Increasing Excitation Power Research Articles

Temperature dependence of the band gap of zinc nitride observed in photoluminescence measurements

We report the photoluminescence properties of DC sputtered zinc nitride thin films in the temperature range of 3.7–300 K. Zinc nitride samples grown at 150 °C exhibited a narrow photoluminescence band at 1.38 eV and a broad band at 0.90 eV, which were attributed to the recombination of free carriers with a bound state and deep-level defect states, respectively. The high-energy band followed the Varshni equation with temperature and became saturated at high excitation powers. These results indicate that the high-energy band originates from shallow defect states in a narrow bandgap. Furthermore, a red-shift of the observed features with increasing excitation power suggested the presence of inhomogeneities within the samples.

Atmospheric pressure-MOVPE growth of GaSb/GaAs quantum dots

This study focuses on the growth of GaSb/GaAs quantum dots (QD) using an atmospheric pressure MOVPE system. For the best uncapped dots, the average dot height, base diameter and density are 5nm, 45nm and 4.5×1010cm−2, respectively. Capping of GaSb QDs at high temperatures caused flattening and formation of thin inhomogeneous GaSb layer inside GaAs resulting in no obvious QD PL peak. Capping at low temperatures lead to the formation of dot-like features and a wetting layer (WL) with distinct PL peaks for QD and WL at 1097nm and 983nm respectively. Some of the dot-like features had voids. An increase in excitation power caused the QD and WL peaks to shift to higher energies. This is attributed to electrostatic band bending leading to triangular potential wells, typical of type-II alignment between GaAs and strained GaSb. Variable temperature PL measurements of the QD sample showed the decrease in the intensity of the WL peak to be faster than that of the QD peak as the temperature increased.

Interlayer exciton dynamics in a dichalcogenide monolayer heterostructure

In heterostructures consisting of different transition-metal dichalcogenide monolayers, a staggered band alignment can occur, leading to rapid charge separation of optically generated electron–hole pairs into opposite monolayers. These spatially separated electron–hole pairs are Coulomb-coupled and form interlayer excitons. Here, we study these interlayer excitons in a heterostructure consisting of MoSe2 and WSe2 monolayers using photoluminescence spectroscopy. We observe a non-trivial temperature dependence of the linewidth and the peak energy of the interlayer exciton, including an unusually strong initial redshift of the transition with temperature, as well as a pronounced blueshift of the emission energy with increasing excitation power. By combining these observations with time-resolved photoluminescence measurements, we are able to explain the observed behavior as a combination of interlayer exciton diffusion and dipolar, repulsive exciton-exciton interaction.

Effect of excitation power on voltage induced local magnetization dynamics in an ultrathin CoFeB film

Voltage or electric field induced magnetization dynamics promises low power spintronics devices. For successful operation of some spintronics devices such as magnetic oscillators and magnetization switching devices a clear understanding of nonlinear magnetization dynamics is required. Here, we report a detailed experimental and micromagnetic simulation study about the effect of excitation power on voltage induced local magnetization dynamics in an ultrathin CoFeB film. Experimental results show that the resonance line-width and frequency remains constant, whereas cone angle of the magnetization precession increases linearly with square-root of excitation power below threshold value, known as linear excitation regime. Above threshold power, the dynamics enters into nonlinear regime where resonance line-width monotonically increases and resonance frequency monotonically decreases with increasing excitation power. Simulation results reveal that a strong nonlinear and incoherent magnetization dynamics are observed in our experiment above the threshold power which reduces dynamic magnetic signal by suppressing large cone angle of magnetization precession. Moreover, a significant transfer of spin angular momentum from uniform FMR mode to its degenerate spin waves outside of excitation area further restrict the cone angle of precession within only few degrees in our device. Our results will be very useful to develop all-voltage-controlled spintronics devices.

Effect of occupation of the excited states and phonon broadening on the determination of the hot carrier temperature from continuous wave photoluminescence in InGaAsP quantum well absorbers

AbstractAn InGaAsP quantum well with a type‐II band alignment is studied using continuous wave power and temperature dependent photoluminescence (PL) spectroscopy. The small energy separation between the ground state and first excited state results in significant thermal carrier redistribution and excited state occupation, particularly, with increasing excitation power and temperature. This state filling is evident as a high‐energy shoulder in the PL spectra, the same energy region where in the simplest Planck‐description the gradient is considered inversely proportional to carrier temperature. The outcome of an excited state occupation in broadening the high‐energy PL tail is to perturb the temperature extracted using this analysis; therefore, the true temperature of carriers is not properly evaluated when significant state filling occurs. In addition, broadening of the PL due to phonons at higher temperatures also distorts (or falsely increases) the non‐equilibrium “hot” carrier temperature and as such should be considered when using Planck's relation. The role of these two effects is considered and their mutual effect on the analysis of the extracted hot carrier temperature discussed. Copyright © 2017 John Wiley & Sons, Ltd.

Influence concentration of Nd3+ ion on the laser induced white emission of Y2Si2O7:Nd3+

The broadband infrared laser induced white emission (LIWE) of sol-gel derived Nd-doped Y2Si2O7 nanocrystals was recorded in vacuum conditions. It was found that LIWE intensity increased exponentially with increasing excitation power density and order of the process was dependent on Nd3+ concentration. Observed emission exhibits a threshold behaviour and a pressure dependence.

Localized states emission in type-I GaAsSb/AlGaAs multiple quantum wells grown by molecular beam epitaxy

As an important candidate for novel infrared semiconductor lasers, the optical properties of GaAsSb-based multiple quantum wells (MQWs) are crucial. The temperature- and excitation power-dependent photoluminescence (PL) spectra of the GaAs0.92Sb0.08/Al0.2Ga0.8As MQWs, which were grown by molecular beam epitaxy, were investigated and are detailed in this work. Two competitive peaks were observed from 40 K to 90 K. The peak located at the low-energy shoulder was confirmed to be localized states emission (LE) and the high-energy side peak was confirmed to be free-carrier emission by its temperature-dependent emission peak position. It is observed that the LE peak exhibited a blueshift with the increase of laser excitation power, which can be ascribed to the band filling effect of localized states. Our studies have great significance for application of GaAsSb-based MQWs in infrared semiconductor lasers.

Microwave-assisted hydrothermal synthesis, temperature quenching and laser-induced heating effect of hexagonal microplate β-NaYF4: Er3+/Yb3+ microcrystals under 1550 nm laser irradiation

Rare earth doped materials can usually achieve high efficiency upconversion luminescence (UCL) under high power density infrared laser excitation, in which process the heat generation cannot be avoided, and then induce temperature dependent UCL quenching. This paper dealt with the temperature sensing property and 1550nm laser irradiation induced thermal effect of Er3+/Yb3+ co-doped NaYF4 microcrystals. The sample was synthesized via a microwave-assisted hydrothermal route, and its crystal structure and microscopic morphology were characterized by means of XRD and FE-SEM. To describe the temperature quenching behaviors for the two typical green emissions of Er3+, a model was proposed and well explained the temperature dependence of two green UCL intensities. Temperature sensing property for NaYF4: Er3+/Yb3+ microcrystals were studied by fluorescence intensity ratio technique, and the temperature sensitivity were also obtained. Furthermore, the 1550nm laser induced thermal effect was studied, and the result showed that the sample temperature increased with increasing excitation power density.

水热法制备Na<sub>3</sub>PO<sub>4</sub>:Ce<sup>3+</sup>/Tb<sup>3+</sup>/Er<sup>3+</sup>纳米荧光粉及其发光性能的研究

本文用水热法制备了Na3PO4:Ce3+/Tb3+/Er3+纳米荧光粉末，并通过吸收光谱、荧光光谱对其进行表征。在980 nm激发光激发下，粉末样品同时发出蓝、绿、红三波段光，对应的发射峰分别位于436 nm，545 nm，692 nm；同时通过调节激发光功率，发现在不同的激发光功率激发下，各波段的发光强度发生明显变化，这也反应出Ce3+与Tb3+/Er3+离子之间的能量传递过程。因此，通过调节激发光激发功率，可获得不同色度的白光样品。此样品可应用于白光LED的光电应用中。 The Na3PO4:Ce3+/Tb3+/Er3+ nanophosphors were prepared by the hydrothermal method. Under the excitation of 980 nm exciting light, the obtained Na3PO4:Ce3+/Tb3+/Er3+ nanophosphors emitted blue, green, red three waves of light, the corresponding emission peaks were at 436 nm, 545 nm and 692 nm, respectively. The increase in white light emission with the increase of excitation power, which indicates the efficient energy inter-ion transfer from Ce3+ to Tb3+/Er3+ ions. Furthermore, the observed the intense white light emission is suggestive exploration for the present phosphor for potential optoelectronic applications such as white light-emitting phosphor for LED.

Large optical Stark shifts in single quantum dots coupled to core-shell GaAs/AlGaAs nanowires.

Nanowire quantum dots (NW-QDs) can be used for future compact and efficient optoelectronic devices. Many efforts have been made to control the QD states by inserting the QDs in doped structures and applying an electric field in a nanowire system. In this paper, we use down-conversion and up-conversion photoluminescence excitations to explore the optical and electronic properties of single quantum dots in GaAs/AlGaAs core-shell nanowires. We investigate a large optical Stark shift in this system as a new method to tune the QD states. When the tunable laser lies within the spectral bandwidth of ZB/WZ GaAs (780 nm-860 nm), we observe an extremely large optical Stark shift of 1.3 nm (0.5 nm) with increasing excitation power at a resonant wavelength of 800 nm (840 nm) in GaAs states. The ability to in situ control the energy states of self-catalyzed NW-QDs should open a new way for quantum light sources and nonlinear optics in a nanowire system.

Nonlinear photoluminescence in monolayer WS2: parabolic emission and excitation fluence-dependent recombination dynamics.

Recombination dynamics during photoluminescence (PL) in two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are complicated and can be easily affected by the surroundings because of their atomically thin structures. Herein, we studied the excitation power and temperature dependence of the recombination dynamics on the chemical vapor deposition-grown monolayer WS2via a combination of Raman, PL, and time-resolved PL spectroscopies. We found a red shift and parabolic intensity increase in the PL emission of the monolayer WS2 with the increasing excitation power and the decay time constants corresponding to the recombination of trions and excitons from transient PL dynamics. We attributed the abovementioned nonlinear changes in the PL peak positions and intensities to the combination of increasing carrier interaction and band structure renormalization rather than to the thermal effect from a laser. Furthermore, the excitation power-dependent Raman measurements support our conclusion. These findings and understanding will provide important information for the development of TMD-based optoelectronics and photonics.

Near-infrared luminescence property of Te-doped zinc phosphate glasses

Broadband near-infrared (NIR) emission has been observed from Te-containing glasses melted under reduced conditions in recent years. However, it has not been paid great attention compared with Bi-containing glasses, and there are few reports on the NIR emission property of Te-containing glasses. In this paper, we report tunable broadband NIR emission from 900 to 1500nm in Te-doped zinc phosphate glasses. The emission intensity increases with increasing melting temperature, indicating the temperature-dependent formation of the related luminescent center. A red shift is observed when increasing the wavelength of excitation. The NIR emission intensity increases linearly with the increase of excitation power. Based on absorption, emission and Raman spectra of the glasses melt at various temperatures, Te2 clusters are suggested to be responsible for the NIR emission.

Up-conversion emission properties and unexpected white light emission from Er3+/Yb3+ doped Gd2O3 nanophosphors

Gd2O3 nanopowders doped with Er3+ and Yb3+ were synthesized using the combustion method. X-ray diffraction and scanning electron microscopy were used to estimate the crystallite size and the morphology of the materials. Scherrer and Warren–Averbach calculations were employed to estimate the mean crystallite size and size distribution of the powders. The crystallite size was found to be <100 nm. The down- and up-conversion emission properties of the materials were studied by laser spectroscopy. Results showed that the 1.5 μm emission of the Er3+ ions broadened with an increasing amount of Yb3+ ions. An n-shape pumping power dependence of the up-conversion emission intensity was observed. Formation of new emission bands and unexpected production of white light emission with increasing excitation power were also observed. The color quality parameters of the obtained visible emission were also measured.

Growth and optical properties of Nb-doped WS2 monolayers

We report the chemical vapor deposition growth of Nb-doped WS2 monolayers and their characterization. Electron microscopy observations reveal that the Nb atom was substituted at the W site at a rate of approximately 0.5%. Unlike Mo doping, Nb-doped samples have photoluminescence (PL) peaks at 1.4–1.6 eV at room temperature. The peak energies are lower than the optical bandgap of 1.8 eV, and a saturation behavior of PL intensity is observed with the increase in excitation power. These results indicate that the observed PL peaks are assignable to the emission from impurity states generated by the substitution of Nb.

Blue cooperative upconversion and white light emission from Y2Si2O7:Yb3+nanopowders due to 975-nm infrared excitation

We report the luminescence properties of undoped and 10% Yb3+-doped Y2Si2O7 nanopowders under the infrared excitation of 975-nm laser diode. A broadband white light (WL) emission is observed in the undoped powder, and its intensity becomes stronger with increasing excitation power. A blue emission centered at ∼475 nm is observed below the excitation power of ∼2 W in the Yb3+-doped sample. The blue emission intensity dependence on the excitation power suggests that this emission is a two-photon absorption process known as the cooperative luminescence of Yb3+ ions. When the excitation power is increased above 2 W, a broadband “white” emission extending from blue into the near infrared appears while the blue cooperative emission band of Yb3+ ions disappears. The excitation power dependence of the WL intensity at 758 nm is due to a multiphoton process that may be attributed to the thermal radiation mechanism. The International Commission on Illumination coordinates (x,y) for the WL emission in the Yb3+-doped Y2Si2O7 nanocrystalline sample is found to be (0.4503, 0.3999) and (0.5035, 0.4096) for 2.005- and 3.68-W excitation powers, respectively. These values lie in the yellowish region and correspond to the color temperature value of 5030 K, with a color rendering index of 85.

Overcoming power broadening of the quantum dot emission in a pure wurtzite nanowire

One of the key challenges in developing quantum networks is to generate single photons with high brightness, purity, and long temporal coherence. Semiconductor quantum dots potentially satisfy these requirements; however, due to imperfections in the surrounding material, the coherence generally degrades with increasing excitation power yielding a broader emission spectrum. Here we overcome this power broadening regime and demonstrate the longest coherence at exciton saturation where the detection count rates are highest. We detect single-photon count rates of 460,000 counts per second under pulsed laser excitation while maintaining a single-photon purity of greater than 99%. Importantly, the enhanced coherence is attained with quantum dots in ultraclean wurtzite InP nanowires, where the surrounding charge traps are filled by exciting above the wurtzite InP nanowire bandgap. By raising the excitation intensity, the number of possible charge configurations in the quantum dot environment is reduced, resulting in a narrower emission spectrum. Via Monte Carlo simulations we explain the observed narrowing of the emission spectrum with increasing power. Cooling down the sample to 300 mK, we further enhance the single-photon coherence two-fold as compared to operation at 4.5 K, resulting in a homogeneous coherence time, T2, of 1.2 ns.

Upconversion photoluminescence properties of SrY2O4:Er3+,Yb3+ under 1550 and 980 nm excitation

Er3+/Yb3+ co-doped SrY2O4 phosphors with high color purity and brightness were successfully synthesized via a solid-state reaction method. Luminescence spectrum studies showed that the main red peaks and the minor green peaks of upconversion emissions were located at approximately 634–681 nm and 543–570 nm, respectively, corresponding to the transitions of 4F9/2?4I15/2 and 4S3/2?4I15/2 of Er3+ ions. Under the excitation of 980 and 1550 nm lasers, the spectra of all of the samples exhibited similar peak positions but different intensities. When excited by the 980 nm laser, the intensity ratio of red to green emission increased with increasing Yb3+ doping concentration and decreased with increasing excitation power. In the case of 1550 nm excitation, the intensity ratio of red to green emission increased with increasing Yb3+ doping concentration and excitation power, thereby, improving the color purity of the red emission. The intensity of red emission was considerably stronger under 1550 nm excitation than that under 980 nm excitation. Therefore, the color of the proposed phosphors could be efficiently tuned by tailoring both the Yb3+ doping concentration and excitation power.

Chemical doping modulation of nonlinear photoluminescence properties in monolayer MoS2

We demonstrate a simple modulation technique of nonlinear optical properties in monolayer (1L) MoS2 via chemical doping. The strong nonlinear behavior of the exciton photoluminescence (PL) intensity is observed with increasing excitation power density for low-electron-density 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ)-doped 1L-MoS2, whereas the exciton PL intensity of as-prepared, heavily electron-doped 1L-MoS2 exhibits weak sublinear behavior. These results are attributable to an enhanced exciton–exciton annihilation rate for the excitons in F4TCNQ-doped 1L-MoS2 as the dominant decay pathway under strong optical excitation conditions.

Photoconductivity of Er-doped InAs quantum dots embedded in strain-relaxed InGaAs layers with 1.5 µm cw and pulse excitation

We fabricated a photoconductive antenna structure utilizing Er-doped InAs quantum dot layers embedded in strain-relaxed In0.35Ga0.65As layers on a GaAs substrate. Mesa-shaped electrodes for the antenna structure were formed by photolithography and wet etching in order to suppress its dark current. We measured the photocurrent with the excitation of ∼1.5 µm cw and femtosecond pulse lasers. Compared with the dark current, the photocurrent was clearly observed under both cw and pulse excitation conditions and almost linearly increased with increasing excitation power in a wide range of magnitudes from 10 W/cm2 to 10 MW/cm2 order.

Lasing from Glassy Ge Quantum Dots in Crystalline Si.

Semiconductor light-emitters compatible with standard Si integration technology (SIT) are of particular interest for overcoming limitations in the operating speed of microelectronic devices. Light sources based on group IV elements would be SIT-compatible, but suffer from the poor optoelectronic properties of bulk Si and Ge. Here we demonstrate that epitaxially grown Ge quantum dots (QDs) in a defect-free Si matrix show extraordinary optical properties if partially amorphized by Ge-ion bombardment (GIB). In contrast to conventional SiGe nanostructures, these QDs exhibit dramatically shortened carrier lifetimes and negligible thermal quenching of the photoluminescence (PL) up to room temperature. Microdisk resonators with embedded GIB-QDs exhibit threshold behavior as well as a superlinear increase of the integrated PL intensity with concomitant line width narrowing as the pump power increases. These findings demonstrate light amplification by stimulated emission in a fully SIT-compatible group IV nanosystem.