Influence Of Pulse Duration Research Articles

Theory of energy- and time-resolved two-photon photoemission from metal surfaces - influence of pulse duration and excitation condition

The theory of energy- and time-resolved two-photon photoemission (2PPE) spectra of metal surfaces is presented using density matrix formulation for a three-level system consisting of an initial occupied, intermediate unoccupied and final photoelectron states. A perturbation expansion method is employed to calculate the energy-resolved 2PPE spectrum for continuous light beams. We have obtained analytical expressions of the 2PPE spectrum corresponding to a step-by-step one-photon process through the intermediate state and a direct two-photon-ionization process via virtual transition. It is demonstrated that the intermediate state can also be populated via the nonresonant virtual process. This indicates an absolute importance of “pure dephasing” associated with the transition between the initial and intermediate states. Evolution of the 2PPE spectrum as a function of the pump photon energy is calculated to demonstrate the conditions under which the intrinsic linewidth (total dephasing time) can be deduced from the lineshape analysis. It is also found that the intensity ratio of the two peaks due to the initial and the intermediate states in 2PPE spectrum can be used to estimate the pure dephasing time. Transient behavior of the excited-state population following pulse excitation is calculated with a focus on how the ultrafast relaxation times of the excited states such as image-potential states of metal surfaces are deduced from the transient 2PPE response observed with a pulse laser with much longer duration. The time-resolved 2PPE spectra are calculated for varying detuning from the resonant excitation from the initial state to the intermediate state. Transient responses of the 2PPE signal due to direct ionization and step-by-step processes are also calculated to demonstrate that the nonresonant former process has an influence on the analysis of the cross-correlation trace of the intermediate state, by which the population relaxation time is estimated. Attempts are also made to apply the present theory to a recent time-resolved 2PPE study of the relaxation dynamics of the image-potential states as well as hot electrons in Cu(100) and Ag(100) surfaces.

Influence of pulse duration on debonding of two layer materials by laser shock technique

By inducing high power laser shocks in the substrate of a two layer material (substrate plus coating), it is possible to generate a tensile stress at the interface and so to use the technique as a test of the level of coating adhesion. The features of the stress history at the interface are governed mainly by the laser shock parameters (related to the laser intensity and the pulse duration) and by the influence of the properties of the target (shock impedance, thickness, strength, etc.) on shockwave propagation. The influence of these parameters on the stress at the interface is discussed on the basis of a simplified analytical model of laser shock propagation in the coated substrate. Some key points such as the interaction of pulse duration and coating thickness are checked by experiment. The study allows some guidelines to be inferred on the use of the laser shock debonding technique as a non-destructive test of adhesion in two layer materials.

発光量評価による放電NOx処理に関するパルス幅の影響分析

発光量評価による放電NOx処理に関するパルス幅の影響分析

Influence of pulse duration on time-resolved sum-frequency generation of surface vibrations

The influence of the pulse duration on time-resolved sum-frequency generation (SFG) is studied on the basis of the density matrix formulation for non-linear optical processes. A general expression is derived for a sum-frequency polarization whose decay as a function of the time delay between the pump (infrared) and probe (visible) pulses provides a direct measure of the vibrational dephasing time of adsorbates. It is clearly demonstrated how the transient SFG intensity is influenced by the pulse duration and under what conditions the temporal resolution is estimated from the rising edge of the SFG signal, and its decay is described by a single exponential with a time constant of the dephasing. The coherent mixing effect between sequent pulses in the transient SFG signal near zero time delay is also discussed.

Energy balance of optical breakdown in water at nanosecond to femtosecond time scales

During optical breakdown, the energy delivered to the sample is either transmitted, reflected, scattered, or ab- sorbed. Pathways for the division of the absorbed energy are the evaporation of the focal volume, the plasma radiation, and the mechanical effects such as shock wave emission and cav- itation. The partition of laser energy between these channels during breakdown in water was investigated for four selected laser parameters typical for intraocular microsurgery ( 6-ns pulses of 1 and 10 mJ focused at an angle of 22 ,a nd30-ps pulses of 50 mJ and 1m Jfocused at 14 ,a ll at1064 nm). Scattering and reflection were found to be small compared to transmission and absorption during optical breakdown. The ratio of the shock wave energy and cavitation bubble energy was approximately constant (between 1.5:1 and 2:1). These results allowed us to perform a more comprehensive study of the influence of pulse duration ( 100 fs- 76 ns )a nd focus- ing angle (4- 32) on the energy partition by measuring only the plasma transmission and the cavitation bubble energy. The bubble energy was used as an indicator for the total amount of mechanical energy. We found that the absorption at the breakdown site first decreases strongly with decreasing pulse duration, but increases again for < 3p s. The conversion of the absorbed energy into mechanical energy is 90% with ns pulses at large focusing angles. It decreases both with de- creasing focusing angle and pulse duration (to < 15 %f or fs pulses). The disruptive character of plasma-mediated laser effects is therefore strongly reduced when ultrashort laser pulses are used.

Influence of pulse duration on mechanical effects after laser-induced breakdown in water

The influence of the pulse duration on the mechanical effects following laser-induced breakdown in water was studied at pulse durations between 100 fs and 100 ns. Breakdown was generated by focusing laser pulses into a cuvette containing distilled water. The pulse energy corresponded to 6-times breakdown threshold energy. Plasma formation and shock wave emission were studied photographically. The plasma photographs show a strong influence of self-focusing on the plasma geometry for femtosecond pulses. Streak photographic recording of the shock propagation in the immediate vicinity of the breakdown region allowed the measurement of the near-field shock pressure. At the plasma rim, shock pressures between 3 and 9 GPa were observed for most pulse durations. The shock pressure rapidly decays proportionally to r−(2⋯3) with increasing distance r from the optical axis. At a 6 mm distance of the shock pressure has dropped to (8.5±0.6) MPa for 76 ns and to &amp;lt;0.1 MPa for femtosecond pulses. The radius of the cavitation bubble is reduced from 2.5 mm (76 ns pulses) to less than 50 μm for femtosecond pulses. Mechanical effects such as shock wave emission and cavitation bubble expansion are greatly reduced for shorter laser pulses, because the energy required to produce breakdown decreases with decreasing pulse duration, and because a larger fraction of energy is required to overcome the heat of vaporization with femtosecond pulses.

Polarization changes in the ferroelectric relaxor lead magnesium niobate

A study is reported of the polarization and depolarization processes in the model ferroelectric relaxor, lead magnesium niobate (PMN), in pulsed and sine electric fields applied along the pseudocubic direction (001) above Td (Td is the phase transition temperature between the induced macrodomain ferroelectric and the nonpolar phases). The study was made by the electroluminescence and polarization-switching current hysteresis techniques. Pulsed electricfield measurements revealed the influence of pulse duration on electroluminescence intensity under polarization and depolarization of a sample, as well as a decrease of delay in the onset of depolarization for pulses shorter than the time required for the polarization to set in. A delay of depolarization in a sine electric field was observed to exist within a certain instantaneous fieldstrength interval. A study has also been made of the increase of the depolarization delay induced by simultaneous application of a sine electric field and a dc bias, or of an axial mechanical stress varying in phase with the electric field. The results obtained indicate the onset and persistence of field-induced local ferroelectric regions within a certain interval of instantaneous electric-field strengths. The effect of domain and domain-like processes on these phenomena is discussed.

Dynamics of laser-induced channel formation in water and influence of pulse duration on the ablation of biotissue under water with pulsed erbium-laser radiation

The ability to use fiber-delivered erbium-laser radiation for non-contact arthroscopic meniscectomy in a liquid environment was studied. The laser radiation is transmitted through a water-vapor channel created by the leading part of the laser pulse. The dynamics of the channel formation around a submerged fiber tip was investigated with time-resolved flash photography. Strong pressure transients with amplitudes up to a few hundreds of bars measured with a needle hydrophone were found to accompany the channel formation process. Additional pressure transients in the range of kbars were observed after the laser pulse associated with the collapse of the vapor channel. Transmission measurements revealed that the duration the laser-induced channel stays open, and therefore the energy transmittable through it, is substantially determined by the laser pulse duration. The optimum pulse duration was found to be in the range between 250 and 350 µS. This was confirmed by histological evaluations of the laser incisions in meniscus: Increasing the pulse duration from 300 to 800 µs leads to a decrease in the crater depth from 1600 to 300 µm. A comparison of the histological examination after laser treatment through air and through water gave information on the influence of the vapor channel on the ablation efficiency, the cutting quality and the induced thermal damage in the adjacent tissue. The study shows that the erbium laser combined with an adequate fiber delivery system represents an effective surgical instrument liable to become increasingly accepted in orthopedic surgery.

放電加工における陽極と陰極の除去量の相違に関する研究

The pulse duration of discharge current greatly influences the removal amount of anode and cathode in EDM process. The purpose of this research is to find out the reason for such a phenomenon. At first, the energy distributed into anode and cathode in the case of the single discharge process is obtained by means of measuring the temperature rise and removal amount of electrodes. Then the influence of the carbon adhesion onto the anode surface on the anode removal amount is investigated. It is found that the energy distributed into the anode is more than that into the cathode, and the influence of pulse duration on the energy distribution is small. Besides, it is concluded from the experimental results that the removal amount difference between anode and cathode in the EDM process is mainly caused by carbon adhesion, not by the energy distribution.

Non-invasive waveform measurements of IC-internal GHz signals in A ps time scale

Electron beam testing has recently started to gain importance in GHz integrated-circuit characterization. It competes in this application with several other techniques. The advantages of the e-beam technique are: 1) its flexibility of device operation - pulses, logic signals or sine waves can be input to the device under test and may be changed in frequency between dc and several GHz, 2) its non-loading probe does not affect the function of the device under test, 3) its capability for probing lines below 1 μm. Currently an effective sampling-gate width of 8 ps is achieved, including the influence of pulse duration, timing jitter and transit time effect of secondary electrons. The system bandwidth is therefore approximately 80 GHz. Signal propagation delays of less than 3.5 ps can be resolved. The noise amplitude is 2 mV/√Hz at a 1GHz pulse repetition rate. This allows typical waveforms to be measured within several seconds.

Influence of pulse duration on the spectra of Ga As lasers above threshold

Influence of pulse duration on the spectra of Ga As lasers above threshold