Gigahertz acoustic vibrations of Ga-doped ZnO nanoparticle array

Small-scale effects on the radial vibration of an elastic nanosphere based on nonlocal strain gradient theory

Nonlocal strain gradient theory is widely used when dealing with micro- and nano-structures. In such framework, small-scale effects cannot be ignored. In this paper a model of radial vibration of an isotropic elastic nanosphere is theoretically investigated. The frequency equation is obtained from a nonlocal elastic constitutive law, based on a mix between local and nonlocal strain. This model is composed of both the classical gradient model and the Eringen's nonlocal elasticity model. To check the validity and accuracy of this theoretical approach, a comparison is made with the literature in certain specific cases, which shows a good agreement. Numerical examples are finally conducted to show the impact of small-scale effects in the radial vibration, which need to be included in the nonlocal strain gradient theory of nanospheres. It reveals that the vibration behavior greatly depends on the nanosphere size and nonlocal and strain gradient parameters. Particularly, when the nanospheres radius is smaller than a critical radius, the small-scale effects play a key role. Thus, the obtained frequency equation for radial vibration is very useful to interpret the experimental measurements of vibrational characteristics of nanospheres.

Time-of-flight secondary ion mass spectrometry fragment regularity in gallium-doped zinc oxide thin films

Time-of-flight secondary ion mass spectrometry fragment analysis remains a challenging task. The fragment appearance regularity (FAR) rule is particularly useful for two-element compounds such as ZnO. Ion fragments appearing in the form of ZnxOy obey the rule [Formula: see text] in the positive secondary ion spectrum and [Formula: see text] in the negative spectrum where the valence of Zn is + 2 and that of O is - 2. Fragment analysis in gallium-doped ZnO (GZO) films can give insights into the bonding of the elements in this important semiconductor. Fragment analysis of 1 and 7 wt% GZO films shows that only the negative ion fragments obey the FAR rule where ZnO‒, 66ZnO‒, 68ZnO‒ and ZnO2‒ ion fragments appear. In the positive polarity, subdued peaks from out-of-the-rule ZnO+, 66ZnO+ and 68ZnO+ ion fragments are observed. The Ga ion peaks are present in both the positive and negative spectra. The secondary ion spectra of undoped ZnO also shows consistency with the FAR rule. This implies that Ga doping even in amounts that exceed the ZnO lattice limit of solubility does not affect the compliance with the FAR rule.

ZnO nanoparticles-based vacuum pressure sensor

ZnO nanoparticles synthesized using the sol-gel technique with an average diameter of 11.5 nm are used to fabricate a vacuum pressure sensor in the range of 1 mbar to 10⁺³ mbar (low vacuum limit). A drastic increase in the current of the drop-casted ZnO on glass with 30 µm separated Au contacts defined by e-beam lithography is observed. The sensor reveals a linear relationship in current versus pressure in a logarithmic plot. In the range of 1 mbar to 10⁺³ mbar, the sensor sensitivity is found be 110. Using the resistance-time plot of the vacuum pressure, the rise (response) and fall (recovery) times of the sensor are determined as 6.6 and 15.6 s, respectively. The power consumption of the sensor is 6.5 [Formula: see text]W. The operational parameters of the proposed sensor are found be much better than those of previously reported ZnO nanostructure-based sensors and, indeed, traditional ones. The sensing mechanism of the sensor is explained by the adsorption/desorption of OH^- ions from the surface of the ZnO nanoparticles, leaving behind oxygen ions combined with oxygen vacancy states.

Improved photoemission and stability of 2D organic-inorganic lead iodide perovskite films by polymer passivation

2D organic-inorganic lead iodide perovskites hold great promise for functional optoelectronic devices. However, their performances have been seriously limited by poor long-term stability in ambient environment. Here, we perform a systematic study for the stability improvement of a typical 2D organic-inorganic lead iodide perovskite (PEA)2PbI4. The degradation of the (PEA)2PbI4 films can be attributed to the interaction with the humidity in environment, which leads to decomposition of the perovskite components. Then, we demonstrate that polymer passivation provides an effective approach for improving the crystal quality and stability of the (PEA)2PbI4 films. Correspondingly, the photoemission of the polymer-passivated (PEA)2PbI4 films has been enhanced due to the decreased trap states. More importantly, a hydrophobic polymer (Poly(4-Vinylpyridine), PVP) will protect the (PEA)2PbI4 films from humidity in ambient environment, which can greatly improve the physical and chemical stability of the 2D perovskite films. As a result, the PVP-passivated (PEA)2PbI4 films can produce a bright emission even after long-term (>15 d) exposure to ambient environment (25 °C, 80% RH) and continuous UV illumination. This work provides a convenient and effective approach for improving the long-term stability of 2D organic-inorganic lead iodide perovskites, which shows great promise for fabricating large-area and versatile optoelectronic devices.

Channel-closing effects of electronic excitation in solids

We investigate the electronic excitation of solids in strong fields by solving the time-dependent Schrödinger equation. The excitation probability exhibits a strong modulation as a function of laser intensity when the initial states fill in the whole valence band. To have a clear insight into the modulation, we further study the electronic excitation from a single eigenstate in solids. A series of resonance-like enhancements of excitation probability are produced by changing the laser intensity and wavelength. We attribute the resonance-like enhancements to the channel-closing effects in solids. It is shown that the excitation probability exhibits enhancements when the value of channel is odd for intracycle interference and an integer for intercycle interference. This is different from the atom that the enhancement occur in the integer channels. We also reveal that the channel-closing effects can be observed by solid high-order harmonic generation.

Two-dimensional photoelectron holography in strong-field tunneling ionization by counter rotating two-color circularly polarized laser pulses

Strong-field photoelectron holography (SFPH), originating from the interference of the direct electron and the rescattering electron in tunneling ionization, is a significant tool for probing structure and electronic dynamics in molecules. We theoretically study SFPH by counter rotating two-color circularly (CRTC) polarized laser pulses. Different from the case of the linearly polarized laser field, where the holographic structure in the photoelectron momentum distribution (PEMD) is clustered around the laser polarization direction, in the CRTC laser fields, the tunneling ionized electrons could recollide with the parent ion from different angles and thus the photoelectron hologram appears in the whole plane of laser polarization. This property enables structural information delivered by the electrons scattering the molecule from different angles to be recorded in the two-dimensional photoelectron hologram. Moreover, the electrons tunneling at different laser cycles are streaked to different angles in the two-dimensional polarization plane. This property enables us to probe the sub-cycle electronic dynamics in molecules over a long time window with the multiple-cycle CRTC laser pulses.

All-optical measurement of high-order fractional molecular echoes by high-order harmonic generation

An all-optical measurement of high-order fractional molecular echoes is demonstrated by using high-order harmonic generation (HHG). Excited by a pair of time-delayed short laser pulses, the signatures of full and high order fractional (1/2 and 1/3) alignment echoes are observed in the HHG signals measured from CO ₂ molecules at various time delays of the probe pulse. By increasing the time delay of the pump pulses, much higher order fractional (1/4) alignment echo is also observed in N ₂O molecules. With an analytic model based on the impulsive approximation, the spatiotemporal dynamics of the echo process are retrieved from the experiment. Compared to the typical molecular alignment revivals, high-order fractional molecular echoes are demonstrated to dephase more rapidly, which will open a new route towards the ultrashort-time measurement. The proposed HHG method paves an efficient way for accessing the high-order fractional echoes in molecules.

Enhanced optoelectronic performance of 2D organic-inorganic hybrid perovskite through light-illumination

Improving the optoelectronic characteristics of organic-inorganic perovskites is crucial for fabrication of functional devices. Herein, we demonstrate that the optoelectronic properties of 2D organic-inorganic perovskites can be greatly improved by UV-light illumination during growth. The photoluminescence emission of the 2D perovskite exhibits a 3.1-folds increase in intensity, with a decreased trap-assisted recombination. The improved optoelectronic characteristics can be attributed to the high-quality crystallization and lattice expansion induced by the UV-light illumination. Moreover, the optimized 2D perovskites enable the fabrication of photoconductive devices with improved optoelectronic responses. This work indicates that light illumination is a novel and convenient approach for engineering the fabrication of 2D organic-inorganic hybrid perovskites, which advocates great promise for achieving high-performance functional devices.

Few-cycle 1.9-μm pulse generation via collinear spectrum synthesis in multiple-crystal OPA

A multiple-crystal optical parametric amplification (OPA) design is reported for efficiently generating few-cycle 1.9-μm laser pulses. Different spectral regions of the idler pulse are successively amplified in three nonlinear crystals with delicately adjusted phase-matching angles, and a broadband spectrum supporting a three-cycle transform-limited (TL) pulse duration is obtained. Near-TL duration of 21.5 fs is realized by simple compression in a silicon window. Owing to sufficient exploitation of the pump energy in the crystals, total conversion efficiency of 31.3% is achieved with idler pulse energy of 65.8 μJ. The gain bandwidth in multiple-crystal OPA is markedly broadened compared to OPA using a single thick crystal; meanwhile, the high efficiency is preserved. Further energy scaling of the proposed scheme is potentially feasible using dual-chirped OPA geometry.

Coherent Perfect Absorption Laser Points in One-Dimensional Anti-Parity–Time-Symmetric Photonic Crystals

We investigate the coherent perfect absorption laser points (CPA-LPs) in anti-parity–time-symmetric photonic crystals. CPA-LPs, which correspond to the poles of reflection and transmission, can be found in the parameter space composed of gain–loss factor and angular frequency. Discrete exceptional points (EPs) split as the gain–loss factor increases. The CPA-LPs sandwiched between the EPs are proved to be defective modes. The localization of light field and the bulk effect of gain/loss in materials induce a sharp change in phase of the reflection coefficient near the CPA-LPs. Consequently, a large spatial Goos–Hänchen shift, which is proportional to the slope of phase, can be achieved around the CPA-LPs. The study may find great applications in highly sensitive sensors.

Influences of Ga Doping on Crystal Structure and Polarimetric Pattern of SHG in ZnO Nanofilms

The second-harmonic generation (SHG) in gallium doped ZnO (GZO) nanofilms was studied. The Ga doping in GZO nanofilms influenced the crystal structure of the films, which affected SHG characteristics of the nanofilms. In our experiments, a strong SHG response was obtained in GZO nanofilms, which was excited by 790 nm femtosecond laser. It was observed that the Ga doping concentrations affected, not only the intensity, but also the polarimetric pattern of SHG in GZO nanofilms. For 5.0% doped GZO films, the SHG intensity increased about 70%. The intensity ratio of SHG between the incident light polarization angle of 90° and 0°changed with the Ga doping concentrations. It showed the most significant increase for 7.3% doped GZO films, with an increased ratio of c/a crystal constants. This result was attributed to the differences of the ratios of d₃₃/d₃₁ (the second-order nonlinear susceptibility components) induced by the crystal distortion. The results are helpful to investigate nanofilms doping levels and crystal distortion by SHG microscopy, which is a non-destructive and sensitive method.