Spectroscopic evidence of tunnel coupling between CdTe quantum wells in the CdTe/ZnTe heterostructures

The photoluminescence (PL) spectra of CdTe/ZnTe double quantum wells (QWs) are studied on a series of samples containing two CdTe layers with nominal thicknesses of 2 and 4 monolayers (ML) in the ZnTe matrix. The QWs were grown in atomic-layer epitaxy and separated by ZnTe spacers with the thicknesses dsp =40−160 ML. The dependences of the relative intensity of shallow QW1 and deep QW2 PL bands (I1 and I2 , respectively) on the pump intensity (J) when excited by the lasers with different radiation wavelengths are investigated. It is found that in the sample with dsp=40 ML, the ratio Y(J)=I1/I2 depends on J and the shape of the Y(J) dependency changes with the excitation wavelength. In the samples with dsp > 70 ML Y(J) also changes with the excitation intensity J, but the shape of this dependence is the same for various excitation wavelengths. It is concluded that the energy relaxation in these samples is influenced not only by the tunneling of charge carriers from QW1 to QW2, but also by carrier relaxation at the nonradiative centers, for which the recombination rate is different for shallow and deep QWs.

Purcell enhancement of fluorescence from silicon-vacancy color centers in Mie-resonant luminescent diamond particles

Over the past two decades, nanosized diamond particles with various luminescent defects have found numerous applications in many areas from quantum technologies to medical science. The size and shape of diamond particles can affect drastically the luminescence of embedded color centers. Here we study diamond particles of 250–450 nm in size containing silicon-vacancy (SiV) centers. Using dark-field scattering spectroscopy, we found that fundamental Mie resonances are excited in the spectral range of interest. We then measured the fluorescence saturation curves under continuous excitation to estimate the effects of the excitation and Purcell factor enhancement on the luminescent properties of the studied particles. The results show that the saturation excitation intensity differs by several times for particles of different sizes which is well explained by the numerical model that takes into account both the Parcell factor enhancement and resonant excitation.

VIBRATONAL MECHANICS AND STOCHASTIC QUASI-RESONANCES

The concept of vibrational mechanics was pioneered in the works by Professor I.I. Blekhman and developed by his numerous disciples and coleagues. It is a powerful tool for the study of such systems with fast excitations, in which slow motion is of primary interest. One important application of this approach is the stochastic resonance, the phenomenon of resonance-like response of slow variables to intensity of stochastic excitation. This phenomenon is considered within the framework of vibrational mechanics as forced lowfrequency oscillations near the natural frequency, which evolves under the influence of changing high-frequency stochastic excitation. We propose a generalization of this approach to the case when the evolution of low-frequency properties of the system leads not to the equality of the natural frequency and the frequency of the external slow force, but to the loss of stability in a certain interval of the stochastic excitation intensity. Since in this case, as for stochastic resonance, the external manifestation of the process is the resonance-like response of the system, the considered effect can be called stochastic quasi-resonance, As an example, we consider a rotor with anisotropy of bending stiffness under the action of stochastic angular velocity oscillations.

Transformation from Self-Focusing to Self-Defocusing of Silver Nanoparticles

The nonlinear refraction of silver nanoparticles (AgNPs) in n-hexane was studied by using the closed-aperture Z-scan technique with a 532 nm nanosecond laser. It was found that, the nonlinear refraction of AgNPs shows the coexistence and transformation from self-focusing to self-defocusing. Specifically, self-focusing occurs at low excitation intensity, self-defocusing occurs at high excitation intensity, and coexistence of self-focusing and self-defocusing occurs at relatively moderate excitation intensity. The experimental results were analysed and discussed in terms of third-order and fifth-order nonlinear refractive effect. Specifically, the self-focusing is caused by the positive third-order nonlinear refraction, the self-defocusing is induced by the negative fifth-order nonlinear refraction, and the transformation from the self-focusing to self-defocusing at medium excitation intensity is caused by the competition of third-order and fifth-order nonlinear refraction. Finally, the third-order refractive index and fifth-order refractive index were obtained.

Excitation-power-dependent upconversion luminescence competition in single β-NaYbF4:Er microcrystal pumped at 808 nm

Controlling the upconversion luminescence (UCL) intensity ratio, especially pumped at 808 nm, is of fundamental importance in biological applications due to the water molecules exhibiting low absorption at this excitation wavelength. In this work, a series of β-NaYbF4:Er microrods were synthesized by a simple one-pot hydrothermal method and their intense green (545 nm) and red (650 nm) UCL were experimentally investigated based on single particle level under the excitation of 808 nm continuous-wave (CW) laser. Interestingly, the competition between the green and red UCL can be observed in highly Yb3+-doped microcrystals as the excitation intensity gradually increases, which leads to the UCL color changes from green to orange. However, the microcrystals doped with low Yb3+ concentration keep green color which is independent on the excitation power. Further investigations demonstrate that the cross-relaxation (CR) processes between Yb3+ and Er3+ ions result in the UCL competition.

Multimodal measurements of phototoxicity in nonlinear optical microscopy

Nonlinear optical imaging modalities, such as two-photon microscopy and stimulated Raman scattering (SRS) microscopy, make use of pulsed-laser excitation with high peak intensity that can perturb the native state of cells. In this study, we investigated the short and long-term effects of pulsed laser induced phototoxicity. We used bulk RNA sequencing, quantitative measurement of cell proliferation, and measurement of the generation of reactive oxygen species (ROS) to assess phototoxic effects, at different time scales, for a range of laser excitation settings relevant to SRS imaging. We define a range of laser excitation settings for which there was no significant ROS generation, differential gene expression, or change in proliferation rates of mouse Neuro2A cells. Changes in proliferation rate and ROS generation were observed under imaging conditions with an excitation intensity of over 600 mW/μm2. Repeated imaging of the same field of view at this excitation intensity of over 600 mWμm2 resulted in visual damage to N2A cells. Laser induced perturbations in live cells may impact downstream measurements of cell state including subsequent imaging or molecular measurements. This study provides guidance for imaging parameters that minimize photo-induced perturbations in SRS microscopy to ensure accurate interpretation of experiments with time-lapse imaging or with paired measurements of imaging and sequencing on the same cells.

Photoluminescence of Ca4Ga2S7:Eu2+ in wide excitation intensity and temperature range

Photoluminescence properties of [Formula: see text] chalcogenide semiconductors have been studied under the impulse laser excitation in the range of 10–105 W/cm2 at room temperature. This study has shown that as a result of excitation, photoluminescence of [Formula: see text] is characterized by the emission in the interval of 450–575 nm with significant domination in the spectra line at 660 nm. Photoluminescence of [Formula: see text] quenches at wavelengths of 560 nm and 660 nm with constant time frames 258 ns and 326 ns, respectively. Moreover, the temperature measurements of photoluminescence were performed on the samples in the temperature range of 10–300 K.

Photoinduced multistage phase transitions in Ta2NiSe5

Ultrafast control of material physical properties represents a rapidly developing field in condensed matter physics. Yet, accessing the long-lived photoinduced electronic states is still in its early stages, especially with respect to an insulator to metal phase transition. Here, by combining transport measurement with ultrashort photoexcitation and coherent phonon spectroscopy, we report on photoinduced multistage phase transitions in Ta2NiSe5. Upon excitation by weak pulse intensity, the system is triggered to a short-lived state accompanied by a structural change. Further increasing the excitation intensity beyond a threshold, a photoinduced steady new state is achieved where the resistivity drops by more than four orders at temperature 50 K. This new state is thermally stable up to at least 350 K and exhibits a lattice structure different from any of the thermally accessible equilibrium states. Transmission electron microscopy reveals an in-chain Ta atom displacement in the photoinduced new structure phase. We also found that nano-sheet samples with the thickness less than the optical penetration depth are required for attaining a complete transition.

Effect of dopant concentration and excitation intensity on the upconversion and downconversion emission of $\beta$-NaYF$_4$:Yb$^{3+}$,Er$^{3+}$ nanoparticles

The dopant concentration of lanthanide ions in photon-upconversion nanoparticles (UCNPs) remains one of the key points to boost these nanomaterials' brightness and, therefore, their application development. Here, we analyzed the...