Absorption Of Pump Light Research Articles

Highly sensitive optical thermometer based on Li+-doped erbium ytterbium silicate films

This work provides a new material, Li+-doped erbium ytterbium silicate, with reliable and high temperature sensing sensitivity for optical thermometers. The Li+-doped erbium ytterbium silicate films were prepared by RF magnetron sputtering. The film shows strong green visible emission, which is related to the radiation transitions from thermally coupled energy levels 2H11/2, 4S3/2 to 4I15/2. Lithium doping reduces the field symmetry around erbium and improves the radiation transition efficiency, and Yb doping improves the pump light absorption. Lithium or ytterbium is advantageous to enhance up-conversion luminescence and temperature sensing sensitivity of erbium ions. Therefore, the temperature dependence of green up-conversion luminescence in the range of 80–460 K of the Li+-doped erbium ytterbium silicate films have been found to be a promising optical thermometer material with a high relative sensitivity of 16.9 % K−1, which is higher than the optimum value of other Er3+ materials. In addition, we have provided a preliminary temperature measurement scheme based on the Li+-doped erbium ytterbium silicate film.

Broad-band-enhanced plasmonic random laser in silver nanostar arrays

As a novel optical device, the plasmonic random laser has unique working principle and emission characteristics. However, the simultaneous enhancement of absorption and emission by plasmons is still a problem. In this paper, we propose a broad-band-enhanced plasmonic random laser. Two-dimensional silver (Ag) nanostar arrays were prepared using a bottom-up method with the assistance of self-assembled nanosphere templates. The plasmon resonance of Ag nanostars contributes to the pump light absorption and photoluminescence (PL) of RhB. Coherent random lasing was achieved in RhB@PVA film based on localized surface plasmon resonance (SPR) dual enhancement and scattering feedback of Ag nanostars. Ag nanostars prepared with different nanosphere diameters affect the laser emission wavelength. In addition, the random laser device achieves wavelength tunability on a flexible substrate under mechanical external force.

Efficient 38.8 W/m2 solar pumped laser with a Ce:Nd:YAG crystal and a Fresnel lens.

Herein, we report a significant improvement in solar-pumped laser collection efficiency based on end-side pumping a 6-mm-diameter 95-mm-length Ce:Nd:YAG/YAG grooved bonded crystal rod. A Fresnel lens, quartz cooling-water tube, and gold-plated conical cavity constituted the solar-energy collection and concentration system, which was designed to maximum pump light absorption and minimize thermal effects in the Ce:Nd:YAG laser medium. To the best of our knowledge, this is the first time that a Ce:Nd:YAG crystal has been pumped by a Fresnel-lens solar-energy collection and concentration system. The 0.69-m2 effective solar-collection area produced 26.93 W of continuous-wave laser power, corresponding to 6.33% slope efficiency. The collection efficiency (38.8 W/m2) was 1.21 times higher than the highest previously reported value for Fresnel-lens solar collection, and is a record for single-beam solar-pumped lasers.

Quantum dot lasing from a waterproof and stretchable polymer film

Colloidal quantum dots (QDs) are excellent optical gain materials that combine high material gain, a strong absorption of pump light, stability under strong light exposure and a suitability for solution-based processing. The integration of QDs in laser cavities that fully exploit the potential of these emerging optical materials remains, however, a challenge. In this work, we report on a vertical cavity surface emitting laser, which consists of a thin film of QDs embedded between two layers of polymerized chiral liquid crystal. Forward directed, circularly polarized defect mode lasing under nanosecond-pulsed excitation is demonstrated within the photonic band gap of the chiral liquid crystal. Stable and long-term narrow-linewidth lasing of an exfoliated free-standing, flexible film under water is obtained at room temperature. Moreover, we show that the lasing wavelength of this flexible cavity shifts under influence of pressure, strain or temperature. As such, the combination of solution processable and stable inorganic QDs with high chiral liquid crystal reflectivity and effective polymer encapsulation leads to a flexible device with long operational lifetime, that can be immersed in different protic solvents to act as a sensor.

10 kW-level Output power from a Tandem- pumped Yb-doped aluminophosphosilicate fiber amplifier

In this paper, we present a 10 kW-level output power fiber amplifier based on tandem-pumping. The characteristics of active fiber for amplifier stage is reported. To ensure adequate absorption of tandem pumping light in relatively short fiber length, a large-scale Yb3+ doped aluminophosphosilicate (Al2O3-P2O5-SiO2, APS) fiber with 50 μm core and 400 μm inner-clad in diameter, i.e., a 50/400 Yb-APS fiber, was theoretically designed and experimentally fabricated by using modified chemical vapor deposition system combined with chelate precursor doping technique. The measurements showed that the dopant concentration of Yb2O3, Al2O3, and P2O5 was estimated to be 0.17 mol%, 1.4 mol%, and 1.5 mol%, respectively, and the absorption coefficient of the fiber was about 0.77 dB/m at 1018 nm. The amplifier stage with this 40 m Yb-APS fiber tandem-pumped by 1018 nm fiber laser, delivered 9.82 kW laser output at 1080.08 nm with an optical-to optical efficiency of 86.8%. The knowledge of the characteristics of the Yb3+ doped APS fiber can usefully guide future design of high output power fiber amplifier based on tandem pumping.

The Relative Importance of Phytoplankton Light Absorption and Ecosystem Complexity in an Earth System Model

AbstractWe investigate the relative importance of ecosystem complexity and phytoplankton light absorption for climate studies. While the complexity of Earth System models (ESMs) with respect to marine biota has increased over the past years, the relative importance of biological processes in driving climate‐relevant mechanisms such as the biological carbon pump and phytoplankton light absorption is still unknown. The climate effects of these mechanisms have been studied separately, but not together. To shed light on the role of biologically mediated feedbacks, we performed different model experiments with the EcoGENIE ESM. The model experiments have been conducted with and without phytoplankton light absorption and with two or 12 plankton functional types. For a robust comparison, all simulations are tuned to have the same primary production. Our model experiments show that phytoplankton light absorption changes ocean physics and biogeochemistry. Higher sea surface temperature decreases the solubility of CO2 which in turn increases the atmospheric CO2 concentration, and finally the atmospheric temperature rises by 0.45°C. An increase in ecosystem complexity increases the export production of particulate organic carbon but decreases the amount of dissolved organic matter. These changes in the marine carbon cycling, however, hardly reduces the atmospheric CO2 concentrations and slightly decreases the atmospheric temperature by 0.034°C. Overall we show that phytoplankton light absorption has a higher impact on the carbon cycle and on the climate system than a more detailed representation of the marine biota.

Simulation and experimental results of a high-gain two-stage and double-pass off-axis Nd:YVO4 picosecond laser amplifier.

A high-gain two-stage double-pass off-axis Nd:YVO4 picosecond laser amplifier has been developed. The comprehensive influence of crystal doping concentration and pump beam quality on the small-signal gain of the Nd:YVO4 amplifier is theoretically analyzed with a model developed by considering energy transfer upconversion and pump light absorption saturation effect. The thermal effect of Nd:YVO4 crystal with different doping concentrations, undoped end cap lengths, and pump beam quality is investigated as well. Based on the theoretical analysis, a high-gain two-stage and double-pass off-axis Nd:YVO4 amplifier based on picosecond fiber seed source is realized by choosing long-composite low-doping Nd:YVO4 crystal and with high brightness laser diode pumping, delivering a gain of 28 dB and an output pulse energy of 67.5 µJ at a repetition rate of 200 kHz.

Surface acoustic wave photonic devices in silicon on insulator

Opto-mechanical interactions in planar photonic integrated circuits draw great interest in basic research and applications. However, opto-mechanics is practically absent in the most technologically significant photonics platform: silicon on insulator. Previous demonstrations required the under-etching and suspension of silicon structures. Here we present surface acoustic wave-photonic devices in silicon on insulator, up to 8 GHz frequency. Surface waves are launched through absorption of modulated pump light in metallic gratings and thermo-elastic expansion. The surface waves are detected through photo-elastic modulation of an optical probe in standard race-track resonators. Devices do not involve piezo-electric actuation, suspension of waveguides or hybrid material integration. Wavelength conversion of incident microwave signals and acoustic true time delays up to 40 ns are demonstrated on-chip. Lastly, discrete-time microwave-photonic filters with up to six taps and 20 MHz-wide passbands are realized using acoustic delays. The concept is suitable for integrated microwave-photonics signal processing.

Plasmon antenna array “patchwork” lasers — towards low etendue, speckle free light sources

In this work, we study plasmon antenna array lasers for solid-state lighting (SSL). Optically pumped plasmon antenna array lasers can provide benefits suitable for SSL including efficient pump-light absorption, high brightness, and good directivity. However, applying lasers in SSL is difficult because of speckle formation. To overcome this issue, we propose two types of lasers based on patchworks of small plasmon lattices with different lattice constants, tessellating an extended surface. The premise is that they could ultimately form a blue-LED pumped broad-area directional emitter with reduced coherence to suppress speckle. An important question is whether different patches couple when assembled together, and how this affects spatial and spectral profiles. In this paper, we show measurement results on the plasmon patchwork lasers, and discuss their modelling and potential application as low etendue and speckle free sources in SSL.

65‐2: Invited Paper: Enabling Technology for MicroLED Display Based on Quantum Dot Color Converter

In this study, red colloidal quantum dots (QDs) were loaded into nanoporous (NP) Gallium Nitride (GaN) with vertical and horizontal nanopores. Strong light scattering effect induced by NP GaN greatly enhanced absorption of blue pumping light by QDs. 74% of light conversion efficiency is achieved.

Externally pumped low-loss graphene-based fiber Mach-Zehnder all-optical switches with mW switching powers.

We propose and experimentally demonstrate an all-optical switch based on a graphene-coated fiber Mach-Zehnder interferometer, where the phase of the signal light in one arm of the interferometer is changed by the heat generated from external pump light absorption by the graphene coating. The external pumping scheme allows efficient pump absorption with multiple layers of graphene coated on an ordinary fiber or a slightly tapered fiber without introducing significant additional signal loss. Without using any wavelength multiplexer/demultiplexer, the switch can be pumped at any convenient wavelength or even with broadband light. Our experimental device, which is based on a standard 125-μm-diameter single-mode fiber with a 5-mm-long graphene coating, can be switched with a pump power of 5.3 mW at an extinction ratio of 19 dB with no additional signal loss. The switching power is insensitive to the graphene coating's length and can be reduced to 4.8 mW, with the fiber tapered to 40 μm. The measured switching powers agree well with the theoretical values obtained by treating the graphene coating as a uniform sheet of heat source without thickness. The switch's response time decreases with the fiber diameter and inversely with the graphene coating's length. The switch's rise and fall times, based on a 40-μm tapered fiber with a 20-mm-long graphene coating, are 30 ms and 50 ms, respectively.

Laser Tuning in van der Waals Crystals.

The great progress that optoelectronic technologies have achieved in van der Waals crystals promises the development of next-generation two-dimensional (2D) integrated optoelectronic systems (IOSs). Here, relying on the anomalous avoidance of pump-light absorption, the harvest of extremely strong Raman scattering, and the achievement of polarization inheritance by the designed optical geometry, we realized laser wavelength tuning with ultrahigh precision (0.01 nm/25 K) in layered MoS2 and WS2 by adjusting the temperature. Our results offer a potential approach for 2D laser tuning, while also building a substantial theoretical foundation, which might be of use in developing future wavelength-division-multiplexing technology in 2D IOSs.

Nitrogen-vacancy ensemble magnetometry based on pump absorption

We demonstrate magnetic field sensing using an ensemble of nitrogen-vacancy centers by recording the variation in the pump-light absorption due to the spin-polarization dependence of the total ground state population. Using a 532 nm pump laser, we measure the absorption of native nitrogen-vacancy centers in a chemical vapor deposited diamond placed in a resonant optical cavity. For a laser pump power of 0.4 W and a cavity finesse of 45, we obtain a noise floor of $\sim$ 100 nT/$\sqrt{\textrm{Hz}}$ spanning a bandwidth up to 125 Hz. We project a photon shot-noise-limited sensitivity of $\sim$ 1 pT/$\sqrt{\textrm{Hz}}$ by optimizing the nitrogen-vacancy concentration and the detection method.

Optical Cooling of CdSe/ZnS Quantum Dots Embedded in PMMA

In 1929, the German physicist Peter Pringsheim proposed that anti-Stokes emission can lead to the cooling of bulk matter. Anti-Stokes emission results in photons of shorter wavelength (higher photon energy) than that of the exciting light due to thermal absorption. In solids, the thermal energy is mostly due to the vibrational modes (phonons) of the lattice. Thus, using lasers to cool a solid, one has to irradiate a sample with laser light in the red tail of the absorption spectrum. The material to be cooled would then absorb a photon and absorb extra energy from a phonon to emit a blue-shifted photon of higher energy. By removing these phonons, the material is cooled. Today, this technique is known as laser cooling of solids or optical refrigeration. It can be used to achieve an all-solid-state cryocooler that is compact, contains no moving parts, has a high reliability, and does not require the use of cryogenic fluids. Laser cooling also allows for the possibility of portable lasers that require no or smaller external cooling systems because the pump wavelength can be adjusted such that spontaneous anti-Stokes luminescence cooling compensates for the stimulated quantum defect heating. A thermally balanced laser such as this would not suffer from thermal defocusing or heat damage; therefore, such solid-state-lasers could achieve higher output powers. Unfortunately, for cooling to occur in solids, the quantum efficiency of the material must be high and nearly all the anti-Stokes luminescence must leave the material without being reabsorbed. Recently, much research has been performed on rare-earth ions for laser cooling. However, the cooling efficiency of rare-earth ions approaches zero as the temperature decreases. As a result, they have a theoretical cooling limit about 90 K. On the other hand, semiconductors provide more efficient pump light absorption, the potential for lower temperatures of 10 K, and the ability to directly integrate the material into electronic and photonic devices. This coupled with the strong electron-phonon coupling of Group II-VI semiconductors and core-shell quantum dot structure minimizing the reabsorption of the anti-Stokes luminescence, makes direct bandgap semiconductors viable laser cooling candidate materials. Owing to the possibility for near-unity quantum efficiency of CdSe/ZnS, we have chosen to investigate CdSe/ZnS quantum dots for laser cooling applications. Recently, we have utilized photothermal deflection techniques to show that CdSe/ZnS can be cooled at a microscopic level. Moreover, we developed a new photothermal technique that is capable of discerning heating or cooling inside a material using nothing but the probe deflection thereby eliminating the need for a Stokes and anti-Stokes laser. Using the information gained from the photothermal results, we have begun to cool CdSe/ZnS polymers. This talk will go over the cooling results as well as our new photothermal technique.

Mechanical oscillations in lasing microspheres

We investigate the feasibility of activating coherent mechanical oscillations in lasing microspheres by modulating the laser emission at a mechanical eigenfrequency. To this aim, 1.5%Nd3+:Barium-Titanium-Silicate microspheres with diameters around 50 μm were used as high quality factor (Q > 106) whispering gallery mode lasing cavities. We have implemented a pump-and-probe technique in which the pump laser used to excite the Nd3+ ions is focused on a single microsphere with a microscope objective and a probe laser excites a specific optical mode with the evanescent field of a tapered fibre. The studied microspheres show monomode and multi-mode lasing action, which can be modulated in the best case up to 10 MHz. We have optically transduced thermally activated mechanical eigenmodes appearing in the 50–70 MHz range, the frequency of which decreases with increasing the size of the microspheres. In a pump-and-probe configuration, we observed modulation of the probe signal up to the maximum pump modulation frequency of our experimental setup, i.e., 20 MHz. This modulation decreases with frequency and is unrelated to lasing emission, pump scattering, or thermal effects. We associate this effect to free-carrier-dispersion induced by multiphoton pump light absorption. On the other hand, we conclude that, in our current experimental conditions, it was not possible to resonantly excite the mechanical modes. Finally, we discuss on how to overcome these limitations by increasing the modulation frequency of the lasing emission and decreasing the frequency of the mechanical eigenmodes displaying a strong degree of optomechanical coupling.

1.2W laser amplification at 1427nm on the 4F3/2 to 4I13/2 spectral line in an Nd3+ doped fused silica optical fiber.

A 9.3dB improvement in optical gain and a 100x improvement in total optical power over prior published experimental results from the 4F3/2 to 4I13/2 transition in an Nd3+ doped fused silica optical fiber is demonstrated. This is enabled via an optical fiber waveguide design that creates high spectral attenuation in the 1050-1120nm-wavelength range, a continuous spectral filter for the primary 4F3/2 to 4I11/2 optical transition. A maximum output power at 1427nm of 1.2W was attained for 43mW coupled seed laser power and 22.2W of coupled pump diode laser power at 880nm a net optical gain of 14.5dB. Reducing the coupled seed laser power to 2.5mW enabled the system to attain 19.3dB of gain for 16.5W of coupled pump power. Four issues limited results; non-optimal seed laser wavelength, amplified spontaneous emission on the 4F3/2 to 4I9/2 optical transition, low absorption of pump light from the cladding and high spectral attenuation in the 1350-1450nm range. Future fibers that mitigate these issues should lead to significant improvements in the efficiency of the laser amplifier, though the shorter wavelength region of the transition from 1310nm to >1350nm is still expected to be limited by excited state absorption.

Designing a Broadband Pump for High-Quality Micro-Lasers via Modified Net Radiation Method.

High-quality micro-lasers are key ingredients in non-linear optics, communication, sensing and low-threshold solar-pumped lasers. However, such micro-lasers exhibit negligible absorption of free-space broadband pump light. Recently, this limitation was lifted by cascade energy transfer, in which the absorption and quality factor are modulated with wavelength, enabling non-resonant pumping of high-quality micro-lasers and solar-pumped laser to operate at record low solar concentration. Here, we present a generic theoretical framework for modeling the absorption, emission and energy transfer of incoherent radiation between cascade sensitizer and laser gain media. Our model is based on linear equations of the modified net radiation method and is therefore robust, fast converging and has low complexity. We apply this formalism to compute the optimal parameters of low-threshold solar-pumped lasers. It is revealed that the interplay between the absorption and self-absorption of such lasers defines the optimal pump absorption below the maximal value, which is in contrast to conventional lasers for which full pump absorption is desired. Numerical results are compared to experimental data on a sensitized Nd3+:YAG cavity, and quantitative agreement with theoretical models is found. Our work modularizes the gain and sensitizing components and paves the way for the optimal design of broadband-pumped high-quality micro-lasers and efficient solar-pumped lasers.

5kW GTWave fiber amplifier directly pumped by commercial 976nm laser diodes.

With home-made fiber perform and special fiber drawing & coating technique, a new-type of (3 + 1) GTWave fiber theoretically designed for bi-directional pump method, was successfully fabricated and justified of integrating multi-kW pump energy from commercial 976nm laser diodes. This (3 + 1) GTWave fiber amplifier demonstrated uniform absorption of pump light and easy thermal management characteristics along the whole fiber length. This amplifier is capable of simultaneously aggregating 5.19kW pump power at 976nm and finally generating 5.07kW laser output at 1066.5nm with an optical-to-optical efficiency of 74.5%, the first publically-reported multi-kW GTWave fiber directly pumped with commercial 976nm laser diodes to the best of our knowledge. No power roll-over was found at 5kW level and further power scaling can be expected with more pump power. The results indicate that GTWave fiber is a competitive integrated fiber device to collect enough pump energy from low-cost commercial laser diodes for multi-kW fiber laser development.

Tuning to a particular acoustic whispering-gallery mode in the GHz range

Surface Acoustic Waves (SAWs) generated with sub-picosecond light pulses are commonly used in non-destructive testing. The absorption of pump light by the medium generates SAWs that are detected by delayed probe light pulses. The spatiotemporal evolution of the SAWs can be imaged by scanning the position and time delay of the focused probe light pulses. Commonly used setups show two main constraints: the pump light is focused to a circular spot that generates SAWs in all directions, which does not allow control of directionality, and the laser repetition rate frep limits accessible frequencies to nfrep (n = 1,2,…). In the case of laser excitation and detection of whispering-gallery modes (WGM) on a disc, only certain frequencies are detectable, and counter-propagating modes cannot be separately excited. Here, we overcome these limitations in experimental GHz SAW imaging of WGMs. To access arbitrary frequencies, we periodically modulate in intensity of the pump and probe beams and make use of both in-phase and in-quadrature lock-in detection [1]-[3]. To generate WGMs propagating in a single direction, we make use of a spatial light modulator and computer-generated holograms. These new results extend the possibilities of SAW imaging by allowing fine control of excited surface acoustic modes. [1] S. Kaneko, M. Tomoda, and O. Matsuda, AIP Adv. 4, 017124 (2014); [2] O. Matsuda et al., IEEE Trans. Ultrason. Ferroelectr. Freq. Control 62, 584-595 (2015); [3] S. Mezil et al., Opt. Lett., 40, 2157-2160 (2015).

Laser Cooling of CdSe/ZnS Quantum Dots

In 1929, the German physicist Peter Pringsheim proposed that anti-Stokes emission can lead to the cooling of bulk matter. Anti-Stokes emission results in photons of shorter wavelength (higher photon energy) than that of the exciting light due to thermal absorption. In solids, the thermal energy is mostly due to the vibrational modes (phonons) of the lattice. Thus, using lasers to cool a solid, one has to irradiate a sample with laser light in the red tail of the absorption spectrum. The material to be cooled would then absorb a photon and absorb extra energy from a phonon to emit a blue-shifted photon of higher energy. By removing these phonons, the material is cooled. Today, this technique is known as laser cooling of solids or optical refrigeration. It can be used to achieve an all-solid-state cryocooler that is compact, contains no moving parts, has a high reliability, and does not require the use of cryogenic fluids. Laser cooling also allows for the possibility of portable lasers that require no or smaller external cooling systems because the pump wavelength can be adjusted such that spontaneous anti-Stokes luminescence cooling compensates for the stimulated quantum defect heating. A thermally balanced laser such as this would not suffer from thermal defocusing or heat damage; therefore, such solid-state-lasers could achieve higher output powers. Unfortunately, for cooling to occur in solids, the quantum efficiency of the material must be high and nearly all the anti-Stokes luminescence must leave the material without being reabsorbed. Recently, much research has been performed on rare-earth ions for laser cooling. However, the cooling efficiency of rare-earth ions approaches zero as the temperature decreases. As a result, they have a theoretical cooling limit of 100 K. On the other hand, semiconductors provide more efficient pump light absorption, the potential for lower temperatures of 10 K, and the ability to directly integrate the material into electronic and photonic devices. This coupled with the strong electron-phonon coupling of Group II-VI semiconductors and core-shell quantum dot structure minimizing the reabsorption of the anti-Stokes luminescence, makes direct bandgap semiconductors viable laser cooling candidate materials. Owing to the possibility for near-unity quantum efficiency of CdSe/ZnS, we have chosen to investigate CdSe/ZnS quantum dots for laser cooling applications. Recently, we have determined that the optimum laser excitation wavelength for CdSe/ZnS anti-Stokes emission is 647 nm. This wavelength indicates that the second-harmonic longitudinal optical phonon (2LO) is strongly coupled to the laser photons. Using this wavelength, we were able to cool a 4-mL colloidal solution containing 2 mg of CdSe/ZnS per 1 mL of toluene by 2.3°C from room temperature. However, it took several hours to cool the liquid owing to the convectional cooling produced by the small-diameter laser beam, which was only cooling 3% of the volume of the solution. To reduce this problem, we added a beam expander to irradiate the entire sample area and speed up the cooling process. In addition, we split the beam to simultaneously measure the changes in temperature of CdSe/ZnS colloid and of pure toluene. Photothermal techniques have also been explored to accurately measure the temperature of the CdSe/ZnS material. In addition, polymer hosts have been prepared for the CdSe/ZnS quantum dots.