Ultrashort Laser Pulse Excitation Research Articles

Model of ultrashort-pulse laser excitation of bulk and thin film dielectrics: Coupling material excitation and electric field propagation

Model of ultrashort-pulse laser excitation of bulk and thin film dielectrics: Coupling material excitation and electric field propagation

229 nm far-ultraviolet second harmonic generation in a vertical polarity inverted AlN bilayer channel waveguide

Far-UV light sources have attracted much attention for human-safe viral inactivation and bacterial disinfection. Due to large optical nonlinearity and transparency to this wavelength region, AlN is a promising material for compact and low-cost far-UV second harmonic generation (SHG) devices. In this study, a transverse quasi-phase-matched AlN channel waveguide with vertical polarity inversion was designed and fabricated. From wavelength spectra and a pump power dependence of an SH intensity, far-UV SHG via the largest nonlinear optical tensor component d 33 was successfully confirmed under ultrashort pulse laser excitation.

Dual-Modal Nanoplasmonic Light Upconversion through Anti-Stokes Photoluminescence and Second-Harmonic Generation from Broadband Multiresonant Metal Nanocavities.

Metal nanocavities can generate plasmon-enhanced light upconversion signals under ultrashort pulse excitations through anti-Stokes photoluminescence (ASPL) or nonlinear harmonic generation processes, offering various applications in bioimaging, sensing, interfacial science, nanothermometry, and integrated photonics. However, achieving broadband multiresonant enhancement of both ASPL and harmonic generation processes within the same metal nanocavities remains challenging, impeding applications based on dual-modal or wavelength-multiplexed operations. Here, we report a combined experimental and theoretical study on dual-modal plasmon-enhanced light upconversion through both ASPL and second-harmonic generation (SHG) from broadband multiresonant metal nanocavities in two-tier Ag/SiO2/Ag nanolaminate plasmonic crystals (NLPCs) that can support multiple hybridized plasmons with high spatial mode overlaps. Our measurements reveal the distinctions and correlations between the plasmon-enhanced ASPL and SHG processes under different modal and ultrashort pulsed laser excitation conditions, including incident fluence, wavelength, and polarization. To analyze the observed effects of the excitation and modal conditions on the ASPL and SHG emissions, we developed a time-domain modeling framework that simultaneously captures the mode coupling-enhancement characteristics, quantum excitation-emission transitions, and hot carrier population statistical mechanics. Notably, ASPL and SHG from the same metal nanocavities exhibit distinct plasmon-enhanced emission behaviors due to the intrinsic differences between the incoherent hot carrier-mediated ASPL sources with temporally evolving energy and spatial distributions and instantaneous SHG emitters. Mechanistic understanding of ASPL and SHG emissions from broadband multiresonant plasmonic nanocavities marks a milestone toward creating multimodal or wavelength-multiplexed upconversion nanoplasmonic devices for bioimaging, sensing, interfacial monitoring, and integrated photonics applications.

Two-temperature model for ultrafast melting of Au-based bimetallic films interacting with single-pulse femtosecond laser: Theoretical study of damage threshold

Ultrashort-pulse laser excitation and associated thermal damage of single-layer metallic films have been extensively studied both theoretically as well as experimentally. However, study of ultrafast heat transfer mechanism in bimetallic films, a topic of immense current interest for designing next-generation optical components with better thermal resilience, is limited to a low fluence regime. Moreover, available results for damage threshold (DT) of bimetallic films have several ambiguities. The current work is an attempt towards development of comprehensive theory for the DT of Au-based bimetallic films, analyzing its dependence on associated physical parameters and bridging the gap in existing literature. This has been achieved by developing fully implicit two-temperature model-based Python code. Excellent agreement of the predicted temperature profiles with recently reported [E. L. Gurevich, Y. Levy, S. V. Gurevich, and N. M. Bulgakova, Phys. Rev. B 95, 054305 (2017)] femtosecond laser irradiated Au film validates the code. Subsequently, the code is utilized to investigate the influence of different metallic substrates, $M$ ($M$ = Ni, Cr, Cu) on ultrafast melting damage of $\mathrm{Au}\text{/}M$ films excited by single-pulse femtosecond laser. Simulations carried out for increasing Au layer thickness revealed a nonmonotonic variation of DT for all substrates. This study also brought out an important observation that for every substrate metal, there exists an optimum Au thickness for maximizing the incipient and the complete melting threshold. Maximum achievable enhancement for $\mathrm{Au}\text{/}M$ films over pure Au are found to be 30%, 22%, and 14% with Cu, Ni, and Cr substrates, respectively. The observed distinction in maximum DT among three different $\mathrm{Au}\text{/}M$ targets is explained by developing appropriate thermophysical models. Finally, an empirical function is proposed here for expressing nonmonotonic variation of two-layer DTs. Very good agreement between the substrate-specific peak positions predicted by the proposed function and effective electron diffusion length arising from thermophysical analysis makes the modeling robust and applicable for a wide range of laser wavelengths as well as arbitrary combinations of bimetallic films.

Quantitatively identifying the upconversion luminescence mechanisms of Er3+:NaYF4 nanocrystals in aluminosilicate glass ceramics under femtosecond laser field

Upconversion luminescence of lanthanide ion doped nanocrystals under ultrashort laser pulse excitation can find significant applications in areas like super-resolution nonlinear microscopic imaging and attracts increasing attention. Though major excitation mechanisms, such as excited state absorption (ESA), energy transfer upconversion (ETU) and multi-photon absorption (MPA), have been analyzed and discussed intensively, quantitative differentiation of their respective contributions with convincing experimental proof has seldom been reported however. Here we report experimental demonstration of a simple but effective way to quantitatively identify simultaneous MPA from other mechanisms like ESA and ETU involved in the Er3+ doped NaYF4 nanocrystals in aluminosilicate glass ceramics under femtosecond laser field excitation. Comparative studies using laser pulses of the same spectral bandwidth but different time duration or repetition rate are performed and convincing experimental evidences for their respective contribution weights are obtained. The dependence of the MPA excitation efficiency on the spectral bandwidth of the femtosecond laser pulse is also explored for maximizing the upconversion luminescence yield. And predictions from a theoretical model agree well with experimental observations.

Complex-valued derivative propagation method with adaptive moving grids for electronic nonadiabatic dynamics

The coupled complex quantum Hamilton–Jacobi equations in the diabatic representation are solved on adaptive moving grids for electronic nonadiabatic dynamics. Substitution of the wave functions in exponential form into the coupled time-dependent Schrödinger equations yields the coupled complex quantum Hamilton–Jacobi equations. Analytic equations of motion are derived for the spatial derivatives of the complex action functions for both surfaces. The equations of motion for the complex actions and their spatial derivatives are converted into the arbitrary Lagrangian–Eulerian frame, allowing us to have many ways to choose the velocities for the moving points. The grid points can be specified to move along with the probability fluid. In contrast, choosing the boundary grid points to follow Lagrangian trajectories and forcing the internal grid points to be equally spaced at each time step, we obtain a moving structured grid of uniform spacing during the wave packet evolution. Several examples are presented to illustrate main features of the adaptive grid method, including a single avoided crossing system, a dual avoided crossing system, an ultrashort laser pulse excitation system, and a three-dimensional nonadiabatic system. Accurate computational results demonstrate that the adaptive grid method provides an alternative way to study electronic nonadiabatic dynamics.

Femtosecond Laser Induced Resonant Tunneling in an Individual Quantum Dot Attached to a Nanotip

Quantized nano-objects offer a myriad of exciting possibilities for manipulating electrons and light that impact photonics, nanoelectronics, and quantum information. In this context, ultrashort laser pulses combined with nanotips and field emission have permitted renewing nano-characterization and control electron dynamics with unprecedented space and time resolution reaching femtosecond and even attosecond regimes. A crucial missing step in these experiments is that no signature of quantized energy levels has yet been observed. We combine in situ nanostructuration of nanotips and ultrashort laser pulse excitation to induce multiphoton excitation and electron emission from a single quantized nano-object attached at the apex of a metal nanotip. Femtosecond induced tunneling through well-defined localized confinement states that are tunable in energy is demonstrated. This paves the way for the development of ultrafast manipulation of electron emission from isolated nano-objects including stereographically fixed individual molecules and high brightness, ultrafast, coherent single electron sources for quantum optics experiments.

Enhanced terahertz radiation from InAs (100) with an embedded InGaAs hole blocking layer.

We demonstrate enhanced THz radiation from p-InAs (100) by advanced heterostructure design. The THz radiation from InAs (100) under ultra-short pulsed laser excitation is due to the photo-Dember effect. Inserting a thin n-InGaAs layer close to the InAs surface effectively blocks the hole diffusion while the electron diffusion is still efficient due to tunneling. Therefore, enhanced photogenerated electron-hole separation and photo-Dember electric field is achieved to enhance the THz emission. The layer structure and doping profile are confirmed by secondary ion mass spectrometry and X-ray diffraction. The blocking of the hole diffusion is independently verified by the surface photovoltage measured by Kelvin probe force microscopy.

Propagation of time-resolved fluorescence in a diffuse medium: complex analytical derivation.

The spatiotemporal evolution of fluorescence in an optically diffusive medium following ultrashort laser pulse excitation is evaluated using complex analytical methods. When expressed as a Fourier integral, the integrand of the time-resolved diffuse fluorescence with embedded fluorophores is shown to exhibit branch points and simple pole singularities in the lower-half complex-frequency plane. Applying Cauchy's integral theorem to solve the Fourier integral, we calculate the time-resolved signal for fluorescence lifetimes that are both shorter and longer compared to the intrinsic absorption timescale of the medium. These expressions are derived for sources and detectors that are in the form of localized points and wide-field harmonic spatial patterns. The accuracy of the expressions derived from complex analysis is validated against the numerically computed, full time-resolved fluorescence signal. The complex analysis shows that the branch points and simple poles contribute to two physically distinct terms in the net fluorescence signal. While the branch points result in a diffusive term that exhibits spatial broadening (corresponding to a narrowing with time in the spatial Fourier domain), the simple poles lead to fluorescence decay terms with spatial/spatial-frequency distributions that are independent of time. This distinct spatiotemporal behavior between the diffuse and fluorescence signals forms the basis for direct measurement of lifetimes shorter than the intrinsic optical diffusion timescales in a turbid medium.

Long-lived electron emission reveals localized plasmon modes in disordered nanosponge antennas

We report long-lived, highly spatially localized plasmon states on the surface of nanoporous gold nanoparticles—nanosponges—with high excitation efficiency. It is well known that disorder on the nanometer scale, particularly in two-dimensional systems, can lead to plasmon localization and large field enhancements, which can, in turn, be used to enhance nonlinear optical effects and to study and exploit quantum optical processes. Here, we introduce promising, three-dimensional model systems for light capture and plasmon localization as gold nanosponges that are formed by the dewetting of gold/silver bilayers and dealloying. We study light-induced electron emission from single nanosponges, a nonlinear process with exponents of n≈5...7, using ultrashort laser pulse excitation to achieve femtosecond time resolution. The long-lived electron emission process proves, in combination with optical extinction measurements and finite-difference time-domain calculations, the existence of localized modes with lifetimes of more than 20 fs. These electrons couple efficiently to the dipole antenna mode of each individual nanosponge, which in turn couples to the far-field. Thus, individual gold nanosponges are cheap and robust disordered nanoantennas with strong local resonances, and an ensemble of nanosponges constitutes a meta material with a strong polarization independent, nonlinear response over a wide frequency range.

Ultrafast laser-collision-induced fluorescence in atmospheric pressure plasma

The implementation and demonstration of laser-collision-induced fluorescence (LCIF) generated in atmospheric pressure helium environments is presented in this communication. As collision times are observed to be fast (~10 ns), ultrashort pulse laser excitation (<100 fs) of the 23S to 33P (388.9 nm) is utilized to initiate the LCIF process. Both neutral-induced and electron-induced components of the LCIF are observed in the helium afterglow plasma as the reduced electric field (E/N) is tuned from <0.1 Td to over 5 Td. Under the discharge conditions presented in this study (640 Torr He), the lower limit of electron density detection is ~1012 e cm−3. The spatial profiles of the 23S helium metastable and electrons are presented as functions of E/N to demonstrate the spatial resolving capabilities of the LCIF method.

Variable ultrafast optical nonlinearity in bacteriorhodopsin achieved through simple chemical treatment

Tunability of the third-order optical nonlinearity and optical limiting action of bacteriorhodopsin (BR) was achieved by altering the native structure through simple surfactant treatment like cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and Triton X-100. The variation in spectral position and intensity of the UV–visible absorption spectrum was due to aggregation (hyperchromic lipid absorption—280 nm), solubilization (redshift in chromophore absorption—600 nm, formation of retinal—396 nm) and monomerization (blueshift in chromophore absorption—553 nm) effects upon interaction with anionic, cationic and non-ionic surfactants. Further, changes in the Raman vibrational modes confirmed the alteration in C=C ethylenic chain and 13-cis retinal on surfactant treatment. Surfactant dose-dependent assay of nonlinear optical parameters was evaluated by using the Z-scan technique with ultrashort laser pulse excitation (800 nm, ~150 fs, 80 MHz). The closed aperture data demonstrated self-defocusing nature and notably higher monomerization (0.15–0.25 mM) effects endorsed sign reversal (self-defocusing to self-focusing) of the nonlinear refractive index coefficient. Bare BR exhibited saturable absorption, while reverse saturable absorption (RSA) was observed for all surfactant-treated samples (expect for 0.20 mM CTAB and 1.0 mM SDS). The observed RSA and optical limiting was ascribed due to two-photon absorption process. Interestingly, we observed that aggregation enhanced the nonlinear refraction, monomerization improved nonlinear absorption (NLA), and solubilization reduced the nonlinear optical (NLO) response of BR. The maximum nonlinear refractive index (n 2 = 8.78 × 10−19 m2/W) was observed for 0.20 mM CTAB, while the NLA coefficient (β = 4.40 × 10−12 m/W) and low optical limiting threshold (538 µJ/cm2) were observed to be highest for 0.25 mM TX-100 sample. The influence of aggregation, monomerization and solubilization effects of purple membrane in achieving the variation of the NLO coefficients is discussed.

Excitation and relaxation dynamics in ultrafast laser irradiated optical glasses

We discuss the dynamics of ultrashort pulsed laser excitation in bulk optical silica-based glasses (fused silica and borosilicate BK7) well-above the permanent modification threshold. We indicate subsequent structural and thermomechanical energy relaxation paths that translate into positive and negative refractive index changes, compression and rarefaction zones. If fast electronic decay occurs at low excitation levels in fused silica via self-trapping of excitons, for carrier densities in the vicinity of the critical value at the incident wavelength, persistent long-living absorptive states indicate the achievement of low viscosity matter states manifesting pressure relaxation, rarefaction, void opening and compaction in the neighboring domains. An intermediate ps-long excited carrier dynamics is observed for BK7 in the range corresponding to structural expansion and rarefaction. The amount of excitation and the strength of the subsequent hydrodynamic evolution is critically dependent on the pulse time envelope, indicative of potential optimization schemes.

Mechanical properties of boron-nitride nanotubes after intense femtosecond-laser excitation

A femtosecond-laser pulse constitutes an unconventional tool to manipulate solids and nanostructures, for it may excite materials in a transient nonthermal state with hot electrons and atoms close to their initial temperature. Here we study the Young’s modulus and the electronic band gap of a (5, 0) zigzag boron-nitride nanotube (BNNT) after an ultrashort laser pulse excitation using density functional theory, where the effect of a femtosecond-laser pulse is modelled by an instantaneous rise of the electronic temperature. At room temperature, before the laser pulse, we obtain a Young’s modulus of 763 GPa, which decreases with increasing electronic temperature. For the band gap we find a value of 2.26 eV at room temperature, which increases with increasing electronic temperature and equals 3.28 eV at 28 420 K. We note that conventional means decrease the band gap of BNNTs and that a femtosecond-laser pulse is, to the best of our knowledge, the first tool that increases it. For comparison, we also present results for a (9, 0) zigzag BNNT.

Probing ultrashort-pulse laser excitation of sapphire: From the initial carrier creation to material ablation

Ultrashort-pulse laser excitation of dielectrics has been investigated over a large span of intensities. Experimentally, single-shot studies on single-crystal sapphire samples combine time-resolved spectral interferometry with time-resolved reflectivity and ablation-rate measurements. The complete development of the excitation from the first creation of conduction-band electrons at low intensities to the formation of a highly excited plasma and associated material fragmentation is observed experimentally and explained by a single theoretical model, which combines material excitation in a multiple-rate equation description with light propagation.

Modeling short-pulse laser excitation of dielectric materials

A theoretical description of ultrashort-pulse laser excitation of dielectric materials based on strong-field excitation in the Keldysh picture combined with a multiple-rate-equation model for the electronic excitation including collisional processes is presented. The model includes light attenuation in a self-consistent manner and changing optical properties described in a Drude picture. The model can be used to calculate the electronic excitation as a function of time and depth, and from these quantities the time-dependent optical parameters as well as the ablation depth can be derived. The simulations provide insight into the excitation and propagation dynamics of short-pulse excitation and show that at increasing fluence the excitation becomes localized near the material surface and gives rise to strong modifications of the optical properties of the material.

Terahertz Radiation from a (113)B GaAs/AlAs Coupled Multilayer Cavity Generated by Ultrashort Laser Pulse Excitation

Terahertz (THz) radiation was demonstrated using a GaAs/AlAs coupled multilayer cavity grown on a (113)B GaAs substrate. Two cavity modes realized in the high-reflection band were simultaneously excited using ultrashort laser pulses for the difference frequency generation (DFG). Oscillations with a period of 0.3 ps were clearly observed in the temporal waveforms of time-resolved THz measurements. The oscillation period well agreed with the difference frequency between the two cavity modes (3.3 THz). We also measured the THz waveforms depending on the polarization direction of the excitation laser pulses, and the anisotropic signal amplitudes also agreed with the calculated anisotropy of the second-order nonlinear polarization on the (113)B GaAs substrate.

Terahertz Radiation from a (113)B GaAs/AlAs Coupled Multilayer Cavity Generated by Ultrashort Laser Pulse Excitation

Terahertz (THz) radiation was demonstrated using a GaAs/AlAs coupled multilayer cavity grown on a (113)B GaAs substrate. Two cavity modes realized in the high-reflection band were simultaneously excited using ultrashort laser pulses for the difference frequency generation (DFG). Oscillations with a period of 0.3 ps were clearly observed in the temporal waveforms of time-resolved THz measurements. The oscillation period well agreed with the difference frequency between the two cavity modes (3.3 THz). We also measured the THz waveforms depending on the polarization direction of the excitation laser pulses, and the anisotropic signal amplitudes also agreed with the calculated anisotropy of the second-order nonlinear polarization on the (113)B GaAs substrate.

Glass waveguides with artificially laser placed scattering centers improve the applicability for medical applications

Objective: Currently applied medical wave guides in medical therapy, e.g. laser induced interstitial thermotherapy (LITT) use a so-called diffuser attached at the distal end. These manually fabricated diffusers are filled with scattering particles embedded in a polymer matrix and are expensive and not very reproducible. The objective is to induce scattering centers directly by utilizing ultra-short laser pulse excitation and selective material reaction inside a quartz waveguide. The laser based processing method allows an integrated diffuser production saving costs and increasing reliability. Furthermore, it is expected that the direction and intensity of the deflected laser light can be strongly influenced by the design and distribution of the scattering centers, creating more selective therapeutic approaches.

Dynamics of electron excitations in densely packed plasmonic Ag/Na3AlF6 nanostructures under pulsed laser action

Differential transient absorption spectra have been studied for planar densely packed Ag/Na3AlF6 nanostructures under ultrashort laser pulse excitation. The nanostructures were fabricated by sequential thermal evaporation of cryolite (Na3AlF6) and silver in vacuo onto glass and quartz substrates. A nonmonotonic variation in relaxation times of induced changes in a surface plasmon resonance band was observed with an increase in the metal surface density that resulted in nanoparticle size growth and structural modification of the densely packed layer. The tendency of the relaxation times to vary nonmonotonically is explained by both features of intrinsic size effects and electron-tunneling processes in plasmonic densely packed nanostructures of various topologies.