Continuous Wave Laser Excitation Research Articles

Light emission from plasmonic nanostructures

The mechanism of light emission from metallic nanoparticles has been a subject of debate in recent years. Photoluminescence and electronic Raman scattering mechanisms have both been proposed to explain the observed emission from plasmonic nanostructures. Recent results from Stokes and anti-Stokes emission spectroscopy of single gold nanorods using continuous wave laser excitation carried out in our laboratory are summarized here. We show that varying excitation wavelength and power change the energy distribution of hot carriers and impact the emission spectral lineshape. We then examine the role of interband and intraband transitions in the emission lineshape by varying the particle size. We establish a relationship between the single particle emission quantum yield and its corresponding plasmonic resonance quality factor, which we also tune through nanorod crystallinity. Finally, based on anti-Stokes emission, we extract electron temperatures that further suggest a hot carrier based mechanism. The central role of hot carriers in our systematic study on gold nanorods as a model system supports a Purcell effect enhanced hot carrier photoluminescence mechanism. We end with a discussion on the impact of understanding the light emission mechanism on fields utilizing hot carrier distributions, such as photocatalysis and nanothermometry.

Dual-Wavelength Continuous Wave Photoacoustic Doppler Flow Measurement

Photoacoustic Doppler flow measurement based on continuous wave laser excitation owns the merit of clearly presenting the Doppler power spectra. Extending this technique to dual wavelengths can gain the spectral information of the flow sample extra to the flow speed information. An experimental system with two laser diodes respectively operated at 405 nm and 660 nm wavelengths is built and the flow measurement with black and red dyed polystyrene beads is performed. The measured Doppler power spectra can vividly reflect the flow speed, the flow direction, as well as the bead color. Since it is straightforward to further apply the same principle to multiple wavelengths, we can expect this type of spectroscopic photoacoustic Doppler flow measurement will be developed in the near future which will be very useful for studying the metabolism of the slowly moving red blood cell inside microvessels.

Notes on thermometric artefacts by Er3+ luminescence band interference

Optical thermometry based on the intensity ratio of the 2H11/2 → 4I15/2 (525 nm) and 4S3/2 → 4I15/2 (545 nm) emission of Er3+ provides a powerful tool for microscale temperature sensing. Crystalline β-NaYF4:Er,Yb is an excellent thermometry material for green upconversion emission upon NIR laser excitation. However, interfering 2H9/2 → 4I13/2 emission is observed around 555 nm for continuous-wave laser excitation intensities above 10 W/cm2. In this study, the green Er3+ emission bands are characterized and it is demonstrated how the true sample temperature is determined circumventing possible artefacts.

Modeling the Induction of Excitons upon the Continuous-Wave Laser Excitation of an Ensemble of Semiconductor Quantum Dots

A theoretical investigation is performed for the possibility and conditions of the stimulated induction of excitons in a thin layer of semiconductor quantum dots upon excitation by continuous-wave laser radiation.

Signature of optical Rabi oscillations in transmission signal of atomic vapor under continuous-wave laser excitation

We have studied the temporal behavior of the atomic absorption signal under resonant excitation with a continuous-wave laser radiation. Measurements done for D2 line of 85Rb with ≈ 1 ns temporal resolution have shown irregular oscillatory behavior of the transmission signal, which becomes well pronounced for high laser power, and disappears when the laser is tuned off-resonance. Application of the fast Fourier transform analysis of the transmission signal reveals power-dependent frequency peaks, which are shown to be associated with Rabi frequency. Possible linkage of the observed results with the phase-to-amplitude noise conversion caused by the phase fluctuations of laser field is discussed.

Spontaneous and Stimulated Emission in Thin Films of Cu(In1 –xGax)(SySe1 –y)2 Solid Solutions in the Сomposition of Solar Cells

The emission spectra of thin nanocrystalline films of Cu(In1 – xGax)(SySe1 – y)2 (CIGSSe) direct-gap solid solutions in the structure of solar cells, recorded upon continuous-wave laser excitation (~0.5 W/cm2) and nanosecond pulsed laser excitation with a power density in the range of 0.1–53 kW/cm2 at temperatures ranging from 10 to 300 K, are analyzed. It is found that stimulated emission (SE) occurs in thin CIGSSe films at temperatures from 10 to 90 K in the spectral range hν = 1.062–1.081 eV, with a minimum threshold-pump level of ~1 kW/cm2. It is shown that the spontaneous emission band is blue-shifted with an increase in the excitation intensity. It is also established that, with an increase in temperature, the photoluminescence (PL) band (upon weak excitation) and the stimulated-emission band are blue-shifted, whereas the PL band is red-shifted upon strong excitation. The possible reasons and mechanisms for the influence of temperature and excitation intensity on the spectral positions of the spontaneous and stimulated emission bands for the solid-solution films are discussed.

Polariton-driven phonon laser

Efficient generation of phonons is an important ingredient for a prospective electrically-driven phonon laser. Hybrid quantum systems combining cavity quantum electrodynamics and optomechanics constitute a novel platform with potential for operation at the extremely high frequency range (30–300 GHz). We report on laser-like phonon emission in a hybrid system that optomechanically couples polariton Bose-Einstein condensates (BECs) with phonons in a semiconductor microcavity. The studied system comprises GaAs/AlAs quantum wells coupled to cavity-confined optical and vibrational modes. The non-resonant continuous wave laser excitation of a polariton BEC in an individual trap of a trap array, induces coherent mechanical self-oscillation, leading to the formation of spectral sidebands displaced by harmonics of the fundamental 20 GHz mode vibration frequency. This phonon “lasing” enhances the phonon occupation five orders of magnitude above the thermal value when tunable neighbor traps are red-shifted with respect to the pumped trap BEC emission at even harmonics of the vibration mode. These experiments, supported by a theoretical model, constitute the first demonstration of coherent cavity optomechanical phenomena with exciton polaritons, paving the way for new hybrid designs for quantum technologies, phonon lasers, and phonon-photon bidirectional translators.

Synthesis, crystal structure and solid-state properties of 4-(3-nitrophenylamino)-4-methylpentan-2-one picrate (3NAP): An efficient cocrystal for χ(3) optics

An organic 4-(3-nitrophenylamino)-4-methylpentan-2-one picrate (3NAP) cocrystal was synthesized using slow solvent evaporation technique and its crystal structure is reported. The 3NAP compound crystallized in monoclinic crystal system with centrosymmetric space group P21/c and the cell parameters are found to be a=8.6339 Å, b=19.1965 Å, c=13.2058 Å, α=90∘, β=106.725∘, γ=90∘ and V=2096.1 Å3. Best agreement between theoretical and experimental value of CHN elemental composition confirmed inclusionless compound formation. The reasonable FWHM value of prominent peak observed from high-resolution X-ray diffraction experiment indicates superior crystallinity of 3NAP. The load-dependent hardness values measured at (010), (010), and (001) – planes of 3NAP single crystal admit its mechanical anisotropy. Further, the work hardening coefficient confirms that the title crystal belongs to the soft material category. The dark and photocurrent of the 3NAP crystal were measured, and it exhibits positive photoconductivity. The laser-induced surface damage threshold of the 3NAP crystal was found to be 7.5 GWcm−2. Nonlinear coupling between the intermolecular charge transfer and an applied optical field enhanced third order nonlinear optical properties. The intermolecular charge transfer and interactions are proved to be true in 3NAP via Hirshfeld surface analysis. The third order nonlinear optical susceptibility (χ(3)) and optical limiting threshold under continuous-wave laser excitation was measured to be 3.96 × 10−6 esu and 3.44 × 103 Wcm−2. The superior physicochemical and nonlinear optical properties promote the title material as a promising candidate for nonlinear optical device applications.

Electromagnetic modeling of excited-state dynamics in the vicinity of metallic nanostructures

Numerical modeling of spectroscopic response, and near-field effects including local field enhancement and local optical chirality induced by metallic non-chiral nanostructures under continuous-wave laser excitations are presented in this paper. Particularly, excited-state dynamics including plasmon resonances and local optical activity is studied. The light-matter interaction problem is described through Maxwell's equations which are numerically solved by the boundary element method. The calculations are verified by the corresponding experiments. Knowledge of excited-state dynamics of metallic nanostructures provides valuable guidelines for designing alternative functional nanomaterials that is benefited from the enhanced light-matter interaction.

Significant effect of film thickness on morphology and third-order optical nonlinearities of Cd1−xZnxO semiconductor nanostructures for optoelectronics

The work presented here reported the thickness dependent structural, linear and nonlinear optical properties of nanostructured Cd1−xZnxO thin films. Thin films were prepared with two different thickness (≈ 0.5 µm and 1 µm) by employing a spray pyrolysis (SP) technique for different Zn-doping levels (Cd1−xZnxO with the x value of 0.00, 0.01, 0.05 and 0.1). X-ray diffraction studies confirm the polycrystalline nature having a cubic crystal structure. In terms of an aspect ratio of the columnar structure and dispersion in hexagonal (1 1 1) basal plane orientation, a thickness dependency of structural evolution was discussed. The Scherrer rule was employed to determine the crystallite size and found to be decreased. FESEM images indicate grains which are uniform and grain size slightly increased with an increase in dopant concentration, annealing and thickness of the films, respectively. The optical energy band gap (Eg) of the prepared films was found to be increased from 2.50 to 2.67 eV. The NLO parameters of the samples were measured from the Z-scan data under DPSS continuous wave laser excitation at 532 nm and the results reveal that reverse saturable absorption (RSA) and self-defocusing natures are the attributed and observed nonlinearity of the nanostructures. The third-order NLO components such as β, $$n_{2}$$ and χ(3) are found to be enhanced with one order of magnitude higher with the influence of thickness from 1.25 × 10–4 to 2.47 × 10–3 (cm W−1), 7.08 × 10–9 to 3.35 × 10–8 (cm2 W−1) and 4.06 × 10–7 to 1.96 × 10–6 (esu) respectively. The inspiring results of NLO parameters are also due to the increasing localized defect states on grain boundaries as the film thickness increases, suggesting the prepared films are a promising material for nonlinear photonic device applications.

Fast three-color single-molecule FRET using statistical inference

We describe theory, experiments, and analyses of three-color Förster resonance energy transfer (FRET) spectroscopy for probing sub-millisecond conformational dynamics of protein folding and binding of disordered proteins. We devise a scheme that uses single continuous-wave laser excitation of the donor instead of alternating excitation of the donor and one of the acceptors. This scheme alleviates photophysical problems of acceptors such as rapid photobleaching, which is crucial for high time resolution experiments with elevated illumination intensity. Our method exploits the molecular species with one of the acceptors absent or photobleached, from which two-color FRET data is collected in the same experiment. We show that three FRET efficiencies and kinetic parameters can be determined without alternating excitation from a global maximum likelihood analysis of two-color and three-color photon trajectories. We implement co-parallelization of CPU-GPU processing, which leads to a significant reduction of the likelihood calculation time for efficient parameter determination.

The structure-property studies and mechanism of optical limiting action of methyl 4-((4-aminophenyl)ethynyl)benzoate crystal under continuous wave laser excitation

This study focuses on the design of the new dye of donor-π-acceptor (D−π−A) system of methyl 4-((4-aminophenyl)ethynyl)benzoate (MAPEB) to highlight its structural-property relation through a combined experimental and quantum chemical calculation approaches for optical limiting applications. MAPEB was synthesized via aerobic condition palladium-catalysed Sonogashira cross-coupling reaction with good yield. This molecular structure was confirmed by single crystal X-ray crystallographic analysis and the crystal exhibits monoclinic non-centrosymmetric space group of Pc. The existence of intermolecular interactions was confirmed by Hirshfeld analysis, showing high contribution of C⋯H contact which suggested the high nonlinear optical response of the compound. The experimental spectroscopic data including Fourier Transform Infrared (FT-IR), 1H and 13C Nuclear Magnetic Resonance, electronic absorption spectra, HOMO-LUMO energy gap were compared with Density Functional Theory (DFT) at 6–311++G (d,p) basis set. The experimental results complement the theoretical findings in supporting anticipated electronic properties of the molecule. Additionally, the z-scan analysis unveils an excellent value of χ3 of MAPEB in the order of 10−6 esu which indicate good nonlinear optical material under 532 nm continuous wave laser. Low optical limiting action was measured at 110 kW/cm2 which is suitable for various continuous wave laser applications. These findings prove that MAPEB has the potential to be employed as NLO materials for photonic applications.

Physicochemical properties of cesium tetroxalate dihydrate single crystal: An efficient material for nonlinear optical applications

Herein we report the tunable optical limiting property of a metal–organic cesium tetroxalate dihydrate (CTD) complex. Temperature and pH were optimized to be 40°C and 3.5 respectively to grow CTD single crystal using the solvent evaporation method. The compound formation was confirmed by its response to single crystal and powder x-ray diffraction analysis. Thermal, mechanical, dielectric, linear optical, photoconductivity and photoluminescence properties of title crystal are reported. Nonlinear coupling between delocalized π-electrons of metal–ligand interaction and the oscillating electric field of a laser beam enhanced the nonlinear optical parameters. The third order nonlinear refractive index (n2), absorption coefficient (β) and susceptibility (χ(3)) of CTD complex under continuous-wave laser excitation are found to be (3.26±0.033)×10−8 cm2W−1, (10.33±0.41)×10−4 cmW−1 and (8.98±0.35)×10−5 esu respectively. Variation in two-photon absorption (2PA) coefficient measured under nanosecond pulsed laser excitation of different intensities confirmed the presence of sequential 2PA in CTD complex. The optical limiting threshold was investigated under different intensities were found to vary from (1.09±0.05)×1012 Wm−2 to (2.49±0.05)×1012 Wm−2 showing the intensity tunable optical limiting property of CTD material.

Bismuth(III)-Doped NaYbF4:Tm3+ Fluorides with Highly Efficient Upconversion Emission under Low Irradiance.

Concentration quenching of upconversion (UC) luminescence (UCL) is a common phenomenon in rare-earth-doped materials that seriously restricts the concentration of the activator and sensitizer and withholds their UC emissions and quantum yields. In particular, it remains a tremendous challenge to develop one novel strategy based on the introduction of trivalent bismuth (Bi3+) ions to exceed the typical thulium (Tm3+) ion concentration and reach high-efficiency UC under low illumination. In this work, the Tm3+ accommodation capacity can be increased from 2.0 to 8.0 mol % in NaYbF4:Tm3+ materials with the assistance of Bi3+ ions, which maintains strong UC emissions with large absolute UC quantum yields under low illumination. Specifically, the total upconversion quantum yield (UCQY) of the as-obtained Na(Tm0.08Yb0.60Bi0.32)F4 (8Tm60Yb32Bi) sample can reach as high as 1.45% upon continuous-wave (CW) laser excitation at 40 W cm-2. Strikingly, the total UCQY still remains at a high level (0.41%) even though the CW power density decreases to 1.5 W cm-2. Moreover, the intrinsic mechanism of the breakthrough in the threshold of concentration quenching of UCL by Bi3+ ions was also fully explored. These advances in enhancing UC emissions and UCQYs under a low pump power density offer exciting opportunities for important photonic applications.

Shifted-excitation rotational Raman spectroscopy and Bayesian inference for in situ temperature and composition determination in laminar flames

Optical, non-invasive techniques for temperature and mole fraction measurements in flames and other reactive flows are often limited to applications under well-defined laboratory environments. These limitations mainly arise from various prerequisites of the techniques e.g. regarding the optical accessibility of the systems under investigation, its environmental conditions such as vibrations and temperature or the need of tracers inside the flow. In order to weaken these constrains, we present a robust, fiber-based sensor system, utilizing tracer-free spontaneous rotational Raman spectroscopy with tunable near-infrared (NIR) continuous-wave laser-excitation capable of a simultaneous point-wise determination of gas temperature and mole fractions in laminar spatially uniform non-sooting reactive flows. Benefits and limits of this method are evaluated investigating a laminar premixed methane/air flat-flame of a McKenna-type burner. In that case, the main rotational Raman-active and, therefore, detectable compounds are nitrogen, oxygen and carbon dioxide. For the subtraction of flame luminosity two different techniques were used: simple background subtraction (BGS) and shifted-excitation Raman difference spectroscopy (SERDS). For both methods, the corrected spectra are matched to simulated spectra via a least-square fit algorithm in order to extract the quantities of interest. Hereby, Bayesian inference enables the determination of the uncertainties of the results. The consideration of the covariance within the multi-variate analysis allows for the combination of the BGS and SERDS method to establish a more advanced analysis routine.

Structural, linear and nonlinear optical properties of Cd1−xAlxS semiconductor nanostructures: Influence of film thickness

The work presented here reports the thickness dependent structural, linear and nonlinear optical properties of nanostructured Al-doped CdS thin films. Thin films were prepared with two different thicknesses(≈0.5 µm and 1 µm) by employing a spray pyrolysis (SP) technique for different Al-doping levels (0, 1, 3, 5 and 10 wt%). Powder X-ray diffraction studies confirm the polycrystalline crystal structure. The Scherrer rule was used to determine the crystallite size which was found to decrease with increasing doping concentration. Field emission scanning electron microscopy images showed uniform grain size which was found to slightly decrease with an increase in thickness of the films. The optical energy band gap (Eg) of the films was found to decrease with an increase of film thickness. The nonlinear optical parameters (NLO) of the samples were measured from the Z-scan data under DPSS continuous wave laser excitation at 532 nm which shows strong two-photon absorption. The increase in film thickness with Al-doping concentration from 0 to 10 wt%, the NLO parameters such as β, n2and χ(3) were enhanced from 4.66 × 10−4 to 3.67 × 10−3 (cm W−1), 5.26 × 10−9 to 3.38 × 10−8 (cm2 W−1) and 4.03 × 10−7 to 1.98 × 10−6 (esu), respectively. These important results of NLO parameters are due to the increase of localized defect states on grain boundaries with the increase of film thickness and suggest that these films are promising material for photonic device applications.

Structural, linear and nonlinear optical characterization of Ni and Al Co-Doped CdO semiconductor nanostructures for nonlinear optical device applications

The effects of metallic doping (Ni and Al) on thin film CdO nanostructures have been investigated and their morphological, structural, linear and nonlinear optical properties have been studied. Thin films were grown using spray pyrolysis method with a substrate temperature of 300 °C for different Ni and Al doping concentrations (0, 1, 3, 5 and 10 wt%). Studies of films with Powder X-ray diffraction confirm the polycrystalline nature with the cubic crystal structure. The Scherer rule was employed to determine the crystallite size and found to be enhanced. The elemental analysis confirms the incorporation of Ni and Al into the host CdO matrix. The surface morphology was analysed using the Field Emission Scanning Electron Microscope and the grain size was found to be altered by increasing the doping content. To understand the characteristics of the defect state, open and closed aperture Z-scan measurement was performed using the DPSS continuous wave (532 nm) laser excitation and nonlinear absorption, refractive index and nonlinear susceptibility values of third order were calculated and found to be increasing with the increased concentration of the dopants. The encouraging findings of the nonlinear optical parameters with the increasing concentration of Ni and Al on CdO nanostructures indicate that doped films are a promising source for nonlinear device applications.

Synthesis and third order optical nonlinearity studies of toluidine tartrate single crystal supported by photophysical characterization and vibrational spectral analysis

The organic nonlinear crystal Toluidine Tartrate (TT) was synthesized, its structure was reinvestigated by X-ray diffraction analysis and investigated theoretically as well as experimentally to examine its nonlinear optical (NLO) behavior. The electron transport mechanism and hydrogen bonding interactions in TT have been explored using the Natural Bond Orbital analysis and optimized geometry calculations. The vibrational spectral investigations of the compound have been carried out using the experimental and computational FT-IR and FT-Raman analyses. UV–vis spectral investigations show the optical absorption of TT crystal at 271 nm and 319 nm and reveal its optical transparency in the visible region. The HOMO LUMO study reveals a 4.5388 eV energy gap which shows its reactivity. Photoluminescence and lifetime studies show a near violet emission with considerable lifetime. The thermal property of the sample was analyzed by TG and DTA analysis and it was found that the TT crystalline compound is stable up to 201 °C. Hyperpolarizability calculations give an augmented value of hyperpolarizability. Z-scan measurements in the pulsed as well as continuous wave laser excitation regimes reveal a substantial decrease in normalized transmittance at higher laser fluences, substantiating the nonlinear behavior and optical limiting application of TT crystals.

Спонтанное и стимулированное излучение в тонких пленках твердых растворов Cu(In-=SUB=-1-x-=/SUB=-Ga-=SUB=-x-=/SUB=-)(S-=SUB=-y-=/SUB=-Se-=SUB=-1-y-=/SUB=-)-=SUB=-2-=/SUB=- в составе солнечных элементов

The results of a study of the emission spectra of thin nanocrystalline films of Cu(In1-xGax)(SySe1-y)2 (CIGSSe) direct-band-gap solid solutions in the structure of solar cells at ~ 0.5 W/cm2 continuous wave and nanosecond pulsed laser excitation in the range of excitation power density 0.1 - 53 kW/cm2 and temperatures of 10-300 K are presented. It was found that stimulated emission (SE) occurs in thin CIGSSe films in the temperature range from 10 K to 90 K in the spectral region h = 1.062 - 1.081 eV with a minimum threshold pump level of about 1 kW/cm2. It was shown, that, with increasing intensity of the exciting emission, the spontaneous emission bands shift toward higher energies. It was found that the photoluminescence bands at low excitation levels and the SE bands shift with increasing temperature toward higher energies, and the PL bands at high excitation levels shift toward low energies. Possible causes and mechanisms of the influence of temperature and excitation intensity on the spectral positions of spontaneous and SE of the films of solid solutions are discussed.

Crystallographic phase changes and damage thresholds of CsPbI3 microwire waveguides through continuous wave photoablation

We investigate waveguide efficiency of CsPbI3 microwire waveguides and their photodegradation over a range of continuous wave laser excitation energies and intensities.