Excitation Laser Power Research Articles

Photoelectrochemical Reaction in an Electric Cell with a Porous Carbon Anode

A photoelectrochemical cell utilizing porous carbon derived from Binchotan charcoal as the anode is demonstrated. Visible pulse laser irradiation of the Binchotan charcoal anode induces the ablation of carbon cations into NaNO3 aqueous solution. Electrons remaining in the anode reduce the potential of the anode with respect to a platinum cathode. As the electrodes are short-circuited, photogenerated electrons are transported from the anode to the cathode as a diffusion current. The photoinduced charge density at the anode exhibits superlinear and linear excitation laser power density dependences at low and high laser power densities, respectively. This difference in excitation power dependence is caused by the valences of the carbon cations generated by absorbing two-photon energy. The revealed reaction mechanism for electron generation at the Binchotan charcoal anode in the photoelectrochemical cell suggests that the sacrificial reagent for the anode can be not only Binchotan charcoal but also any materi...

Characterization of Copper–Graphite Composites Fabricated via Electrochemical Deposition and Spark Plasma Sintering

In the present study, we have demonstrated a facile and robust way for the fabrication of Cu-graphite composites (CGCs) with spatially-aligned graphite layers. The graphite layers bonded to the copper matrix and the resulting composite structure were entirely characterized. The preferential orientation and angular displacement of the nano-sized graphite fiber reinforcements in the copper matrix were clarified by polarized Raman scattering. Close investigation on the change of the Raman G-peak frequency with the laser excitation power provided us with a manifestation of the structural and electronic properties of the Cu-graphite composites (CGCs) with spatially-distributed graphite phases. High resolution transmission electron microscopy (TEM) observation and Raman analysis revealed that reduced graphite oxide (rGO) phase existed at the CGC interface. This work is highly expected to provide a fundamental way of understanding how a rGO phase can be formed at the Cu-graphite interface, thus finally envisioning usefulness of the CGCs for thermal management materials in electronic applications.

Probing third-order nonlinearity in serotonin: A Z-scan study.

Serotonin (5-hydroxytryptamine, 5-HT) is a crucial endogenous monoamine neurotransmitter that modulates neurotransmission, gastrointestinal motility, hemostasis, and cardiovascular integrity. There have been numerous attempts to study the biochemical and photophysical properties of serotonin to carry out its molecular imaging and quantitative estimation. Here, we investigate the properties of serotonin at physiological concentration and pH using a continuous wave (CW) laser excitation closed-aperture (CA) Z-scan technique. Serotonin is packaged at high concentration inside the acidic environment of vesicles, and upon release gets diluted at the release sites in a neutral pH environment. Our solution-based measurements indicate that serotonin showed negative refractive nonlinearity and positive absorptive nonlinearity at a neutral pH. However, in the acidic medium, it showed negative refractive nonlinearity and mostly negative absorptive nonlinearity. The effect of excitation laser power on the observed nonlinearity is also verified. We attribute the origin of the nonlinearity in serotonin to the thermal lensing effect. Our robust and straightforward strategy to probe the monoamine neurotransmitter properties will provide new avenues to investigate serotonergic processes.

GCaMP6 ΔF/F dependence on the excitation wavelength in 3-photon and 2-photon microscopy of mouse brain activity

A fundamental challenge in calcium imaging is to minimize the excitation laser power while still maintaining a sufficient signal-to-noise ratio to distinguish individual transients in the fluorescence traces. It is important to characterize relative fluorescence (i.e., ΔF/F) dependence on the excitation wavelength in vivo where the environment cannot be controlled effectively during imaging. Leveraging time division multiplexing of two excitation beams to achieve nearly simultaneous 2-photon and 3-photon imaging of the calcium transients, we measured systematically the ΔF/F dependence on the excitation wavelength in 2-photon and 3-photon in vivo imaging of neuronal activity in mouse brain labeled with GCaMP6s. The technique can be applied to in vivo measurements of other fluorescence sensors.

Physical Properties of Photonic Cooper Pairs Generated via Correlated Stokes–anti‐Stokes Raman Scattering

The theoretical and experimental background for the creation of photonic Cooper pairs (PCPs) via correlated Stokes–anti‐Stokes Raman scattering is introduced, followed by experimental results on the second‐order photon time correlation g2(0) in diamond. It is shown how g2(0) changes with energy (Stokes–anti‐Stokes Raman shift), momentum (scattering angle) and excitation laser power. Both real and virtual phonon‐mediated scattering processes are addressed.

The effect of excitation laser power in Raman spectroscopic measurements of the degree of conversion of resin composites

ObjectivesTo evaluate the effect of excitation laser power in Raman spectrometry by comparing the spectra and the degree of conversion (DC) values obtained using excitation powers between 300 and 1000mW. MethodsFive commercial and three experimental resin composites were light cured at 1200mW/cm2 for 10–20s from a commercial blue-violet LED dental curing unit. Raman spectra were collected from composite specimens within 9min after light-curing. The excitation laser (1064nm) was focused on the spot of 0.4mm in diameter. The following powers were used for specimen excitation (mW): 300, 400, 600, 800, and 1000. From Raman spectra, the DC values were calculated and compared among different laser powers. Also, vector-normalized Raman spectra collected using the lowest excitation power (300mW) were compared to those collected using the maximum excitation power (1000mW). ResultsVarying the excitation laser power between 300 and 1000mW resulted in statistically significant differences in both the DC values and the intensity of particular spectral features. The effect of varying laser power on Raman spectra and obtained DC values was material-dependent. The DC values measured within an individual material using different laser powers varied between 3.2 and 7.2% (absolute DC difference). The spectral bands affected by variations in laser power were assigned to symmetric and asymmetric stretching of −CH2 (2900-3100cm−1), symmetric stretching of aliphatic CC (1640cm−1) and scissoring of C–H (1458cm−1). SignificanceThe DC can be artificially elevated through increasing excitation laser power. This effect should be considered in Raman spectroscopic evaluations of DC in specimens during ongoing post-cure polymerization.

Tunable dual emission in type-I/type-II CdSe/CdS/ZnSe nanocrystals

We have used wet chemical methods to fabricate colloidal CdSe/CdSm/ZnSen core/intermediate/shell (C/I/S) nanocrystals (NCs), where m = 1 and 2 and n = 1–4 are the number of monolayers. The growth was monitored by using Raman and UV–vis spectroscopy, which demonstrated the formation of correct CdSe/CdSm/ZnSen C/I/S NCs. X-ray diffraction studies proved single-phase NCs crystallized in the zincblende-type structure. Photoluminescence (PL) studies have indicated that after photoexcitation C/I/S NCs generate simultaneously type-I and type-II emissions, namely EI and EII, associated with CdSe/CdSm and CdSm/ZnSen structures, respectively. For a specific value of m = 2, we have found the redshift of both EI and EII emissions when n is increased from 1 to 4. The PL studies versus the laser-excitation power (Pex) up to about four orders of magnitude allow us to identify the origin of two emissions. A large blueshift of the EII peak is ascribed to the band bending effect resulting from the spatially separated photoexcited carriers in type-II NCs. It appears that the dependence of the EII peak on the cube root of Pex (Pex1/3) is linear while that of the EI peak on Pex1/3 is almost constant. In this work, we also point out that the emission intensity (I) of EI and EII can be tuned by changing the values of n and Pex, where I obeys a power law I∝Pexk, with k = 0.7–1.0 dependent on the emission type. The nature of these phenomena is discussed in comparison with previous studies on C/S and C/I/S nanoheterostructures.

Unusual near-infrared luminescence from Ti-doped MgSiF6·6H2O powder

Abstract We prepared Ti-doped MgSiF6·6H2O powder through a chemical synthesis method and observed Ti-related near-infrared (NIR) photoluminescence (PL), resulting from the dehydration due to vacuum evacuation. The NIR PL characteristics have unusual features: its emission intensity nonlinearly increases with the excitation laser power and a slow (∼1 s) increase of the emission intensity is observed under the continuous irradiation of the excitation laser light. These results indicate that the NIR PL band is activated by a heating effect due to the laser irradiation.

Nonlinear two-layer model for photocarrier radiometry of ion-implanted silicon wafers

A nonlinear two-layer model was developed to describe and analyze Photocarrier Radiometric (PCR) signals of ion-implanted Si wafers which are intrinsically nonlinear with excitation laser power. The thickness of the implantation layer and the optical/electronic damage threshold for different implantation doses were estimated using the Monte Carlo method and the effective medium approximation theory, respectively, which can provide key parameter values for the model to calculate the nonlinearity coefficient, defined as the slope of PCR amplitude versus excitation power in log-log scale. Experimentally, the nonlinearity coefficients of seven c-Si wafers with implantation doses from 1011 to 1016 cm-2 were measured at two different excitation wavelengths (830 and 405 nm), and good agreement between theory and experiment was found. Results show that the nonlinearity coefficient has a negative correlation with the implantation dose, and the coefficient measured at 405 nm is consistently smaller than that measured at 830 nm for each sample. Compared with the conventional PCR models, the nonlinear two-layer model proposed here is more coincident with experimental facts, thus enabling PCR to provide more accurate quantitative characterization of the carrier recombination and transport properties of ion-implanted semiconductor wafers.

Managing optical heating via Al3+-doping in Er3+:SrF2 powder phosphors prepared by combustion synthesis.

Quenching of photoluminescence due to optical heating generated by high power laser sources has been identified as a major concern for photonics applications that relies on inorganic phosphor materials. Here we investigate how erbium-doped strontium fluoride (Er3+:SrF2) powders prepared by combustion synthesis respond to intense optical heating. We found that the near-infrared to visible photon up-conversion (UC) luminescence from Er3+ was quenched and the internal temperature of the sample increased from 298 to 695 K when the excitation power of a CW diode laser operating at 808 nm was increased from 0.1 to 2.1 W. However, when SrF2 was co-doped with Al3+, we observed an increase in the UC intensity and an unexpected internal temperature reduction of up to 155 K for an excitation power of 2.1 W. Our analysis suggests that Al3+ decreases the phonon energy and increases the local symmetry of the environment of the rare-earth ion in SrF2.

The photoluminescence, field emission and femtosecond nonlinear absorption properties of Al-doped ZnO nanowires, nanobelts, and nanoplane-cone morphologies.

Al-doped ZnO (AZO) nanowires, nanobelts and nanoplane-cone nanostructures have been successfully synthesized. The structural, photoluminescence (PL) and field emission (FE) properties of AZO nanowires have been characterized. The dependence of the PL properties of AZO nanostructures versus excitation laser power in the range from 1 to 12 mW and temperature in the range of 10–273 K was discussed. The PL measurement results demonstrated that the ultraviolet emission came from a near band edge emission, and two peaks in visible light region were due to deep-level emission. Moreover, the AZO nanowires have a relatively stronger ultraviolet emission than other kinds of samples. The FE measurements indicate that the turn-on field for the nanoplane-cone structure is 2.52 V μm−1, which is smaller than 4.42 V μm−1 for nanowires and 5.28 V μm−1 for nanobelts. In addition, the nonlinear absorption properties of AZO nanowires were measured using a femtosecond Z-scan technique. The effect of morphology on the nonlinear optical absorption properties of AZO nanowires was studied. From the results, the AZO nanowires show reverse saturable absorption (RSA) behavior. Furthermore, the results show that the order of magnitude of the nonlinear absorption coefficient for AZO nanowires is ∼10−2 cm3 GW−2. Our results show that AZO films are a promising candidate in further optoelectronic device applications.

Enhancing the upconversion luminescence properties of Er3+–Yb3+ doped yttrium molybdate through Mg2+ incorporation: effect of laser excitation power on temperature sensing and heat generation

Upconversion luminescence was enhanced by incorporating Mg2+ into Er3+–Yb3+-doped yttrium molybdate and the effect of laser excitation power on temperature sensing and nanoheating was investigated.

Raman spectroscopic determination of hole concentration in undoped GaAsBi

Raman spectra of undoped GaAs1−xBix (0 < x < 0.037) grown on GaAs by molecular beam epitaxy were investigated. With an increase of Bi component, we find that the longitudinal optical phonon-hole-plasmon-coupled (LOPC) mode first appears in the vicinity of the unscreened longitudinal optical (ULO) phonon frequency, and then shifts towards the transverse optical (TO) phonon frequency. A new vibrational mode (∼287 cm−1) between the TO and the ULO phonons was verified by use of low temperature polarized Raman measurement and the corresponding scattering intensities are found to be linearly proportional to the composition of Bi in GaAsBi. The hole concentrations determined by using the LOPC/ULO Raman intensities ratio increase from ∼6.5 × 1016 to ∼2.8 × 1017 cm−3 with increased Bi content and the measured results are in agreement with Hall measurements. Furthermore, the influence of excitation laser power on the estimation of the hole densities is discussed with the help of power dependent Raman spectroscopy.

Laser-induced bulk modification in KDP crystal analyzed by optical spectroscopy under high-power laser excitation

Combining high-power laser irradiation and a measurement method for emission spectra, a spectral analysis technique under high power laser excitation was developed. The emission spectra of undamaged and damaged sites in the bulk of a KDP crystal under high-power laser irradiation at 355 nm and 532 nm were studied in detail. By comparing emission peaks detected from undamaged sites, it’s believed that Raman scattering dominated the undamaged sites. As for the damaged sites, the Raman scattering features still existed with unchanged intensities, but a strong emission peak centered at 467 nm was detected under laser irradiation both at 355 nm and 532 nm. The difference between undamaged and damaged sites in KDP crystals was interpreted as a result of laser-induced modification. It’s believed that micron-level damage had little influence on the molecular structure of the crystal, but the intense local vibration caused by the effect of stimulated Raman scattering led to the generation of a new defect energy level.

Effect of LiNbO3 polarity on the structural, optical and acoustic properties of epitaxial ZnO and MgxZn1−xO films

The effect of LiNbO3 substrate polarization on a charge density at the interface with epitaxial ZnO and MgxZn1−xO films was studied in comparison with ZnO films on dielectric sapphire substrates. An excitonic emission and optical properties such as band gap energy, refraction index, absorption and reflectivity of the ZnO/LiNbO3 structures were studied. The effect of interface charges on the performance of surface acoustic waves with frequency of 2.7 GHz was investigated, as well. The stimulated cavity mode emission was observed due to attained quasi-phase matching conditions for thin ZnO films on sapphire at high laser excitation power. The positive polarization of the ferroelectric substrate induced the band bending of the semiconductor layer, resulting in the accumulation of the negative charges at the ferroelectric/semiconductor interface, which induced an increased reflectivity, a generation of oxygen emission lines in PL spectra under laser excitation, and an increased electromechanical coupling of surface acoustic waves. The negative polarization of the substrate increased atomic peening during sputtering deposition, which induced deteriorated quality of texture and in-plane orientation and increased the residual stresses in ZnO and MgxZn1−xO films.

Radiation dose detection using a high-power portable optically stimulated luminescence real-time reading system

Optically stimulated luminescence (OSL) reading systems are becoming smaller and capable of real-time detection. To improve real-time and multipurpose radiation dosimetry readings, we built a real-time continuous-wave (RCW) OSL reading system. This system is both small and lightweight, and it employs powerful laser excitation (478 mW/cm2) at the dosimetry probe location. We investigate the possibility of using the RCW mode to read the radiation luminescence (RL) or OSL by using a single-crystal Al2O3:C dosimeter in a low-dose-rate 137Cs γ field. Our results indicate that the RL/OSL follows a stable and uniform distribution. The minimum detected doses associated with the RL, OSL, and RL + OSL signals are 2.1 × 10−2, 3.17 × 10−1, and 5.7 × 10−2 μGy, respectively. This device provides a framework for the future development of applications for practical radiation dose measurements.

Correlation between photoluminescence and morphology for single layer self-assembled InGaAs/GaAs quantum dots

Single layer self-assembled InGaAs quantum dots (QDs) are manipulated by using different arsenic species on GaAs (100) surface. The As4 molecules are experimentally observed to be more promising than As2 to promote the formation of one-dimensionally-aligned QD-chain arrays. The lateral alignment of QDs and the corresponding formation of dot chains are explained by the anisotropic surface kinetics in combination with the different reactivities of the two molecules with bonding sites on the GaAs (100) surface. Photoluminescence (PL) measurements demonstrate that the spectra of the QD-chains broaden to higher energy and increases in intensity with increasing excitation laser power. The PL band of the QD-chains also exhibits a 9 meV reduction in linewidth as temperature increases starting from 8 K. These observations confirm an efficient lateral coupling between neighboring QDs and thereafter polarized QD emission, whereas the randomly distributed QDs grown with As2 show no preferential polarization. Such QD-chains exhibiting anisotropic properties have the potential for nanophotonics applications like electro-optic modulators with very low drive voltage and ultra-wide bandwidth operation.

Development of plasmonic U‐bent plastic optical fiber probes for surface enhanced Raman scattering based biosensing

AbstractEvanescent wave‐based excitation of noble metal nanostructures on fiber core surface allows simultaneous excitation over a larger area in comparison with free space coupling, and hence highly suitable for development of surface‐enhanced Raman scattering (SERS)‐based sensor applications. This study describes the development of plasmonically active U‐bent plastic optical fiber (POF) probes for SERS‐based sensing. The SERS activity of gold nanostructures on the fiber probe surface obtained by means of electroless deposition, sputtering, and chemisorption of nanoparticles are thoroughly characterized using a confocal microscopic setup. Influence of excitation wavelength and laser excitation power on SERS activity and stability of the plasmonic U‐bent probes is evaluated. Gold‐sputtered POF probes and gold nanoparticle immobilized POF probes show higher enhancement factor in the order to ~108. A biosensor application of these POF‐based SERS sensors is demonstrated by realizing a sandwich immunoassay with 4‐MBA as Raman label.

Surface-Enhanced Raman Spectroscopy Analysis of Human Breast Cancer via Silver Nanoparticles: An Examination of Fabrication Methods

Breast cancer in a traditional way is diagnosed using mammography, computer tomography, ultrasounds, biopsy, and finally, histopathological analysis. Histopathological analysis is a gold standard in breast cancer diagnostics; however, it is time consuming and prone to the human interpretations. That is why new methods based on optical properties of analyzed human tissue samples are needed to be introduced to the clinical practice objective, costless and fast diagnostic protocols. Nowadays, Raman spectroscopy-based methods are gaining more and more importance. Raman spectroscopy and imaging allow to characterize human tissue samples using an electromagnetic radiation from a safe range, and simultaneously, a minimal sample preparation is required. During measurements, a natural differentiation in tissues components’ scattering cross sections is used to build 2D and 3D maps of the chemical component distribution. The paper presents the application of SERS (surface-enhanced Raman spectroscopy) measurements for analysis of human breast cancer (adenocarcinoma). The advantages of SERS application in cancer diagnostics are also discussed. Moreover, the detailed chemical composition of human breast cancer tissue based on Raman bands of DNA/RNA, amino acids, lipids, and proteins which are significantly enhanced is presented. Three different methods of NP preparation are presented, and the effectiveness of Raman signal enhancement of Ag nanoparticles synthetized by these methods is compared. The enhancement effect of NPs synthetized by reduction of silver nitrate with sodium borohydride (method no. 1) and silver nitrate-hydroxylamine hydrochloride reduction (method no. 2) was stronger when compared with the polyol method (method no. 3). Presented SERS results confirmed that the clearly resolved and high-intensity Raman spectra of cancer human breast tissue can be recorded using integration times of the order of fractional seconds and one milliwatt of the excitation laser power.

Reduced potential fluctuation in a surface sulfurized Cu(InGa)Se2

Cu(InGa)Se2 (CIGSe) that was grown under Cu-deficient conditions exhibited potential fluctuations in its band structure. When the donor and acceptor density is high and compensated for in CIGSe, the local deviations in the spatial distribution of donors and acceptors result in potential fluctuations. This study demonstrates the reduced potential fluctuation of CIGSe using the surface sulfurization process. We performed photoluminescence (PL) analysis on non-sulfurized and sulfurized CIGSe in the temperature range 15–100 K with varying excitation laser power values (1.7–180.6 mW/cm2). The results state that sulfurized CIGSe exhibited reduced potential fluctuations in comparison with non-sulfurized CIGSe. The depth of potential wells in sulfurized CIGSe was 5 meV, which was significantly reduced compared to that of non-sulfurized CIGSe (13 meV). This implies that surface sulfurization led to the substitution or reduction of donors (VSe2− and/or InCu2+) due to reaction with S. Subsequently, the compensation was moderated in sulfurized CIGSe.