Pump Laser Energy Research Articles

Förster energy-transfer signatures in optically driven quantum dot molecules

eld dependent opti- cal spectrum of realistic QDMs. In Sec. II the model for the QDM is introduced. An excitonic population map, as function of applied bias and pump laser energy is constructed, resembling the dressed LACS spectra of the QDM. Our main results are presented for two dif- ferent cases. In Sec. III we consider the LACS for a QDM dressed spectrum involving Forster coupling of di- rect single excitons. We show how the coupling is man- ifested in the variation of the amplitude (brightness) of a FRET split satellite as a function of the applied elec- tric

Cascade processes in inelastic light scattering in ZnSe nanowire structures

The resonance Raman scattering of light in MBE-grown structures with ZnSe nanowires (10–20 nm in diameter) with an Au film deposited on the substrate used as a catalyst was investigated. The thicknesses of the Au layers were 2, 10, and 100 A. The photon energy of the He-Cd pump laser (λ = 441.6 nm) was in excess of the band gap of bulk ZnSe, and the measurements were conducted at room temperature. Under these conditions, the Raman spectra are defined by a cascade process in which the electron interacting with a longitudinal optical phonon transfers between real band states with a certain probability of radiative recombination at each step. The blue shift of the luminescence maximum associated with the quantum confinement of carriers in the nanowire has been observed. The average nanowire diameter derived from the magnitude of this shift agrees well with electron microscopy measurements.

Strong terahertz radiation from air plasmas generated by an aperture-limited Gaussian pump laser beam

Terahertz radiation generated by focusing the fundamental laser pulse and its second harmonic into ambient air strongly saturates with increasing pump laser energy. We demonstrate a simple method to control the Gaussian pump laser beam to improve the output of terahertz radiation with an adjustable aperture. With the optimal aperture-limited pump laser beams, the terahertz wave amplitudes can be enhanced by more than eight times depending on the pump laser parameters than those of aperture-free cases.

Time resolved resonance Raman, transient diffuse reflectance and kinetic studies of species generated by UV laser photolysis of biphenyl occluded within dehydrated Y-faujasite zeolites

Transient diffuse reflectance UV–vis absorption (TDRUV) and time resolved resonance Raman (TR 3) spectroscopy (370 nm exciting laser line, 8 ns) were performed to investigate the species generated by UV pulsed laser photolysis (248 nm, 20 ns) of biphenyl (BP) occluded in cavities of dehydrated silica-rich Y-faujasitic (M n FAU) zeolites with M n (AlO 2) n (SiO 2) 192− n formulae per unit cell. The TDRUV spectra were recorded in the time range 0.5–340 μs and the TR 3 spectra were recorded in the 0.05–100 μs as functions of aluminum content ( n = 0, 56), extra framework cations (M = Na +, K +, Cs +), BP loading (1, 2, 4, 8, 16 BP/UC) and laser energy (0.15–1.5 mJ). Specific spectra of BP(T 1) triplet state, BP + radical cation, trapped electron as Na 4 3+ cluster and BP − radical anion were resolved by Multivariate curve resolution (MCR) of TDRUV and TR 3 data sets. The RR spectra of BP + and BP(T 1) correspond to nearly planar structures. The concentration decays fit a model of dispersed heterogeneous kinetics. At lower laser energy, BP(T 1) was generated as major transient species. The energy transfer via T 1 occurs with rates not greater than 10 −6 s −1. Photoionization is found to be the dominant phenomenon at higher pump laser energy. BP +, Na 4 3+ and BP − exhibit distinct decay behaviors with rates not lower than 10 −3 s −1. In polar environment of Na 56FAU supercage, the ejected electron is trapped as Na 4 3+ cluster at low loading and as BP − at high loading. In the non-polar environment of FAU supercage, the ejected electron is captured by BP(S 0) even at low loading. The effects of Si/Al ratio, extraframework cation and BP loading upon the energy and electron transfer rates were discussed according to BP photochemical behavior in solvents.

Two-photon pumping of random lasers by picosecond and nanosecond lasers

We experimentally demonstrated two-photon pumping of random lasers using picosecond and nanosecond pump lasers. The picosecond laser pumping experiment was performed with 400 ps laser pulses at 770 nm, and the gain media was a Coumarin 480D dye solution doped with TiO2 nanoparticles. Onset of laser action was observed at a pump laser pulse energy below 500 μJ. The nanosecond laser pumping experiment was performed with 7 ns laser pulses at 1064 nm, and the gain media was a Rhodamine 640 dye solution doped with TiO2 nanoparticles. Onset of laser action was observed at a pump laser energy ∼18 mJ. Our results suggest that there exists an optimal pulse duration of the pumping laser in two-photon pumped random lasing that leads to minimum photodamage of the gain media and still keeps a high pumping efficiency.

Effect of Thermal Defocusing on Backward Stimulated Raman Scattering in CH4

The conversion efficiency of stimulated Raman scattering (SRS) in CH4 is studied by using a single longitudinal mode second-harmonic Nd:YAG laser (532 nm, linewidth 0.003 cm1, pulse-width (FWHM) 6.5 ns). Due to the heat release from vibrationally excited particles, SRS processes often suffer from the thermal defocusing effect (TDE). In view of 6.5 ns laser pulse width is much shorter than the vibrational relaxation time of CH4 molecules, TDE can only affect the SRS processes afterwards. In the cases of low laser repetition, TDE will be not serious, because it will be removed by the thermal diffusion in Raman medium before the next pulse arrives. At the laser repetition rate 2 Hz, CH4 pressure 1.1 MPa and pump laser energy 95 mJ, the quantum conversion efficiency of backward first-Stokes (BS1) has attained 73%. This represents the highest first-stokes conversion efficiency in CH4. Furthermore, due to the relaxation oscillation, the BS1 pulses are narrowed to about 1.2 ns. As a result, the BS1 peak power turns out to be 2.7 times that of the pump. Its beam quality is also much better and is only slightly affected by TDE. This reason is that BS1 represents a wave-front-reversed replica of the pump beam, which can compensate the thermal distortions in Raman amplify process. Under the same conditions, but pump laser repetition rate as 10 Hz, the conversion efficiency of BS1 goes down to 36% due to TDE. From this study, we expect that a well-behaved 630 nm Raman laser may be designed by using a closed CH4/He circulating-cooling system, which may have some important applications.

Experiments and Simulations of Short-Pulse Laser-Pumped Extreme Ultraviolet Lasers

Recent experimental work on the development of extreme ultraviolet lasers undertaken using as the pumping source the VULCAN laser at the Rutherford Appleton Laboratory is compared to detailed simulations. It is shown that short duration (/spl sim/picosecond) pumping can produce X-ray laser pulses of a few picosecond duration and that measurement of the emission from the plasma can give an estimate of the duration of the gain coefficient. The Ehybrid fluid and atomic physics code developed at the University of York is used to simulate X-ray laser gain and plasma emission. Two postprocessors to the Ehybrid code are utilized: 1) to raytrace the X-ray laser beam amplification and refraction and 2) to calculate the radiation emission in the kiloelectronvolt photon energy range. The raytracing and spectral simulations are compared, respectively, to measured X-ray laser output and the output of two diagnostics recording transverse X-ray emission. The pumping laser energy absorbed in the plasma is examined by comparing the simulations to experimental results. It is shown that at high pumping irradiance (>10/sup 15/ Wcm/sup -2/), fast electrons are produced by parametric processes in the preformed long scalelength plasmas. These fast electrons do not pump the population inversion and so pumping efficiency is reduced at high irradiance.

Study of diode-laser pumped self-Q-switched intracavity-doubled laser of <inline-formula><math display="inline" overflow="scroll"><msup><mrow><mi>Cr</mi></mrow><mrow><mn>4</mn><mo>+</mo></mrow></msup><msup><mrow><mi>Nd</mi></mrow><mrow><mn>3</mn><mo>+</mo></mrow></msup><mo>:</mo><mi>YAG</mi></math></inline-formula> plus <inline-formula><math display="inline" overflow="scroll"><msub><mi>KNbO</mi><mrow><mn>3</mn></mrow></msub></math></inline-formula> material

A diode-laser pumped blue intracavity frequency-doubled self-Q-switched microchip laser of a chromium Cr4+ Neodymium Nd3+ codoped yttrium aluminum garnet crystal (Cr4+Nd3+:YAG) combined with a potassium niobate (KNbO3) crystal is developed. By coating the cavity mirrors with the films that suppress the 1064-nm operation and enhance the 946-nm laser, the 4F3/24I9/2 transition of the Nd3+ ion is facilitated to achieve the 946-nm laser oscillation. The 946-nm laser of the Cr4+Nd3+:YAG is self-Q-switched due to the saturable absorption of the Cr4+ ion. The full width at half maximum (FWHM) of the laser pulse at a 964-nm wavelength is about 5.07 ns. A self-Q-switched 473-nm laser pulse is sequentially obtained through intracavity frequency doubling of the 946-nm laser by the KNbO3 crystal, whose FWHM is about 4.30 ns. The 473-nm intracavity doubled laser has a good fundamental transverse TEM00 mode, because the self-Q-switched 946-nm laser has a good TEM00 mode that results from the absorption bleaching established by both the 808-nm pump laser and 946-nm oscillating laser. The constant FWHM results from both the response time of the self-Q-switching and the establishing time of the oscillating laser being much faster than the accumulated time of the pump laser energy. The constant peak power of the 946-nm self-Q-switched laser mainly depends on the modulation ability of the self-Q-switching.

Comparison of Tm : ZBLAN and Tm : silica fiber lasers; Spectroscopy and tunable pulsed laser operation around 1.9 μm

Tm-doped ZBLAN and Tm-doped silica glass are compared spectroscopically and the fiber lasing of the Tm 3 F 4 →3 H 6 transition around 1.9 μm in ZBLAN and silica fibers is compared. The spectroscopy of these materials indicates that Tm:ZBLAN possesses advantages over Tm:silica glass due to the lower phonon energies. The phonon energy in these glass hosts influences both the pump manifold lifetime, the Tm 3 H 4, and the upper laser manifold lifetime, the Tm 3 F 4. The maximum phonon energy in Tm:ZBLAN, ∼500 cm-1 , compared to Tm:silica, ∼1100 cm-1, leads to better Tm–Tm self quenching towards populating the Tm 3 F 4, as well as better Tm 3 F 4→3 H 6 quantum efficiency. A spectroscopic analysis using the Judd–Ofelt theory and measured lifetimes are used to assess the merits of Tm:ZBLAN over Tm:silica as a fiber laser material. Diode-pumped fiber lasing experiments show that Tm:ZBLAN possesses advantages over Tm:silica that are believed to be due to a lower phonon energy. Data is presented for launched pump energy versus laser energy, fiber length versus slope efficiency, and output mirror reflectivity versus slope efficiency. Tm:ZBLAN is demonstrated to possess higher slope efficiencies and lower thresholds, than Tm:silicate. A grating tuned Tm:ZBLAN laser is also demonstrated for tunable operation between 1.893 μm and 1.955 μm.

Generation of UV laser light by stimulated Raman scattering in D 2 , D 2 /Ar and D 2 /He using a pulsed Nd:YAG laser at 355nm

A pulsed Nd:YAG laser at 355nm is used to pump Raman cell filled with D2, D2/Ar and D2/He. With adequately adjusted parameters, the maximum photon conversion efficiency of the first-order Stokes light (S1, 396.796nm) reaches 33.33% in D2/Ar and the stability of S1 in pure D2 is fairly high, the energy drift being less than 10% when the pump energy drifts in the range of 5%. The conversion efficiency and stability, which are functions of the composition and pressure of the Raman medium and the energy of pump laser, are investigated. The result has been used to optimize the laser transmitter system for a differential absorption lidar system to measure NO2 concentration profiles.

Investigation of stimulated Raman scattering of ν1 and ν2 modes in CH 4

Using 355 nm excitation laser radiation, the stimulated Raman scattering (SRS) lines of methane between 14 000 and 26 000 cm −1 at different experimental conditions were recorded. Two sets of successive SRS lines manifesting the Raman shift in the ν 1 and ν 2 modes of vibration in the 1A 1 electronic state of CH 4 have been assigned. The evolution of SRS lines at different pressures has been investigated. The energy dependence of SRS lines has revealed that SRS line due to higher order components become stronger than the fundamental at higher laser pump energy.

Highly directive 18.9 nm nickel-like molybdenum x-ray laser operating at 150 mJ pump energy.

We experimentally demonstrate that by longitudinally pumping 2 mm long molybdenum preformed plasma with high-intensity 475 fs duration laser pulse, a highly directive soft-x-ray laser at 18.9 nm wavelength is generated. The divergence of the beam is evaluated to be of the submilliradian order, and only requires a pump laser energy of 150 mJ.

Studies of advanced soft X-ray laser and coherent higher harmonic generation using intense subpicosecond laser produced high density plasma

Current activities on our research of soft X-ray lasers and higher harmonic generations using intense, subpicosecond lasers are reviewed. Especially for soft X-ray lasers we experimentally demonstrate that by longitudinally pumping 2-mm-long molybdenum preformed plasma with high-intensity 475 fs duration laser pulse, a highly directive soft-X-ray laser at 18.9 nm wavelength is generated. The divergence of the beam is evaluated to be of the submilliradian order, and only requires pump laser energy of 150 mJ . Simulations show that the pedestal in the main pump pulse can generate electron density and gain profiles with large spatial gradients, which result in the selective amplification of low-order transverse modes. The present result is the demonstration of an efficient and alternative method of improving the spatial coherence of X-ray lasers with amplified spontaneous emission medium, with possibilities of becoming an excellent tool to explore various application experiments. In higher harmonic generation, topics related to observation of blue shift due to collisionless absorption process is described.

Absorption and emission dynamics in concentrated optical ensembles under laser excitation

A new theoretical model describing the emission and absorption dynamics in an ensemble of molecules under intense coherent pulsed pumping is developed on the basis of the concepts of cooperative light-induced luminescence (CLIL). The CLIL development is described within the framework of formalism of the system density matrix in the space of photon wave functions. It is shown that the fast growth of CLIL relates to the development of coherent states of the quantum field in the area of efficient cooperative interactions of molecules (coherence volume). A system of equations for the calculation of CLIL energy, population of excited states, and optical absorption of the system in dependence on the laser pump energy density is solved. The theoretical results obtained are in good agreement with the experimental data.

Pump Angle and Laser Energy Dependence of Stimulated Scattering in Microcavities

We show that stimulated polariton scattering in a semiconductor microcavity is observable over a wide range of laser pump angles. The ratio of idler to signal beams is found to vary strongly with angle, consistent with the expected variation of the photon fraction of the idler states, and the increasing probability of scattering of idler polaritons to the exciton reservoir with increasing wave-vector. The observed variation of threshold with angle is in good agreement with a semi-classical treatment of the parametric scattering process.

Pump Angle and Laser Energy Dependence of Stimulated Scattering in Microcavities

Pump Angle and Laser Energy Dependence of Stimulated Scattering in Microcavities

Analyses of the short pulse laser pumped transient collisional excited X-ray lasers

The soft x-ray gain of the transient collisional excited (TCE) Ni-like Ag laser is investigated using the plasma hydrodynamics and atomic kinetics codes. The gain is calculated for a plasma produced from two 100ps laser irradiated solid target to show qualitative agreement with the experiment. The calculation shows significant improvement of the gain using a thin foil target pumped by two short laser pulses, because of a better coupling of the pump laser energy into the gain region of the plasma. The codes will provide performance prediction as well as optimization of the experimental studies of the TCE x-ray lasers.

Room temperature adsorption of Na on Si(1 1 1)-7×7 as studied by resonant optical second harmonic generation

Resonantly enhanced optical second harmonic generation at a pump laser energy of 1.17 eV was applied to investigate the Na-adsorption process on the 7×7 reconstructed Si(1 1 1) surface at room temperature. The change of second harmonic (SH) intensity as a result of Na deposition at normal incident pump radiation varies significantly from that measured at 45° angle of incidence. The observed difference can be explained in terms of electronic transitions from the rest-atom state to the adatom state of the 7×7 reconstructed Si(1 1 1) surface as well as of the features of Na growth on the silicon surface. By measuring the SH polarization dependence for different stages of Na deposition, a change of the interface symmetry in both directions, namely, parallel and perpendicular to the (1 1 1) plane, was clearly observed at the very beginning of the adsorption process.

Nonlinear Optical Response of Colloidal Gold Nanoparticles Studied by Hyper-Rayleigh Scattering Technique

Abstract Hyper-Rayleigh scattering (HRS) technique is used to study the nonlinear optical response of nanocrystalline gold particle suspensions. The signal intensity measured at 400 nm has a cubic dependence on the pump laser energy at 1064 nm, indicating that the measured signal is due to a third-order nonlinear optical process, namely three-photon excited fluorescence (3PF). To the best of our knowledge, this is the first experimental observation of 3PF for colloidal gold nanoparticle suspensions.

Investigations into Trace Detection of Nitrocompounds by One- and Two-Color Laser Photofragmentation/Fragment Detection Spectrometry

Trace concentrations of nitrogen dioxide (NO2), nitromethane (CH3NO2), and 2,4,6-trinitrotoluene (TNT) are detected by both one- and two-color laser photofragmentation/fragment detection (PF/FD) spectrometry using one or two lasers. The PF/FD methods studied are (1) one-laser, one-color photofragmentation of the analyte molecule at 227 or 454 nm with subsequent detection of the characteristic nitric oxide (NO) photofragment by one- or two-photon laser-induced fluorescence using its A2Σ+ –X2π (0,0) transitions near 227 nm; (2) one-laser, two-color PF/FD, where a 355 nm laser beam is used for additional analyte photofragmentation and NO is detected by both one- and two-photon LIF as in the previous case; (3) two-laser, two-color PF/FD, where the pump and probe beams are time delayed; and (4) one-laser, one-color PF/FD at 355 nm, where the 355 nm beam photofragments the target molecule and the prompt emission from electronically excited NO (A2Σ+) is monitored in the range of 200–300 nm. PF/FD excitation and emission spectra are recorded and also simulated with the use of a computer program based on a Boltzmann distribution analysis with transition probabilities, rotational energies, and rovibrational temperatures as input parameters. The effects of laser wavelength, laser pump energy, time delay between pump and probe beams, and analyte concentration on PF/FD signal are investigated and reported. Limits of detection [signal-to-noise (S/N) = 3] for the nitrocompounds range from low ppbv to ppmv for 10 s integration time and laser energies of ∼ 5 mJ and 100 μJ for the pump and probe beams, respectively. These results are presented and compared to other PF/FD methods for nitrocompound monitoring.