Excitation Laser Power Research Articles

Laser-Induced Hydrogen Generation from Methanol with Graphene Aerogel as the Target

The present work demonstrates a new concept of the efficient generation of hydrogen from methanol by the continuous wave laser diode irradiation of an immersed graphene aerogel (GA) scaffold as the target. It was observed that the process occurred very intensively when it was assisted by bright white light emission in the spot of a laser-irradiated GA scaffold. The yield of hydrogen emission increased exponentially with the applied laser power. The light emission was assisted by the intense production of H2, CH4, and CO gases. It was found that with increasing excitation laser power, the H2 generation increased at the expense of CO. It is shown that the volume of CO decreases because of the formation of C2 molecules and CO2 gases. The mechanism of the laser-driven dissociation of methanol was discussed in terms of the violent ejection of hot electrons from the GA surface as a result of the laser-induced light emission of the graphene target.

Magnetic and power tuning of spin-asymmetric multiple excitons in a GaAs quantum well

Abstract A comprehensive study of excitonic complexes in high-quality GaAs quantum wells has been performed. The polarization-resolved photoluminescence results show exciton, biexciton, and trion emissions tuned by temperature, excitation power, and by an external magnetic field. Excitons and biexcitons show important differences in their dependence on the external field. While the Zeeman splitting of biexcitons depends monotonically on the magnetic field, with a nearly constant g-factor, the behaviour of the exciton energy splitting is nonmonotonic and includes an inversion of sign as a function of the magnetic field. More importantly, we observe that a trion resonance emerges at a finite magnetic field, and only appears at low laser excitation power and for σ+ polarization emission. The non-trivial dependence of the energy levels on the external magnetic field, together with the polarization selective trion emission, leads to an intricate exciton and trion dynamics, which can be reproduced by a set of coupled rate equations. Based on this theoretical approach, we show that a fast spin-flip process can lead to the spin-asymmetric emission, which reveals a contrasting spin-dynamics of excitonic complexes ruled by their charge.

Enhancing photostriction in KNN-based ceramics by constructing the morphotropic phase boundary and narrowing the energy band gap

Lead-free ferroelectric ceramics exhibiting photostriction effect has attracted a lot of attention in the past decade. Herein, we fabricated a series of (1−x)K0.5Na0.5NbO3-x(La0.51Na0.49)(Zr0.54Ni0.46)O3 ((1−x)KNN-xLNNZ) ceramics by traditional solid-state synthesis method aiming to improve photostriction. The addition of LNNZ in KNN has led to significant changes in phase structure and grain size. The ceramics with a composition of 0.97KNN-0.03LNNZ has shown a narrow bandgap ~2.43 eV, large piezoelectric coefficient (~209 pC/N), low dielectric loss (0.021 at 1 kHz) and high remnant polarization (~24 μC/cm2). Further, 0.97KNN-0.03LNNZ also exhibits a considerable photostrictive coefficient (~1.83 × 10−9 m2W−1), which is attributed to the combined effect of significantly narrow bandgap along with the morphotropic phase boundary. Further, all the substituted samples show distinct red shift of v1 mode compared to the pure KNN, which suggest a new kind of distortion introduced in (Nb, Zr, Ni)O6 octahedron for KNN-based ceramics under variable power laser excitation. The obtained results indicate that LNNZ-substituted KNN ceramics can serve as a potential material for the fabrication of optomechanical devices.

High resolution imaging with anomalous saturated excitation

The nonlinear fluorescence emission has been widely applied for high spatial resolution optical imaging. Here, we studied the fluorescence anomalous saturating effect of the nitrogen vacancy defect in diamond. The fluorescence reduction was observed with high power laser excitation. It increased the nonlinearity of the fluorescence emission, and changed the spatial frequency distribution of the fluorescence image. We used a differential excitation protocol to extract the high spatial frequency information. By modulating the excitation laser’s power, the spatial resolution of imaging was improved approximately 1.6 times in comparison with the confocal microscopy. Due to the simplicity of the experimental setup and data processing, we expect this method can be used for improving the spatial resolution of sensing and biological labeling with the defects in solids.

Multiple Hydrogen Bond-Induced Structural Distortion for Broadband White-Light Emission in Two-Dimensional Perovskites

Low-dimensional hybrid lead-halide perovskites with broadband white-light emission upon near-UV excitation have attracted immense scientific interest due to their potential application for the next...

Lotka-Volterra population dynamics in coherent and tunable oscillators of trapped polariton condensates

We demonstrate a regime in which matter-wave condensates of exciton-polaritons trapped in an elliptically shaped two-dimensional potential appear as a coherent mixture of ground and first-excited state of the quantum harmonic oscillator. This system resembles an optically controllable two-level system and produces near terahertz harmonic oscillations of the condensate's center of mass along the major axis of the elliptical trapping potential. The population ratio between the two trap levels is tunable through the excitation laser power and is shown to follow Lotka-Volterra dynamics. We demonstrate coherence formation between two spatially displaced trapped condensate oscillators - the polaritonic analogue of Huygen's clock synchronization for coupled condensate oscillators.

Adjustable charge states of nitrogen-vacancy centers in low-nitrogen diamond after electron irradiation and subsequent annealing

In this work, we investigate the photoluminescence spectra of nitrogen-vacancy (NV) centers in low-nitrogen diamond under 200 keV electron irradiation. We discuss the dependence of NV center charge states on annealing temperature, laser excitation power, and measurement temperature. The results show that the NV centers in low-nitrogen diamond are more likely to exist in the form of NV0 centers. Annealing breaks the charge balance between two charged NV centers, and, as the annealing temperature increases (300–800 °C), NV− centers are converted into NV0 centers. Meanwhile, the NV intensities significantly increase when the nitrogen atoms capture the vacancies after electron irradiation and subsequent annealing. With increases in laser power, the NV− centers are more prone to Auger recombination, and thus, the NV− centers are converted into NV0 centers. In addition, the NV centers are quenched by increases in measurement temperature, but the measurement temperature does not affect their intensity ratio. This result indicates that low-nitrogen diamond becomes more likely to form stable NV0 and NV− centers at different measurement temperatures.

Photothermal excitation efficiency enhancement of cantilevers by electron beam deposition of amorphous carbon thin films

In recent years, the atomic force microscope has proven to be a powerful tool for studying biological systems, mainly for its capability to measure in liquids with nanoscale resolution. Measuring tissues, cells or proteins in their physiological conditions gives us access to valuable information about their real ‘in vivo’ structure, dynamics and functionality which could then fuel disruptive medical and biological applications. The main problem faced by the atomic force microscope when working in liquid environments is the difficulty to generate clear cantilever resonance spectra, essential for stable operation and for high resolution imaging. Photothermal actuation overcomes this problem, as it generates clear resonance spectra free from spurious peaks. However, relatively high laser powers are required to achieve the desired cantilever oscillation amplitude, which could potentially damage biological samples. In this study, we demonstrate that the photothermal excitation efficiency can be enhanced by coating the cantilever with a thin amorphous carbon layer to increase the heat absorption from the laser, reducing the required excitation laser power and minimizing the damage to biological samples.

Homogeneous NaYF4 dual-functioned core-shell nanostructures with NaYF4:Er3+/Yb3+ core as temperature probe and NaYF4:Sm3+/Yb3+ shell as photothermal converter

In recent years, multi-functional nano-structures potentially used as photothermal conversion agents, biomedical imaging platforms, drug delivery carriers and temperature probes have been widely investigated. In this paper, NaYF4:Er3+/Yb3+@NaYF4:Sm3+/Yb3+ core-shell nanoparticles (NPs) which combined the functions of both the photothermal conversion and temperature detection were synthesized. Temperature sensing properties of the core-shell particles were investigated, and it was found that NaYF4:Er3+/Yb3+ self-coating or NaYF4:Sm3+/Yb3+ coating did not cause substantial change in the temperature sensing properties of Er3+. Furthermore, the photothermal conversion was investigated, and it was found that the photothermal conversion can be achieved by the designed core-shell particles. Moreover, the photo-to-heat conversion capacity increases with increasing Yb3+ concentration and 980 nm laser excitation power, which was also verified by the laser-induced ethanol evaporation experiments.

Strong photocharging effect in CsPbBr3 nanocrystal films

In this study, a strong photocharging effect has been detected by a spectroscopic study of colloidal CsPbBr3 quantum dot film. Photocharging induces the formation of charged excitons, even with the application of a very low excitation laser power. Thus, it can introduce a drastic problem on the optical properties of quantum dots when the charges are involved. The charge exciton then can absorb another photon from the next laser pulse, leading to a trion state. The presence of the trion state was proven by studying its behavior against three effects, namely laser power, electric field (EF) and temperature (T) dependence. The trion area of the peak increases linearly with the increasing of the laser power, which we consider to be an indication of the influence of a strong photocharging effect, suggesting that long-lived charges can be kept trapped in quantum dots (QDs). It was also found the trion is affected by an external electric field, due to the weak Stark effect. It is more sensitive than the excitonic state to the electric field due to the presence of electron–electron repulsion forces. A red-shift behavior was observed when heating the sample above 180 K, which is a strong evidence of the presence of a trapping state.

Light-Emitting Diode Excitation for Upconversion Microscopy: A Quantitative Assessment

Lanthanide-based upconversion nanoparticles (UCNPs) generally require high power laser excitation. Here, we report wide-field upconversion microscopy at single-nanoparticle sensitivity using incoherent excitation of a 970 nm light-emitting diode (LED). We show that due to its broad emission spectrum, LED excitation is about 3 times less effective for UCNPs and generates high background compared to laser illumination. To counter this, we use time-gated luminescence detection to eliminate the residual background from the LED source, so that individual UCNPs with high sensitizer (Yb3+) doping and inert shell protection become clearly identified under LED excitation at 1.18 W cm-2, as confirmed by correlated electron microscopy images. Hydrophilic UCNPs are obtained by polysaccharide coating via a facile ligand exchange protocol to demonstrate imaging of cellular uptake using LED excitation. These results suggest a viable approach to bypassing the limitations associated with high-power lasers when applying UCNPs and upconversion microscopy to life science research.

Measurement of non-linear optical properties in graphene oxide using the Z-scan technique.

We report the value of non-linear optical parameters in graphene oxide (GO) using the Z-scan technique. The value of the non-linear refractive index η2, non-linear absorption coefficient β and the complex value of third order susceptibility χ(3) in samples with different concentrations of GO diluted in distilled water were determined. The value of these parameters were obtained directly from the measurement of normalized transmittance of each sample as a consequence of its non-linear response when are exposed to electromagnetic radiation from a Nd:YAG laser emitting at 532nm in CW mode. In addition, we found that the parameters η2, β and χ(3) growth by increasing excitation laser power, where β showed an high order of magnitude around ~10-2 cm/W.

Influence of graphene flakes on upconversion spectra of Y2O3:Yb3+,Er3+ nanocrystalline powders

Abstract The effect of graphene flakes (GF) admixing to the Er3+,Yb3+:Y2O3 nanocrystalline powders on upconversion (UC) emission was investigated. It was found that GFs significantly reduce not only the overall intensity of UC transitions but also change their intensity distribution. In particular the red band corresponding to the 2F9/2 → 4I15/2 transition decreases in comparison to the green band associated with the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions which were significantly weakened. The green to red ratio of transition band intensities (G/R) varied with a content of graphene flakes. It was observed that G/R ratio was dependent on excitation laser power. For larger concentration of GFs there was observed the broadband white emission centered at 650 nm that increased exponentially with excitation power with simultaneous dumping of UC transitions.

Photo-induced doping effect and dynamic process in monolayer MoSe2

Dynamic processes of electron transfer by optical doping in monolayer MoSe2 at 6 K are investigated via measuring time resolved photoluminescence (PL) traces under different excitation powers. Time-dependent electron transfer process can be analyzed by a power-law distribution of t−α with α = 0.1–0.24, depending on the laser excitation power. The average electron transfer time of approximately 27.65 s is obtained in the excitation power range of 0.5 to 100 μW. As the temperature increases from 20 to 44 K, the energy difference between the neutral and charged excitons is observed to decrease.

Optically tunable charge carrier injection in monolayer MoS2

We report on excitation laser power-dependent Raman and photoluminescence (PL) measurements on gold nanoparticles–monolayer MoS2 hybrid nanostructures. Excitation of localized surface plasmon resonances in gold nanoparticles and their subsequent non-radiative relaxation inject charge carriers (electrons) into the adjacent monolayer MoS2. Due to the non-centrosymmetric nature of monolayer MoS2, the localized electron doping induces lattice compression via inverse piezoelectric response. The resultant lattice distortion manifests as a shift as well as broadening of $${A}_{1g}(\Gamma )$$ and $${E}_{2g}^{1}\left(\Gamma \right)$$ MoS2 Raman modes. A splitting of $${E}_{2g}^{1}\left(\Gamma \right)$$ mode is also observed. The PL spectra reveal power-dependent enhancement of trion feature, which again is a signature of an increasing electron doping. The observed effects are confirmed to be doping related as they are absent in monolayer MoS2 without gold nanoparticles. Our observations reveal that charge carrier injection is effectively controlled by varying the excitation laser power, which may aid in optically tuning physical response of MoS2 hybrid nanostructures and devices.

Near Infrared Photoluminescence of N<sub>C</sub>V<sub>Si</sub><sup>-</sup> Centers in High-Purity Semi-Insulating 4H-SiC Irradiated with Energetic Charged Particles

This paper reports optical propertites of negatively charged NCVSi- centers in silicon carbide (a nitrogen substituting for a carbon atom adjacent to a silicon vacancy) whose emission wavlength is 1100-1500 nm at room temperature. High-purity semi-insulating (HPSI) 4H-SiCs are implanted with high energy N ion beams and subsequently thermally annealed to form NCVSi centers. We investigated a wide range of N ion implantation dose using a micro ion beam implantation technique and observed the photoluminescence intensity from the SiC-NV centers. We show that under conditions of heavy implantation, the excitation laser power excites residual defects and their fluorescences intereferes with the emission from the NCVSi- centers. These results allow us to clarify the requirements to optically detect isolated single NCVSi- centers at lightly implanted conditions.

Competition between two- and three-photon upconversion in Er3+-doped microcrystals

The emission spectrum of lanthanide-rich upconverters consists of optically or thermally excited transition bands involving different pathways. We investigate multiphoton processes in Yb3+–Er3+ codoped β−NaGdF4 to demonstrate that the emission from the phosphor involves both two- and three-photon mechanisms, where the sample temperature, and the laser excitation power play crucial roles in determining the leading pathway. The two-photon processes dominate at room temperature, and the effect of higher-order (three-photon) processes becomes increasingly important as the temperature is lowered. The Yb3+–Er3+ systems are reported to be among the best performing ratiometric luminescent thermometers in a very wide range. Our experimental findings demand a careful elimination of errors that may appear from the laser-induced heating of the sample and contributions from higher-order multiphoton processes when using such thermometers below room temperature.

Diffraction ring patterns and nonlinear measurements of the Tris(2′,2-bipyridyl)iron(II) tetrafluoroborate

The nonlinear optical properties of the Tris(2,2′-bipyridyl)iron(II) tetrafluoroborate in aqueous solution was investigated by the spatial self-phase modulation (SSPM) technique using CW laser at λ = 532 nm. The experimental results show large nonlinear refractive index (n2) and the thermo-optic coefficient (dn/dT) equal to −3.17 × 10−6 cm2/W and the 3.17 × 10−3 k−1, respectively. The effect of input power of excitation laser and the temporal evolution on the shape of the diffraction patterns were studied. Consequently, the collapse effect of the diffraction ring patterns was explained.

Emission Efficiency of Terahertz Antennas with Conventional Topology and Metal Metasurface: A Comparative Analysis

Characteristics of photoconductive antennas (PCAs)—radiation sources with conventional topology and a metal metasurface in the shape of a plasmonic grating made on the basis of InGaAs/InAlAs superlattice heterostructures—have been studied experimentally. The photocurrents and THz power spectra of samples of PCAs have been measured, the energy characteristics of terahertz (THz) radiation, and the optical-to-THz conversion efficiency at different bias voltages and average laser excitation power have been determined. For PCAs with a metasurface, the integrated THz radiation power of 10 μW and a conversion efficiency of up to 0.2%, unattainable for antennas with conventional topology due to their thermal breakdown, have been demonstrated experimentally. Therefore, it can be stated that PCAs with a metasurface are efficient sources of THz radiation and can become an element base for constructing THz spectroscopy systems associated with solving medical diagnostic problems.

High-quality nanodisk of InGaN/GaN MQWs fabricated by neutral-beam-etching and GaN regrowth: towards directional micro-LED in top-down structure

A nanodisk array of blue InGaN/GaN multiple quantum wells was made using neutral beam etching (NBE) followed by GaN regrowth. The NBE-fabricated nanodisk presented a vertical and highly smooth sidewall surface where the InGaN well layers were easily distinguished even with a scanning electron microscope. A high interface quality without any voids or obvious defects was obtained between the nanodisk and the regrown-GaN layer. The nanodisk after regrowth presented a smaller blueshift of photoluminescence emission energy (12 meV) and a substantially higher and almost constant internal quantum efficiency of ∼50% over three orders of magnitude of excitation laser power when compared to the nanodisk before regrowth. This study shows that the process of NBE nanodisk etching followed by GaN regrowth represents a promising step forward in the development of truncated cone-shaped directional micro-LEDs with a buried active region in a top-down structure.