Optical Properties of InGaN/GaN QW with the Same Well-Plus-Barrier Thickness

Optical properties of wurtzite violet InGaN/GaN quantum well (QW) structures, with the same well-plus-barrier thickness, grown by metal-organic chemical vapor deposition (MOCVD) on c-plane sapphire substrates, were investigated using temperature-dependent photoluminescence (TDPL) and excitation-power-dependent photoluminescence (PDPL). Two samples were compared: one had a thicker well (InGaN/GaN 3/5 nm); the other had a thicker barrier (InGaN/GaN 2/6 nm). It was found that the GaN barrier thickness in the InGaN/GaN MQWs plays an important role in determining the optical characteristics of the MQWs. The peak energy of the two samples varied with temperature in an S-shape. The thicker-barrier sample had a higher turning point from blueshift to redshift, indicating a stronger localization effect. From the Arrhenius plot of the normalized integrated PL intensity, it was found that the activation energy of the nonradiative process also increased with a thicker barrier thickness. The radiation recombination process was dominated in the sample of the thicker barrier, while the non-radiation process cannot be negligible in the sample of the thicker well.

Multiphoton Bleaching of Red Fluorescent Proteins and the Ways to Reduce It

Red fluorescent proteins and biosensors built upon them are potentially beneficial for two-photon laser microscopy (TPLM) because they can image deeper layers of tissue, compared to green fluorescent proteins. However, some publications report on their very fast photobleaching, especially upon excitation at 750–800 nm. Here we study the multiphoton bleaching properties of mCherry, mPlum, tdTomato, and jREX-GECO1, measuring power dependences of photobleaching rates K at different excitation wavelengths across the whole two-photon absorption spectrum. Although all these proteins contain the chromophore with the same chemical structure, the mechanisms of their multiphoton bleaching are different. The number of photons required to initiate a photochemical reaction varies, depending on wavelength and power, from 2 (all four proteins) to 3 (jREX-GECO1) to 4 (mCherry, mPlum, tdTomato), and even up to 8 (tdTomato). We found that at sufficiently low excitation power P, the rate K often follows a quadratic power dependence, that turns into higher order dependence (K~Pα with α > 2) when the power surpasses a particular threshold P*. An optimum intensity for TPLM is close to the P*, because it provides the highest signal-to-background ratio and any further reduction of laser intensity would not improve the fluorescence/bleaching rate ratio. Additionally, one should avoid using wavelengths shorter than a particular threshold to avoid fast bleaching due to multiphoton ionization.

Dynamic Response of Bridge-Landslide Parallel System under Earthquake

In order to further understand the instability mechanism and geohazard causation when the main sliding path of the slope body is parallel to the path of the bridge, the corresponding bridge-landslide parallel system is constructed for shaking table tests. This paper summarizes the combination forms of bridge-landslide model under different position and focused on the slope body located above the bridge deck. Firstly, based on the shaking table test results of El Centro (1940), the failure behavior of bridge-landslide parallel system was evaluated, and the changes of acceleration and deformation of bridge pile were subsequently analyzed. Then, the interaction bridge structure and sliding body were explained by the spectral features. The main conclusions are as follows. First, in the model test, the landslide belongs to the thrust-type landslide. Due to the barrier function of the bridge, the main failure site of landslide occurs in the middle and trailing edge of slope body. At the same time, the acceleration value of earthquake waves is 0.3 g, which is the key to this variation. Second, the acceleration response of the measuring points on the bridge pile and landslide increases with the increase of ground elevation. If the slope structure is damaged severely, the deformation response of weak interlayer is inconsistent with the surrounding soil structure. Third, with the increase of excitation power, the dominant frequency of bridge-landslide parallel system gradually transitions from low to high frequency rate, and the interaction of the parallel system weakens the influence of river direction on frequency. Finally, under the same working condition, the dynamic response of the measuring points has obvious regularity with the change of situation. But the response of the same points is not regular due to the different earthquake excitation intensity.

Single-photon emission and multi-photon emission from single CdTeSe/ZnS quantum dots at room temperature

In this report, single-photon emission and ordered multi-photon emission from single colloidal CdTeSe/ZnS quantum dots at near-infrared wavelength were realized at room temperature. The fluorescence lifetime, blinking, and anti-bunching effect of this single quantum dots at different excitation powers were measured. The relationship between the excitation power and the emission of exciton, biexciton and multiexciton of single quantum dots at a wavelength of 800nm was obtained. The physical mechanism of photoluminescence of the single quantum dots was analyzed.

Assessment results of soil properties by engineering-geophysical studies

В статье излагаются результаты инженерно-геофизических исследований на территории новой компрессорной станции Специального подземного хранилища газа (СПХГ) в г. Абовян (Армения). Учитывая то, что Республика Армения находится в сейсмоактивной зоне, проведение данных исследований является весьма актуальными, а новая компрессорная станция Специального подземного хранилища газа является особо важным объектом. Целью работы является выявление для данной территории особенности удельного электрического сопротивления грунтов, наличие блуждающих токов (БТ), оценка коррозийной агрессивности грунтов, изучение сейсмических условий и оценка ожидаемой сейсмической интенсивности на территории, намеченной под строительство. Методы работы. Определение удельного (кажущегося) электрического сопротивления грунтов исследуемой территории производилось методом электропрофилирования с использованием симметричной четырехэлектродной установки Веннера. В статье представлены пункты измерения удельного электрического сопротивления грунтов на территории новой компрессорной станции Абовянского СПХГ, выявлено наличие блуждающих токов, приведены таблицы степени коррозионной агрессивности грунтов, указаны места наличия опасного уровня блуждающих токов. Также проведены работы по сейсмическому микрорайонированию для определения величины ожидаемой сейсмической интенсивности конкретной территории компрессорной станции. Ожидаемая сейсмическая интенсивность на данной территории определена на основе анализа инженерно-геологических материалов с учетом результатов полевых инженерно-сейсмометрических инструментальных исследований. С помощью малоглубинной сейсморазведки были определены скорости распространения сейсмических волн. Измерения проводились горизонтально-ориентированным сейсмоприемником СМ-3 (вертикальный удар). Ударные волны создавались импульсным возбуждением. Для обеспечения необходимой мощности возбуждения импульсное воздействие создавалась с помощью падающего груза. Результаты работы. В статье приведены сейсмические условия территории, результаты сейсмометрических исследований,данные по наблюдениям микросейсм, спектры Фурье преобладающих периодов по некоторым точкам наблюдений, схема инженерно-сейсмометрических наблюдений. В результате проведенных исследований установлено, что грунтовые условия данной территории относятся к грунтам первой категории по сейсмическим свойствам. Ожидаемую сейсмическую опасность исследованной территории необходимо характеризовать следующими значениями: I=7 баллов или PGA=0,24 g. The article presents the results of engineering-geophysical studies on the territory of the new compressor station of the Special Underground Gas Storage (SPGS) in the city of Abovyan (Armenia). Considering that the Republic of Armenia is located in a seismically active zone, these studies are highly relevant, and the new compressor station of the Special Underground Gas Storage is especially important object. The Aim of the presented work is to identify the features of the specific electrical resistivity of soils, the presence of stray currents, assess the corrosive aggressiveness of soils, seismic conditions of the territory and the expected seismic intensity in the area designated for construction. Methods. The determination of the specific (apparent) electrical resistivity of the soils of the study area was carried out by the method of electrical profiling using symmetrical four-electrode Wenner setup. The article presents the points of measuring the electrical resistivity of soils in the territory of the new compressor station of the Abovyan SPGS, revealed the presence of stray currents, tables of the degree of corrosiveness of soils are given, the locations of the presence of a dangerous level of stray currents are indicated. Also works on seismic microzoning were carried out, the values of the expected seismic intensity of the compressor station territory were determined. The expected seismic intensity in this area has been determined based on analysis of engineering-geological materials, taking into account the results of field engineering-seismometric instrumental studies. By using shallow seismic surveys have determined the speed of seismic velocity. Measurements were carried out horizontally oriented seismic receiver SM-3 (vertical impact). Shock waves were generated by pulsed excitation. To provide the required excitation power the impulse action was created using a falling weight. Results. The article presents the seismic conditions of the territory, the results of seismometric studies, data on observations of microseisms, Fourier spectra of prevailing periods for some observation points, a scheme of engineering seismometric observations. As a result of the research carried out, it was found that that the soil conditions composing this territory belong to the soils of the first category in terms of seismic properties, the expected seismic hazard of the investigated area should be taken as I = 7 points or PGA = 0.24 g.

A unifying perspective of the ultrafast photo-dynamics of Orange Carotenoid Protein from Synechocystis: peril of high-power excitation, existence of different S* states and influence of tagging

A substantial number of Orange Carotenoid Protein (OCP) studies have aimed to describe the evolution of singlet excited states leading to the formation of photo-activated form, OCPR. The most recent one suggests that three picosecond-lived excited states are formed after the sub-100 fs decay of the initial S2 state. The S* state which has the longest reported lifetime of a few to tens of picoseconds is considered to be the precursor of the first red photoproduct P1. Here, we report the ultrafast photo-dynamics of the OCP from Synechocystis PCC 6803, carried out using Visible-NIR femtosecond time-resolved absorption spectroscopy as a function of the excitation pulse power and wavelength. We found that a carotenoid radical cation can form even at relatively low excitation power, obscuring the determination of photo-activation yields for P1. Moreover, the comparison of green (540 nm) and blue (470 nm) excitations revealed the existence of an hitherto uncharacterized excited state, denoted as S~, living a few tens of picoseconds and formed only upon 470 nm excitation. Since neither the P1 quantum yield nor the photo-activation speed over hundreds of seconds vary under green and blue continuous irradiation, this S~ species is unlikely to be involved in the photo-activation mechanism leading to OCPR. We also addressed the effect of His-tagging at the N- or C-termini on excited state photo-physical properties. Differences in spectral signatures and lifetimes of the different excited states were observed, at variance with the usual assumption that His-tagging hardly influences protein dynamics and function. Altogether our results advocate for careful consideration of the excitation power and His-tag position when comparing the photo-activation of different OCP variants, and beg to revisit the notion that S* is the precursor of photoactivated OCPR.

Influence of in volatilization on photoluminescence in InGaN/GaN multiple quantum wells

Two multiple quantum well (MQW) InGaN/GaN structures emitting green light, without (A) and with (B) an indium (In) volatilization suppression technique (IVST) during growth of the active region, were fabricated. The dependencies of the photoluminescence (PL) spectra upon temperature at different levels of excitation power were investigated. The results indicate that an IVST can increase the In content while suppressing the phase separation caused by volatilization of that In incorporated in the well layers. Also, compared with Structure B with IVST, which contains one phase structure, Structure A without IVST, which contains two separate phases (i.e., an In-rich phase and an In-poor phase), exhibits higher internal quantum efficiency (IQE) at low excitation power and lower IQE at high excitation power. The former is mainly attributed to the stronger In-rich phase-related localization effect of Structure A, because the In-rich phase-related emission dominates the PL spectra of Structure A at a low excitation power; the latter is mainly due to the In-poor phase-related weaker localization effect of Structure A, because the In-poor phase-related emission dominates the PL spectra of Structure A at high excitation power because localized states in this In-rich phase are saturated.

Lipid droplets as endogenous intracellular microlenses

Using a single biological element as a photonic component with well-defined features has become a new intriguing paradigm in biophotonics. Here we show that endogenous lipid droplets in the mature adipose cells can behave as fully biocompatible microlenses to strengthen the ability of microscopic imaging as well as detecting intra- and extracellular signals. By the assistance of biolenses made of the lipid droplets, enhanced fluorescence imaging of cytoskeleton, lysosomes, and adenoviruses has been achieved. At the same time, we demonstrated that the required excitation power can be reduced by up to 73%. The lipidic microlenses are finely manipulated by optical tweezers in order to address targets and perform their real-time imaging inside the cells. An efficient detecting of fluorescence signal of cancer cells in extracellular fluid was accomplished due to the focusing effect of incident light by the lipid droplets. The lipid droplets acting as endogenous intracellular microlenses open the intriguing route for a multifunctional biocompatible optics tool for biosensing, endoscopic imaging, and single-cell diagnosis.