Short Pulse Excitation Research Articles

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.

155-μm VCSEL with polarization-independent HCG mirror on SOI

We designed and fabricated a vertical-cavity surface-emitting laser (VCSEL) incorporating a polarization-independent high-index-contrast subwavelength grating (HCG) mirror on silicon-on-insulator (SOI) for a novel polarization-bistable device on a silicon substrate. The VCSEL consists of the HCG mirror, an active layer with InGaAsP quantum wells having optical gain around 1.55 μm, and an Al0.9Ga0.1As/Al0.16Ga0.84As DBR. We used direct wafer bonding for the bonding between the active layer and the AlGaAs DBR, and benzocyclobutene (BCB) bonding for the bonding between the active layer and the polarization-independent HCG mirror. The reflectivity of the HCG embedded with BCB was measured, resulting in a 200-nm-high reflectivity band with reflectivity higher than 99% and a small polarization dependence of ± 1%. We achieved lasing of the fabricated HCG-VCSEL at 1527 nm under an optical short pulse excitation with an average power of 50 mW (~0.2 mJ/cm2) at 240 K.

Berry phases of quantum trajectories of optically excited electron–hole pairs in semiconductors under strong terahertz fields

The quantum evolution of particles under strong fields can be essentially captured by a small number of quantum trajectories that satisfy the stationary phase condition of the Dirac–Feynmann path integral. The quantum trajectories are a key concept in understanding extreme nonlinear optical phenomena, such as high-order harmonic generation (HHG) and high-order terahertz sideband generation (HSG). In contrast to HHG in atoms and molecules, HSG in semiconductors can have interesting effects due to nontrivial ‘vacuum’ states of band materials. We find that, in a semiconductor with non-vanishing Berry curvature in its energy bands, the cyclic quantum trajectories of an electron–hole pair under a strong elliptically polarized terahertz field can accumulate a Berry phase. Taking monolayer MoS2 as a model system, we show that the Berry phase appears as a Faraday rotation angle in the pulse emission from the material under short-pulse excitation. This finding reveals an interesting transport effect in the extreme nonlinear optics regime.

Continuous-wave laser damage of uniform and nanolaminate hafnia and titania optical coatings

The laser-damage thresholds of single material and nanolaminate thin films were compared under continuous-wave (CW) illumination conditions. Nanolaminate films consist of uniform material interrupted by the periodic insertion of one or more atomic layers of an alternative material. Hafnia and titania were used as the base materials, and the films were deposited using atomic-layer deposition. The nanolaminates were less polycrystalline than the uniform films, as quantified using x-ray diffraction. It was found that the nanolaminate films had reduced laser-damage thresholds on smooth and patterned substrates as compared to uniform single-material films. This behavior is unusual as prior art indicates that amorphous (less polycrystalline) materials have higher laser-damage thresholds under short-pulse excitation. It is speculated that this may indicate that local thermal conduction affects breakdown more strongly under CW excitation than the dielectric properties that are important for short-pulse excitation.

Ultrafast optical nonlinearity in nanostructured selenium allotropes

In this letter we investigate nonlinear optical properties of amorphous selenium (a-Se) nanoparticles and trigonal selenium (t-Se) nanowires using the open aperture Z-scan, employing 100fs and 5ns laser pulses, at 800nm and 532nm, respectively. In the former case (ultrafast excitation) optical nonlinearity arises largely from absorption saturation and three-photon absorption, whereas in the latter (short-pulse excitation) it occurs mostly due to excited state absorption. Nonlinearity parameters for both regimes have been numerically evaluated from experimental data. Both allotropes are found to be efficient optical limiters, particularly for short-pulse excitation, with potential applications in laser safety devices.

Time-Dependent Theory of Angular Correlations in Sequential Double Ionization

In this work, we emphasize the importance of the bound-state dynamics to the two-electron ejection in double ionization processes. The conclusions of the present study are pertinent to all excitation or decay processes that proceed via well-defined intermediate states. A general strong-field time-dependent density matrix theory is established and applied to the case of neon, allowing us to analyze the role of the ionizing field in the interpretation of reported angular patterns [M. Kurka et al., J. Phys. B 42, 141002 (2009); A. S. Kheifets, J. Phys. B 42, 134016 (2009)] and in the dynamic ionic alignment. The present analysis reveals that short-pulse coherent excitation of the neon ionic doublet 2P(1/2,3/2) leads to quantum beats in the two-electron angular correlation patterns.

On the Electromagnetic Susceptibility of Hot Wire-Based Electroexplosive Devices to RF Sources

We present an analysis of the thermal response of a hot-wire electroexplosive device (EED) excited with different transient signals. First-order and second-order analytical models to calculate the thermal response of an EED are assessed taking as reference numerical simulations obtained using ANSYS. For the early-time response, when the time is much smaller than the thermal constant of the EED, the best approach corresponds to a first-order differential model in which the thermal capacitance is calculated with short-pulse excitations. A linear simplification to calculate the maximum temperature due to short excitations is also shown to be adequate. On the other hand, the most appropriate model for the late-time response is a second-order model. The models are used to assess the electromagnetic susceptibility of a wired EED for different electromagnetic pulsed environments. Radiated signals produced by a mesoband radiator, two types of radars, and a hyperband radiator are considered. The radar signal proved to be the most disturbing source because of its highest duty cycle and its flat spectral response around a specific frequency. Even the temperature firing threshold can be exceeded with the radiated field produced by a radar of 200 kW of output power located at a distance of 5 m.

Towards pump–probe experiments of defect dynamics with short ion beam pulses

A novel, induction type linear accelerator, the Neutralized Drift Compression eXperiment (NDCX-II), is currently being commissioned at Berkeley Lab. This accelerator is designed to deliver intense (up to 3×1011ions/pulse), 0.6 to ∼600ns duration pulses of 0.05–1.2MeV lithium ions at a rate of about 2 pulses per minute onto 1–10mm scale target areas. When focused to mm-diameter spots, the beam is predicted to volumetrically heat micrometer thick foils to temperatures of ∼30,000°K. At lower beam power densities, the short excitation pulse with tunable intensity and time profile enables pump–probe type studies of defect dynamics in a broad range of materials. We briefly describe the accelerator concept and design, present results from beam pulse shaping experiments and discuss examples of pump–probe type studies of defect dynamics following irradiation of materials with intense, short ion beam pulses from NDCX-II.

Inner membrane dynamics in mitochondria

Combining the use of cells with sparse cristae marked with IMP-EGFP and short pulsed sub-saturating fluorescence excitation (non-saturation fluorescence microscopy/NSFM) revealed inhomogeneous fluorescence distribution along mitochondria in living cells. Also the matrix located TMRE was distributed non-uniformly and at least in part filling the gaps between the IMP-EGFP fluorescence: fluorescence intensities are modulated in space and time in part in an antidromic manner. The spatial modulations can be interpreted to represent cristae/matrix distributions. The temporal fluctuations of fluorescence vary within 0.3-3s. Because most peak positions of IMP fluorescence remain stationary up to at least several minutes, temporal intensity modulations may result from varying emissions related to the degree of excitation and/or represent wobbling of cristae, i.e. lateral movements, bending or size changes. Modulations by noise and non-saturated excitation have been reduced by 3 steps of deconvolution followed by averaging 4 images. This allowed a final temporal resolution of 150ms. Disappearance of cristae or formation of new ones takes place within a few seconds, but these are rare events. Thus position of cristae seems to be rather stable, but they regularly disassemble close to fission sites. Treatment with oligomycin strongly reduces "wobbling" activity.

Band tail absorption saturation in CdWO4 with 100 fs laser pulses

The decay kinetics of the excitonic emission of CdWO4 scintillators was studied under excitation by powerful 100 fs laser pulses in the band tail (Urbach) absorption region. A special imaging technique possessing both spatial and temporal resolution provided a unique insight into the Förster dipole–dipole interaction of self-trapped excitons, which is the main cause of the nonlinear quenching of luminescence in this material. In addition, the saturation of phonon-assisted excitonic absorption due to extremely short excitation pulses was discovered. A model describing the evolution of electronic excitations in the conditions of absorption saturation was developed and an earlier model of decay kinetics based on the Förster interaction was extended to include the saturation effect. Compared to the previous studies, a more accurate calculation yields 3.7 nm as the Förster interaction radius. It was shown that exciton–exciton interaction is the main source of scintillation nonproportionality in CdWO4. A quantitative description using a new model of nonproportionality was presented, making use of the corrected value of the Förster radius.

Spectroscopic properties of ytterbium, praseodymium-codoped fluorozirconate glass for laser emission at 36 μm

Excitation of the F27/2→F25/2 transition of the Yb3+ ion in Yb3+, Pr3+-doped fluorozirconate glass at 974 nm results in efficient excitation of the G41 level of Pr3+ ion that in turn emits in the middle infrared at ∼3.6  μm. The energy transfer (ET) process Yb3+(F25/2)→Pr3+(G41) is assisted by fast excitation migration among the Yb3+ ions. An upconversion process involving ET from the F25/2 level to the G41 excited state populates the P03 excited state that produces emission at 481, 521, 603, 636, and 717 nm. A study of the behavior of the fluorescence from the G41 level at 1325 nm and from the P03 level at 603 nm allowed the estimation of the ET rate constants for the processes involved after short-pulsed laser excitation at 974 nm. A rate-equation model was employed to evaluate the population inversion relating to the G41→F43 transition of the Pr3+ ion at 3.6 μm under continuous wave pumping.

Investigation on the inertial cavitation threshold of micro-bubbles

Experimental measurements and numerical analyses were performed to investigate the IC thresholds of two commercialized UCAs, albumin-shelled KangRun® and lipid-shelled SonoVue®. The IC thresholds of these two UCAs were measured at varied acoustic pulse lengths and bubble concentrations, according to the IC dose quantifications based on passive cavitation detection (PCD). Then, the shell properties of UCAs were estimated by fitting the measured acoustic attenuation data. Finally, the influences of acoustic pulse length and UCA shell properties on the microbubble nonlinear behaviors were discussed based on numerical simulations, which would give us better understanding of the dependence of microbubble IC threshold on the sonication condition and physical structure properties of the coating shells. The experimental results show that: (1) the IC threshold of UCAs is dependent on the acoustic driving conditions, the shell properties of UCAs and the bubble concentration; (2) for both the lipid- and albumin-shelled UCAs, the IC threshold generally decreases with the increasing UCA volume concentration; (3) IC threshold is observed higher for short-pulse excitation, then its value decreases as the acoustic pulse length increases from 5 to 20 cycles and finally tends to reach a steady state for even longer pulsed exposures.

Ultrafast optical nonlinearity, electronic absorption, vibrational spectra and solvent effect studies of ninhydrin

FT-IR, FT-Raman and UV–Vis spectra of the nonlinear optical molecule ninhydrin have been recorded and analyzed. The equilibrium geometry, bonding features, and harmonic vibrational wavenumbers have been investigated with the help of B3LYP density functional theory method. A detailed interpretation of the vibrational spectra is carried out with the aid of normal coordinate analysis following the scaled quantum mechanical force field methodology. Solvent effects have been calculated using time-dependent density functional theory in combination with the polarized continuum model. Natural bond orbital analysis confirms the occurrence of strong intermolecular hydrogen bonding in the molecule. Employing the open-aperture z-scan technique, nonlinear optical absorption of the sample has been studied in the ultrafast and short-pulse excitation regimes, using 100fs and 5ns laser pulses respectively. It is found that ninhydrin exhibits optical limiting for both excitations, indicating potential photonic applications.

Electronic circuit design for reciprocal operation of transit-time ultrasonic flow meters

The reciprocal operation theory has been used in the circuit design of transit-time ultrasonic flow meters (USMs) to promote their zero flow performances. In this paper, the authors analyzed the transfer functions of the inter-conversion between acoustic and electric signals on each transducer, indicated that the equivalent load consistency for each transducer between the transmitting and the receiving phases would guarantee the transmitting and the receiving transfer functions equal, and thus guarantee the reciprocal operation of the USM system. In order to achieve the “load consistency” condition in practical systems with non-ideal parameters, a method of applying short pulse excitation and fixing the equivalent load in the “free oscillation period” of the transmitting transducer is proposed. Based on this method, several circuit designs are shown in the paper, the impedance preferences are also analyzed. Practical experiments done with these circuits show clear improvement in both promoting the accordance of the signals received in both directions and reducing the zero flow error in the measurement.

Identification and Reconstruction of Cracks in Ultrasonic Infrared Thermography

Ultrasonic infrared thermography is a novel nondestructive detection technique, which combines a short ultrasonic pulse excitation and infrared imaging to detect defects, such as crack, in materials and structures. A simplified one-dimension heat-conduction model excited by ultrasonic pulses is put forward in this paper. Based on this model, a serial of image processing methods for recognition and reconstruction of cracks were presented. Results obtained show that the proposed method is creditable and applicable.

Modeling of transient dynamics in two-dimensional circular microresonators using the pulsed complex source point beam concept

An analytical method for transient dynamics description in microresonators is used to characterize and visualize the transient effects. In the frame of this method, the pulsed complex source point concept is used to simulate an incident transient beam. The excited fields in the microcavity are described by means of a rigorous mathematical approach that is based on the analytical solution in the Laplace transform domain and accurate evaluation of residues at singular points corresponding to the excited modes. The effects of transient mode beating and ultrashort pulse splitting inside the microresonator for short pulse excitation are discussed.

Scanning internal photoemission microscopy for the identification of hot carrier transport mechanisms

Linear and nonlinear internal photoemission in a thin-film metal-insulator-metal heterosystem, i.e., a Ta-TaOx-Ag junction, together with surface reflectivity are mapped with a lateral resolution of better than 5 μm. The spatial correlation of the different signals and time-resolved internal photoemission spectroscopy reveal excitation mechanisms and ballistic hot carrier injection. The internal photoemission yield variation with Ag layer thickness is quantitatively explained by above-barrier injection. The hot-spot-like behavior of the two-photon induced internal photoemission observed for short pulse excitation is attributed to local field enhancements because of Ag-film thickness reduction and plasmonic effects at structural defects.

Detection of domain wall eigenfrequency in infinity-shaped magnetic nanostructures

The dynamics of a magnetic infinity-shaped nanostructure has been experimentally studied by two different techniques such as the sinusoidal resonance excitation and the damped short pulse excitation to measure the eigenfrequency of domain walls. Direct observation of the magnetic domain wall nucleation has been measured in the frequency domain. Electrical measurements of the domain wall dynamics in the frequency domain reveal the existence of multi-eigenmodes for large excitation amplitudes. The time-resolved measurements show that the frequency of the damped gyration is similar to that of the frequency domain and indicate coexistence of spin wave excitations.

Analysis of characteristics of grating lobes generated with Gaussian pulse excitation by ultrasonic 2-D array transducer

Characteristics of ultrasonic beams generated by two-dimensional periodic array transducers have been analyzed through simulations based on the Rayleigh–Sommerfeld model. An excitation waveform is treated as a Gaussian pulse in the simulations with an enhanced model for a continuous wave. Acoustical fields for focal beams, waveforms and spectra are calculated and compared among Gaussian pulses with various pulse lengths. Theoretical generation conditions for propagation directions of grating lobes with the center frequency of the excitation waveform are derived from the first order term of the delay law for an angled focal beam. The theoretical results show good agreement with the simulations. Furthermore, the theoretical condition expressions are extended for high-frequency grating lobes as a specific phenomenon for short pulse excitation.

Dynamics and microscopic origin of fast 1.5μm emission in Er-doped SiO2sensitized with Si nanocrystals

We investigate the origin of fast 1.5 $\ensuremath{\mu}$m photoluminescence from Er-doped SiO${}_{2}$ sensitized with silicon nanocrystals, which appears and decays within the first microsecond after a short laser excitation pulse. Time-resolved and temperature-dependent measurements on the 1.5 $\ensuremath{\mu}$m emission from Er-doped and Er-free samples reveal that the major part of this emission is Er related. A possible contribution from other photoluminescence bands, specifically of the defect-related band centered around 1.3 $\ensuremath{\mu}$m, has also been considered. All the results obtained indicate the dominant contribution of Er${}^{3+}$ ions to the fast 1.5 $\ensuremath{\mu}$m emission in the investigated materials. We propose two possible mechanisms behind the fast excitation and quenching of Er${}^{3+}$ 1.5 $\ensuremath{\mu}$m emission, which are both facilitated by Er-related trap centers with ionization energy of $E$${}_{A}$ $\ensuremath{\approx}$ 60 meV.