Pump Power Density Research Articles

Ho:SSO solid-state saturable-absorber Q switch for pulsed Ho:YAG laser resonantly pumped by a Tm:YLF laser

A Ho:SSO crystal was employed as an efficient saturable absorber Q switch for the pulsed Ho:YAG laser for the first time. The characteristics of the passive Q-switch Ho:YAG laser were studied with different incident pump power and different laser energy density around the Ho:SSO. The single pulsed energy varied from 8.3 μJ to 76.6 μJ with the increasing of the incident power from 2.2 W to 8.2 W. The maximum repetition rate was 42.1 kHz, and the minimum pulsed duration was 48 ns. The maximum single-pulse peak power was 1.53 kW. The output center wavelength shifted from 2090.1 nm (the continuous-wave mode) to 2091.3 nm (the passive Q-switch mode).

Influence of 980 nm pump power on optical thermometry based on NaYF4:Yb3+/Er3+ nanoparticles

For optical thermometry, the pump light source has an important influence on the temperature measurement performance. In this study, we investigated the influence of 980 nm pump power on fluorescence intensity ratio (FIR) thermometry based on NaYF4:5% Yb3+/2% Er3+ nanoparticles. The thermally coupled level (TCL) of 2H11/2 and 4S3/2 of Er3+ was selected to evaluate the temperature. The correspondence between the FIR and temperature of this sample near room temperature was observed. In addition, the relationship between the FIR and pump power was analysed. The temperature increment induced by the laser thermal effect and the uncertainty of the FIR of the sample were studied with an increasing power density. The results indicate that the uncertainty ratio of the FIR (ΔFIR/FIR) decreases rapidly with the pump power density, which implies that the precision of the temperature measurement is improved. While the temperature increment induced by the pump power increases linearly with a slope of 6 K/(W/cm2), this will reduce the temperature sensing accuracy. Clearly, this contradiction in utilizing a 980 nm pump power should be balanced in practical applications of optical thermometry.

Calculation of single-pass gain for laser ceramics with losses

Rate equations describing the single-pass gain in an active medium with losses are analytically solved. The found relations illustrate the dependences of the amplification efficiency of Nd : YAG ceramics on the pump power density and specific losses. It is concluded that specific losses can be estimated from comparative measurements of unsaturated and saturated gains.

Manipulation of microstructures and the stability of white emissions in NaLuF4:Yb3+, Ho3+, Tm3+ upconversion crystals

A series of Yb3+, Ho3+, Tm3+ doped NaLuF4-based nanostructures and microstructures with controllable crystalline phases and morphologies were designed via adjusting the parameters of the oleic acid-assisted hydrothermal method. It is found that increasing the NH4F content and prolonging the reaction time to conduce the formation of β-NaLuF4 and the possible evolution process was proposed. Besides, the dependence of upconversion emission on sensitizer (Yb3+) concentration and pump power in Yb3+-Ho3+ and Yb3+-Tm3+ codoped β-NaLuF4 was investigated. On this basis, a tunable multicolor was achieved in NaLuF4:Yb3+/Ho3+/Tm3+ microrods. It is worth mentioning that white-light output with the calculated CIE color coordinate of (0.333, 0.330) was realized in the microrods, and the triply doped samples presented an outstanding capacity for color controlling and satisfactory color stability in the white region as well as lower threshold pump power density and lower optimal excitation pump power density for white-emission, which present great potential applications in the field of color display and white light-emitting diodes.

Influence of template properties and quantum well number on stimulated emission from Al0.7Ga0.3N/Al0.8Ga0.2N quantum wells

AlGaN multiple quantum well laser heterostructures for emission around 240 nm have been grown by metalorganic vapor phase epitaxy on epitaxially laterally overgrown (ELO) AlN/sapphire templates. The edge emitting laser structures showed optically pumped lasing with threshold power densities in the range of 2 MW cm−2. The offcut angle of the sapphire substrates as well as the number and the width of the quantum wells were varied while keeping the total thickness of the gain region constant. A larger offcut angle of 0.2° leads to step bunching on the surface as well as Ga accumulation at the steps, but also to an increased inclination of threading dislocations and coalescence boundaries resulting in a reduced dislocation density and thus a reduced laser threshold in comparison to lasers grown on ELO with an offcut of 0.1°. For low losses, samples with fewer QWs exhibited a lower lasing threshold due to a reduced transparency pump power density while for high losses, caused by a higher threading dislocation density, the quadruple quantum well was favorable due to its higher maximum gain.

Thickness optimization for an anti-ASE cap in a thin disk laser considering dioptric power and aberration-induced loss

A numerical model is developed to analyze the optical path difference (OPD) of a capped disk with different thickness anti-ASE caps. The spherical and aspherical parts of the OPD are described by dioptric power and diffraction loss, which are used to evaluate the stability range of the resonator and output power of a thin disk laser, respectively. The analytical results show that the anti-ASE cap increases thermal aberration and diffraction loss compared with the uncapped disk. However, with a particular thickness anti-ASE cap, the dioptric power of the capped disk almost keeps constant with the increase of pump power density, which will be beneficial to designing a stable resonator with large fundamental mode size. At the same time, the diffraction loss will reach the maximum value at this thickness, too. Thus, weighing the thermal lens effect and diffraction loss will be important for designing an anti-ASE cap with an optimal thickness to realize higher-power laser generation.

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.

Zirconia based Ho3+–Yb3+ codoped upconverting nanophosphors for green light emitting devices applications

Photoluminescence study of the Ho3+–Yb3+ codoped ZrO, AlZrO and YZrO nanophosphors, synthesized by chemical co-precipitation method, upon excitation at 450 and 980 nm radiations have been performed. An improvement of about ∼4.5 times in the downconversion emission intensity of green band corresponding to the 5F4, 5S2 → 5I8 transition for codoped YZrO nanophosphors compared to ZrO codoped nanophosphors has been observed. On varying the pump power density upon 980 nm excitation the colour tunability in the codoped YZrO nanophosphors has been observed. UC emission intensity of the green band arising from the Ho3+ ion in the codoped YZrO nanophosphors is enhanced about ∼22 times compared to that of the codoped ZrO nanophosphors. The absorption and UC emission study for the codoped YZrO nanophosphors dispersed in different biologically compatible solvents viz. water, methanol, ethanol and dimethyl sulfoxide (DMSO) has been performed. The green UC emission intensity of about ∼1.3 and ∼1.7 times for the efficient codoped YZrO nanophosphors dispersed in methanol compared to that dispersed in water and DMSO respectively has been observed. The absorption spectra of an efficient upconverting YZrO nanophosphors dispersed in methanol exhibit no change with the passage of time.

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.

Amplified Spontaneous Emission on Sodium D-lines Using Nonresonant Optical Pumping

We describe experiments on the excitation of amplified spontaneous emission on the D-lines of sodium (D2 = 588.9 nm; D1 = 589.6 nm) with longitudinal optical pumping with a large detuning from the D2 line toward shorter wavelengths. The radiation spectra are measured, showing gain on both lines, at a Na concentration of (1–5) × 1014 cm–3, buffer gas (helium) pressure of 600 Torr at the working temperature, and pumping power density of more than 1.5 MW/cm2. The time characteristics show that the output radiation varies versus the concentration of sodium atoms in the active medium. The dependences of radiation absorption at the D-lines of sodium on the concentration of Na atoms and buffer gas pressure are given.

Optical thermometry based on up-conversion emission behavior of Ba2LaF7 nano-crystals embedded in glass matrix

Ba2LaF7:Yb3+, Er3+ nano-crystals embedded in a transparent aluminium silicate glass matrix was successfully fabricated by a traditional high temperature melt-quenching technique and the subsequently heat treatment process. The formation of Ba2LaF7 nano-crystals was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The enhancement of the up-conversion (UC) emission intensity of Er3+ in the glass ceramic (GC) was attributed to the crystallization of Ba2LaF7 nano-crystals with a low phonon energy compared to the precursor glass counterpart. The temperature-depended fluorescence intensity ratio (FIR) of thermally coupled emitting states (4S3/2, 2H11/2) behavior of Er3+ was studied under the excitation of 980nm. The maximum thermal sensitivity was approximately 0.43% K−1 at 298K. Besides, the temperature of Ba2LaF7 GC rose from 336 to 407K, when the laser pump power density increased from 1.01 to 9.59W/cm2. Moreover, the excellent thermal stability of the sample for temperature detection in the range of 398-573K was proved. The results indicate that the Yb3+/Er3+ co-doped Ba2LaF7 GCs could be potential candidates for integrating heaters and thermometers.

Frequency upconversion and fluorescence intensity ratio method in Yb3+-ion-sensitized Gd2O3:Er3+-Eu3+ phosphors for display and temperature sensing

Near infrared (NIR) to visible frequency upconversion emission studies in Er3+-Eu3+/Er3+-Eu3+-Yb3+ co-doped/tri-doped Gd2O3 phosphors prepared by the co-precipitation technique have been explored under 980 nm laser diode radiation. The developed phosphors were characterized with the help of XRD, FE-SEM and FTIR analysis. No upconversion (UC) emission was found in the Eu3+-doped Gd2O3 phosphor. UC emission from Eu3+ ions along with Er3+ ions was observed in Er3+-Eu3+ and Er3+-Eu3+-Yb3+ co-doped/tri-doped phosphors. The UC emission arising from the Er3+ and Eu3+ ions was enhanced several times due to the incorporation of Yb3+ ions. The processes involved in the UC emission were obtained on the basis of the effect of energy transfer/sensitization through the Yb3+ → Er3+ → Eu3+ process. The red/green intensity ratio was improved from 0.16 to 1.50 and 1.01 to 1.50 for Er3+-Eu3+-Yb3+ tri-doped phosphors as compared to the Er3+-doped and Er3+-Yb3+ co-doped phosphors, respectively, at a fixed pump power density. A UC fluorescence intensity ratio (FIR)-based temperature sensing study was performed in the prepared Er3+-Eu3+-Yb3+ tri-doped Gd2O3 phosphors for green upconversion emission bands in the 300 K–443 K temperature range. A maximum sensor sensitivity of about ∼0.0043 K−1 at 300 K was achieved for the synthesized tri-doped phosphors upon excitation with a 980 nm laser diode. The colour coordinates lying in the green–yellow region are invariant, with variation in pump power density and temperature. The observed results support the utility of the prepared tri-doped phosphors in optical temperature sensing, display devices and NIR to visible upconverters.

Study on optical properties and upconversion luminescence of Er^3+/Yb^3+ co-doped tellurite glass for highly sensitive temperature measuring

Er3+/Yb3+ co-doped tellurite glass is prepared by conventional melt-quenching method. The thermal and optical properties of the glass are investigated for temperature sensing. The Judd-Ofelt theory is applied for spectral analysis and the obtained intensity parameters (Ω2 = 6.48 × 10−20 cm2, Ω4 = 1.82 × 10−20 cm2, Ω6 = 1.27 × 10−20 cm2) are used to estimate the spectroscopic parameters of Er3+ ions in the glass. The upconversion (UC) luminescence of the glass is investigated under the laser diode (LD) excitation at 976 nm. The absolute quantum yield for UC luminescence is determined to be 0.0049% when the pump power density is 10 W/cm2. The dependences of green and red UC emission intensities on pump power indicate that both of the red and green UC emissions of Er3+ ions are mostly contributed by the co-existing two and three-photon involved energy transfer processes between the Yb3+ and Er3+ ions. Furthermore, the temperature-dependent green UC emissions of the glass are studied and the results show that the glass is an excellent candidate for construction of temperature sensors based on the self-referenced fluorescence intensity ratio (FIR) technique.

Effect of the Electrode Radii on the Pump Discharge width and KrF Laser Radiation

The effect of the electrode shapes on the pump discharge width and the energy of radiation of the KrF laser with pulse duration of 30 ns is investigated by the method of computer modeling. The pump and laser power density distributions across the discharge cross section are obtained as functions of the electrode curvature radii. It is shown that an optimum is observed in the dependence of the laser radiation energy on the electrode radius given that the excitation parameters and the mixture composition remain unchanged. The optimum is reached with the electrodes for which the maximum excitation power density in the discharge does not exceed 8 MV/cm3.

Manipulable and Hybridized, Ultralow‐Threshold Lasing in a Plasmonic Laser Using Elliptical InGaN/GaN Nanorods

Manipulating stimulated‐emission light in nanophotonic devices on scales smaller than their emission wavelengths to meet the requirements for optoelectronic integrations is a challenging but important step. Surface plasmon polaritons (SPPs) are one of the most promising candidates for sub‐wavelength optical confinement. In this study, based on the principle of surface plasmon amplification by the stimulated emission of radiation (SPASER), III‐Nitride‐based plasmonic nanolaser with hybrid metal–oxide–semiconductor (MOS) structures is designed. Using geometrically elliptical nanostructures fabricated by nanoimprint lithography, elliptical nanolasers able to demonstrate single‐mode and multimode lasing with an optical pumping power density as low as 0.3 kW cm−2 at room temperature and a quality Q factor of up to 123 at a wavelength of ≈490 nm are achieved. The ultralow lasing threshold is attributed to the SPP‐coupling‐induced strong electric‐field‐confinement in the elliptical MOS structures. In accordance with the theoretical and experimental results, the size and shape of the nanorod are the keys for manipulating hybridization of the plasmonic and photonic lasing modes in the SPASER. This finding provides innovative insight that will contribute to realizing a new generation of optoelectronic and information devices.

Whispering gallery mode lasing from InGaN/GaN quantum well microtube.

In this work, we have successfully fabricated microtubes by strain-induced self-rolling of a InGaN/GaN quantum wells nanomembrane. Freestanding quantum wells microtubes, with a diameter of 6 µm and wall thickness of 50 nm, are formed when the coherently strained InGaN/GaN quantum wells heterostructure is selectively released from the hosting substrate. Periodic oscillations due to whispering-gallery modes resonance were found superimposed on photoluminescence spectra even at low optical excitation power. With increasing pumping power density, the microtube is characterized by a stimulated emission with a threshold as low as 415 kW/cm2. Such emission shows predominant TM polarization parallel to the microtube axis.

Temporal profiles for measuring threshold of random lasers pumped by ns pulses

The working threshold is an important parameter to assess the performance of cavity-free random lasers. Here, the temporal profile measurement is proposed as an alternative method to determine the thresholds of the surface plasmon based random lasers pumped by ns pulses based on analyzing the delay time (tDelay) and rising time (tR) of the emission signal. The obvious and slight inflection points of the curves of tDelay and tR varying with the pump power density are observed as indicators for the thresholds of random lasing and for the transition of lasing mode, respectively. The proposed method supplies consistent values to those supplied by traditional methods in frequency-domain for the random systems with different gain length. The demonstrated temporal profile approaches are free from the spectrometers and may be as a candidate for measuring the threshold of random lasers in ultrafast optics, nonlinear optics and bio-compatible optoelectronic probes.

Defect Emission and Optical Gain in SiCxOy:H Films.

Luminescent SiCxOy:H films, which are fabricated at different CH4 flow rates using the plasma-enhanced chemical vapor deposition (PECVD) technique, exhibit strong photoluminescence (PL) with tuning from the near-infrared to orange regions. The PL features an excitation-wavelength-independent recombination dynamics. The silicon dangling bond (DB) defects identified by electron paramagnetic resonance spectra are found to play a key role in the PL behavior. The first-principles calculation shows that the Si DB defects introduce a midgap state in the band gap, which is in good agreement with the PL energy. Moreover, the band gap of a-SiCxOy:H is found to be mainly determined by Si and C atoms. Thus, the strong light emission is believed to result from the recombination of excited electrons and holes in Si DB defects, while the tunable light emission of the films is attributed to the substitution of stronger Si-C bonds for weak Si-Si bonds. It is also found that the light emission intensity shows a superlinear dependence on the pump intensity. Interestingly, the film exhibits a net optical gain under ultraviolet excitation. The gain coefficient is 53.5 cm-1 under a pumping power density of 553 mW cm-2. The present results demonstrate that the SiCxOy system can be a very competitive candidate in the applications of photonics and optoelectronics.

Lateral carrier diffusion in InGaAs/GaAs coupled quantum dot-quantum well system

The lateral carrier diffusion process is investigated in coupled InGaAs/GaAs quantum dot-quantum well (QD-QW) structures by means of spatially resolved photoluminescence spectroscopy at low temperature. Under non-resonant photo-excitation above the GaAs bandgap, the lateral carrier transport reflected in the distorted electron-hole pair emission profiles is found to be mainly governed by high energy carriers created within the 3D density of states of GaAs. In contrast, for the case of resonant excitation tuned to the QW-like ground state of the QD-QW system, the emission profiles remain unaffected by the excess kinetic energy of carriers and local phonon heating within the pump spot. The lateral diffusion lengths are determined and present certain dependency on the coupling strength between QW and QDs. While for a strongly coupled structure the diffusion length is found to be around 0.8 μm and monotonically increases up to 1.4 μm with the excitation power density, in weakly coupled structures, it is determined to ca. 1.6 μm and remained virtually independent of the pumping power density.

Yb3+ concentration on emission color, thermal sensing and optical heater behavior of Er3+ doped Y6O5F8 phosphor

Abstract Up-conversion phosphor is a potential candidate as non-contact temperature sensor because of its unjammable and unique detection abilities. In this work, we investigate the influence of Yb 3+ concentration on the emission color, thermal sensing and optical heater behavior of Er 3+ doped Y 6 O 5 F 8 phosphor. Our results show that the emission color of Er 3+ and Yb 3+ co-doped Y 6 O 5 F 8 powder changes from green to yellow with the Yb 3+ concentration increasing. Importantly, the temperature sensing sensitivities of Er 3+ and Yb 3+ co-doped Y 6 O 5 F 8 powder reach 0.008, 0.009, 0.010 and 0.011 K −1 as the sample doped with 2%, 5%, 8% and 11% Yb 3+ at 476 K, respectively. Moreover, the temperature of high Yb 3+ concentration sample shows preferable optical heating behavior, whose temperature is ascended by a large value of 94 K when the excitation pump power density changes from 1.0 to 13.1 W cm −2 . These results suggest Er 3+ and Yb 3+ co-doped Y 6 O 5 F 8 powder has great potential in colorful display, temperature sensing and optical heating.