On-chip ultrafast stackable dielectric laser positron accelerator

We present a first on-chip positron accelerator based on dielectric laser acceleration. This innovative approach significantly reduces the physical dimensions of the positron acceleration apparatus, enhancing its feasibility for diverse applications. By utilizing a stacked acceleration structure and far-infrared laser technology, we are able to achieve a seven-stage acceleration structure that surpasses the distance and energy gain of using the previous dielectric laser acceleration methods. Add...     

Transient response analysis of photoconductive detector under ultra-short laser pulse radiation

The transient response characteristics of HgCdTe photoconductive detector under the radiation of ultra-short laser pulse have been discussed in detail. Specifically, the transient effect of pulse width to the temperature of electronics and crystal lattice, and corresponding resistance changes of detector are mainly discussed. Based on traditional drift-diffusion model, considering that the temperature of electronics and crystal lattice are different under the ultra-short laser pulse, the double-temperature equation is joined to describe the semiconductor carriers’ dynamics features. Using the numerical method, the transient response characteristics of detector in the case of ultra-short pulse have been worked out. The calculation results show that: When the pulse width is greater than nanosecond pulse, the temperature of electronics will be equal to which of crystal lattice. If the pulse width is less than nanosecond pulse, the temperature of electronics is higher than which of the latter. After the end of a pulse, the rebound resistance of detector will be higher than the dark resistance because of the thermal effect. The heat effect is more obvious when a pulse with narrower width and higher energy density incident to the detector.     

介质加载天线阵的瞬态辐射特性

提高天线增益和辐射效率是瞬态天线研究的重要内容之一。通过设定高斯脉冲激励波形,导出了瞬态辐射的能量阵列因子,分析了1维对称均匀天线阵列的瞬态辐射特性。结果表明:阵列的辐射波形与观察时间区间和角度有关,瞬态与稳态特性差异明显,时域辐射方向图与脉冲宽度有关,阵列可实现时域波束扫描。设计了介质加载天线阵,并采用3维电磁场分析软件进行了时域仿真计算,分析了介质加载对天线辐射特性的影响。研究表明:有介质加载的天线前向辐射电场峰值比没有介质加载的天线增加逾1倍,即瞬态辐射功率增大了3倍多,通过对计算和仿真结果的分析比较,验证了分析和设计方法的正确性和有效性。     

介质加载天线阵的瞬态辐射特性

 提高天线增益和辐射效率是瞬态天线研究的重要内容之一。通过设定高斯脉冲激励波形,导出了瞬态辐射的能量阵列因子,分析了1维对称均匀天线阵列的瞬态辐射特性。结果表明:阵列的辐射波形与观察时间区间和角度有关,瞬态与稳态特性差异明显,时域辐射方向图与脉冲宽度有关,阵列可实现时域波束扫描。设计了介质加载天线阵,并采用3维电磁场分析软件进行了时域仿真计算,分析了介质加载对天线辐射特性的影响。研究表明:有介质加载的天线前向辐射电场峰值比没有介质加载的天线增加逾1倍,即瞬态辐射功率增大了3倍多,通过对计算和仿真结果的分析比较,验证了分析和设计方法的正确性和有效性。     

Phase transitions in erbium-doped silicon exposed to laser radiation

The dynamics of phase transitions induced by nanopulsed ruby laser radiation (80 nsec, 2 J/cm²) both in silicon layers doped with erbium ions and in those containing doped erbium and oxygen have been studied by an optical probing method. It is shown that the reflectivity behavior of structures under pulsed irradiation is governed by phase transitions (melting and crystallization) of implanted silicon and also by interference effects at the interfaces of the resulting phases. It is established that the profiles of erbium distribution change under nanosecond laser irradiation and that the dopant is forced out to the surface due to a segregation effect at small implantation doses. As the implanatation dose increases, diffusion deep into the sample tends to prevail over segregation. A considerable increase in the photoluminescence peak intensity at 0.81 eV is found after both the pulsed laser processing and thermal post-annealing of doped samples as opposed to spectra of samples subjected either to thermal annealing or to pulsed laser irradiation. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 2, pp. 225–231, March–April, 2009.     

Fluorescence and absorption of polystyrene exposed to UV laser radiation

We show that when polystyrene is exposed (for 15–60 sec) to a UV laser light beam (λ = 248 nm), its absorption and luminescent properties change significantly. In the irradiated polymer, optical centers are formed with absorption bands in the 280–460 nm region and fluorescence bands in the 330–520 nm region. We have established the chemical structure of the optical centers for fluorescence of polystyrene. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 1, pp. 54–58, January–February, 2006.     

Direct ultrafast laser micro-structuring of materials using programmable beam shaping

Using active and programmable focal-spot shaping, we demonstrate direct femtosecond laser micro-structuring of materials. Surface marking as well as drilling with user-defined geometrical shapes of small dimensions (20 μm) are performed, without moving the laser beam nor the material target.     

Oxyhemoglobin photodissociation efficiency in biological tissue exposed to laser radiation

We have obtained quantitative data on the differential (with respect to depth) and the integrated oxyhemoglobin photodissociation efficiency in the dermis when the skin surface is exposed to a light beam in the wavelength range 300–650 nm. With this aim, we have used our own previously developed optical model for skin tissue and analytical procedure for calculating the characteristics of optical fields in a medium. We have estimated the number of oxygen molecules formed at different depths in the medium, and also their integrated number over the entire thickness of the dermis as a function of the irradiation wavelength. We consider models for a dermis that is homogeneous with respect to depth and a dermis that has a layered structure. We show that the spectral photodissociation efficiency has a number of maxima associated with the absorption spectrum of oxyhemoglobin and the optical properties of all the layers of skin tissue. We discuss the effect of the epidermis on these maxima.     

Kinetics of oxygenation of skin tissue exposed to low-intensity laser radiation

We present the results of modeling the action spectrum and an experimental investigation of the effect of laser-induced photodissociation of oxyhemoglobin in vivo on the increase in the degree of oxygenation of skin tissue in the exposed area. We have shown that controlling the local concentration of free molecular oxygen in biological tissues together with the possibility of eliminating tissue hypoxia using laser radiation makes it possible to stimulate aerobic cell metabolism and to achieve the needed therapeutic effect. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 120–125, January–February, 2007.     

Enhanced photosensitivity in germanosilicate fibers exposed to CO2 laser radiation

We report a novel method to increase the UV photosensitivity of GeO(2): SiO(2) optical fibers based on exposure to CO(2) laser irradiation before grating writing. Fibers treated with a CO(2) laser can produce gratings with refractive-index modulation two times greater and a Bragg wavelength that can be 2 nm longer than those of untreated fibers. Experiments on GeO(2): SiO(2) preform samples treated with a CO(2) laser in a way similar to the fibers showed a marked increase of the 242-nm absorption band, which is associated with an increase of germanium oxygen-deficient centers and is believed to be responsible for the higher photorefractive response.     

Multiple beam scattering effects in biological tissues exposed to laser radiation

Summary A mathematical model is presented for the computation of intensity of radiation in biological tissues irradiated by laser. The time-independent transport radiative equation is solved in terms of a Neumann series. Each term of the series is associated with a different order of magnitude of the internal-scattering effect. The importance of these effects has been recognized during the irradiation by ion-argon and ND: YAG laser used as coagulting or cutting instruments in surgery applications. Agreement is found with the single-scattering picture.

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Riassunto Si presenta un modello matematico per calcolo dell'intensità della radiazione nei tessuti biologici esposti a radiazione laser. L'equazione del trasporto radiativo indipendente dal tempo è risolta con la serie di Neumann. Ogni termine della serie rappresenta ordini di grandezza differenti dell'effetto di scattering interno. Questi effetti divengono importanti durante l'irraggiamento con laser ad argon e ND: YAG utilizzati in applicazioni mediche per la coagulazione e l'incisione. Il modello è in accordo al primo ordine con il termine di scattering singolo.

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Резюме Предлагается математическая модель для вычисления интенсивности излучения в биологических тканях, подверженньх лазерному облучению. С помощью рядов Неймана решается не зависящее от времени уравнение переноса излучения. Каждый член ряда связан с различным порядком величнины эффекта внутреннего рассеяния. Определяется важность эффектов многократного рассеяния при облучении ион-аргоновым лазером и ND: YAG лазером, которые ирользуются как инструменты коагуляции или резания в хирургии. Показывается, что процесс взаимодействия соответствует картине однократного рассеяния.

     

Fluorescence and absorption of a polystyrene-based scintillator exposed to UV laser radiation

We have established that exposure of polystyrene-based scintillator samples to UV laser radiation (248 nm) leads to a significant decrease in the fluorescence intensity. We have carried out a spectral analysis of the luminescent and absorption properties of the scintillator, which allowed us to determine the major factor in the decrease in luminescence intensity of the samples exposed to UV radiation. We propose a new hypothesis for the mechanism of the processes leading to the decrease in light output of the scintillator during operation. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 6, pp. 744–749, November–December, 2007.     

Femtosecond opto‐magnetism: ultrafast laser manipulation of magnetic materials

This review summarizes the recent progress in the study of ultrafast nonthermal effects of light on magnetic materials. It is demonstrated that due to opto‐magnetic phenomena an intense 100 fs circularly polarized laser pulse acts on the spins similar to an equivalently short effective magnetic field pulse up to 1 T. The review shows that using such opto‐magnetic phenomena one may selectively excite different modes of magnetic resonance, realize quantum control of magnons, trigger magnetic phase transitions and switch spins on a subpicosecond time‐scale. All these findings open new insights into the understanding of ultrafast magnetic excitation and, considering recent progress in the development of compact ultrafast lasers, may provide new prospects for applications of ultrafast opto‐magnetic phenomena in magnetic storage and information processing technology.     

Femtosecond laser ablation from dielectric materials: Comparison to arc discharge erosion

After explosive ablation from sapphire crystals by linearly polarised laser pulses, regular structures are observed on the bottom of the ablation pit. These structures do not comply with conventional ripple patterns. Instead, they more nearly resemble wickerwork, aligned perpendicular to the laser beam polarisation. A similar morphology is obtained by arc discharge erosion at AgCdO electrodes, suggesting that an explosive laser ablation may be characterised by high electric field effects and self organisation in the ablation craters.     

Timescales in the response of materials to femtosecond laser excitation

The interaction of ultrashort laser pulses with materials involves a number of special features that are different from laser–matter interaction for longer pulse durations. For femtosecond laser excitation the fundamental physical processes such as energy deposition, melting, and ablation are separated in time. By choosing proper time windows, the various processes can be investigated separately. We present selected examples of theoretical studies of free electron excitation in metals, timescales of different melting processes, and peculiarities of near-threshold ablation. Depending on the timescales and intensity ranges, the discussed processes are combined in an overall picture of possible pathways of the material from excitation to ablation. PACS 61.80.-x; 64.70.-p     

Application of VUV laser harmonic radiation to the measurement of porous silicon dielectric function

Vacuum ultra-violet (VUV) laser harmonics have been generated in a noble gas jet, which, combined with a standard spectrophotometer, have allowed measurements of the reflectance of porous silicon over a wide energy spectral range from 1 to 16 eV. Porous silicon dielectric function was, then, deduced from reflectance measurements by Kramers–Kronig analysis. Data are found to be in good agreement with those reported in literature, thus showing that laser harmonics represent a new, alternative, and suitable VUV source for optical characterisation of materials such as semiconductors and thin films.     

Interface response theory of electromagnetism in composite dielectric materials

It is shown how a recent interface theory can be used for solving the Maxwell equations in any composite dielectric material. General expressions for the corresponding response functions are given. These new results are illustrated by a general application to layered isotropic dielectrics and a simple derivation of the response function of two different isotropic dielectrics separated by a planar interface.