High Pump Power Density Research Articles

High-efficiency generation of harmonics in polythiophene

High-efficiency harmonic generation is observed in a semiconducting polymer RR-P3HT [regioregular poly(3-hexylthiophene)], which exhibits a high nonlinear susceptibility [χ (2) > 10−6 m V−1] under pumping by a pulsed fibre laser. The harmonic generation efficiency in RR-P3HT is comparable or higher than that observed for liquid crystals, which are used in reference experiments to estimate the susceptibility [the quadratic susceptibility of a nematic liquid crystal NLC 1289 is χ (2) ≈ 2 × 10−6 m V−1]. No generation is observed in polythiophene with a random structure, RRa-P3HT [regiorandom poly(3-hexylthiophene)], at the same pump power. It is experimentally demonstrated that a necessary condition for generating the second and third harmonics in a polymer, along with a high pump power density, is the presence of a large power density gradient (exceeding 1013 W m−3). Based on a preliminary theoretical analysis, we can suggest that the quadrupole mechanism, which is a consequence of the regularity of RR-P3HT structure in the thin near-wall layer, may contribute significantly to the nonlinear radiation conversion in semiconducting polymers.

Optical temperature sensing properties of KLu2F7: Yb3+/Er3+/Nd3+ nanoparticles under NIR excitation

Yb3+/Er3+/Nd3+ tri-doped KLu2F7 nanoparticles were fabricated via a typical co-precipitation method, and their phase structure, up-conversion (UC) emission and temperature sensing properties have been investigated in this work. An intense UC green emission originated from Er3+ ions is observed under the near infra-red (NIR) excitation. The maximum sensitivity of temperature is approximately 0.0044 K−1 at 533 K and 0.0041 K−1 at 413 K under the excitation of 980 and 808 nm, respectively. Meanwhile, KLu2F7: Yb3+/Er3+/Nd3+ nanoparticles exhibit excellent thermal stability under the NIR excitation. However, the thermal effect induced by 808 nm laser excitation is negligible compared with that of induced by 980 nm excitation, especially under a high pump power density excitation. It indicates that the employment of 808 nm excitation on KLu2F7: Yb3+/Er3+/Nd3+ system circumvents the laser-induced thermal effect as well as improves the accuracy of temperature sensing. Thus, it was proved that these nanoparticles provide a potential multifunctional application as thermometers and biosensors.

Two-LP-Mode Six-Core Cladding Pumped EDFA With High Pump Power Density

We report the design, fabrication, and evaluation of a cladding pumped two-LP-mode six-core moderately coupled erbium-doped fiber (EDF) amplifier. In this study, we show that moderately coupled multi-core EDF can realize a high pump power density in a cladding by allowing core-to-core power coupling. We also experimentally evaluate the amplification characteristics and confirm an average mode-core gain of 18 dB, a differential mode-core gain of 4 dB, and an average noise figure of 7 dB in the C-band. Moreover, we confirm an average mode-core gain of 15 dB, a differential mode-core gain of 5 dB, and an average noise figure of 7.5 dB in the L-band.

Cooling in Er3+:BaMoO4 phosphor on codoping with Yb3+ for elevated temperature sensing

The tetragonal BaMoO4 phosphors having scheelite structure codoped with Er3+-Yb3+ have been synthesized by chemical co-precipitation method. The cooling on codoping with Yb3+ in Er3+ doped BaMoO4 phosphor has been achieved under excitation at 808nm CW diode laser. The pump power density dependence fluorescence intensity ratio (FIR) variation study shows that the FIR remains almost constant (FIR=1.2) as the pump power density increases from 6.49W/cm2 to 72.07W/cm2. For FIR=1.2 the calculated temperature is observed to be ∼270K which is lower than the room temperature. This shows that the presence of the Yb3+ ions upto certain extent in the codoped phosphor causes cooling effect around the Er3+ ions. This is basically due to the back energy transfer from the Er3+ to Yb3+ ions. The cooling efficiency of the prepared phosphor is found to be ∼80.97%. The phosphors codoped with Yb3+ ions up to 2.0wt% do not show optical heating, whereas the strong optical induced heating is observed in the Er3+ singly doped phosphor. As the concentration of the Yb3+ is increased above 2.0wt% the codoped phosphor starts showing optical heating. The temperature dependent study performed at high pump power density (72.07W/cm2) for the codoped phosphor reveals that the phosphors codoped with Yb3+ ions, where the optical heating is not observed (i.e., cooling is observed), shows temperature sensing behaviour and hence may be suitable for high temperature sensing with better sensitivity.

An abnormal fluorescence intensity ratio between two green emissions of Er3+ caused by heating effect of 980 nm excitation

An abnormal fluorescence intensity ratio (FIR) between two green emissions of Er3+ at room temperature, which is larger than a normal value, emerged in many reported articles. However, up to now detailed work has seldom been done to clarify this abnormal phenomenon. In this paper, green upconversion luminescence of the β-NaLuF4:20%Yb3+,2%Er3+ powder sample was investigated under 980 nm excitation at different circumstances, different pump power densities and different temperatures as well as different air pressures. The corresponding local temperature calculated using FIR technique increased gradually with the enhancement of the pump power density. It was demonstrated that high pump power density of 980 nm laser led to the increase of local temperature of the luminescent material, which further gave the abnormal FIR.

Decrease in particle size and enhancement of upconversion emission through Y3+ions doping in hexagonal NaLuF4:Yb3+/Er3+nanocrystals

The upconversion (UC) enhancement with size decrease has been realized through Y3+ ions doping in β-NaLuF4:Yb3+/Er3+ nanocrystals (NCs) by a solvothermal process. X-Ray diffraction (XRD), scanning electron microscopy (SEM) and UC luminescence spectra were used to characterize the resulting samples. With an increase of the Y3+ doping concentration, the NCs size is continuously decreased with no phase transition. Meanwhile, the UC intensity is firstly increased and then it decreases when the Y3+ ions concentration is over 30 mol%. The NCs (β-NaLu0.48Y0.3Yb0.2Er0.02F4) with the optimum Y3+ ions doping concentration (30 mol%) for UC emission have a diameter of about 80 nm. Compared with β-NaLu0.78Yb0.2Er0.02F4 and β-NaY0.78Yb0.2Er0.02F4 prepared under the same conditions, the green UC emission is enhanced by a factor of 1.8 and 16, respectively, for β-NaLu0.48Y0.3Yb0.2Er0.02F4. The variation of the UC intensity with the increasing Y3+ ions concentration is attributed to the changing of the symmetry of the local crystal field induced by Y3+ ions doping, which has been proved by the structural probe Eu3+ ions. For β-NaLu0.78Yb0.2Er0.02F4 and β-NaLu0.48Y0.3Yb0.2Er0.02F4, a three-photon green UC process and a three-photon red UC process occur simultaneously at high pump power density, which can be described as 4G11/2 (Er3+) + 4I15/2 (Er3+) → 2H11/2/4S3/2 (Er3+) + 4I13/2 (Er3+) and 4G11/2 (Er3+) + 2F7/2 (Yb3+) → 4F9/2 (Er3+) + 2F5/2 (Yb3+), respectively.

Organo-erbium systems for optical amplification at telecommunications wavelengths

Modern telecommunications rely on the transmission and manipulation of optical signals. Optical amplification plays a vital part in this technology, as all components in a real telecommunications system produce some loss. The two main issues with present amplifiers, which rely on erbium ions in a glass matrix, are the difficulty in integration onto a single substrate and the need of high pump power densities to produce gain. Here we show a potential organic optical amplifier material that demonstrates population inversion when pumped from above using low-power visible light. This system is integrated into an organic light-emitting diode demonstrating that electrical pumping can be achieved. This opens the possibility of direct electrically driven optical amplifiers and optical circuits. Our results provide an alternative approach to producing low-cost integrated optics that is compatible with existing silicon photonics and a different route to an effective integrated optics technology.

Simultaneous collinear and non-collinear parametric generation in 1D single grating periodically poled lithium tantalate

We report a detailed study of multiple wavelengths optical parametric generation in a single grating one-dimensional periodically poled lithium tantalate crystal. Simultaneous collinear and non-collinear generations are observed around the pump collinear direction. Similar spectra are found in continuous spatial positions symmetrical to the collinear direction, with decreasing signal and increasing idler wavelengths, associated with decreasing powers when the far-field angle increases. A phase matching scheme is proposed to describe these interactions. Numerical simulations emphasize the agreement of our phase matching scheme with the experimental results. Single-pass gain and high pump power density are very likely at the origin of the simultaneous collinear and non-collinear QPM interactions.

Evaluating upconversion materials developed to improve the efficiency of solar cells: comment

The upconversion efficiency depends on the pump power density. Materials exhibiting a high upconversion efficiency under high pump power density do not necessarily have a reasonable upconversion efficiency under the pump power density around that of the concentrated sunlight. Also, materials which have an upconversion efficiency not so high under high pump power density do not necessarily have no practical upconversion efficiency under the pump power density of concentrated sunlight. To avoid the inconveniences in measuring the upconversion efficiency under the pump power density of concentrated sunlight which is very weak, a method which can evaluate the significance of upconversion materials for solar cells is proposed.

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.

Formation of a volume discharge in air at atmospheric pressure upon application of nanosecond high-voltage pulses

Volume self-sustained discharges in gases at high pressure are formed with the use of pre-ionization sources and are widely used for pulsed laser pumping [1, 2]. In a pulsed nanosecond discharge [3, 4], the conditions can be realized in which the time of discharge channel formation is less than the voltage pulse duration. Then the volume discharge is formed under conditions of multielectron initiation when the initial voltage exceeds the static breakdown voltage. In [5–9], the volume pulsed discharge was initiated in air and other gases at atmospheric pressure without preionization source. To this end, nanosecond high-voltage pulses (~100 kV and more) with a leading pulse edge duration of ~1 ns and less were applied to the gap in [5–9]. The anode was flat, and the cathode had a small radius of curvature. With increase in the interelectrode gap, a pulse corona discharge was observed under these conditions. The main difference between volume and corona pulsed discharges is that in the corona discharge, the gas is ionized only in a part of the gap with the highest electric field strength. In the volume discharge, the gas is ionized in the entire gap, which allows high current densities on the anode (~3 kA/cm), high specific deposited power (~1 J/cm), and high specific pump power density (~400 MW/cm) to be obtained [6–9]. The diffusion discharge with a brighter central channel was realized in [10] when the voltage pulse duration increased up to 170 ns and the leading edge pulse duration increased up to 10 ns. This means that the discharge constriction is observed with increasing pulse duration. However, in [5–10] much attention was given to the study of the discharge initiated in gaps with the potential cathode having a small radius of curvature, and the influence of voltage pulse polarity and a decrease in the strength of electric field applied to the gap on the formation of the volume discharge was not investigated. Only in [9] the volume discharge was realized in air at atmospheric pressure and near the tip anode (the electrode of a shear discharger). The volume discharges initiated in comparatively short interelectrode gaps or with preionization sources were investigated in [1–4]. The present study investigates the influence of polarity and radius of curvature of the electrodes on the formation of the volume pulsed discharge without pre-ionization source under application of nanosecond voltage pulses of comparatively high amplitude (~100 kV). In our experiments we used the pulse generator of the ARINA x-ray apparatus [4]. The open-circuit gap voltage was about 150 kV and the leading pulse edge duration was less than 1 ns. The voltage pulse polarity could be both positive and negative. The pulse duration at half-maximum was ~2 ns for the volume discharge. Three types of the potential metal electrodes were used, including a sphere 40 mm in diameter (electrode No. 1), a tube 6 mm in diameter fabricated from a foil 50 μm thick (electrode No. 2), and a tip (electrode No. 3). The electrode was connected through a metal rod to a peaking spark-gap of the ARINA apparatus and was placed into a chamber with an inner diameter of 160 mm fabricated from a copper foil. This chamber had a flat bottom used as the second electrode. The interelectrode gap could be increased from 20 to 70 mm. All investigations of the discharge formation were conducted in air at a pressure of 1 atm. During our experiments, the total discharge glow (Fig. 1) was registered together with current and voltage waveforms. As in [5–9], the volume discharge (Fig. 1a) was stably recorded when the voltage pulse of negative polarity was applied to the electrodes with a small radius of curvature (Nos. 2 and 3). Bright spots are seen only on the cathode; volume plasma jets started from these spots. In [6] the discharge of this type was called VADIEB (volume avalanche discharge

Pump-power dependence of THz radiation from InAs surfaces under magnetic fields excited by ultrashort laser pulses

THz radiation from InAs surfaces excited by ultrashort laser pulses has been studied under low (∼0.6 μJ/cm2 fluence) and high (∼6 mJ/cm2 fluence) pump power densities. The THz radiation amplitude is affected in both cases. We found that the radiation wave form is broadened by applying magnetic fields for the low-density-excitation case whereas it is not for the high-density-excitation case. This result is explained by the difference of the effect of the magnetic field on the THz radiation due to the transient photocurrent and optical rectification effects.

Blue Upconversion Characteristics of Thulium-doped Silica Fiber with High Germania Content

Intense blue upconversion luminescences were observed in a high-NA silica fiber doped with Tm3+ and 25mol% GeO2 by pumping with a 630-670nm laser. The intensities of two bands due to the 1D2 → 3F4 at 455nm and 1G4 → 3H6 transition at 480nm were changed with the pumping wavelength, power, and the fiber length. These phenomena were ascribed to the population inversion of the3F4 excited state to the ground state and large population of the 3H4, that are the intermediate excited levels for the 3G4 and 1D4 upconversions, respectively. The nonlinear power dependence of the population could be due to the relatively low-phonon-energy, high Tm3+-concentration and high pump-power density in the core of this high-NA fiber. The peak shift of the 480nm-band with the fiber length and a two-photon process of 1.2 µm luminescence band are also discussed.

Erbium-doped phosphate glass waveguide on silicon with 4.1 dB/cm gain at 1.535 μm

Erbium-doped multicomponent phosphate glass waveguides were deposited by rf sputtering techniques. The Er concentration was 5.3×1020 cm−3. By pumping the waveguide at 980 nm with a power of ∼21 mW, a net optical gain of 4.1 dB at 1.535 μm was achieved. This high gain per unit length at low pump power could be achieved because the Er–Er cooperative upconversion interactions in this heavily Er-doped phosphate glass are very weak [the upconversion coefficient is (2.0±0.5)×10−18 cm3/s], presumably due to the homogeneous distribution of Er in the glass and due to the high optical mode confinement in the waveguide which leads to high pump power density at low pump power.

Numerical simulation of the temperature dependence of photoluminescence in strained-Si1?xGex/Si heterostructures

In previous work it has been shown that the decay in photoluminescence from Si/strained-Si1−xGex/Si quantum wells at temperatures over 100 K is controlled by surface recombination and that the photoluminescence intensity can be increased by over an order of magnitude by surface passivation. These results had been explained only by a simple phenomenological model, which could not explain why at high pump power density the observed luminescence was constant from 77 to 250 K. This paper uses a two-carrier heterojunction device simulator to determine the carrier profiles during optical pumping. The profiles are used to understand quantitatively luminescence as a function of temperature and pump power density without making the over-simplifying assumptions required for analytical modeling. Surface recombination velocities over 103 cm/s drastically affect the results, and Auger recombination plays an important role at high pump power density.

Scalable concept for diode-pumped high-power solid-state lasers

A new, scalable concept for diode-pumped high-power solid-state lasers is presented. The basic idea of our approach is a very thin laser crystal disc with one face mounted on a heat sink. This allows very high pump power densities without high temperature rises within the crystal. Together with a flat-top pump-beam profile this geometry leads to an almost homogeneous and one-dimensional heat flux perpendicular to the surface. This design dramatically reduces thermal distortions compared to conventional cooling schemes and is particularly suited for quasi-three-level systems which need high pump power densities. Starting from the results obtained with a Ti:Sapphire-pumped Yb:YAG laser at various temperatures, the design was proved by operating a diode-pumped Yb:YAG laser with an output power of 4.4 W and a maximum slope efficiency of 68%. From these first results we predict an exctracted cw power of 100 W at 300 K (140 W at 200 K) with high beam quality from a single longitudinally pumped Yb: YAG crystal with an active volume of 2 mm3. Compact diode-pumped solid-state lasers in the kilowatt range seem to be possible by increasing the pump-beam diameter and/or by using several crystal discs.

Growth of Erbium Doped PbF2 - SrF2 Epitaxial Layers on GaAs(111)B for Upconversion Waveguide Laser Applications

AbstractWaveguide structures offer the possibility of making an upconversion laser operating at room temperature. By reducing the optical mode cross section can lead to a very high pump power density which allows modest pumping powers to overcome non-radiative decay processes. We have reported earlier on the observation of upconversion luminescence from both planar and channel waveguides of Erbium doped ZnF2 on MgF2(001) and Erbium doped PbF2 on GaAs(100). The epitaxial fluoride layers were all grown by Molecular Beam Epitaxy. Fluoride films preferentially grow on the (111) GaAs surface. In order to exploit this fact, we have grown Erbium doped PbF2 on GaAs(111)B with an intervening SrF2 cladding layer. The SrF2 and PbF2 growth conditions have been optimized on GaAs(111)B using X-ray rocking curve analysis. The crystalline quality of the films grown on GaAs(111)B are far superior to those grown on GaAs(100). Upconversion luminescence has been observed in the PbF2: Er/SrF2/GaAs(111)B planar waveguide structures. The guide ends were formed by cleaving the semiconductor wafer and the 800nm and 980nm pump light was introduced from a Ti-Sapphire laser by end pumping using a microscope objective.

Experimental and theoretical investigation of a pulsed laser utilizing the CF4molecule

An experimental investigation was made of the characteristics of a pulsed CF4 laser at high pump power densities. A radiation intensity of 0.5 MW/cm2 was obtained at a frequency of 615 cm−1 when the pump power conversion efficiency was 12%. Calculations are made of the laser parameters using steady-state and complete models. It is found that two-photon processes may play a decisive role in the lasing mechanism. The feasibility of using amplifiers to improve the radiation power in the 16μ range is determined.

Power-scaling effects in dye lasers under high-power laser excitation

Eight laser dyes emitting in the spectral region from 550 to 915 nm were investigated to determine their power scalability under excitation by Q-switched frequency-doubled Nd : YAG lasers possessing up to 480 mJ pulse energy at 10 Hz pulse repetition frequency (PRF). Conversion efficiencies for some dyes begin to be limited at pump-power densities as low as 14 MW/cm2 due to excited-state absorption of the pumping radiation. The dependence on resonator length of dye spectral emission, conversion efficiency, and temporal modulation are reported. Divergence of the beam emerging from the dye laser was found to be adversely affected by measures taken to avoid output power saturation at high pump-power densities.