Intense Femtosecond Laser Excitation Research Articles

Third-Order Nonlinear Optical Response near the Plasmon Resonance Band of Cu2-x Se Nanocrystals**Supported by the National Natural Science Foundation of China under Grant Nos 11274302, 11474276 and 11674240.

The third-order nonlinear optical properties of water-soluble Cu2-xSe nanocrystals are studied in the near infrared range of 700–980 nm using a femtosecond pulsed laser by the Z-scan technique. It is observed that the nonlinear optical response of Cu2-xSe nanocrystals is sensitively dependent on the excitation wavelength and exhibits the enhanced nonlinearity compared with other selenides such as ZnSe and CdSe. The W-shaped Z-scan trace, a mixture of the reversed saturated absorption and saturated absorption, is observed near the plasmon resonance band of Cu2-xSe nanocrystals, which is attributed to the state-filling of free carriers generated by copper vacancies (self-doping effect) of Cu2-xSe nanocrystals as well as the hot carrier thermal effect upon intense femtosecond laser excitation. The large nonlinear optical response and tunable plasmonic band make Cu2-xSe nanocrystals promising materials for applications in ultra-fast all-optical switching devices as well as nonlinear nanosensors.

Structure, Reactivity, and Fragmentation of Small Multi-Charged Methane Clusters.

Small methane clusters (CH4)n are irradiated using intense femtosecond laser excitation at 624 nm. The ionized species and those resulting from their fragmentation are detected via time-of-flight mass spectrometry (TOF MS). We find evidence of bound, multiply charged methane molecules and clusters resulting from Coulomb explosion upon exposure to highly energetic, ultrafast radiation. The assignment of the mass spectra is done after first-principles calculations (at the (R)MP2/aug-cc-pVXZ (X = D,T) level) on the charged (CH4)n(q+) clusters (n = 1-4, q = 1-4). We also considered the cluster stabilities and fragments that may result from intracluster molecular reactivity. Complex intracluster ion-molecule reactions induced by photoionization are expected to occur. Interestingly, we show that multi charged small methane clusters undergo intracluster reactions and fragmentations which are different from those observed for isolated methane ions or for large ionized methane clusters.

Spherical gold nanoparticles and core/shell microparticles under intense femtosecond laser excitation: relaxation dynamics of gold nanoparticles and nanostructuring of borosilicate glass using microparticles

This paper reports surface nanostructuring of borosilicate glass covered with a water layer and the production of diameter pits using core/shell microparticles under excitation with pulses using the optical scheme of an inverted microscope with a objective. We compare the thresholds for hole formation in glass with the use of and uncoated microparticles. The threshold is for and for microparticles, which coincides with the threshold for nanostructuring by a focused femtosecond pulse without microparticles: . Femtosecond pump – probe spectroscopy has been used to study the relaxation dynamics of laser pulse energy absorbed in a Au nanoparticle and the dynamics of energy dissipation to the ambient medium. The threshold for cavitation bubble formation in water with has been determined to be , which is a factor of 30 lower than the bubble formation threshold in the case of uncoated microparticles.

Ultrafast dynamics of the dielectric functions of ZnO and BaTiO3 thin films after intense femtosecond laser excitation

The ultrafast carrier dynamics of epitaxial ZnO and BaTiO3 thin films after intense excitation at 3.10 eV and 4.66 eV photon energy has been studied by femtosecond absorption spectroscopy. Modelling the transient transmission changes on the basis of spectroscopic ellipsometry data and pertinent equilibrium model dielectric functions extended by additional terms for the effects at high carrier density (P-band luminescence and stimulated emission from electron-hole-plasma), a self-consistent parameterized description was obtained for both materials. Excited carrier lifetimes in the range of ≈2 to ≈60 ps and long-lived thermal effects after several hundred ps have been identified in both materials. These findings form a reliable basis to quantitatively describe future femtosecond studies on ZnO/BaTiO3 heterolayer systems.

Energy splitting of CdSe quantum dots induced by intense femtosecond laser excitation

Microscopic photoluminescence (PL) spectra of self-assembled CdSe quantum dots (QDs) grown by molecular beam epitaxy were investigated under excitation of intense femtosecond laser. Two samples with different QD sizes were fabricated. One had a single layer of larger CdSe QDs while the other had three layers of smaller QDs. The second harmonic radiation at 420 nm obtained from a mode-locked tunable Ti-Sapphire laser was used as the excitation source. The laser power density was in the order of kW cm−2 and the peak power density was in the order of GW cm−2 for the 150 fs laser pulse with a repetition rate of 78 MHz. The intense femtosecond laser pulses generated strong surface acoustic waves and modulated energy bands of electrons and holes of CdSe QDs. Increasing of the laser power resulted in the PL peak of the CdSe QDs splitting into four peaks for both QD samples: two peaks shifted to a lower energy side and the other two shifted to a higher energy side. The strong strain fields led to the mixing of heavy-hole state and light-hole state in the quantum dots. The strain fields further modulated the energy bands of electrons and holes and produced splitting of both electron–heavy hole (e-hh) transition and electron–light hole (e-lh) transition. For the sample with a single layer of smaller QDs, the energy splitting for both e-hh and e-lh transitions reached 23.5 meV at a peak power density of 0.32 GW cm−2. For the sample with three layers of larger QDs, the energy splitting was 19.9 meV for e-hh transition and 17.9 meV for e-lh transition at a peak power of 1.1 GW cm−2.

Oxygen-containing gas-phase diatomic trications and tetracations: ReOz+, NbOz+ and HfOz+ (z = 3, 4)

Three oxygen-containing gas-phase diatomic trications ReO(3+), NbO(3+) and HfO(3+) as well as the diatomic tetracation NbO(4+) have been observed by mass spectrometry at non-integer m/z values. These unusual triply charged molecular ion species, together with the corresponding diatomic dications ReO(2+), NbO(2+) and HfO(2+), were produced by energetic, high-current oxygen ((16)O(-)) ion beam sputtering of rhenium, niobium and hafnium metal samples, respectively, whose surfaces were dynamically oxidized by oxygen primary ion incorporation. In addition, NbO(z+) (z≤ 4) were generated by intense femtosecond laser excitation and photofragmentation (Coulomb explosion) of Nb(x)O(y) clusters and were detected through Time-of-Flight Mass Spectrometry (TOF). Our experimental results confirm previous reports on the detection of NbO(4+), NbO(3+), NbO(2+), HfO(3+) and HfO(2+) with Atom Probe mass spectrometry, whereas ReO(3+) and ReO(2+) apparently had not been observed before. In addition, these multiply charged molecular ions have been studied theoretically for the first time. Ab initio calculations of their electronic structures show that the diatomic trications ReO(3+), NbO(3+) and HfO(3+) are long-lived metastable gas-phase species, with bond lengths of 1.61 Å, 1.62 Å and 1.86 Å, respectively. They present large potential barriers with respect to dissociation of more than 2.7 eV. The corresponding diatomic dications are thermochemically stable molecules with very large dissociation energies (>3.5 eV). Our calculations predict the diatomic tetracation ReO(4+) to be a metastable ion species in the gas phase. We compute a potential barrier toward fragmentation of 0.6 eV; its formation requires a quadruple adiabatic ionization energy of 85.7 eV. Even though our calculations show that NbO(4+) is a weakly bound (dissociation barrier ∼0.1 eV) metastable molecule, it is here identified via linear time-of-flight mass spectrometry.

Three-photon absorption saturation in ZnO and ZnS crystals

We reported the observation of the saturation of interband three-photon absorption (3PA) in wide-gap semiconductors under intense femtosecond laser excitation. Theoretically, we developed a 3PA saturation model that is in agreement with the Z-scan experimental results. The characteristic saturation intensities were determined to be 44 and 210GW∕cm2 for ZnO and ZnS crystals, respectively. The 3PA saturation model is also consistent with the ultrafast dynamics of 3PA-generated charge carriers in ZnO and ZnS crystals, obtained from the femtosecond transient absorption measurements.

Yellow-green upconversion luminescence of Dy3+ ion in LiNbO3 crystal heavily codoped with ZnO

The observed Dy3+ ion upconversion luminescence in LiNbO3 crystal heavily codoped with ZnO was spectrally and temporally analyzed by microsecond time-resolved spectrum under 806nm intense femtosecond laser excitation at room temperature. Absorption spectrum and modified Judd-Ofelt approach were used to investigate its spectroscopic properties. The bright blue and intense yellow emissions are assigned to transitions F9∕24→H15∕26 and F9∕24→H13∕26, respectively. It is concluded that the main upconversion mechanism is excited state absorption by pump power dependence in combination with luminescence intensity temporal evolvements.

Ultrafast Bond Softening in Bismuth: Mapping a Solid's Interatomic Potential with X-rays

Intense femtosecond laser excitation can produce transient states of matter that would otherwise be inaccessible to laboratory investigation. At high excitation densities, the interatomic forces that bind solids and determine many of their properties can be substantially altered. Here, we present the detailed mapping of the carrier density-dependent interatomic potential of bismuth approaching a solid-solid phase transition. Our experiments combine stroboscopic techniques that use a high-brightness linear electron accelerator-based x-ray source with pulse-by-pulse timing reconstruction for femtosecond resolution, allowing quantitative characterization of the interatomic potential energy surface of the highly excited solid.

Multiphoton route to ZnO nanowire lasers

With intense femtosecond laser excitation, multiphoton absorption-induced stimulated emission and laser emission in ZnO bulk crystal and nanowires have been demonstrated at room temperature. UV-stimulated emission peaks appeared in both bulk crystal and nanowires when the excitation exceeded certain thresholds, and a sharp lasing peak with a linewidth of ~0.5 nm was observed from ZnO nanowires. The emission properties were attributed to the band-edge emission of the recombination of carriers excited by two- and three-photon absorption processes in the wide-bandgap semiconductor.

Observation of Interband Two-Photon Absorption Saturation in CdS Nanocrystals

We report the observation of interband two-photon absorption (TPA) saturation in cadmium sulfide nanocrystals (CdS NCs) under intense femtosecond laser excitation with 1.6 eV photon energy. The observation has been compared to interband TPA saturation in bulk CdS under the same experimental conditions. By using both Z-scan techniques and transient absorption measurements, the saturation intensity has been determined to be 190 GW/cm2 for CdS NCs of 4-nm diameter, which is 2 orders of magnitude greater than that for CdS bulk crystal. The results are in agreement with an inhomogeneously broadened, saturated TPA model.

Concerted elimination of CH 2I 2 and CH 2ICl under intense femtosecond laser excitation

The interaction of CH 2I 2 and CH 2ICl with 60 fs laser pulses at 800 and 400 nm has been studied using TOF-MS. Molecular detachment of I 2 + and ClI + are observed. The generation of I 2 + and ClI + can be understood by a concerted elimination mechanism. The kinetic energy released is estimated from the peak splitting of I 2+ obtained in the TOF-MS, originated from Coulomb explosion. According to the results, multiphoton ionization is excluded from the possible mechanisms. The observed phenomena are attributed to the interaction of the intense laser field with the molecular potential energy surfaces.

Cooperative Photochemical Reaction in Molecular Crystal Induced by Intense Femtosecond Laser Excitation: Photochromism of Spironaphthooxazine

Photochromism of indolinospironaphthooxazine (SNO) microcrystalline powder was investigated by steady state and time-resolved diffuse reflectance spectroscopy using a femtosecond laser as a light source. When laser intensity is weak, the photoinduced ring-opening reaction in a picosecond time region and subsequent thermal ring-closure one in a nanosecond scale take place in the crystal, not leading to permanent photocoloration of the powder. The same sample shows photochromism upon intense femtosecond laser excitation. The absorption spectra after laser excitation are similar to those of the photocolored form in solution. The spectral shape is independent of the laser fluence, whereas the yield increases nonlinearly with the fluence. To reveal the photocoloration mechanism, we examined the excitation fluence dependence of the transient absorption spectra and elucidated the photocoloration behavior by femtosecond double pulse excitation varying the delay of the two pulses. These experimental results show t...