Pump Power Density Research Articles

Visible upconversion luminescence of Tb 3+ ions in Y 2O 3 nanoparticles induced by a near-infrared femtosecond laser

Y 2O 3:Tb 3+ nanoparticles were synthesized using Pechini-type sol–gel method, and characterized by TEM, XRD, and Raman spectra. Their spectroscopic properties are investigated using optical absorption, spectral- and time-resolved upconversion emission spectra. The dependence of Tb 3+ emission intensity on pump power density reveals that it is a three-photon excitation process. These results above indicate that upconversion excitation practically originates from the parity-allowed 4 f–5 d transition via a three-photon simultaneous absorption of Tb 3+ ions. The upconversion luminescence intensities of Y 2O 3:Tb 3+ nanoparticles decrease with Tb 3+ doping concentrations in the range of 1∼4 mol%. The measured average lifetimes of the 5D 4 level of Tb 3+ ions also decrease with increasing Tb 3+ concentrations at room temperature.

Optimization of electron-beam sustained discharge laser electrodes system

One of the basic problems at creation of powerful electron-beam sustained discharge lasers is achievement of the greatest possible intensity of an electric field in the charge volume limited to the aperture of the resonator and uniformity of power pumping on this volume. In our opinion known types of electrodes, for example,, etc. are not optimum, and the recommendations hallmarking a ratio of electrodes width to interelectrodes distance as 4:1, are a little bit overestimated.It is shown, what even at high uniformity of an electric field in the interelectrodes volume covered with the optical resonator, heterogeneity of pumping power density in cross section of the charge can reach 100%.

Pump and amplification dynamics of gamma rays in a nuclear medium with the hidden population inversion

The features of the pump dynamics of isomeric nuclei excited by X-rays of a repetitively pulsed relativistic electron beam followed by the production of a medium with the negative absorption of gamma quanta are analysed. In the extended nuclear medium, the pump excites a travelling hidden-population-inversion wave with the anisotropic gamma amplification, which becomes positive in the case of the excess over the critical pump parameter equal to the product of the peak spectral power density of the X-ray source and the relative duration of an ultrashort relativistic electron bunch. In the alternative scheme with orthogonal directions of pumping X-rays and a flux of amplified gamma quanta, the absence of the amplification anisotropy opens up the possibility for constructing a standard two-mirror resonator with Bragg single-crystal reflectors. The critical peak value of the spectral pump power density is compared with the known characteristics of relativistic-electron X-ray sources by examples of some nuclides.

Measurement of stimulated Brillouin scattering (SBS) threshold based on waveform variation of SBS optical limiting

This paper proposes a method for measuring the stimulated Brillouin scattering (SBS) threshold based on waveform variation of SBS optical limiting. The output waveforms for different pump power densities are numerically simulated, and validated in the Nd:YAG seed-injected laser system. The results indicate that SBS does not take place in the case of a low pump power density and thus the output power scales up linearly with pump power. Once the pump power density exceeds the SBS threshold, SBS takes place and thereby the energies are transferred from pump to Stokes. As a result, a small shoulder appears in the trailing edge of the output waveform, which provides another method to determine the SBS threshold.

Effect of excitation power density on the upconversion luminescence of LaF 3:Yb 3+, Er 3+ nanocrystals

The upconversion fluorescent Yb 3+, Er 3+ ions co-doped LaF 3 nanocrystals were synthesized and studied. The nanocrystals display green and red visible emissions under 980 nm near infrared excitation, which is suitable for biological application. The effect of excitation power density on the upconversion luminescence properties of nanocrystals are investigated detailed. Under lower pump power density irradiation, the upconversion luminescence shows a typical two photon involved process. When the pump power density turns to higher region, the dependence of upconversion emission intensity versus the excitation power density exhibits as the hysteresis loops which does not accord with the normal two-photon process. This phenomenon may be caused by the thermal effect which can increase the temperature of nanocrystals, and then enhance the visible emissions greatly.

High repetition rate laser pulses amplified by Nd/Cr:YAG ceramic amplifier under CW arc-lamp-light pumping

Laser pulses with high repetitive rate generated from a Q-switch microchip Nd:YAG oscillator were amplified by Active mirror composed of Nd/Cr:YAG ceramic pumped by CW arc-lamp. The laser pulses were amplified, and saturation of averaged output laser power was observed. Repetitive ratio of injected laser pulses was changed from 20 to 100 kHz. Averaged output laser power and gain were measured, and gain coefficient and power-extraction efficiency were evaluated. Output laser power was calculated and compared to experimentally measured one, and the calculated results are consistent with the experimental ones. For amplification of laser pulses with high repetitive ratio, this laser material can realize very high laser gain with low pumping power density and high optical–optical conversion efficiency under CW-lamp-light pumping.

Coulomb effects in type‐II Ga(As)Sb quantum dots

AbstractWe present a detailed explanation of the blue‐shift that is observed in Ga(As)Sb quantum dots (QD) with increasing pump power density by employing an 8‐band k · p model with the Coulomb attractive potential added into the Hamiltonian. Two types of structures are considered: Ga(As)Sb QDs in a GaAs matrix and the same dots overgrown with an InGaAs quantum well (QW). We show that with increasing charge density the electron and hole energy levels shift relative to each other, which results in an increase of the optical transition energy (i.e. a blue‐shift). The major contribution to the effect arises from the holes and is a result of their 3D confinement inside the dot. Additionally, with increasing charge density the electrons move closer to the dot, which improves the oscillator strength of the ground‐state optical transition. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Power scaling of Nd:YVO_4 and Nd:GdVO_4 disk lasers using synthetic diamond as a heat spreader

A newly developed low-birefringence synthetic diamond is shown to be an effective intracavity heat spreader in Nd:YVO4 and Nd:GdVO4 disk lasers. A cw output power of 25.7 W from only one double pass of the pump is reported. The diamond heat spreader is shown to increase the pump power density at which fracture occurs.

Defect-selective photothermal imaging: a sensitive tool for damage mapping in optical coatings

A defect-selective photothermal imaging system for the diagnostics of optical coatings is demonstrated. The instrument has been optimized for pump and probe parameters, detector performance, and signal processing algorithm. The imager is capable of mapping purely optical or thermal defects efficiently in coatings of low damage threshold and low absorbance. Detailed mapping of minor inhomogeneities at low pump power has been achieved through the simultaneous action of a low-noise fiber optic photothermal beam defection sensor and a common-mode-rejection demodulation (CMRD) technique. The linearity and sensitivity of the sensor have been examined theoretically and experimentally, and the signal to noise ratio improvement factor is found to be about 110 compared to a conventional bicell photodiode. The scanner is so designed that mapping of static or shock sensitive samples is possible. In the case of a sample with absolute absorptance of 3.8 × 10−4, a change in absorptance of about 0.005 × 10−4 has been detected without ambiguity, ensuring a contrast parameter of 760. This is about 1085% improvement over the conventional approach containing a bicell photodiode, at the same pump power. The merits of the system have been demonstrated by mapping two intentionally created damage sites in a MgF2 coating on fused silica at different excitation powers. Amplitude and phase maps were recorded for thermally thin and thick cases, and the results are compared to demonstrate a case which, in conventional imaging, would lead to a deceptive conclusion regarding the type and location of the damage. Also, a residual damage profile created by long term irradiation with high pump power density has been depicted.

Violet-light spontaneous and stimulated emission from ultrathin In-rich InGaN/GaN multiple quantum wells grown by metalorganic chemical vapor deposition

We investigated the spontaneous and stimulated emission properties of violet-light-emitting ultrathin In-rich InGaN/GaN multiple quantum wells (MQWs) with indium content of 60%–70%. The Stokes shift was smaller than that of In-poor InGaN MQWs, and the emission peak position at 3.196 eV was kept constant with increasing pumping power, indicating negligible quantum confined Stark effect in ultrathin In-rich InGaN MQWs despite of high indium content. Optically pumped stimulated emission performed at room temperature was observed at 3.21 eV, the high-energy side of spontaneous emission, when the pumping power density exceeds ∼31 kW/cm2.

Effective Fluorescence Lifetime and Stimulated Emission Cross-Section of Nd/Cr:YAG Ceramics under CW Lamplight Pumping

Remarkable improvements in the lifetime of the Nd upper level and in the effective stimulated emission cross-section of Nd/Cr:YAG ceramics have been theoretically and experimentally studied. Until recently, it had been thought that the long energy transition time from Cr ions to Nd ions of Nd/Cr:YAG adversely affects laser action, degrading optical–optical conversion efficiency under CW and flash lamp pumping. However, current research showed that high-efficiency energy transition has a positive effect on laser action. The effective lifetime is increased from 0.23 to 1.1 ms and the emission cross-section is effectively increased to three times for that of the conventional Nd:YAG. A small signal gain is significantly improved, and the saturation power density is reduced to 1/10 that of the Nd:YAG for the same pumping power density. A CW laser light generated in a laser diode (LD)-pumped 1064 nm Nd:YAG laser oscillator was amplified, and the measured output power was saturated. The output laser power calculated using theoretical saturation power density was consistent with the experimental results.

Temperature-stable operation of a quantum dot semiconductor disk laser

We demonstrate temperature-independent output characteristics of an optically pumped semiconductor disk laser (SDL) based on quantum dots (QDs) grown in the Stranski-Krastanow regime. The gain structure consists of a stack of 7×3 QD layers, each threefold group being located at an optical antinode position. The SDL emits at 1210nm independent of the pump power density. Threshold and differential efficiency do not dependent on heat sink temperature. Continuous-wave operation close to 300mW output power is achieved using the ground-state transition of the InGaAs QDs.

Nuclear-pumped laser operating in the master oscillator—amplifier regime

The efficiency of a master oscillator—amplifier scheme and the possibility of its using in multichannel nuclear-pumped laser systems are studied. The small-signal gain and saturation parameter are measured for the He:Ar:Xe = 380:380:1 mixture at a pressure of 1 atm at a wavelength of 2.03 μm. It is shown that the small-signal gain increases linearly with the specific pump power density and achieves 1.1—1.2 m-1 at a pump density of 40 W cm-3. The saturation parameter is almost independent of the pump power and is equal to 70—90 W cm-2. It is found that at the pump-pulse maximum (40 W cm-3) the laser radiation power increases after amplification by 100%.

Propagation characteristics of fast light in an erbium-doped fiber amplifier

A perturbation method is used to study the interactions among the signal power, pump power, and metastable population density for fast light in an erbium-doped fiber amplifier. The impact of temporal pump depletion (TPD) on fast light is investigated in which TPD is the response of the pump power to the temporal variation of the metastable population density. It is found that the gain coefficient and the absolute value of the negative group velocity are overestimated without considering the TPD. The effects of high-order dispersions on fast light are also shown.

Three-photon-excited upconversion luminescence of niobium ions doped silicate glass by a femtosecond laser irradiation

We report on the bluish green upconversion luminescence of niobium ions doped silicate glass by a femtosecond laser irradiation. The dependence of the fluorescence intensity on the pump power density of laser indicates that the conversion of infrared irradiation to visible emission is dominated by three-photon excitation process. We suggest that the charge transfer from O(2-) to Nb(5+) can efficiently contribute to the bluish green emission. The results indicate that transition metal ions without d electrons play an important role in fields of optics when embedded into silicate glass matrix.

A compact three-electrode discharge system for long-pulse KrCl excimer lasers

Spatially very homogeneous gas discharges with long-pulse duration have been realized in HCl-based rare-gas halide gas mixtures at over-atmospheric pressures. A low inductive three-electrode prepulse–mainpulse configuration with two discharge volumes has been used as excitation circuit. The energy transfer efficiency of the pump circuit is very high and reaches 87%. Furthermore, the rise time of the discharge current is very short, in order to improve the discharge stability for high pump powers. Experiments in Xe/HCl/Ne mixtures reveal spatially very homogeneous discharges for up to 370 ns (FWHM) with a power deposition of 260 kW cm−3. For discharges in Kr/HCl/Ne mixtures, we also observe very homogeneous discharges for pump power densities of more than 500 kW cm−3. Discharges in mixtures with a krypton concentration of 100 mbar and 0.5 mbar HCl are homogeneous for 300 ns against 200 ns when 1 mbar of HCl is used.

Photo-operated two-photon absorption effects in the GeSe 2–Ga 2S 3–PbI 2 glasses

The possibility to operate the two-photon absorption (TPA) of newly synthesized GeSe 2–Ga 2S 3–PbI 2 glasses using the CO laser beam ( λ=5.5 μm) as a photoinducing one has been demonstrated. As the fundamental laser beam we have used the illumination of 10.6 μm passively modulated 0.5 ns CO 2 laser with a rate repetition of about 10 Hz. We have established that the maximal photoinduced TPA is observed for the 8% doped samples (up to 14 cm/GW), which is achieved at a pump CO laser pump power density equal to about 0.6 GW/cm 2. The undoped PbI 2 samples show the TPA maximum at a pump power density of about 0.2 cm/GW. The minimal TPA values were observed for the samples with 5% of PbI 2. The obtained results show that these materials can be used as effective optically operated optical limiters.

Spontaneous and stimulated emission of vertically aligned ZnO nanorods of different lengths

The effect of the length of zinc oxide nanorods on their photoluminescence (PL) has been studied in the visible through UV spectral region. Hexagonally faceted columnar nanorods grown on (100) Si substrates have been shown to be aligned almost vertically. The chemistry of point defects in the nanorods depends on their position in the reactor during growth. The room-temperature PL spectrum of the nanorods shows narrow peaks due to stimulated free-exciton emission. The threshold optical pump power density for lasing in the longer ZnO nanorods is 8000 kW/cm2, and the laser radiation is directed predominantly along their axis.

Gain of a long-pulse KrCl excimer laser

The net gain is measured for various pump power densities in a discharge-pumped long-pulse KrCl laser (222 nm). For this, a three-electrode laser system is employed, which is operated at high gas pressures of around 3.5 bar with pump power densities of 300−650 kW cm−3. The net gain reaches a maximum of 1.6% cm−1. Using the experimental results, values are calculated for the small-signal gain, the losses, and the saturation intensity. The losses and the saturation intensity are found to increase much stronger with the excitation rate than the small-signal gain. Fluorescence measurements at 222 nm show that the upper laser level depopulates faster when the power density increases. The influence of collisional quenching and the halogen consumption on the laser performance are discussed.

Blue lasing at room temperature in an optically pumped lattice-matched AlInN/GaN VCSEL structure

Laser action with low threshold average pump power density (∼50 W · cm−2) at room temperature is reported for a crack-free planar vertical cavity surface emitting laser (VCSEL) structure based on a bottom lattice-matched AlInN/GaN distributed Bragg reflector (DBR) and a top dielectric DBR. The cavity region, formed by n- and p-type GaN layers surrounding only three InGaN/GaN quantum wells, corresponds to a typical active region suitable for an electrically driven VCSEL. In addition to low threshold, a spontaneous emission coupling factor β∼2×10−3 is derived for this ready-to-be-processed laser structure.