁻¹⁴ m²/W, almost six orders of magnitude larger than that of bulk dielectrics. This finding suggests that the TI:Bi₂Se₃ is indeed a promising nonlinear optical material and thus can find potential applications from passive laser mode locker to optical Kerr effect based photonic devices.

Multiphoton photoelectron emission microscopy of single Au nanorods: combined experimental and theoretical study of rod morphology and dielectric environment on localized surface plasmon resonances

Multiphoton photoelectron emission from individual Au nanorods deposited on indium tin oxide (ITO) substrates is studied via scanning photoionization microscopy, based on femtosecond laser excitation at frequencies near the rod longitudinal surface plasmon resonance (LSPR). The observed resonances in photoemission correlate strongly with plasmon resonances measured in dark field microscopy (DFM), thus establishing a novel scheme for wavelength-resolved study of plasmons in isolated metallic nanoparticles based on highly sensitive electron counting methods. In this work, we explore experimental and theoretical effects of (i) morphology and (ii) aspect ratio (AR) for longitudinal plasmon resonance behavior in Au nanorods. A quasilinear dependence between LSPR and aspect ratio (AR) is experimentally determined [Δλ≈ +100(10) nm/AR unit] for Au nanorods on ITO, in excellent agreement with the first principles value from finite element computer modeling [Δλ = +108(5) nm/AR unit]. Interestingly, however, LSPR values for larger vs. smaller diameter rods (w≈ 20 nm and 10 nm) are systematically red-shifted [ΔE≈-0.03(1) eV; Δλ≈ +15(5) nm at λ≈ 800 nm], indicating that electromagnetic retardation effects must also be considered for highest accuracy in LSPR position. To augment these results, the influence of the dielectric environment on the rod LSPR has been explored both experimentally and numerically. In particular, detailed finite-element simulations for ITO supported Au nanorods are found to yield plasmon resonances in near quantitative agreement (ΔE≈±0.04 eV) with experiment, with residual differences arising from uncertainty in the refractive index of the ITO thin film. Furthermore, the results indicate that plasmon resonance predictions based on infinitely thick ITO substrates are reliable to a few meV for film thicknesses larger than approximately twice the rod width.

Blue two-photon fluorescence metal cluster probe precisely marking cell nuclei of two cell lines

A bifunctional peptide was designed to in situ reduce Cu ions and anchor a Cu cluster. The peptide-Cu cluster probe, mainly composed of Cu14, emitted blue two-photon fluorescence under femtosecond laser excitation. Most important, the probe can specifically mark the nuclei of HeLa and A549 cells, respectively.

Measurement of boron isotopic ratio with non-gated molecular spectroscopy of femtosecond laser-produced plasma

Laser-induced breakdown spectroscopy can be augmented by concurrent spectroscopic measurements of molecules and molecular ions resulting from recombination of laser-ablated plasmas with the ablated sample and surrounding environment. Molecular emissions can exhibit significantly greater isotopic shifts than atomic emissions, making this approach attractive for isotopic characterization of nuclear materials. We describe a measurement of boron isotopic ratios in four boron-containing samples utilizing the emission from boron monoxide radicals formed in the expanding laser-induced ablation plume. Femtosecond laser excitation is used and the emission in the 532–540nm region of interest is studied, where a linear superposition of characteristic spectra for two boron isotopes has been applied for reconstruction of the boron isotopic ratio. It is also demonstrated that the use of non-gated measurements of emission spectra suffices for accurate isotopic characterization using this method. The application of multivariate regression to deconvolve the isotopic ratio from the measured emission spectra is discussed in detail, including the limitations and subtleties of this approach.

Water splitting by infrared femtosecond laser excitation of surface plasmon

Gold nanoparticles supported on soda-lime glass exhibit a photochemical water splitting activity under infrared femtosecond laser excitation. Both H2 and hydroxyl radicals productions were characterized. The hydroxyl radicals production mechanism was identified by comparison with three prototypal mechanisms, photoionization of organic compound in the UV, VUV dissociation of water and water gamma radiolysis. The mechanisms involved in the case of laser femtosecond seem to be water ionization events occurring at distance from the gold particles.

Spin precession modulation in a magnetic bilayer

We report on modulation of the spin precession in a Co/garnet bilayer by femtosecond laser excitation using time-resolved magneto-optical tools. Damped oscillations in the Faraday rotation transients representing precessional motion of the magnetization vector are observed in both the 2 nm Co layer and 1.8 μm garnet of the bilayer with distinct frequencies differing by about a factor of two. The excitation efficiency of these precessions strongly depends on the out-of-plane magnetic field. The modulation effect with the coupling in a magnetic bilayer can be useful for non-thermally controlling the magnetization of nanomagnets and ultrafast switching in magnetic nanodevices.

Ultrafast dynamics of thermionic emission on Au film under femtosecond laser excitation

A high-fidelity numerical model for investigations of the ultrafast heating is highly desirable for simulating the pulsed laser damage and the ultrafast electron emission characteristics. However, realization of accurate predictions of thermal dynamics and thermionic electron emission remains challenging due to the high non-equilibrium state, in which the equilibrium heating parameters are invalid. Here, we report an axisymmetric two-dimensional (2-D) high-fidelity numerical model for predictions of the thermionic emission with respect to the temperature-dependent dynamics parameters. The spatio-temporal temperature evolution dynamics and the thermionic emission rate characteristics on Au film target are demonstrated, whose credibility is approved by the Au film ablation threshold experiments.

Photoemission electron microscopy of a plasmonic silver nanoparticle trimer

We present a combined experimental and theoretical study to investigate the spatial distribution of photoelectrons emitted from silver-coated polystyrene nanoparticles. We use two-photon photoemission electron microscopy (2P-PEEM) to image electron emission from a silver-capped aggregate trimer. Finite difference time domain (FDTD) simulations are performed to model the intensity distributions of the electromagnetic near fields resulting from femtosecond laser excitation of localized surface plasmon oscillations in the trimer structure. We demonstrate that the predicted FDTD near-field intensity distribution reproduces the 2P-PEEM photoemission pattern.

Ultrafast third order nonlinear optical properties of water soluble zinc-octacarboxy-phthalocyanine

Synthesis and measurements of third order nonlinear optical (NLO) coefficients of water soluble zinc octacarboxy phthalocyanine (ZnOCPc) are reported here using different laser pulse durations. Nonlinear absorption and refraction behaviour in the nanosecond (ns), picosecond (ps) and femtosecond (fs) time domains were studied in detail. Three-photon absorption was the prevalent mechanism with femtosecond laser excitation whereas strong reverse saturable absorption due to dominant two-photon absorption (TPA) was observed with picosecond and nanosecond excitations. We have evaluated the sign and magnitude of the third order nonlinearity in fs, ps, and ns regimes. We observed large off-resonant second order hyperpolarizability (γ) with ultrafast nonlinear optical response in the femtosecond domain using degenerate four wave mixing (DFWM) technique. We also report the optical limiting characteristics and figure of merit (FOM) of ZnOCPc for optical switching.

Femtosecond laser excitation of dielectric materials: experiments and modeling of optical properties and ablation depths

Modeling of the interaction between a dielectric material and ultrashort laser pulses provides the temporal evolution of the electronic excitation and the optical properties of the dielectric. Experimentally determined reflectances and ablation depths for sapphire are compared to the calculations. A decrease in reflectance at high fluences is observed experimentally, which demonstrates the necessity of a temperature-dependent electron scattering rate in the model. The comparison thus provides new constraints on the optical parameters of the model.

Femtosecond Laser Excitation Drives Ferromagnetic Gadolinium out of Magnetic Equilibrium

The temporal evolution of the exchange-split Δ(2)-like Σ valence bands of the 4f-ferromagnet gadolinium after femtosecond laser excitation has been studied using angle-resolved photoelectron spectroscopy based on high-order harmonic generation. The ultrafast drop of the exchange splitting reflects the magnetic response seen in femtosecond magnetic dichroism experiments. However, while the minority valence band reacts immediately, the response of the majority counterpart is delayed by 1 picosecond and is only half as fast. These findings demonstrate that laser excitation drives the valence band structure out of magnetic equilibrium.

Synergistic effect of trimethylsilane for photoinduced electron transfer on 1,8-naphthalimides in polar solvent

Photoinduced electron-transfer (PET) process of 1,8-naphthalimidelinkerTMS (NISOSTMS, where SOS = dithiaoxaundecyl and TMS = trimethylsilane) has been investigated using transient absorption measurements in CH3CN and CH3CN/H2O (v/v = 9:1). The femtosecond pulsed laser excitation of NISOSTMS produced the NI radical anion (NI−) with a transient absorption band around 415 nm, via the intramolecular PET from heteroatom nearby TMS to NI in the singlet excited (S1) state. However, in case of NISOS, the transient band at around 415 nm increased concomitantly with the decay of 1NI* (or 3NI*) at around 470 nm. This is implied that NI− is primarily generated via the intermolecular quenching of 1NI* (or 3NI*) by NI. In contrast, in a protic polar solvent mixture of CH3CN/H2O, a proton abstraction process occurred from NI− to generate the NI ketyl radical (NIH), which showed a transient absorption band around 405 nm. The decay time constants of NIH were quite long compared to those of NI− in CH3CN. Both the rates of charge transfer and deactivation processes of transient species largely depended on the protic polar solvent.

Modelling of decay kinetics of self-trapped exciton luminescence in CdWO4 under femtosecond laser excitation in absorption saturation conditions

Abstract The analytical expressions for the three-dimensional spatial distribution of excitons and luminescence decay kinetics of wide band-gap crystals were derived taking into account the saturation effect for absorption near the centre of the incident laser beam with Gaussian transversal profile. The model is verified for the excitonic emission of CdWO4 crystals under excitation by ultra-short laser pulses in the region of weak excitonic absorption.

Photodynamics of Free and Solvated Tyrosine

We present a theoretical simulation of the ultrafast nonadiabatic photodynamics of tyrosine in the gas phase and in water. For this purpose, we combine our TDDFT/MM nonadiabatic dynamics (Wohlgemuth et al. J. Chem. Phys. 2011, 135, 054105) with the field-induced surface hopping method (Mitrić et al. Phys. Rev. A 2009, 79, 053416) allowing us to explicitly include the nonadiabatic effects as well as femtosecond laser excitation into the simulation. Our results reveal an ultrafast deactivation of the initially excited bright ππ* state by internal conversion to a dark nπ* state. We observe deactivation channels along the O-H stretching coordinate as well as involving the N-H bond cleavage of the amino group followed by proton transfer to the phenol ring, which is in agreement with previous static energy path calculations. However, since in the gas phase the canonical form of tyrosine is the most stable one, the proton transfer proceeds in two steps, starting from the carboxyl group that first passes its proton to the amino group, from where it finally moves to the phenol ring. Furthermore, we also investigate the influence of water on the relaxation processes. For the system of tyrosine with three explicit water molecules solvating the amino group, embedded in a classical water sphere, we also observe a relaxation channel involving proton transfer to the phenol ring. However, in aqueous environment, a water molecule near the protonated amino group of tyrosine acts as a mediator for the proton transfer, underlining the importance of the solvent in nonradiative relaxation processes of amino acids.

Theory of laser-induced ultrafast superdiffusive spin transport in layered heterostructures

Femtosecond laser excitation of a ferromagnetic material creates energetic spin-polarized electrons that have anomalous transport characteristics. We develop a semiclassical theory that is specifically dedicated to capture the transport of laser-excited nonequilibrium (NEQ) electrons. The randomly occurring multiple electronic collisions, which give rise to electron thermalization, are treated exactly and we include the generation of electron cascades due to inelastic electron-electron scatterings. The developed theory can, moreover, treat the presence of several different layers in the laser-irradiated material. The derived spin-dependent transport equation is solved numerically and it is shown that the hot NEQ electron spin transport occurs neither in the diffusive nor ballistic regime, it is superdiffusive. As the excited spin majority and minority electrons in typical transition-metal ferromagnets (e.g., Fe, Ni) have distinct, energy-dependent lifetimes, fast spin dynamics in the femtosecond (fs) regime is generated, causing effectively a spin current. As examples, we solve the resulting spin dynamics numerically for typical heterostructures, specifically, a ferromagnetic/nonmagnetic metallic layered junction (i.e., Fe/Al and Ni/Al) and a ferromagnetic/nonmagnetic insulator junction (Fe or Ni layer on a large band-gap insulator as, e.g., MgO). For the ferromagnetic/nonmagnetic metallic junction where the ferromagnetic layer is laser-excited, the computed spin dynamics shows that injection of a superdiffusive spin current in the nonmagnetic layer (Al) is achieved. The injected spin current consists of screened NEQ, mobile majority-spin electrons and is nearly 90$%$ spin-polarized for Ni and about 65$%$ for Fe. Concomitantly, a fast demagnetization of the ferromagnetic polarization in the femtosecond regime is driven. The analogy of the generated spin current to a superdiffusive spin Seebeck effect is surveyed.

Perfectly registered multiphoton and reflectance confocal video rate imaging of in vivo human skin

We present a multimodal in vivo skin imaging instrument that is capable of simultaneously acquiring multiphoton and reflectance confocal images at up to 27 frames per second with 256 × 256 pixel resolution without the use of exogenous contrast agents. A single femtosecond laser excitation source is used for all channels ensuring perfect image registration between the two-photon fluorescence (TPF), second harmonic generation (SHG), and reflectance confocal microscopy (RCM) images. Images and videos acquired with the system show that the three imaging channels provide complementary information in in vivo human skin measurements. In the epidermis, cell boundaries are clearly seen in the RCM channel, while cytoplasm is better seen in the TPF imaging channel, whereas in the dermis, SHG and TPF channels show collagen bundles and elastin fibers, respectively. The demonstrated fast imaging speed and multimodal imaging capabilities of this MPM/RCM instrument are essential features for future clinical application of this technique.

Intravital multiphoton imaging of rhodamine 123 in the rat liver after intravenous dosing

Intravital imaging with multiphoton microscopy was used to investigate the hepatic disposition of rhodamine 123 (RH123) in the exposed liver of anesthetized rats after intravenous dosing. The role played by the biliary canalicular transporter P-glycoprotein (P-gp) on the disposition of RH123 was explored by administering a P-gp inhibitor, cyclosporine A prior to RH123 administration. The fluorescence intensity of RH123 was defined by multiphoton microscopy using a femtosecond laser excitation wavelength of 900 nm whereas the autofluorescence at an excitation wavelength of 740 nm was used to define the morphology of the liver acini. Intravital imaging showed that RH123 was rapidly taken up from the sinusoids into hepatocytes but slowly eliminated from the cells (half-life 2.89 ± 1.37 h). The presence of cyclosporine A did not affect the uptake of RH123 but markedly increased the fluorescence intensity of RH123 in the liver and was associated with a slower elimination of RH123 from the liver (half-life 11.5 ± 4.24 h). In conclusion, the spatial disposition of RH123 in the rat liver and the effects of a transporter inhibitor on its disposition have been monitored over time using intravital imaging.