Ns Time Delay Research Articles

Transient Absorption of N719 and its Electron Transfer Kinetics on ZnO Nanoparticles Surface

The transient absorption and emission spectra of ruthenium complex sensitizer N719 under excitation in different solvents were studied. Isopropanol was found to stabilize the singlet excited state of N719. An emission band centered at 460 nm resulting from the singlet excited state of N719 was observed at 1 ns time delay, which is much longer than the reported observation time of the singlet excited state of N719. The triplet excited state of N719 undergoes photophysical decay in acetonitile and isopropanol with lifetimes of around 40 ns, while it may encounter photochemical reactions in water resulting in long living intermediate. The sensitizer was introduced to the surface of three types of ZnO nanoparticles with different morphology, which were used as the electron acceptors upon photoexcitation. The electron transfer dynamics between sensitized N719 and ZnO interface both in the presence and absence of electrolyte were studied by time-correlated single photon counting technique, nanosecond transient absorption and emission spectroscopies. It was revealed that the electrolyte has a significant impact upon the electron transfer dynamics at the N719-ZnO interface. In the absence of electrolyte, the electron transfer process at the N719-ZnO interface is dependent on the depth of defects in ZnO nanoparticles. Conversely, in the presence of electrolyte, ZnO defects show no impacts on the electron transfer process at the N719-ZnO interface and effective electron injection happens similarly from the excited N719 to ZnO in spite of ZnO particle sizes.

A SNAP coupled microresonator delay line

A delay line fabricated of a chain of SNAP (Surface Nanoscale Axial Photonics) coupled microresonators is demonstrated. In contrast to resonant delay lines demonstrated to date, the slow light in this structure is enhanced by the 2R (Rotation + Reflection) effect realized due to the 3D propagation of light along the surface of a SNAP fiber. Here, the delay line coupled to a single input/output waveguide (i.e., operating in the reflection mode) is considered. Depending on the coupling parameters and loss, the delay time in this device is either proportional to the density of resonances averaged over the pulse spectrum or tends to zero. The delay line is fabricated of 20 coupled microresonators with the total length of 1.2 mm and footprint area of 0.05 mm2. It exhibits the record low insertion loss (< 3 dB), small speed of light (<c/250), and large (>1 ns) delay time along the 0.1 nm bandwidth achieved for the miniature microresonator delay lines. The feasibility of significant improvement of the SNAP delay line characteristics (larger delay time and bandwidth, smaller losses and dimensions, and anti-reflecting apodization) is discussed.

Laser-driven high-frequency vibrations of metal blister surface

Time-resolved interferometric microscopy was applied to investigate laser-induced blistering of a titanium film on a silica substrate. Ablation of the titanium/silica interface by single 0.7 ns pulses within a certain fluence range results in local exfoliation of the metal film from the substrate avoiding, however, complete film destruction. Time-dependent transformation of the metal surface profile was reconstructed from the interference patterns within 0–13 ns time delay range. Transverse annular waves with typical amplitude of one hundred of nanometers and estimated traveling speed of few kilometers per second were revealed on the blister surface. The wave occurrence was attributed to fast inhomogeneous bending of the film covering the expanding blister. The resultant high-frequency (∼1 GHz) vibrations of the metal surface provide intensive inertial forces when such metalized target is used for blister-based laser-induced forward transfer of nanopowders and organic molecules.

Influence of doping profiles on coherent acoustic phonon detection and generation in semiconductors

The doping profile in different n-doped GaAs homoepitaxial structures grown by molecular beam epitaxy is investigated in the time domain by employing a laser based picosecond ultrasound technique in a contactless and noninvasive way. Experiments based on asynchronous optical sampling employ two femtosecond lasers, which allow us to detect changes in the optical reflectivity over a 1 ns time delay with a signal-to-noise ratio of 107 and 100 fs time resolution in &amp;lt;1 min of acquisition time. We show that the doping profile with doping densities of the order of 1018 cm−3 can be detected with picosecond ultrasound, although there is no difference in the acoustic properties of the doped and undoped region. The detection mechanism is based on a different sensitivity function for a coherent strain pulse in the doped and undoped regions. These results are corroborated by experiments at room temperature and 10 K.

Time-Resolved Shadowgraphs of Material Ejection in Intense Femtosecond Laser Ablation of Aluminum

The dynamic process of intense 50 fs laser ablation of aluminum is investigated by ultrafast time-resolved microscopy. A stripe pattern preceding phase explosion is clearly seen in the shadowgraph of 1 ns time delay. Intermittent material ejections are observed within the ejected plume after 2.5 and 7 ns time delay, respectively, which may be attributed to the material response to the generation of an extremely strong thermoelastic wave. Similar processes are also recorded in the ablation of silicon and glass samples, except for the glass samples, the intermittent material ejections are not found.

Photexcitation of Aqueous Ruthenium(II)-tris-(2,2‘-bipyridine) with High-Intensity Femtosecond Laser Pulses

We report a femtosecond pump-probe study on the photochemistry of concentrated aqueous solutions of [RuII(bpy)3]2+, as a function of pump power (up to 2 TW/cm2) at 400 nm excitation. The transient absorption spectra in the 345-660 nm range up to 1 ns time delay enable the observation of the following photoproducts: the triplet 3MLCT (metal-to-ligand-charge-transfer) excited state, the reduced form [RuII(bpy)3]+, the oxidized species [RuIII(bpy)3]3+, and the solvated electron e(aq). The 3MLCT state is formed within the excitation pulse and undergoes vibrational relaxation in 3-5 ps, as evidenced by the shift of the ligand-centered (LC) absorption band below 400 nm. Even at the highest pump powers, the majority of e(aq) originates from multiphoton ionization of [RuII(bpy)3]2+ and not from the solvent, generating [RuIII(bpy)3]3+ as a byproduct. At 10 ps time delay, the total concentration of the three product species is balanced by the depleted concentration of [RuII(bpy)3]2+, even at the highest fluences used, indicating that no further reaction products significantly contribute to the overall photochemistry. On the 100 ps time scale, most probably diffusion-controlled reduction of ground-state [RuII(bpy)3]2+ by solvated electrons occurs, next to recombination between e(aq) and [RuIII(bpy)3]3+.

Femtosecond time-resolved optical pump-probe spectroscopy at kilohertz-scan-rates over nanosecond-time-delays without mechanical delay line

We demonstrate a technique for femtosecond time-resolved optical pump-probe spectroscopy that allows to scan over a nanosecond time delay at a kilohertz scan rate without mechanical delay line. Two mode-locked femtosecond lasers with approximately 1 GHz repetition rate are linked at a fixed difference frequency of ΔfR=11kHz. One laser delivers the pump pulses, the other provides the probe pulses. The relative time delay is linearly ramped between zero and the inverse laser repetition frequency at a rate ΔfR, enabling high-speed scanning over a 1 ns time delay. The advantages of this method for all-optical pump-probe experiments become evident in an observation of coherent acoustic phonons in a semiconductor superlattice via transient reflectivity changes. A detection shot-noise limited signal resolution of 7×10−8 is obtained with a total measurement time of 250 s. The time resolution is 230 fs.

Protein conformational relaxation and ligand migration in myoglobin: a nanosecond to millisecond molecular movie from time-resolved Laue X-ray diffraction.

A time-resolved Laue X-ray diffraction technique has been used to explore protein relaxation and ligand migration at room temperature following photolysis of a single crystal of carbon monoxymyoglobin. The CO ligand is photodissociated by a 7.5 ns laser pulse, and the subsequent structural changes are probed by 150 ps or 1 micros X-ray pulses at 14 laser/X-ray delay times, ranging from 1 ns to 1.9 ms. Very fast heme and protein relaxation involving the E and F helices is evident from the data at a 1 ns time delay. The photodissociated CO molecules are detected at two locations: at a distal pocket docking site and at the Xe 1 binding site in the proximal pocket. The population by CO of the primary, distal site peaks at a 1 ns time delay and decays to half the peak value in 70 ns. The secondary, proximal docking site reaches its highest occupancy of 20% at approximately 100 ns and has a half-life of approximately 10 micros. At approximately 100 ns, all CO molecules are accounted for within the protein: in one of these two docking sites or bound to the heme. Thereafter, the CO molecules migrate to the solvent from which they rebind to deoxymyoglobin in a bimolecular process with a second-order rate coefficient of 4.5 x 10(5) M(-1) s(-1). Our results also demonstrate that structural changes as small as 0.2 A and populations of CO docking sites of 10% can be detected by time-resolved X-ray diffraction.

M.S.W. nondispersive, electronically tunable time delay elements

A nondispersive, electronically tunable (165-180 ns) time delay unit based on magnetostatic wave (m.s.w.) propagation with 6% bandwidth at 3 GHz has been demonstrated. The unit is implemented by cascading magnetostatic surface wave and magnetostatic backward volume wave delay lines designed so that the dispersion slopes are equal and opposite in slope over the common operating bandwidth; this produces an element with constant time delay over the bandwidth which can be adjusted by a bias field variation of one of the lines.