Ultrafast Laser Pulses Research Articles

Evolution of laser-induced strain in a Ge crystal for the [111] and [100] directions probed by time-resolved X-ray diffraction

Ultra-short laser-pulse-induced strain propagation in a Ge crystal is studied in the [111] and [100] directions using time-resolved X-ray diffraction (TXRD). The strain propagation velocity is derived by analysis of the TXRD signal from the strained crystal planes. Numerical integration of the Takagi–Taupin equations is performed using open source code, which provides a very simple approach to estimate the strain propagation velocity. The present method will be particularly useful for relatively broad spectral bandwidths and weak X-ray sources, where temporal oscillations in the diffracted X-ray intensity at the relevant phonon frequencies would not be visible. The two Bragg reflections of the Ge sample, viz. 111 and 400, give information on the propagation of strain for two different depths, as the X-ray extinction depths are different for these two reflections. The strain induced by femtosecond laser excitation has a propagation velocity comparable to the longitudinal acoustic velocity. The strain propagation velocity increases with increasing laser excitation fluence. This fluence dependence of the strain propagation velocity can be attributed to crystal heating by ambipolar carrier diffusion. Ge is a promising candidate for silicon-based optoelectronics, and this study will enhance the understanding of heat transport by carrier diffusion in Ge induced by ultra-fast laser pulses, which will assist in the design of optoelectronic devices.

Ultrafast coherent excitation of an ytterbium ion with single laser pulses

Experimental realizations of two qubit entangling gates with trapped ions typically rely on addressing spectroscopically resolved motional sidebands, limiting gate speed to the secular frequency. Fast entangling gates using ultrafast pulsed lasers overcome this speed limit. This approach is based on state-dependent photon recoil kicks from a sequence of counter-propagating, resonant, ultrafast pulse pairs, which can allow sub-microsecond gate speeds. Here, we demonstrate a key component of the ultrafast gate protocol, the coherent excitation of a 171Yb+ ion across the 2S1∕2-2P1∕2 transition with a single near-resonant short optical pulse at 369.53 nm. We achieve a maximum population transfer of 94(1)% using a picosecond pulsed laser that can be tuned across the 2S1∕2-2P1∕2 transition and 42.5(9)% with 190(7) GHz detuning.

Thermal Effect in a 3-D Simulation within Multilayer Thin Film of Ultrafast-Pulsed Laser

Hard disk drive (HDD) and storage media have the potential to revolutionize future information technology. Heat-assisted magnetic recording (HAMR) is a promising method for increasing hard disk storage density and it is one of the applications of this study. The essential component of nanoscale spintronic devices is spin current. The simulation of a thermal gradient to generate a pure spin current using an ultrafast femtosecond (fs) laser in a multilayer thin film is presented. The trilayer sample (ferromagnetic/spacer/magnetic insulator) is the candidate to achieve the spin current. Ultrafast laser with fs pulse width simulated to creates a spin diffusion spark. These pulses penetrate within the trilayer reaching the magnetic insulator due to the penetration depth that record the effect. COMSOL Multiphysics software® is used to simulate the thermal behavior within the trilayer with three-dimensional (3-D) view. The thickness of the ferromagnetic layer (Ni81Fe19) has been shown to generate a high-temperature gradient within the magnetic insulator and therefore a larger spin current.

Time-ordering in Heisenberg\u2019s equation of motion as related to spontaneous radiation

Despite many years of research into Raman phenomena, the problem of how to include both spontaneous and stimulated Raman scattering into a unified set of partial differential equations persists. The issue is solved by formulating the quantum dynamics in the Heisenberg picture with a rigorous accounting for both time- and normal-ordering of the operators. It is shown how this can be done in a simple, straightforward way. Firstly, the technique is applied to a two-level Raman system, and comparison of analytical and numerical results verifies the approach. A connection to a fully time-dependent Langevin operator method is made for the spontaneous initiation of stimulated Raman scattering. Secondly, the technique is demonstrated for the much-studied two-level atom both in vacuum and in a lossy dielectric medium. It is shown to be fully consistent with accepted theories: using the rotating wave approximation, the Einstein A coefficient for the rate of spontaneous emission from a two-level atom can be derived in a manner parallel to the Weisskopf–Wigner approximation. The Lamb frequency shift is also calculated. It is shown throughout that field operators corresponding to spontaneous radiative terms do not commute with atomic/molecular operators. The approach may prove useful in many areas, including modeling the propagation of next-generation high-energy, high-intensity ultrafast laser pulses as well as spontaneous radiative processes in lossy media.

Ultrafast plasmon dynamics in asymmetric gold nanodimers

We theoretically investigate the effect of symmetry breaking on the ultrafast plasmon responses of Au nanodisk (ND) dimers by varying the diameter of one of the constituent nanodisks. In the case of a single ultrafast laser pulse, we demonstrate that the ultrafast responses of Au ND homodimer can be significantly modified due to the effect of symmetry breaking. The symmetric dimer shows a single broad spectral peak, whereas the size-asymmetric dimer shows three spectral peaks. The first system displays at most one temporal maximum and no beats in ultrafast temporal, whereas the second system may have three temporal maxima and two beats due to a combination of broken symmetry and the coherent superposition between various plasmon modes induced by the ultra-short laser pulse. Moreover, the shape of temporal dynamics of the size-asymmetric dimer is significantly deformed due to the excitation of local plasmon modes with different wavelength components. Furthermore, the decay time of the amplitude of the local field is longer and oscillates with a high frequency due to the narrower linewidth and red-shifted spectral peaks. We show that the ultrafast plasmon responses of both dimers can be controlled by varying the relative phase and time delays between a pair of two pulses. Our results will open new paths to understanding ultrafast plasmon responses in asymmetric heterodimers with suitable properties for different applications.

Laser-assisted formation of luminescent domains in metal- or semiconductor-doped silicate and phosphate glasses

In this study, silicate and phosphate glasses doped with Ag or CdS were exposed to femtosecond laser pulses and photoluminescence properties of the laser-written domains were investigated. Laser writing in phosphate glass doped with CdS was found to induce very weak photoluminescence, while laser-written domains in silicate glass had a comparatively high photoluminescence intensity, that was assigned to the formation of the sulphur vacancies in the CdS nanocrystals precipitated under the ultrafast laser pulses. Observed photoluminescence bands in Ag-containing glasses we assigned to the formation of different silver nanospecies which provide photoluminescence bands with the maxima at 685 and 600 nm in Ag-doped silicate and phosphate glasses, respectively.

Analysis of a higher-energy structure in nanotip enhanced fields

We investigate strong field ionization of an atomic gas in a plasmonically enhanced field resulting from the illumination of a nanometer-sized structure with ultrafast laser pulse. We use perturbation theory to derive an approximate solution for electron’s motion following ionization. These analytical estimates are corroborated by the time-dependent Schrödinger equation and classical trajectory Monte Carlo simulations. Notably, our approach can be used to obtain electron energy spectra without having to rely on numerical simulations. This allows for a deeper study of the dependence of electron energy spectrum on the properties of the near-field, suggesting electric field sensor applications. We derive an analytical expression for the location of the peak of the higher-energy structure (HES) as a function of laser parameters and near-field decay length. We find a particularly strong dependence of the energy peak on laser frequency, with lower frequencies causing a significant upward shift in the final electron energies. Combined with control of the width of the HES, which can be done by changing the size of the nanostructure, this points to the possibility of using nanotips as sources of ultrashort electron beams of tunable energy.

Fast and direct optical dispersion estimation for ultrafast laser pulse compression.

In ultrashort pulse laser applications, optical dispersion seriously affects the energy concentration in the laser pulse duration and lowers the peak power. Accordingly, this study proposes a rapid dispersion estimation mechanism to facilitate the compensation of optical dispersion using a closed-loop control system. In the proposed approach, the optical dispersion information of the laser pulse is estimated directly from a frequency-resolved optical gating trace without the need for an iterative pulse-retrieval algorithm. In particular, the group delay dispersion (GDD) is determined from frequency and delay marginals, which are related to the laser spectrum and intensity autocorrelation, respectively, using a simple lookup table approach. The accuracy of the estimated GDD results is confirmed via a comparison with the spectral phase distribution of the electric field reconstructed using the principal component generalized projections algorithm. It is shown that the computation time of the proposed direct estimation method is around 13 times faster than that of the traditional iterative algorithm. It thus provides a feasible approach for enabling the real-time compensation of ultrafast laser pulse compression. Moreover, in a multiphoton-excited fluorescence imaging application, the proposed pulse compression mechanism yields an effective improvement in the intensity and contrast of the reconstructed image due to the increased nonlinear optical excitation efficiency of the optimized laser pulses.

Time-resolved relaxation and fragmentation of polycyclic aromatic hydrocarbons investigated in the ultrafast XUV-IR regime

Polycyclic aromatic hydrocarbons (PAHs) play an important role in interstellar chemistry and are subject to high energy photons that can induce excitation, ionization, and fragmentation. Previous studies have demonstrated electronic relaxation of parent PAH monocations over 10–100 femtoseconds as a result of beyond-Born-Oppenheimer coupling between the electronic and nuclear dynamics. Here, we investigate three PAH molecules: fluorene, phenanthrene, and pyrene, using ultrafast XUV and IR laser pulses. Simultaneous measurements of the ion yields, ion momenta, and electron momenta as a function of laser pulse delay allow a detailed insight into the various molecular processes. We report relaxation times for the electronically excited PAH*, PAH+* and PAH2+* states, and show the time-dependent conversion between fragmentation pathways. Additionally, using recoil-frame covariance analysis between ion images, we demonstrate that the dissociation of the PAH2+ ions favors reaction pathways involving two-body breakup and/or loss of neutral fragments totaling an even number of carbon atoms.

Perspectives on all-optical Kerr switching for quantum optical applications

We offer a perspective on recent advances in picosecond-timescale all-optical switching with applications in quantum optics. The switch is based on polarization rotation in standard single-mode fiber via the optical Kerr effect. By using ultrafast laser pulses and short (∼10 cm) fibers, this technique can achieve a switching duration of ≲1 ps, at the repetition rate of 80 MHz or above. This high repetition rate is well-suited to quantum optics where experiments operate in the photon-counting regime. The switch efficiency can be ≳99% with a noise floor of just ∼10−4 photons/pulse, enabling high fidelity operations on quantum states of light, with negligible generation of spurious noise photons. We highlight the capabilities of this technique in four early applications: switching of heralded single photons, time-bin to polarization conversion of photonic qubits, noise gating for quantum key distribution, and pulse carving.

Direct precipitation of CdS nanocrystals in glass by ultrafast laser pulses

In this study, we investigated the structure of microdomains directly laser-written in the bulk of glass and containing CdS nanoparticles by transmission electron microscopy. The CdS nanocrystals have a multidisperse size distribution while quantum dots with a minimal size of ∼ 3–4 nm are registered near the edge of a microdomains. The size of CdS nanocrystals sweepingly increases up to ∼340 nm from the periphery to the center of the laser-written microdomain. It has been found that microdomains possess laser polarization-sensitive optical properties providing opportunities for the development of advanced optical data encoding and security tagging.

Performance of Graphene Nanopowder-Polyvinyl Alcohol in Optical Pulse Generation at 1.5 Micron Region

This paper explains about the performance of graphene nanopowder (GNP) based saturable absorber (SA) at 1.5-micron region which is prepared by dissolution in polyvinyl alcohol (PVA) polymer. Two different GNP flakes thickness (AO2-8 nm and AO4-60 nm) are tested. By applying a solution casting method, three weight ratio of GNP to PVA (12.04, 8.03 and 3.11 wt.%) have been prepared and fabricated as a composite thin film. To characterize for the SA performance, 4 mm2 area of GNP-PVA thin film is embedded in a 14 meters long ring cavity with 3 meters Erbium doped fiber (EDF) as a gain medium. Our characterization results show that the GNP-PVA thin film act as a Q-switcher which produce stable laser pulses for 12.04 wt.% with maximum repetition rate of 39.22 kHz and shortest pulse width of 11.79 µs. Meanwhile, unstable Q-switched pulses of 8.03 wt.% and 3.11 wt.% have been observed with recorded signal to noise ratio (SNR) of only 21 dB and 17 dB, respectively. The threshold pumping power for Q-switched lasing to emerge is recorded as low as 30 mW. Apparently, it shows that GNP concentration and flakes thickness in fabricated SA composite plays vital role in the performance of generated Q-switch laser, particularly at 1.5 µm region. Full Text: PDF ReferencesT. Hasan, Z. Sun, F. Wang, F. Bonaccorso, P.H. Tan, A.G. Rozhin, A.C. Ferrari, "Nanotube–Polymer Composites for Ultrafast Photonics", Adv. Mater. 21, 3874 (2009). CrossRef Q. Bao, H. Zhang, Y. Wang, Z. Ni, Y. Yan, Z.X. Shen, K.P. Loh, D.Y. Tang, "Atomic-Layer Graphene as a Saturable Absorber for Ultrafast Pulsed Lasers", Adv. Funct. Mater. 19, 3077 (2009). CrossRef Z. Luo, M. Zhou, J. Weng, G. Huang, H. Xu, C. Ye, Z. Cai, Opt. Lett. 35(21), 3709 (2010). CrossRef D. Popa, Z. Sun, T. Hasan, F. Torrisi, F. Wang, A.C. Ferrari, "Graphene Q-switched, tunable fiber laser", Appl. Phys. Lett. 98, 3106 (2011). CrossRef Y.M. Chang, H. Kim, J.H. Lee, Y. Song, "Multilayered graphene efficiently formed by mechanical exfoliation for nonlinear saturable absorbers in fiber mode-locked lasers", Appl. Phys. Lett. 97, 211102 (2010). CrossRef M. Jiang, Z. Ren, Y. Zhang, B. Lu, R. Zhang, J. Guo, Y. Zhou, J. Bai, "Passive Q-Switching with Graphene Saturable Absorber in Nd:YAG Operating at 1064nm", Mater. Sci. Forum 694, 700 (2011). CrossRef N. Hussin, M.H. Ibrahim, F. Ahmad, H. Yahaya, S.W. Harun, "Graphene Nanoplatelets (GnP)-PVA Based Passive Saturable Absorber", Telkomnika 15(2), 814 (2017). CrossRef F.C. Mat, M. Yasin, A.A. Latiff, S.W. Harun, Photonics Letters of Poland 9, 100 (2017). CrossRef E.K. Ng, K.Y. Lau, H.K. Lee, N.M. Yusoff, A.R. Sarmani, M.F. Omar, M.A. Mahdi, "L-band femtosecond fiber laser based on a reduced graphene oxide polymer composite saturable absorber", Opt. Mater. Express 11, 59 (2021). CrossRef N.H.M. Apandi, S.N.F. Zuikafly, N. Kasim, M.A. Mohamed, S.W. Harun, F. Ahmad, "Observation of dark and bright pulses in q-switched erbium doped fiber laser using graphene nano-platelets as saturable absorber", Bull. Electr. Eng. Inform. 8, 1358 (2019). CrossRef N.U.H.H.B. Zalkepali, N.A. Awang, Y.R. Yuzaile, Z. Zakaria, A.A. Latif and F. Ahmad, "Graphene Nanoplatelets as Saturable Absorber for Mode-locked Fiber Laser Generation", J. Adv. Res. Dyn. Control Syst. 12(2), 602 (2020). CrossRef X. Zhu and S. Chen, "Autoencoder-Based Transceiver Design for OWC Systems in Log-Normal Fading Channel", IEEE Photonics J. 11, 7105109 (2019). CrossRef

Soliton Generation in Negative Thermal Expansion Materials

Strain solitons have been observed statically in several 2D materials and dynamically in substrate materials using ultrafast laser pulses. The latter case relies on lattice relaxation in response to ultrafast heating in a light-absorbing transducer material, a process which is sensitive to the thermal expansion coefficient. Here we consider an unusual case where the sign of the thermal expansion coefficient is negative, a scenario which is experimentally feasible in light of rapid and recent advances in the discovery of negative thermal expansion materials. We present numerical solutions to a nonlinear differential equation which has been repeatedly demonstrated to quantitatively model experimental data and discuss the salient results using realistic parameters for material linear and nonlinear elasticity. The solitons that emerge from the initial value problem with negative and positive thermal expansion are qualitatively different in several ways. The new case of negative thermal expansion gives rise to a nearly-periodic soliton train with chirped profile and free of an isolated shock front. We suggest this unanticipated result may be realized experimentally and assess the potential for certain applications of this generic effect.

X-ray scattering from light-driven spin fluctuations in a doped Mott insulator

Manipulating spin fluctuations with ultrafast laser pulses is a promising route to dynamically control collective phenomena in strongly correlated materials. However, understanding how photoexcited spin degrees of freedom evolve at a microscopic level requires a momentum- and energy-resolved characterization of their nonequilibrium dynamics. Here, we study the photoinduced dynamics of finite-momentum spin excitations in two-dimensional Mott insulators on a square lattice. By calculating the time-resolved resonant inelastic x-ray scattering cross-section, we show that an ultrafast pump above the Mott gap induces a prompt softening of the spin excitation energy, compatible with a transient renormalization of the exchange interaction. While spin fluctuations in a hole-doped system (paramagnons) are well described by Floquet theory, magnons at half filling are found to deviate from this picture. Furthermore, we show that the paramagnon softening is accompanied by an ultrafast suppression of d-wave pairing correlations, indicating a link between the transient spin excitation dynamics and superconducting pairing far from equilibrium.

The effects of ultrafast laser pulse duration on the optical properties of polymers

Dramatic morphological and optical changes in polypropylene are measured during ultrafast laser micromachining.

Long-range propagation of ultrafast ionizing laser pulses in a resonant nonlinear medium

We study the propagation of 0.05--1 TW power, ultrafast laser pulses in a 10-m-long rubidium vapor cell. The central wavelength of the laser is resonant with the ${D}_{2}$ line of rubidium, and the peak intensity is in the ${10}^{12}$--${10}^{14}\phantom{\rule{4pt}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$ range, enough to create a plasma channel with single-electron ionization. We observe the absorption of the laser pulse for low energy, a regime of transverse confinement of the laser beam by the strong resonant nonlinearity for higher energies and the transverse broadening of the output beam when the resonant nonlinearity ceases due to the valence electrons all being removed during ionization. We compare experimental observations of the transmitted pulse energy and the transverse fluence profile with the results of computer simulations modeling pulse propagation. We find a qualitative agreement between theory and experiment that corroborates the validity of our propagation model. While the quantitative differences are substantial, the results show that the model can be used to interpret the observed phenomena in terms of self-focusing and channeling of the laser pulses by the saturable, resonant nonlinearity.

Large ultrafast-modulated Voigt effect in noncollinear antiferromagnet Mn3Sn

The time-resolved magneto-optical (MO) Voigt effect can be utilized to study the Néel order dynamics in antiferromagnetic (AFM) materials, but it has been limited for collinear AFM spin configuration. Here, we have demonstrated that in Mn3Sn with an inverse triangular spin structure, the quench of AFM order by ultrafast laser pulses can result in a large Voigt effect modulation. The modulated Voigt angle is significantly larger than the polarization rotation due to the crystal-structure related linear dichroism effect and the modulated MO Kerr angle arising from the ferroic ordering of cluster magnetic octupole. The AFM order quench time shows negligible change with increasing temperature approaching the Néel temperature (TN), in markedly contrast with the pronounced slowing-down demagnetization typically observed in conventional magnetic materials. This atypical behavior can be explained by the influence of weakened Dzyaloshinskii–Moriya interaction rather than the smaller exchange splitting on the diminished AFM order near TN. The temperature-insensitive ultrafast spin manipulation can pave the way for high-speed spintronic devices either working at a wide range of temperature or demanding spin switching near TN.

All-Optical and Label-Free Stimulation of Action Potentials in Neurons and Cardiomyocytes by Plasmonic Porous Metamaterials.

Optical stimulation technologies are gaining great consideration in cardiology, neuroscience studies, and drug discovery pathways by providing control over cell activity with high spatio‐temporal resolution. However, this high precision requires manipulation of biological processes at genetic level concealing its development from broad scale application. Therefore, translating these technologies into tools for medical or pharmacological applications remains a challenge. Here, an all‐optical nongenetic method for the modulation of electrogenic cells is introduced. It is demonstrated that plasmonic metamaterials can be used to elicit action potentials by converting near infrared laser pulses into stimulatory currents. The suggested approach allows for the stimulation of cardiomyocytes and neurons directly on commercial complementary metal‐oxide semiconductor microelectrode arrays coupled with ultrafast pulsed laser, providing both stimulation and network‐level recordings on the same device.

Liquid–liquid phase transition in simulations of ultrafast heating and decompression of amorphous ice

• Ultrafast heating experiments on amorphous ice clarified by simulations. • High density amorphous ice heated to liquid state by ultrafast laser pulse. • Decompression of high density liquid reveals liquid–liquid transition. • Simulations detect structural transition along out-of-equilibrium paths. A recent experiment [K. H. Kim, et al. , Science 370 , 978 (2020)] showed that it may be possible to detect a liquid–liquid phase transition (LLPT) in supercooled water by subjecting high density amorphous ice (HDA) to ultrafast heating, after which the sample reportedly undergoes spontaneous decompression from a high density liquid (HDL) to a low density liquid (LDL) via a first-order phase transition. Here we conduct computer simulations of the ST2 water model, in which a LLPT is known to occur. We subject various HDA samples of this model to a heating and decompression protocol that follows a thermodynamic pathway similar to that of the recent experiments. Our results show that a signature of the underlying equilibrium LLPT can be observed in a strongly out-of-equilibrium process that follows this pathway despite the very high heating and decompression rates employed here. Our results are also consistent with the phase diagram of glassy ST2 water reported in previous studies.

Fabrication of fused silica microstructure based on the femtosecond laser

Ultrafast pulsed lasers can facilitate the manufacturing of complex microstructures. However, previous laser processing applications have focused on static components. Consequently, the application of laser processing in high-performance resonant structures has received less attention. Ultrafast lasers provide a unique opportunity for realizing the structure detachment from the planar fused silica substrates. However, the processing quality has a considerable influence on resonant structures. High-quality and high-efficiency laser manufacturing methods are critical for processing resonant structures. In this study, we demonstrate a method for processing fused silica microstructures based on the femtosecond laser. We studied the influence of different laser parameters on the processing quality and determined the optimal laser parameters suitable for the microstructure. The resonant structure of the butterfly gyroscope was used to verify the manufacturing method. A steepness of 86.6° and a roughness of 653.2 nm were achieved by using the optimized laser parameters. These are expected to provide technical support for the development of high-performance fused silica dynamic devices in the future.