Pump Power Density Research Articles

Demonstration of UV-A stimulated emission from optical pumping with a nano-porous cladding layer

Abstract We report room-temperature ultraviolet-A (UV-A) stimulated emission from a multiple-quantum-well laser diode featuring a nano-porous bottom cladding layer on the GaN substrate. For a 1500×15 µm ridge-type edge-emitting laser, we achieved 372.8 nm emission under optical pumping, with a full-width-half-maximum (FWHM) of less than 2 nm and a threshold optical pumping power density less than 1.2 MW/cm2. The integration of a nano-porous cladding layer effectively minimizes lattice mismatch, enhances confinement factor and maintains electrical conductivity. This demonstration expands the potential for developing high-performance UV laser diodes on GaN substrates, overcoming limitations previously imposed by critical thickness contrasts.

Double-Cycle Alternating-Flow Diode Pumped Potassium Vapor Laser

A novel double-cycle alternating-flow diode-pumped potassium vapor laser is proposed, theoretically modeled and simulated. The results show that the optical-to-optical efficiency of the laser increases with increasing gas flow rates, although at high flow rates the rate of increase in efficiency decreases. The optical-to-optical efficiency reaches 74.8% at a pump power density of 30 kW/cm2 and a gas flow rate of 50 m/s. The optical-to-optical efficiency of the laser is greater with a narrow linewidth pump and high buffer gas pressure. The optical-to-optical efficiency of a flow gas cell is higher than that of a static gas cell. There is an optimal gas cell length that provides the highest optical-to-optical efficiency. At higher pump power densities, higher flow rates are required to obtain higher optical-to-optical efficiencies.

Compact terahertz harmonic generation in the Reststrahlenband using a graphene-embedded metallic split ring resonator array

Harmonic generation is a result of a strong non-linear interaction between light and matter. It is a key technology for optics, as it allows the conversion of optical signals to higher frequencies. Owing to its intrinsically large and electrically tunable non-linear optical response, graphene has been used for high harmonic generation but, until now, only at frequencies < 2 THz, and with high-power ultrafast table-top lasers or accelerator-based structures. Here, we demonstrate third harmonic generation at 9.63 THz by optically pumping single-layer graphene, coupled to a circular split ring resonator (CSRR) array, with a 3.21 THz frequency quantum cascade laser (QCL). Combined with the high graphene nonlinearity, the mode confinement provided by the optically-pumped CSRR enhances the pump power density as well as that at the third harmonic, permitting harmonic generation. This approach enables potential access to a frequency range (6-12 THz) where compact sources remain difficult to obtain, owing to the Reststrahlenband of typical III-V semiconductors.

Giant enhancement of third harmonic generation via guided mode resonances in notched silicon waveguides

We report a giant enhancement of the third harmonic generation (THG) at 343 nm by periodically notched silicon waveguide arrays supporting guided mode resonances (GMRs) at 1030 nm. Maximum efficiency of the third harmonic generation η = 7.71 × 10−5 is achieved with a peak pump power density of 5.31 GW/cm2. The enhancement factor of the THG from the GMR metasurface reaches up to 1.3 × 107 compared to a flat silicon film with the same thickness. This observation demonstrates a promising approach to design high-efficiency nonlinear optical metasurfaces.

Modeling of flowing gas diode pumped rare gas atoms laser

Diode-pumped rare gas lasers (DPRGLs) have the potential for high optical conversion efficiency and excellent beam quality, positioning them as promising candidates for directed energy lasers. This study proposes a semi-analytical model for a flowing gas diode-pumped rare gas laser considering the influence of temperature elevation resulting from increased pump power on the output. Furthermore, we explore the relationship between laser output power density and gas temperature with respect to gas flow velocity. The results of numerical simulation agree well with those of Rawlins et al.’s experiment. The model addresses the existing gap in DPRGL dynamic models by incorporating temperature considerations, emphasizing the crucial role of a gas flow device in achieving high-energy laser. With the best set of parameters, the output power of DPRGL reaches 100 kW/cm2, at pumping power density of 300 kW/cm2 and gas flow velocity of 140 m/s. Parameter optimization is helpful for design of a relatively high-power DPRGL system.

Thermal sensitivity of Ho3+ fluorescence in oxyfluoride nanocrystals and derivative embedded fibers

(Y1-x-yHoxYby)OF/polyacrylonitrile (YOF-HuYv/PAN) composite fibers prepared by electrostatic spinning have been demonstrated to possess effective upconversion (UC) luminescence and sensitive temperature feedback properties for laser display and real-time temperature monitoring in complex environments. When the excitation power density is increased to 73 mW/mm2, the total luminous flux reaches 3.35 mlm, demonstrating that the Ho3+/Yb3+ co-doped YOF nanocrystals embedded in the composite fibers are efficient luminous materials. The quantum yields (QYs) of green and red emissions from Ho3+ are derived to be 2.97 × 10−5 and 1.40 × 10−5 respectively when the pump power density arrives at 73 mW/mm2 under 977 nm laser, and the high photon generation efficiency ensures sufficient fluorescence intensity for tracing temperature feedback. The maximum relative sensitivities appear at 303 K, while the absolute sensitivity (SA) and relative sensitivity (SR) of the nanofibers retain 13.77% K−1 and 0.34% K−1 at 433 K respectively, demonstrating that the nanofibers have superior optical temperature sensing properties. The composite fibers with effective luminescence and high temperature sensitivity as luminescent materials provide a potential option for the field of laser display and temperature sensing.

High-power and low-threshold 1200 nm InGaAs/GaAs quantum-wells VECSEL grown by MOCVD

Here, we demonstrate an InGaAs/GaAs quantum-wells (QWs) based vertical-external cavity surface emitting laser (VECSEL) with multi-watt emitting beyond 1200 nm. The development is motivated by the need for high-performance 1200 nm laser sources, which are located in the C–H bond photoacoustic tomography window and atmospheric transmission band. In the exploration, strain compensation, epitaxy growth, and package strategy are all optimized for effective performance improvement. Highly strained InGaAs/GaAs QWs epitaxy structures are grown on GaAs substrate with high crystalline quality by metal-organic chemical vapor deposition (MOCVD) technology. With effective thermal management, the thermal impedance of the chip is as low as about 0.92 K/W. As a result, the 1200 nm VECSEL can emit a power of 9.4 W under continuous wave (CW) operation, and the threshold pump power density is as low as about 1.61 kW/cm2. The VECSEL can also operate at a quasi-continuous wave (QCW) emission with tunable pulse width and repetition rates. Under a duty cycle of ∼9.5%, the VCSEL is with a peak power of 16.6 W and a slope efficiency of about 15.5%, and the corresponding pulse energy is as high as about 1.57 mJ. No thermal rollover is observed. This exploration significantly broadens the VECSEL performance and can be regarded as an extension and enrichment of the earlier works on this topic.

Randomized whispering-gallery-mode microdisk laser arrays via cavity deformations for anti-counterfeiting labels

Optical physical unclonable functions (PUFs) have emerged as a promising strategy for effective and unbreakable anti-counterfeiting. However, the unpredictable spatial distribution and broadband spectra of most optical PUFs complicate efficient and accurate verification in practical anti-counterfeiting applications. Here, we propose an optical PUF-based anti-counterfeiting label from perovskite microlaser arrays, where randomness is introduced through vapor-induced microcavity deformation. The initial perovskite microdisk laser arrays with regular positions and uniform sizes are fabricated by femtosecond laser direct ablation. By introducing vapor fumigation to induce random deformations in each microlaser cavity, a laser array with completely uneven excitation thresholds and narrow-linewidth lasing signals is obtained. As a proof of concept, we demonstrated that the post-treated laser array can provide fixed-point and random lasing signals to facilitate information encoding. Furthermore, different emission states of the lasing signal can be achieved by altering the pump energy density to reflect higher capacity information. A threefold PUF (excited under three pump power densities) with a resolution of 5×5 pixels exhibits a high encoding capacity (1.43×1045), making it a promising candidate to achieve efficient authentication and high security with anti-counterfeiting labels.

A 2 μm Dual-Wavelength Laser at Cryogenic Temperature with Balanced Simultaneous Emission

A Tm,Ho:YAP laser at cryogenic temperature is demonstrated for the first time with simultaneous emission at 2000 nm and 2119 nm. The feasibility of switching wavelength and achieving balanced output powers at two widely separated wavelengths has been confirmed by investigating the temperature dependence of the laser spectra. The optimal temperature for balanced output evidently diminishes as the pump power density increases, thereby manifesting a rate of change quantified at 1.19 K/W. At the optimal temperature of 43.1 K, the optical-to-optical conversion efficiency of the Tm,Ho:YAP simultaneous dual-wavelength laser (SDWL) with a pump power of 11.8 W is 12.7%, corresponding to a slope efficiency of 15.8%.

Novel design of flow path spacers for reverse electrodialysis cell

AbstractReverse electrodialysis (RED) is one of the technologies used to harness ‘Blue Energy’, which is generated from the controlled separation of ions between salt water and fresh water through cation and anion exchange membranes (CEM/AEM) stack with end electrodes. The spacers present in between CEM and AEM allows the flow of salt and fresh water significantly affecting the fouling and concentration polarization in the RED cell. The present work focuses on improvement in flow path design, which may be used in place of mesh spacers in order to reduce pressure drop and enhance shear stress on the surface of the membrane to reduce concentration polarization. A three‐dimensional direct numerical simulation of the Navier–Stokes equation is conducted using Fluent 14.0 to analyze four different flow field designs, including serpentine, criss‐cross, rhombus, diamond, and standard mesh spacers. The simulation predicted closely the experimental data on pressure drop for the mesh spacers available in the literature. The present study points out that the diamond type flow field design, which combines characteristics of mesh spacers and flow field plates, gives lesser pressure drop per unit length with the increase in velocity within the same range of shear stress generated in mesh spacers. In other words, net power density of RED would improve with the use of diamond type spacer flow field with the decrease in concentration polarization loss and pumping power density.

Bloch-Surface Plasmon Polariton Enhanced Amplified and Directional Spontaneous Emission from Plasmonic Hexagonal Nanohole Array.

The light-matter interactions at nanoscale can be enhanced by Bloch-surface plasmon polariton (Bloch-SPP) on the plasmonic lattice. An Ag nanohole array in hexagonal arrangement served as an optical cavity to realize the directional and polarized amplified spontaneous emission (ASE) of R6G. A 100-fold enhanced ASE was observed at 15° emission angle under TM polarization when the pump power density exceeded the threshold of 198 W/cm2 based on the degenerated high state density modes. Moreover, a specific polarization dependence of ASE was modulated by the Bloch-SPP modes, and the degree of polarization was enhanced from 1.3 to 2.1 when the pump power density exceeded the threshold of ASE. This work clarifies the interaction between the gain media and plasmonic systems, which lays a foundation for the plasmonic device designing.

Stokes linewidth narrowing by stimulated Brillouin scattering in liquid media

As an effective means to obtain a narrow-linewidth laser, stimulated Brillouin scattering (SBS) has not only the advantages of pulse compression but also controllable Stokes linewidth output. However, most research thus far has been focused on continuous-wave lasers, with little emphasis on short-pulse lasers. This work demonstrates that the Brillouin gain linewidth and pump power density are the primary factors affecting the linewidth of the Stokes pulse. As the pump power density increases, the Stokes linewidth tends to narrow and approaches the pump linewidth. This is the first study to reveal that the pump linewidth is the limiting factor in narrowing the Stokes linewidth. The Stokes linewidths of different liquid media were compared, and it was found that media with a wide Brillouin gain linewidth can be used to obtain lasers with a wider range of linewidths.

Understanding Yb3+-sensitized photon avalanche in Pr3+ co-doped nanocrystals: modelling and optimization.

Among different upconversion processes where the emitted photon has higher energy than the one absorbed, photon avalanche (PA) is unique, because the luminescence intensity increases by 2-3 orders of magnitude in response to a tiny increase in excitation intensity. Since its discovery in 1979, PA has been observed in bulk materials but until recently, obtaining it at the nanoscale has been a significant challenge. In the present work, the PA phenomenon in β-NaYF4 colloidal nanocrystals co-doped with Pr3+ and Yb3+ ions was successfully observed at 482 nm (3P0 → 3H4) and 607 nm (3P0 → 3H6) under excitation at 852 nm. The impact of Pr3+ ion concentration and pump power dependence on PA behavior was investigated, i.e. PA non-linearity slopes of luminescence intensity curves as a function of pump power density as well as PA thresholds. The highest slopes, namely 8.6 and 9.0, and the smallest thresholds equal to 286 kW cm-2 and 281 kW cm-2, observed for emission bands at 607 nm and 482 nm, respectively, were obtained for NaYF4:0.5%Pr3+,15%Yb3+@NaYF4 colloidal nanocrystals. Besides experimental research, simulations of PA behavior in Pr3+, Yb3+ co-doped materials were performed based on differential rate equations describing the phenomena that contribute to the existence of PA. The influence of different processes leading to PA, e.g. the rates of nonradiative and radiative transitions as well as energy transfers, on PA performance was simulated aiming to understand their roles in this complex sensitized system.

Surface lattice resonances enhanced directional amplified spontaneous emission on plasmonic honeycomb nanocone array.

Plasmonic arrays have emerged as a promising platform for investigating light-matter interactions enhanced by surface lattice resonance (SLR) at the nanoscale, which exhibit superior properties in localized field enhancement, narrow linewidth, and effective radiation loss suppression. In this study, an Al nanocone array in a honeycomb arrangement served as an optical cavity with a tip effect to realize the directional and polarized amplified spontaneous emission (ASE) of R6G. Based on the optical feedback between the degenerated SLR mode of high local density of states (LDOS) and the emission of gain media, 140-fold enhanced ASE was observed at an emission angle of 25° under TM polarization when the pump power density exceeded the threshold of 197.8 W cm-2. Moreover, polarization-resolved iso-frequency images indicated that a specific polarization dependence of ASE was modulated by the SLR mode. This study clarifies the interaction between the gain media and plasmonic system, which is beneficial for the generation of nanolasing with directional emission and lays a foundation for the plasmonic device.

Markov Noise in Atomic Spin Gyroscopes: Analysis and Suppression Based on Allan Deviation

Atomic spin gyroscopes (ASGs) based on Spin-Exchange Relaxation-Free co-magnetometer have ultra-high rotation sensitivity and low gyro drift. However, the noise characteristics of ASGs have not been comprehensively studied. In this paper, the primary noise in ASGs which weakens the gyro’s long-term stability is analyzed and identified with Allan deviation. We discovered for the first time that the slow convergence rate of the spin-coupled ensemble brings a long correlation time of the gyro output and the Markov noise introduced by this effect is the primary noise that impacts the long-term stability of ASGs. Allan deviation is used to assess the characteristics of Markov noise and its impact on long-term stability. The correlation time is adjusted by changing the pumping power density to suppress the Markov noise. Comparing the Allan deviations under different correlation time, it’s concluded that the Markov noise caused by long correlation time restricts the improvement of the long-term stability of ASGs. Moreover, Markov noise at low frequencies is suppressed by increasing the pumped optical power density, enhancing the rotation sensitivity of ASGs. A relatively low gyro drift 5.76×10-3 °/h@100s is acquired after the suppression of the Markov noise. Different from traditional amplitude-frequency response analysis, this paper focuses on the time correlation of noise, which provides a new idea for the investigation of ASG’s noise characteristics.

Temperature distribution and the laser performance of LD end-pumped LuYSGG/Er:LuYSGG composite crystal

The laser performance of the LuYSGG/Er:LuYSGG crystal operated in continuous wave and pulsed modes is greatly enhanced by thermal bonding technology, and mid-infrared laser is obtained with excellent beam quality.

Synthesis and upconversion emission studies of CaYF5:Ho3+/Yb3+ phosphor and its applications in optical thermometry, fingerprint detection, and security ink.

In this work, a CaYF5:Ho3+/Yb3+ upconversion phosphor was synthesized and its structural, morphological, and optical properties were studied. The upconversion emission study was performed at an excitation pump power density of 5 W cm-2 and emissions at 544 nm, 650 nm and 747 nm due to the 5F4(5S2) → 5I8, 5F5 → 5I8 and 5F4(5S2) → 5I7 transitions of the Ho3+ ion, respectively were observed. From temperature-dependent upconversion spectra temperature sensitivity was calculated and sensitivity is found to be 14 × 10-3 K-1 for the synthesized upconversion phosphor. The photothermal conversion efficiency of the prepared sample in ethanol medium was tested. Moreover, the sample was used to develop latent fingerprints on various surfaces and good contrast in recorded images is observed. The invisible ink was prepared using the upconversion phosphor and then written words were recorded in upconversion emission mode.

Stimulated emission in heavily doped Al0.68Ga0.32N:Si structures with external cavity

Room temperature broad-band stimulated emission with transverse optically pumping was demonstrated from heavily doped Al0.68Ga0.32N:Si structures with external selective and non-selective cavities. The Al0.68Ga0.3N:Si films with thickness 1.2 μm were grown by molecular beam epitaxy on (0001) oriented sapphire substrates. Stimulated emission was observed in the continuous wavelength range from 400 to 650 nm under pulse optical pumping of the YAG:Nd3+ laser with a wavelength 266 nm, 10 Hz repetition rate and 8 ns pulse width. Low optical pumping threshold power of 5 kW/cm2 at 490 nm in the external non–selective cavity has been observed. The nature of stimulated emission has been studied by means time–resolved spectroscopy. This broad band stimulated emission in the visible region is attributed with two main recombination mechanisms: slow donor–acceptor pair and fast free electron-acceptor transitions. The relative contributions of these processes are experimentally measured at all wavelength range of stimulated emission. The optical gain coefficients of (0.1–17.3) × 103 cm−1 have been determined at pump power density from 12 to 1040 kW/cm2 by direct measurement the gain of a probe radiation in the Al0.68Ga0.32N:Si structures.

Heat Loss Analysis of a 2D Pump’s Transmission

Highly enhanced pump power density inevitably results in a profound rise in pump temperature, which seriously influences both power loss and service performance. Heat loss analysis is an important part of analyzing the mechanical and cooling efficiency of a 2D piston pump. This paper focuses on heat loss analysis of this pump’s transmission. Firstly, theoretical and experimental studies are carried out on the thermal–hydraulic model to investigate the heat loss of the pump’s transmission. A pump test rig is developed and thermal experiments are conducted, from 1000 rpm to 6000 rpm. Furthermore, its transient thermal simulation model is implemented with Ansys software to capture the pump’s thermal status. The test convective heat transfer coefficients and temperature data are set in the model, and the simulation results are mutually validated with the experimental ones. Finally, the transmission’s heat loss is compared with its reference churning loss formula. The distribution of the transient heat loss is 49.66% into the end cap, 27.74% into the cylinder head, 13.30% into the inner cylinder, and 9.30% into the oil. The heat loss simulation results agree with the churning loss below 4000 rpm; therefore, the transmission thermal model is accurate and efficient.

High-speed and high-contrast two-channel all-optical modulator based on solution-processed CdSe/ZnS quantum dots

Recently, all-optical modulators are potentially the most promising candidate to achieve high-bit rate modulation in high-speed all-optical communication technologies and signal processing. In this study, a two-channel all-optical modulator based on a solution-processed quantum dot structure is introduced for two sizes of quantum dots to operate at two wavelengths of MIR spectra (3 µm and 5 µm). To perform numerical and theoretical analysis and evaluate the optical behavior of the proposed all-optical modulator, the coupled rate and propagation equations have been solved by considering homogeneous and inhomogeneous broadening effects. The modulation depth at the 50 GHz frequency and 3 mW probe power is attained, about 94% for channel-1 with the wavelength of 559 nm at 300 Wcm−2 pump power density as well as approximately 83.5% for channel-2 with the wavelength of 619 nm at 500 Wcm−2 pump power density. The introduced two-channel all-optical modulator can operate simultaneously at two wavelengths during the modulation process in which information could be transmitted through both signals from the control light. This approach can present the practical device as a high-contrast and high-speed two-channel all-optical modulator with a high modulation depth in numerous applications such as thermal imaging in night vision cameras, wavelength de-multiplexing, signal processing, free-space communication.