Ultrafast Pulse Excitation Research Articles

Focusing Surface Acoustic Waves with a Plasmonic Hypersonic Lens.

Plasmonic nanoantennas have proven to be efficient transducers of electromagnetic to mechanical energy and vice versa. The sudden thermal expansion of these structures after an ultrafast optical pulsed excitation leads to the emission of hypersonic acoustic waves to the supporting substrate, which can be detected by another antenna that acts as a high-sensitivity mechanical probe due to the strong modulation of its optical response. Here, we propose and experimentally demonstrate a nanoscale acoustic lens comprised of 11 gold nanodisks whose collective oscillation at gigahertz frequencies gives rise to an interference pattern that results in a diffraction-limited surface acoustic beam of about 340 nm width, with an amplitude contrast of 60%. Via spatially decoupled pump-probe experiments, we were able to map the radiated acoustic energy in the proximity of the focal area, obtaining a very good agreement with the continuum elastic theory.

Modeling of Charge Injection, Recombination, and Diffusion in Complete Perovskite Solar Cells on Short Time Scales.

A model of charge population decay upon ultrafast optical pulse excitation in complete, working perovskite solar cells is proposed. The equation, including charge injections (extractions) from perovskite to contact materials, charge diffusion, and charge recombination via first-, second-, and third-order processes, is solved using numerical simulations. Results of simulations are positively verified by broadband transient absorption results of mixed halide, triple-cation perovskite (FA0.76MA0.19Cs0.05Pb(I0.81Br0.19)3). The combined analytical and experimental findings reveal the best approaches for the proper determination of the crucial parameters that govern charge transfer dynamics in perovskite solar cells on picosecond and single nanosecond time scales. Measurements from both electron and hole transporting layer sides under different applied bias potentials (zero and close to open circuit potential) and different pump fluence (especially below 5 μJ/cm2), followed by fitting of parameters using numerical modeling, are proposed as the optimal methodology for describing the processes taking place in efficient devices.

Coherent Phonons in Antimony: An Undergraduate Physical Chemistry Solid-State Ultrafast Laser Spectroscopy Experiment

Ultrafast laser pump-probe spectroscopy is an important and growing field of physical chemistry that allows the measurement of chemical dynamics on their natural timescales, but undergraduate laboratory courses lack examples of such spectroscopy and the interpretation of the dynamics that occur. Here we develop and implement an ultrafast pump-probe spectroscopy experiment for the undergraduate physical chemistry laboratory course at the University of California Berkeley. The goal of the experiment is to expose students to concepts in solid-state chemistry and ultrafast spectroscopy via classic coherent phonon dynamics principles developed by researchers over multiple decades. The experiment utilizes a modern high-repetition rate 800 nm femtosecond Ti:Sapphire laser, split pulses with a variable time delay, and sensitive detection of transient reflectivity signals. The experiment involves minimal intervention from students and is therefore easy and safe to implement in the laboratory. Students first perform an intensity autocorrelation measurement on the femtosecond laser pulses to obtain their temporal duration. Then, students measure the pump-probe reflectivity of a single-crystal antimony sample to determine the period of coherent phonon oscillations initiated by an ultrafast pulse excitation, which is analyzed by fitting to a sine wave. Due to the disruption of in-person instruction caused by the COVID-19 pandemic, during those semesters students were provided the data they would have obtained during the experiment to analyze at home. Evaluation of student written reports reveals that the learning goals were met, and that students gained an appreciation for the field of ultrafast laser-induced chemistry.

TRANSDUCCIÓN OPTOMECÁNICA Y BIESTABILIDADES ÓPTICAS ENMICRORRESONADORES BASADOS EN REFLECTORES DE BRAGG

In cavity optomechanics experiments with ultrashort laser pulses, the temporal dynamics of the optical modes of the system play a fundamental role in the generation and detection of coherent phonons. In this work, we analyze the feasibility of using a second continuous emission laser to optimize the detection efficiency, by setting the fundamentalótical mode to a specific spectral position. Under certain conditions, and through this continuum excitation, it is possible to modify the recovery dynamics of the optical modes after an ultrafast pulsed excitation, to the point of stopping it on a quasiequilibrium. This phenomenon is directly related to the optical bistability of the resonator.

Improved near‒infrared nonlinear optical properties of Sm3+ containing borate glasses: Effect of silver nanoparticles concentration

Efficacy of silver nanoparticles on tuning the optical nonlinear characteristics of Sm3+ doped borate‒based glasses has been investigated comprehensively and the outcomes are provided. The nonlinear optical (NLO) attributes were retrieved using the Z‒scan technique under an ultrafast pulse excitation with the wavelength ranging from 700 nm to 1000 nm. Reverse saturable absorption (RSA), ascribed to two‒photon absorption, was realized in open aperture Z‒scan profiles whereas refraction nonlinearity of positive type due to the self‒focusing was observed in closed aperture Z‒scan data. Based on the Z‒scan results along with the figure of merit condition (higher than unity), it was identified that the silver nanoparticles embedded Sm3+ containing glass hosts are competing materials for NLO device applications to function in near‒infrared region.

Controlling optical trapping of metal-dielectric hybrid nanoparticles under ultrafast pulsed excitation: a theoretical investigation.

Crucial to effective optical trapping is the ability to precisely control the nature of force/potential to be attractive or repulsive. The nature of particles being trapped is as important as the role of laser parameters in determining the stability of the optical trap. In this context, hybrid particles comprising of both dielectric and metallic materials offer a wide range of new possibilities due to their tunable optical properties. On the other hand, femtosecond pulsed excitation is shown to provide additional advantages in tuning of trap stiffness through harnessing optical and thermal nonlinearity. Here we demonstrate that (metal/dielectric hybrid) core/shell type and hollow-core type nanoparticles experience more force than conventional core-type nanoparticles under both continuous-wave and, in particular, ultrafast pulsed excitation. Thus, for the first time, we show how tuning both materials properties as well as the nature of excitation can impart unprecedented control over nanoscale optical trapping and manipulation leading to a wide range of applications.

Atomic-level ablation of Au@Ag NRs using ultrafast laser excitation.

Metallic nanorods (NRs) are an important class of materials with widespread applications because of their appealing tunable plasmon resonances, high photothermal conversion efficiency, and chemical stability. It is essential to control the shape and atomic structures of metallic NRs for practical applications. Laser processing of metallic NRs relying on light-matter interactions provides many opportunities. However, the atomic-level fabrication of NRs remains a challenge, and the understanding of laser-induced ablation is still limited. Here, we proposed the atomic-level ablation of Au@Ag NRs using ultrafast laser excitation, which suggests that the near-field effect plays a key role in comparison with thermal evaporation. Through ultrafast laser pulse excitation, abundant atomic steps are fabricated in Au@Ag NRs, which can enhance the surface activity. We suggest that this study highlights the role of the laser near-field effect and also provides a facile strategy to tailor the external shape of metallic NRs at the atomic level, opening a pathway to design metallic NRs for energy and environmental applications.

Generalized Lorenz-Mie theory for the reversal of optical force in a nonlinear laser trap

Hybrid nanoparticles have gained intense attention in the field of optical trapping due to their potential wide-ranging applications, for example, in drug delivery. The utility of the optical Kerr effect in modulating trapping force and potential under high-repetition-rate ultrafast pulsed excitation has recently been realized for dielectric and metallic particles ranging from micron to nanometer. However, in the context of hybrid nanoparticles, the mechanism of trapping is yet to be fully explored. Here, we present a comparative study of trapping force and potential on conventional, hybrid, and hollow-core type nanoparticles using generalized Lorenz-Mie theory and dipole approximation incorporating third-order optical nonlinearity. We find an explicit advantage of using pulsed excitation over continous-wave excitation which can have potentially far-reaching practical applications.

Ti2CTx MXene‐based all‐optical modulator

AbstractMXenes, a new class of 2D transition metal carbides, nitrides, and carbonitrides, have attracted much attention due to their outstanding properties. Here, we report the broadband spatial self‐phase modulation of Ti2CT x MXene nanosheets dispersed in deionized water in the visible to near‐infrared regime, highlighting the broadband nonlinear optical (NLO) response of Ti2CT x MXene. Using ultrafast pulsed laser excitation, the nonlinear refractive index n2 and the third‐order nonlinear susceptibility of Ti2CT x MXene were measured to be ∼10−13 m2/W and ∼ 10−10 esu, respectively. Leveraging the large optical nonlinearity of Ti2CT x MXene, an all‐optical modulator in the visible regime was fabricated based on the spatial cross‐phase modulation effect. This work suggests that 2D MXenes are ideal broadband NLO materials with excellent prospects in NLO applications.image

Pulsed spin wave propagation in a magnonic crystal

Amplitude, frequency, and time domain characteristics have been mapped for short spin wave pulses inside a magnonic crystal. A space- and time-resolved magnetoinductive probing system has been used to detail the spin wave spectral, propagation, and evolution characteristics in a geometrically structured yttrium iron garnet film. Experiments have been performed using magnetostatic surface spin waves excited in a chemically-etched magnonic crystal, ultrafast pulsed excitation of the spin waves, and direct spin wave detection using a scannable magnetoinductive probe connected to a synchronized fast oscilloscope. The results show how the frequency discriminating effect of a magnonic bandgap decreases as the excitation pulse width decreases. They also show how the use of rectangular pulses compromise the magnonic crystal performance because of the high frequency components of such pulses. Space and time maps show how these components are transmitted without additional damping.

Simultaneous detection of two-photon fluorescence and backscatter in laser trapping of dielectric nanoparticles

In recent past, high-repetition-rate, ultrafast pulse excitation has been identified to play an important role in stable trapping of dielectric nanoparticles assisted by optical nonlinearity due to its high peak power. We experimentally demonstrate trapping of 100-nm fluorescent polystyrene particles by simultaneous detection of both two-photon fluorescence (TPF) and backscattered signals. Here we show that TPF signal decays over time due to photobleaching, but this signal is useful to know whether a particle is dragged toward the trap while backscattered signal provides detailed information about the particle’s dynamics inside the trap. We also discuss pros and cons of moving-averaging method (used to smooth noisy experimental data). We conclude that both TPF and backscattered detection methods are needed to explore the dynamics of the particle inside the potential well.

Disappearance and reappearance of an optical trap for silver nanoparticles under femtosecond pulsed excitation: A theoretical investigation

Recently, the role of ultrafast pulsed excitation in laser trapping of dielectric nanoparticles has been explored and it was observed that the optical Kerr effect (OKE) plays an important role in determining the stability of the trap. Here, we theoretically investigate the trapping behaviour of metallic (silver) nanoparticles and study the effect of OKE (up to sixth order) under high repetition rate femtosecond pulsed excitation. We observe that the trapping potential is first stabilized, then destabilized and again stabilized with an increase in laser power. This work shows how one can fine-tune the stability of an optical trap for metallic nanoparticles through OKE.

Measurements of cesium mixing and quenching cross sections in methane gas: understanding sources of heating in cesium vapor lasers.

Accurate modeling of the operation of diode-pumped alkali lasers is a critical step toward the design of high-powered devices. We present precision measurements for the Cs-CH4 62P3/2 → 62P1/2 mixing cross section and the 62P3/2,1/2 → 62S1/2 quenching cross section, which are important parameters in understanding the operation and, in particular, the heat generated in a cesium vapor laser. Measurements are carried out using ultrafast laser pulse excitation and observation of fluorescence due to collisional excitation transfer in time is done using the technique of time-correlated single-photon counting. Mixing rate measurements are acquired over methane pressures of 10 - 40 Torr, resulting in a Cs-CH4 62P3/2 → 62P1/2 mixing cross section of (1.40 ± 0.08) × 10-15 cm2, while quenching rate measurements are carried out over methane pressures of 500 - 4000 Torr, resulting in a 62P3/2,1/2 → 62S1/2 quenching cross section of (1.57 ± 0.03) × 10-18 cm2. These results suggest only a slight contribution to the heating of a cesium vapor laser is due to Cs 62P quenching, contrary to previous studies. We also discuss additional possible sources of energy transfer from upper excited states of Cs.

Collisional excitation transfer and quenching in Rb(5P) -methane mixtures

We have examined fine-structure mixing between the rubidium $5^{2}P_{3/2}$ and $5^{2}P_{1/2}$ states along with quenching of these states due to collisions with methane gas. Measurements are carried out using ultrafast laser pulse excitation to populate one of the Rb $5^{2}P$ states, with the fluorescence produced through collisional excitation transfer observed using time-correlated single-photon counting. Fine-structure mixing rates and quenching rates are determined by the time dependence of this fluorescence. As Rb($5^{2}P$) collisional excitation transfer is relatively fast in methane gas, measurements were performed at methane pressures of $2.5 - 25$ Torr, resulting in a collisional transfer cross section ($5^{2}P_{3/2} \rightarrow 5^{2}P_{1/2}$) of $(4.23 \pm 0.13) \times 10^{-15}$ cm$^{2}$. Quenching rates were found to be much slower and were performed over methane pressures of $50 - 4000$ Torr, resulting in a quenching cross section of $(7.52 \pm 0.10) \times 10^{-19}$ cm$^{2}$. These results represent a significant increase in precision compared to previous work, and also resolve a discrepancy in previous quenching measurements.

Alternate analytic formulation of optical force on a dielectric sphere in the ray optics limit

In the past, the optical force on a micrometer-sized dielectric sphere in a single-beam gradient laser trap was formulated by considering 2D distribution of rays for a plane-wave excitation. However, laser beams usually have a Gaussian transverse intensity profile, which, upon tight focusing, leads to a 3D optical trap. Here, we systematically formulate a generalized ray/geometric optics formalism for estimating force (and potential) for both flat-top and Gaussian laser beams using 2D distribution of light rays as well as 3D distribution of light cones. We also compare our method with the exact Mie theory. In addition, we present a detailed discussion on the nature of force (and potential) considering the optical Kerr effect under high-repetition-rate ultrafast pulsed excitation.

Theoretical estimation of nonlinear optical force on dielectric spherical particles of arbitrary size under femtosecond pulsed excitation

Experimental evidence indicates that high-repetition-rate ultrafast pulsed excitation is more efficient in optical trapping of dielectric nanoparticles as compared with continuous-wave excitation at the same average power. The physics behind the different nature of force under these two excitation conditions remained deceptive until quite recently when it was theoretically explained, in the dipole limit, as a combined effect of (1) repetitive instantaneous momentum transfer and (2) optical Kerr nonlinearity. The role of optical Kerr effect was theoretically studied for larger dielectric spherical particles, in the ray optics limit, also. However, a theoretical underpinning is yet to be established as to whether the effect of optical nonlinearity is omnipresent across different particle sizes, which we investigate here. Using localized approximation of generalized Lorenz-Mie theory, we theoretically analyze the nature of force (and potential) and provide a detailed comparative discussion between this generalized scattering formulation with dipole scattering formulation for dielectric nanoparticles.

Plasmon assisted control of photo-induced excitation energy transfer in a molecular chain

The strong and ultrafast laser pulse excitation of a molecular chain in close vicinity to a spherical metal nano-particle (MNP) is studied theoretically. Due to local-field enhancement around the MNP, pronounced excited-state formation has to be expected for the part of the chain which is in proximity to the MNP. Here, the description of this phenomenon will be based on a uniform quantum theory of the MNP–molecule system. It accounts for local-field effects due to direct consideration of the strong excitation energy transfer coupling between the MNP and the various molecules. The molecule–MNP distances are chosen in such a way as to achieve a correct description of the MNP via dipole–plasmon excitations. Short plasmon life-times are incorporated in the framework of a density matrix approach. By extending earlier work the present description allows for multi-exciton formation and multiple dipole–plasmon excitation. The region of less intense and not-too-short optical excitation is identified as being best suited for excitation energy localization in the chain.

Theoretical investigation on optical Kerr effect in femtosecond laser trapping of dielectric microspheres

Stable trapping of individual dielectric nanoparticles under high-repetition-rate ultrafast pulsed excitation has been demonstrated and theoretically explained based on repetitive instantaneous trapping as well as optical nonlinearity assisted trapping. Here we estimate the force exerted on a micron-sized spherical dielectric particle including optical Kerr effect. Using geometric optics approximation, we show how inclusion of optical Kerr effect results in significant change of the force curves along axial direction under femtosecond pulsed excitation compared with continuous-wave excitation. The results show excellent agreement with previous experimental findings. Most importantly, similar to the optical trapping of nanoparticles under ultrafast pulsed excitation, we show that the efficiency of trapping of micron-sized particles is also governed by the barrier height (and not the absolute depth) of the axial trapping potential.

Optomechanics of Single Aluminum Nanodisks.

Aluminum nanostructures support tunable surface plasmon resonances and have become an alternative to gold nanoparticles. Whereas gold is the most-studied plasmonic material, aluminum has the advantage of high earth abundance and hence low cost. In addition to understanding the size and shape tunability of the plasmon resonance, the fundamental relaxation processes in aluminum nanostructures after photoexcitation must be understood to take full advantage of applications such as photocatalysis and photodetection. In this work, we investigate the relaxation following ultrafast pulsed excitation and the launching of acoustic vibrations in individual aluminum nanodisks, using single-particle transient extinction spectroscopy. We find that the transient extinction signal can be assigned to a thermal relaxation of the photoexcited electrons and phonons. The ultrafast heating-induced launching of in-plane acoustic vibrations reveals moderate binding to the glass substrate and is affected by the native aluminum oxide layer. Finally, we compare the behavior of aluminum nanodisks to that of similarly prepared and sized gold nanodisks.

Theoretical investigation on nonlinear optical effects in laser trapping of dielectric nanoparticles with ultrafast pulsed excitation.

The use of low-power high-repetition-rate ultrafast pulsed excitation in stable optical trapping of dielectric nanoparticles has been demonstrated in the recent past; the high peak power of each pulse leads to instantaneous trapping of a nanoparticle with fast inertial response and the high repetition-rate ensures repetitive trapping by successive pulses However, with such high peak power pulsed excitation under a tight focusing condition, nonlinear optical effects on trapping efficiency also become significant and cannot be ignored. Thus, in addition to the above mentioned repetitive instantaneous trapping, trapping efficiency under pulsed excitation is also influenced by the optical Kerr effect, which we theoretically investigate here. Using dipole approximation we show that with an increase in laser power the radial component of the trapping potential becomes progressively more stable but the axial component is dramatically modulated due to increased Kerr nonlinearity. We justify that the relevant parameter to quantify the trapping efficiency is not the absolute depth of the highly asymmetric axial trapping potential but the height of the potential barrier along the beam propagation direction. We also discuss the optimal excitation parameters leading to the most stable dipole trap. Our results show excellent agreement with previous experiments.