Power-dependent Spectra Research Articles

Selective Transition Enhancement in a g-Engineered Diradical.

A diradical with engineered g-asymmetry was synthesized by grafting a nitroxide radical onto the [Y(Pc)2]⋅ radical platform. Various spectroscopic techniques and computational studies revealed that the electronic structures of the two spin systems remained minimally affected within the diradical system. Fluid-solution Electron Paramagnetic Resonance (EPR) experiments revealed a weak exchange coupling with |J| ~ 0.014 cm-1, subsequently rationalized by CAS-SCF calculations. Frozen solution continuous-wave (CW) EPR experiments showed a complicated and power-dependent spectrum that eluded analysis using the point-dipole model. Pulse EPR manipulations with varying microwave powers, or under varying magnetic fields, demonstrated that different resonances could be selectively enhanced or suppressed, based on their different tipping angles. In particular, Field-Swept Echo-Detected (FSED) spectra revealed absorptions of MW power-dependent intensities, while Field-Swept Spin Nutation (FSSN) experiments revealed two distinct Rabi frequencies. This study introduces a methodology to synthesize and characterize g-asymmetric two-spin systems, of interest in the implementation of spin-based CNOT gates.

Anomalous layer-dependent photoluminescence spectra of supertwisted spiral WS2.

Twisted stacking of two-dimensional materials with broken inversion symmetry, such as spiral MoTe2 nanopyramids and supertwisted spiral WS2, emerge extremely strong second- and third-harmonic generation. Unlike well-studied nonlinear optical effects in these newly synthesized layered materials, photoluminescence (PL) spectra and exciton information involving their optoelectronic applications remain unknown. Here, we report layer- and power-dependent PL spectra of the supertwisted spiral WS2. The anomalous layer-dependent PL evolutions that PL intensity almost linearly increases with the rise of layer thickness have been determined. Furthermore, from the power-dependent spectra, we find the power exponents of the supertwisted spiral WS2 are smaller than 1, while those of the conventional multilayer WS2 are bigger than 1. These two abnormal phenomena indicate the enlarged interlayer spacing and the decoupling interlayer interaction in the supertwisted spiral WS2. These observations provide insight into PL features in the supertwisted spiral materials and may pave the way for further optoelectronic devices based on the twisted stacking materials.

Up-conversion properties of Ba3Lu2Zn5O11:Yb3+,Er3+ phosphors for optical thermometer based on FIR technique

In this work, Ba3Lu2Zn5O11:Yb3+,Er3+ phosphors were successfully elaborated. The structures, morphology features and up-conversion characteristics were fully investigated through X-ray powder diffraction patterns, scanning electron microscopy and photoluminescence spectra, respectively. The optimal green emission sample Ba3Lu2Zn5O11:0.4Yb3+,0.02Er3+ was determined through up-conversion spectra excited by 980 nm. Based on power-dependent spectra and energy level schematics of Er3+ and Yb3+, the up-conversion luminescence behaviors and corresponding up-conversion mechanisms were interpreted in detail. Ba3Lu2Zn5O11:Yb3+,Er3+ phosphors show excellent temperature sensing performances based on fluorescence intensity ratio technique. The maximal relative sensitivity is 1.18 %K−1@303 K and the minimal temperature resolution is 0.26 K@303 K. More importantly, calculated temperature sensing repeatability at 333 K and 573 K is higher than 98%. All results suggest the great promise of Ba3Lu2Zn5O11:Yb3+,Er3+ phosphors as future precise thermometers.

Up-conversion luminescence properties and temperature sensing performances of Ba5Y8Zn4O21:Yb3+, Er3+ phosphors

A serials of Er3+ single-doped and Yb3+, Er3+ co-doped Ba5Y8Zn4O21 phosphors were manufactured through high-temperature solid-state reaction technique to explore the application for optical thermometer. In the course of experiment, the samples were designed based on the orthogonal experimental method to find the optimal sample with the strongest green emission that can bring in high measurement accuracy. The relationship between doping content and luminescent intensity was systematically explored and the variation tendencies of green and red emission intensity were presented intuitively. Er3+ single-doped phosphors exhibit bright up-conversion emission under stimulation of 980 nm laser. With the addition of Yb3+, the characteristic emission intensity of Er3+ was greatly enhanced. Based on the luminescent features, the power dependent spectra and the energy structure of Yb3+ and Er3+, conceivable luminescent mechanism was deduced. Meanwhile, the temperature sensing behaviors were explored by the temperature dependent up-conversion spectra. The maximum relative sensitivity (SR) is 1.36 × 10−2 K−1 (at 293 K). The above investigations manifest that the Yb3+, Er3+ co-doped Ba5Y8Zn4O21 phosphors have potential applications for optical thermometer.

Doppler-free intermodulated fluorescence spectroscopy of 4He 23P–31,3D transitions at 588 nm with a 1-W compact laser system

We have demonstrated Doppler-free intermodulated fluorescence spectroscopy of helium 23P–31,3D transitions in an rf-discharged sealed-off cell using a compact laser system at 588 nm. An external cavity diode laser at 1176 nm was constructed to seed a Raman fiber amplifier. Laser power of more than 1 W at 588 nm was produced by frequency doubling of the fiber amplifier output using a MgO:PPLN crystal. A doubling efficiency of 23 % was achieved. The power-dependent spectra of the 23P–33D transitions were investigated. Furthermore, the Doppler-free spectrum of the spin-forbidden 23P–31D transitions was observed for the first time. Our results are crucial toward precision test of QED atomic calculations, especially for improving the determination of the helium 31D–33D separation.

Measurement of an Exciton Rabi Rotation in a SingleGaN/AlxGa1−xNNanowire-Quantum Dot Using Photoluminescence Spectroscopy: Evidence for Coherent Control

Experimental observation of excited state exciton Rabi rotation in a single GaN quantum dot is presented. The dot is embedded in a site-controlled GaN/AlGaN nanowire. Damped oscillation is observed in the power-dependent spectra of the quantum-dot ground state upon resonant pumping of an excited state that had been identified by photoluminescence excitation spectroscopy. A discussion on the origins of the damping is given.

Lanthanide upconversion within microstructured optical fibers: improved detection limits for sensing and the demonstration of a new tool for nanocrystal characterization

We investigate a powerful new sensing platform based on the excitation of upconversion luminescence from NaYF(4):Yb/Er nanocrystals loaded in solution within a suspended-core microstructured optical fiber. We demonstrate a substantial improvement in the detection limit that can be achieved in a suspended-core fiber sensor for solution-based measurements using these nanocrystals as an alternative to more traditional fluorophores, with sensing of concentrations as low as 660 fM demonstrated compared with the 10 pM obtained using quantum dots. This nanocrystal loaded suspended core fiber platform also forms the basis for a novel and robust nanoscale spectrometry device capable of capturing power-dependent spectra over a large dynamic range from 10(3) W cm(-2) to 10(6) W cm(-2) using a laser diode. This serves as a useful tool to study the multiple energy levels of rare earth luminescent nano-materials, allowing the two sharp emission bands to be studied in detail over a large dynamic range of excitation powers. Thus, in addition to demonstrating a highly sensitive dip sensor, we have devised a powerful new approach for characterizing upconversion nanoparticles.

Fine Structure of Ultraviolet Photoluminescence of Tin Oxide Nanowires

Photoluminescence (PL) properties of tin oxide (SnO2) nanowires are studied in detail using high spectral resolution spectroscopy in a temperature range of 10-300 K. The nanowires have an average diameter of 86 nm. The high quality of the nanowires enables the observation of rich fine structures in the ultraviolet PL spectra at low temperatures. By carefully analyzing the temperature and excitation power dependent spectra, the following emissions are identified: recombination of donor-acceptor pairs, excitons bound to neutral and ionized donor impurities and optical transitions from free electrons to neutral acceptor impurities. Moreover, it is believed that the emission from recombination of free excitons is observed, which is unusual for SnO2, a dipole forbidden direct band gap semiconductor.

Efficient all-optical 2R regeneration using self-phase modulation in bidirectional fiber configuration

An efficient wavelength-conversion-free all-optical 2R regenerator is proposed. The regeneration is based on power-dependent spectrum broadening in normal-dispersion fiber and subsequent off-centered filtering. Twofold regeneration is performed in a bidirectional configuration where the signal is transmitted twice along the fiber in opposite directions. Experiment at 10Gb/s shows no penalty arising from the bidirectional high-power signal transmission in the fiber and demonstrates strong improvement of extinction ratio of the input signal by the regenerator.

Study of light emission from GaN/AlGaN quantum wells under power-dependent excitation

We have performed a study of excitation power-dependent spectra of GaN/AlGaN single quantum wells (QWs). First, the experimental “blueshift” of the emission energy, due to screening of internal piezoelectric fields, was compared with the model calculations based on self-consistent solution of Schroedinger and Poisson equations. We found that, even for the highest applied levels of excitation power (2.5 MW/cm2), only 0.5×1012 cm−2 carriers were present in the QW layers. Second, we analyzed the evolution of power-dependent spectra of two single QW having different widths. For the thinner QW (2.1 nm), the peak corresponding to a QW photoluminescence (PL) emission dominates the entire spectrum in the whole range of the used excitation power. In the case of the wider QW (4.4 nm), for sufficiently high excitation power, we observe the effect of PL quenching. Using the rate equation model we show that the observed effect of the PL quenching can be associated with the reduction of exciton binding energy due to the many body interactions in the QW.

Laser diode coupled to a single-delay external Kerr cavity

We investigate the steady state and the dynamic behavior of a laser diode coupled to an external single-delay, 2 ns, Kerr cavity throughout the typical five feedback levels. We show that over a large range of feedback levels, for various Kerr constants, the steady state power dependent spectrum consists of one frequency which is shifted about 0.03 GHz from the solitary laser frequency. This is manifested by suppressing mode hopping at moderate feedback levels corresponding to level II. Similarly, coherence collapse is suppressed and its onset is shifted to higher feedback levels.