Excitation Laser Power Research Articles

Effects of the temperatures on photoluminescence intensity in self-assembled InAs quantum dots coated with gold nanoparticles

The effect of the temperature on the photoluminescence intensity of self-assembled InAs quantum dots (QDs) coated with gold nanoparticles was studied in this work. The sample was grown by MBE and optical properties are studied by photoluminescence. The result shows an improvement of photoluminescence intensity for the samples. This improvement of intensity is associated to surface Plasmon (SP) for coupling effects. The SP-QD coupling behavior of InAs QD coated by gold nanoparticles was studied at different temperatures and fixed laser excitation power of 50 mW. The plasmonic enhancement factor increases with increasing temperature showing that the temperature improves the SP-QD coupling efficiency.

Probing Thermal Flux in Twinned Ge Nanowires through Raman Spectroscopy.

We report a noninvasive optical technique based on micro-Raman spectroscopy to study the temperature-dependent phonon behavior of normal (nondefective) and twinned germanium nanowires (Ge-NWs). We studied thermophysical properties of Ge-NWs from Raman spectra, measured by varying excitation laser power at ambient condition. We derived the laser-induced temperature rise during Raman measurements by analyzing the Raman peak position for both the NWs, and for a comparative study we performed the same for bulk Ge. The frequency of the Ge-Ge phonon mode softens for all the samples with the increase in temperature, and the first-order temperature coefficient (χT) for defected NWs is found to be higher than normal NWs and bulk. We demonstrated that apart from the size, the lamellar twinning and polytype phase drastically affect the heat transport properties of NWs.

Low Temperature Photoluminescence Kinetics of Double-Ring Structured GaAs Quantum Dots.

This work is focused on spectroscopically characterizing kinetic properties of concentric quantum-ring complexes of GaAs quantum dots. Quantum-ring (or double-ring) GaAs quantum dots, embedded in an Al0.3Ga0.7As barrier layer, were grown by a droplet epitaxy method during molecular beam epitaxy on a GaAs (001) substrate. Emission peaks of photoluminescence spectra with different excitation power, were measured as 702 nm at 0.17 mW and 690 nm at 400 mW, were blue-shifted as the excitation power increased. In addition, excitation laser power dependence of time-resolved photoluminescence of double-ring GaAs quantum dots at 10 K under 400 nm excitation wavelength was performed, revealing that photoluminescence lifetime slowly decreased in comparison to that of single disc-like quantum dots as excitation power increased, implying that carrier transfer between inner ring and outer ring could slow down the decay process. The luminescence lifetime at 10 K increased from 245 to 409 ps in the range from 0.17 to 400 mW of excitation power.

Nonlinear effects in optical pumping of a cold and slow atomic beam

By photoionizing hyperfine (HF) levels of the Cs state $6{\phantom{\rule{0.16em}{0ex}}}^{2}{P}_{3/2}$ in a slow and cold atom beam, we find how their population depends on the excitation laser power. The long time (around $180\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s})$ spent by the slow atoms inside the resonant laser beam is large enough to enable exploration of a unique atom-light interaction regime heavily affected by time-dependent optical pumping. We demonstrate that, under such conditions, the onset of nonlinear effects in the population dynamics and optical pumping occurs at excitation laser intensities much smaller than the conventional respective saturation values. The evolution of population within the HF structure is calculated by numerical integration of the multilevel optical Bloch equations. The agreement between numerical results and experiment outcomes is excellent. All main features in the experimental findings are explained by the occurrence of ``dark'' and ``bright'' resonances leading to power-dependent branching coefficients.

Generation of nonclassical light upon resonant excitation of a semiconductor microcavity

Strong correlations in the fluctuations of the intensity of emission from a semiconductor microcavity under resonant laser excitation are observed. The intensity correlation function exhibits an unusual oscillatory behavior with an unexpectedly long oscillation period and decay time. The visibility of the correlation function reaches 0.81. Long oscillation times are attributed to the Rabi frequency characterizing weak coupling between the electromagnetic field of the semiconductor microcavity mode and long-lived exciton states localized by the random potential of the quantum well. For a laser excitation power density of 400 W/cm2, the power density of the radiation emitted by the microcavity is 12 W/cm2, which corresponds to the total flux of nonclassical light of 1.5 × 1015 photons/s from an excited spot 50 μm in diameter. Thus, a microcavity can serve as a bright emitter of nonclassical light.

Stokes and anti-Stokes Raman spectra of the high-energy C-C stretching modes in graphene and diamond

The Stokes and the anti-Stokes Raman spectra from the high energy carbon–carbon stretching modes in graphene and diamond are studied, with a focus on their intensity dependence on the excitation laser power. For diamond, the experiment was performed on a 50 μm slice of chemical vapour deposition grown single crystal using a 785 nm laser, in both the continuous wave (CW) and pulsed mode-locked (ML—130 fs pulses) excitation regimes. For graphene, the experiments were performed on suspended samples, including single layer, bilayer with AB-staking, twisted bi-layer and a sample with 4–6 layers, using λ = 532 and 633 nm CW lasers. The Stokes and anti-Stokes intensities vary largely from sample to sample, and the anti-Stokes/Stokes intensity ratio shows different excitation laser power behaviours. These differences can be explained on the basis of laser heating or correlated Stokes–anti-Stokes scattering phenomena.

Laser Power Dependent Optical Properties of Mono- and Few-Layer MoS2

We report on the exponential decay of the red-shift of the photoluminescence A-exciton peak in monolayer molybdenum disulfide (MoS2) with the excitation laser power. The linear relationship found for the thermal variation of the same peak suggests that the laser power effect goes beyond the exciton dynamics associated to temperature variations. Laser exitation power effect on the broadening and red-shifting of the A(1g) and E(2g)1 phonon peaks observed by Raman spectroscopy reflect the damping of vibration due local thermal heating induced by the laser. Our results point out the laser excitation power dependence on the photoluminescence properties of monolayer MoS2.

Dependence of laser-induced fluorescence on exciting-laser power: partial saturation and laser – plasma interaction

In recent publications on laser-induced fluorescence (LIF), the measurements are usually constricted to the region of weak exciting-laser power – the so called linear LIF. In this work, a practical formula F(E_L) = (alpha) *E_L / (1+(beta)*E_L) describing the dependence of partially saturated fluorescence on the exciting-laser power is derived, together with practical implementation suggestions and comments on its limitations. In the conclusion, the practical formula is proposed with the limitation for validity (beta)*EL = 0.4, where (alpha)*EL is the hypothetical linear fluorescence without saturation effects, and a more general formula is derived, which is valid for higher values of (alpha)*EL as well. Extending the range of exciting laser power to the region of partial saturation enhances the signal-to-noise ratio. Such measurements in a surface dielectric barrier discharge further reveal discharge disruption by photoelectrons emitted from the dielectric surface. Methods of control and solution of this problem are discussed.

A light-intensity-controlled microfiber-assisted Mach–Zehnder interferometer based on ethyl orange solution under 532 nm laser excitation

A compact light-intensity-controlled Mach–Zehnder interferometer has been proposed by exploiting the photosensitivity of ethyl orange solution in combination with the evanescent field property of a microfiber-assisted Mach–Zehnder structure. We have experimentally investigated the influences of irradiation laser power density as well as weight percentage ratio of ethyl orange solution on the transmission spectral characteristics of the proposed microfiber-assisted fiber-optic interferometer. Experimental results indicate that the interference transmission spectra are highly sensitive to the applied excitation laser power density. An optimal ethyl orange weight percentage of 0.5wt% has been experimentally acquired, with which the largest interference dip wavelength sensitivity reaches 0.02576nm/(mWcm−2). Moreover, experimental observation on the transmission spectrum shows that the proposed MZI possesses good spectral reversibility, which ensures its applicability for practical applications.

Investigation of surface-plasmon coupled red light emitting InGaN/GaN multi-quantum well with Ag nanostructures coated on GaN surface

Surface-plasmon (SP) coupled red light emitting InGaN/GaN multiple quantum well (MQW) structure is fabricated and investigated. The centre wavelength of 5-period InGaN/GaN MQW structure is about 620 nm. The intensity of photoluminescence (PL) for InGaN QW with naked Ag nano-structures (NS) is only slightly increased due to the oxidation of Ag NS as compared to that for the InGaN QW. However, InGaN QW with Ag NS/SiO2 structure can evidently enhance the emission efficiency due to the elimination of surface oxide layer of Ag NS. With increasing the laser excitation power, the PL intensity is enhanced by 25%–53% as compared to that for the SiO2 coating InGaN QW. The steady-state electric field distribution obtained by the three-dimensional finite-difference time-domain method is different for both structures. The proportion of the field distributed in the Ag NS for the GaN/Ag NS/SiO2 structure is smaller as compared to that for the GaN/naked Ag NS structure. As a result, the energy loss of localized SP modes for the GaN/naked Ag NS structure will be larger due to the absorption of Ag layer.

Charged two-exciton emission from a single semiconductor nanocrystal

Here, we study the photoluminescence (PL) time trajectories of single CdSe/ZnS nanocrystals (NCs) as a function of the laser excitation power. At the low laser power, the PL intensity of a single NC switches between the “on” and “off” levels arising from the neutral and positively charged single excitons, respectively. With the increasing laser power, an intermediate “grey” level is formed due to the optical emission from a charged multiexciton state composed of two excitons and an extra electron. Both the inter-photon correlation and the PL decay measurements demonstrate that lifetime-indistinguishable photon pairs are emitted from this negatively charged two-exciton state.

Polarization and time-resolved photoluminescence spectroscopy of excitons in MoSe2 monolayers

We investigate valley exciton dynamics in MoSe2 monolayers in polarization- and time-resolved photoluminescence (PL) spectroscopy at 4 K. Following circularly polarized laser excitation, we record a low circular polarization degree of the PL of typically ≤5%. This is about 10 times lower than the polarization induced under comparable conditions in MoS2 and WSe2 monolayers. The evolution of the exciton polarization as a function of excitation laser energy and power is monitored in PL excitation experiments. Fast PL emission times are recorded for both the neutral exciton of ≤3 ps and for the charged exciton (trion) of 12 ps.

Natrium-Teide experiment. Sodium layer observations at Teide Observatory (Canary Is.)

With the aim of characterizing the atmospheric Na layer, the Teide Observatory facilities have been used in the last years to carry out a long term monitoring experiment on LGS generation by low power laser excitation. The LGSs are generated by a Rodamine 6G Dye tunable laser launched from the Optical Ground Station with a typical output power of 400 — 500 mW. The LGS images are obtained in a bistatic configuration with a baseline of 165 m. The experiment has been improved in several aspects in order to boost the Na photon flux return. The natrium-Teide experiment has generated a dataset of more than 400 images of Na LGS traces obtained in 37 nights. The main results are going to be been published soon.

Gate-controlled generation of optical pulse trains using individual carbon nanotubes.

In single-walled carbon nanotubes, electron–hole pairs form tightly bound excitons because of limited screening. These excitons display a variety of interactions and processes that could be exploited for applications in nanoscale photonics and optoelectronics. Here we report on optical pulse-train generation from individual air-suspended carbon nanotubes under an application of square-wave gate voltages. Electrostatically induced carrier accumulation quenches photoluminescence, while a voltage sign reversal purges those carriers, resetting the nanotubes to become luminescent temporarily. Frequency-domain measurements reveal photoluminescence recovery with characteristic frequencies that increase with excitation laser power, showing that photoexcited carriers provide a self-limiting mechanism for pulsed emission. Time-resolved measurements directly confirm the presence of an optical pulse train synchronized to the gate voltage signal, and flexible control over pulse timing and duration is also demonstrated. These results identify an unconventional route for optical pulse generation and electrical-to-optical signal conversion, opening up new prospects for controlling light at the nanoscale.

Optically investigating Nd(3+)-Yb(3+) cascade sensitized upconversion nanoparticles for high resolution, rapid scanning, deep and damage-free bio-imaging.

The rapid development of upconversion nanoparticles (UCNPs) has been facing with a great challenge: intense emission, fast scanning, and deep imaging require high-power light irradiation with minimized heating effect (the intrinsic 975-nm excitation of Yb(3+)-sensitized UCNPs have overheating problem). By shifting the excitation peak from 975 nm to 795 nm, Nd(3+)-Yb(3+) cascade sensitized upconversion nanoparticles (Nd-UCNPs) with minimized heating effect were reported as the new generation UCNPs. For the first time, within two optically modeled applications in vitro and in vivo, the damage outcomes under long time high power laser excitation were solidly calculated, complementing the damage-free study of Nd-UCNPs. The higher resolution (20% improvement) and five times faster scanning microscopy were successfully performed using Nd-UCNPs under safety laser power level. The computational results showed the Nd(3+)-Yb(3+) energy transfer efficiency would not compromise the deep imaging ability, and the red (650-nm) emission is worth to be enhanced for deep tissue imaging.

Laser-induced white-light emission from graphene ceramics–opening a band gap in graphene

Graphene ceramics can have their band gaps opened and emit white light when excited by visible or infrared light, report scientists in Poland. Wieslaw Strek and co-workers from the Polish Academy of Sciences and Wroclaw University of Technology say that the broadband emission is centred at 650 nm and has an intensity that is strongly influenced by the excitation laser power with a clear threshold. Their analysis suggests that the origin of the emission is entirely electronic in nature and is not related to incandescence or blackbody radiation. White-light emission is observed for cryogenically cooled samples with temperatures as low as 10 K. The graphene ceramic was fabricated by placing graphene flakes in a calcium carbonate toroid and subjecting them to a high temperature (550 °C) and a pressure of 8 GPa for 1 min.

Laser Terahertz Emission Spectroscopy of Graphene/InAs Junctions

ABSTRACTWe applied laser THz emission spectroscopy to study the effects of monolayer graphene on the THz emission from InAs. THz emission from graphene/InAs varies linearly with the laser excitation power in the low-intensity excitation regime. We found that unlike in graphene/SI-InP junctions, graphene and O2 adsorbates on graphene have no significant effect on the THz emission from graphene/InAs junctions because the THz radiation mechanism in InAs is by the photo-Dember effect, whereas for SI-InP is by the surge current effect. There is also a slight enhancement in the THz emission from both bare InAs and graphene/InAs by UV illumination, which is probably due to the additional photoexcited carriers by UV that somehow enhances the photo-Dember field.

Fluorescence resonance energy transfer between CdTe quantum dots and copper phthalocyanine

The fluorescence resonance energy transfer in CdTe quantum dots (QDs)-copper phthalocyanine (CuPc) is investigated by ultrafast time-resolved spectroscopy technique equipped with femtosecond laser (780 nm, 76 MHz, 130 fs). The results show that the fluorescence lifetime of CdTe QDs decreases with the increase of CuPc concentration, and the energy transfer efficiency is found to increase with the increase of CuPc concentration. Moreover, the influence of the laser excitation power on the energy transfer efficiency is also studied. It is found that transfer efficiency decreases as excitation laser power increases, the physical mechanism is the thermal activation in the high power and the excited state transitions of high order induced by two-photon. The energy transfer efficiency can reach 43.8%, when the laser power is 200 mW, via two-photon excitation. This study indicates that the CdTe QDs-CuPc composite system has high potential as the third generation of photosensitizers.

Beam-Scanning Broadband Cars Microscopy for Rapid Tissue Imaging

A beam-scanning approach is demonstrated to improve signal-to-noise ratio and to reduce photo-induced sample damages in nonlinear hyperspectral imaging, such as broadband coherent anti-Stokes Raman scattering (BCARS) microscopy. Signals generated on a fast scanning axis are transferred to the slit of a monochromator, then onto the vertical axis of a CCD. This configuration acquires the whole BCARS spectra on the fast scanning axis at once, reducing the overall data transfer time. The fast scanning speed by a resonant beam scanner at 5 kHz greatly reduces sample damage from accumulative photo excitation, allowing for a higher excitation laser power to generate a stronger nonlinear signal. The optical characteristics related with this configuration and parameters are discussed and a few examples that demonstrate the advantage of the beam-scanning method for BCARS tissue imaging are presented.

Accounting for Photophysical Processes and Specific Signal Intensity Change in Fluorescence-Detected Sedimentation Velocity Analytical Ultracentrifugation

Fluorescence detected sedimentation velocity (FDS-SV) analytical ultracentrifugation has emerged as a powerful technique for the study of macromolecular interactions, particularly high-affinity protein interactions, with hydrodynamic resolution exceeding that of diffusion-based techniques, and with sufficient sensitivity for binding studies at low picomolar concentrations. In order to fit the FDS data structure, in the quantitative analysis it is essential to adjust the conventional sedimentation models for detailed description of the sedimentation boundaries. A key consideration is the change in the macromolecular fluorescence intensity during the course of the experiment, caused by slow drifts of the excitation laser power, and/or by photophysical processes. In the present work we demonstrate that FDS-SV data have inherently a reference for the time-dependent macromolecular signal intensity, resting on a geometric link between boundary migration and plateau signal. We show how this new time-domain can be exploited to study molecules exhibiting photobleaching and photoactivation. This expands the application of FDS-SV to proteins tagged with photoswitchable fluorescent proteins, organic dyes, or nanoparticles, such as those recently introduced for sub-diffraction microscopy. At the same time, we find conventional fluorophores undergo minimal photobleaching under standard illumination in the FDS. These findings support the application of a high laser power density for the detection, which we demonstrate can further increase the data quality.