Laser transitions under resonant optical pumping of donor centres in Si:P

Terahertz stimulated emission of phosphorus donors in silicon optically excited by radiation from the free-electron laser FELIX has been studied. It is found that a spectral line of the Si:P laser emission depends on pump frequency. Stimulated emission arises on the 2p₀→1s(E) intra-centre transition (21.2 meV) under resonant pumping of the 2p₀ state and on the 2p₀→1s(T₂) transition (22.3 meV) under pumping of the 2p_± or higher odd-parity donor states. The line shift is attributed to the Auger redistribution of the 1s(E)- and 1s(T₂)-state populations. Received: 7 November 2002 / Revised version: 7 April 2003 / Published online: 14 May 2003 RID="*" ID="*"Corresponding author. Fax: +49-30/6705-55-07, E-mail: sergeij.pavlov@dlr.de RID="**" ID="**"On leave from: The Institute for Physics of Microstructures, Nizhny Novgorod, Russia     

Molecular nanocluster fluorescence in microwave infrared-radiation fields

The paper considers a donor-acceptor nanocluster fluorescing in a microwave infrared radiation field. It is assumed that the nanocluster consists of two dipole-dipole interacting organic molecules. It is shown that the fluorescence process of the nanocluster occurs if the donor molecule contains a substructure of identical diatomic pairwise interacting bonds of dipoles (an IR antenna). This antenna is capable of accumulating vibrational energy as a sum of collective vibrational quanta (excimols). The acceptor molecule has no permanent dipole moment and cannot be excited by microwave IR radiation. This molecule is polarized in the dipole moment field of the donor IR antenna and can receive energy accumulated in the IR antenna of the donor molecule. If the acceptor molecule has an electronically excited state in the long-wavelength visible region of its absorption spectrum, then after receiving energy equal to the energy of this state from the donor antenna, the electronic excitation of the acceptor molecule and its fluorescence is possible. As an example, the fluorescence of a nanocluster is considered whose donor molecule has a C _n H_2n IR antenna. The acceptor molecule is aromatic and the external infrared frequency, 1.1 × 10¹⁴ s^?1, is equal to the frequency of the excimol in the donor infrared antenna.     

电多极相互作用引起的静态能量传递中施主发光衰减与受主浓度的关系

本文指出,通常用于描述电多极相互作用引起的静态传递模型中时间足够长时施主发光衰减规律的表示式exp[-γt-CA(X0t)3/s)Г(1-3/s)]应该用exp[-γt-CA·A(CA)(X0t)2/sГ(1-3/s)]/(1-CA)来代替;文中还指出,静态有序阶段的时间上限由X0t相似文献   

Generation of tunable infrared radiation in rubidium titanyl phosphate crystal for the optical measurement of number density, absorption cross-section of methane gas

Tunable near-infrared radiation has been generated in a rubidium titany1 phosphate (RTP) crystal by employing non-collinear difference-frequency mixing (DFM) technique. The input radiation sources are Nd:YAG laser radiation and its second harmonic pumped dye laser radiation. For the generation of 2.0 radiation, the maximum value of the conversion efficiency (quantum) obtained in the process is 49% from the dye (0.6945 μm) to the infrared (2.0 μm) radiation in the 7.9-mm-long crystal. The generated tunable mid-infrared radiation has been used to measure the number density, absorption cross-section and minimum detectable concentration of methane gas in its 2ν₃ band in a multi-pass cell at 30.075 Torr pressure. The number density and column density of the methane molecules are found to be 1.068×10¹⁸ cm⁻³ and 3.02×10²¹ cm⁻², respectively, whereas the minimum still-detectable concentration at 1.658 μm wavelength is estimated to be 4.523×10¹⁷/cm³.     

Ultraviolet KrCl excilamps pumped by a pulsed longitudinal discharge

Results are presented from experimental studies of KrCl excilamps pumped by pulsed longitudinal discharges. The conditions for achieving the maximum efficiency and average radiation power are analyzed. At wavelengths <250 nm an average radiation power of ∼60 W and an efficiency of ∼10% are obtained. It is shown that the maximum energy parameters of the UV radiation are reached at low powers and long durations (>1 ms) of the pump pulses and also with the use of donor chloride Cl₂. Zh. Tekh. Fiz. 67, 78–82 (January 1997)     

Fir spectroscopy on shallow donor states in GaAs:Si

Far infrared spectroscopy experiments on the shallow donor states of GaAs:Si are presented. Transitions between shallow donor levels are induced by F.I.R. radiation from an optically pumped molecular gas laser. The measurements are performed by measuring the electrical conductivity of the GaAs:Si sample at fixed laser frequencies by sweeping an external applied magnetic field up to 12 T. In total about 30 different transitions are observed using radiation between 60 m and 1.8 mm. In low magnetic fields and at high frequencies a series of photoconductivity signals are observed, which we ascribe to transitions from the ground state towards bound donor levels with hydrogen quantum numbers n, l=n–1, and m=n–1.     

Ultrafast resonance energy transfer in bio-molecular systems

In this article, we present our consistent efforts to explore thedynamical pathways of the migration of electronic radiation by usingultrafast (picosecond/femtosecond time scales) Förster resonance energytransfer (FRET) technique. The ultrafast non-radiative energy migration froman intrinsic donor fluorophore (Tryptophan, Trp214) present in domain IIA ofa transporter protein human serum albumin (HSA) to variousnon-covalently/covalently attached organic/inorganic chromophores includingphotoporphyrin IX (PPIX), polyoxovanadate[V₁₅As₆O₄₂(H₂O)]^-6clusters (denoted as V₁₅) and CdS quantum dots (QDs) has beenexplored. We have also used other covalently/non-covalently attachedextrinsic fluorogenic donors (NPA, ANS) in order to exploit the dynamics ofresonance energy migration of an enzyme α-chymotrypsin (CHT). Theuse of extrinsic donor instead of intrinsic Trp in CHT avoids ambiguity inthe location of the donor molecule as seven tryptophans are present in theenzyme CHT. We have labeled CHT with ANS (1-anilinonaphthalene-8-sulfonate)and NPA (4-nitrophenyl anthranilate) and studied FRET. Labeling of DNA hasalso been done in the context that the DNA bases have very low quantum yieldfor fluorescence. We have also validated FRET model over nano-surface energytransfer technique (NSET) in the case of quantum clusters and applied thefindings to other QDs. The use of QDs over organic fluorophore was justifiedby least photo-bleaching of QDs compared to organic fluorophore. Our studiesmay find relevance in the exploration of alternate pathway for ultrafastmigration of electronic radiation through FRET to minimize the detrimentaleffect of UV radiation in living organism.     

Facile synthesis, optical properties and theoretical calculation of two novel two-photon absorption chromophores

Two heterocycle-based derivatives that can be used as two-photon absorption chromophore, 9-butyl-3-(2, 6-diphenylpyridin-4-yl)-9H-carbazole (BDPYC) and 9-butyl-3-(4-(2, 6-diphenylpyridin-4-yl)styryl)-9H-carbazole (BDPSC) have been successfully synthesized and fully characterized by elemental analysis, IR, ¹H NMR, ¹³C NMR and MS. The molecules possess D-π-A structures, but have different π bridge. The 9-butylcarbazole is used as a donor (D), and the pyridine ring is used as an acceptor (A). One- and two-photon absorption and excited fluorescence properties in various solvents were experimentally investigated. Two-photon initiated optical data recording experiments have been carried out under 740 nm laser radiation, and the possible mechanism of optical data storage is discussed based on theoretical calculations.     

Excited states of the donor-exciton complex in CdS

The excitation spectrum of the I₂ recombination line of the donor-exciton complex (D⁰, X) in CdS exhibits a set of nine distinct resonances. They represent transitions into excited (D⁰, X) states as shown by polarization, temperature, and magnetic field dependent measurements. The investigation of several sample series involving chemically different donor centers indicates that each donor center is responsible for a shift of the whole set of (D⁰, X) states but its influence is weakened for higher excited states. The analysis of the resonances reported requires an extended (D⁰, X) term scheme and a renewed assignment of the optical transitions to the possible electronic excited states of this complex.     

Asymmetry of the angular distribution of radiation of channeled relativistic electrons in optically transparent crystals

It has been shown that optical and ultraviolet radiation from relativistic electrons at planar channeling in optically transparent crystals is characterized by an unusual dependence on the polar and azimuth angles. A fraction of radiation with the frequency ω near which the derivative of the refractive index is nonzero, n'(ω) = dn(ω)/dω ≠ 0, should be observed at an angle close to π/2 with respect to the electron beam. For normal dispersion (n'(ω) > 0), this angle is smaller than π/2, whereas for anomalous dispersion (n'(ω) < 0), it is larger than π/2 (“backward” radiation). A pronounced dependence of the radiation intensity on the azimuth angle φ, i.e., azimuthal asymmetry, appears beyond the region of normal and anomalous dispersion at a fixed polar angle θ. In particular, the ratio of radiation intensities at angles φ = 0 and π/2 at θ = π/2 reaches a maximum value of about the square of the refractive index.     

Numerical simulation of nonlinear properties of fullerene-containing one-dimensional photonic crystal

A model of interaction between a one-dimensional photonic crystal (PC) containing a defect (nonlinear optical layer of C₆₀ fullerene) and radiation with a wavelength ??₀= 1064 nm under steady-state conditions has been considered. This structure is a Fabry-Perot microcavity: a fullerene layer (with a thickness multiple of ??₀/2) is placed between the interference mirrors formed by alternating layers ??₀/4 thick. The PC under consideration (1) has a narrow transmission band in the vicinity of ??₀ against the background of a relatively wide 100%-reflection band (photonic band gap) in the linear mode and (2) provides multiple amplification of the radiation intensity in the intermediate layer with respect to the external radiation intensity. Since C₆₀ fullerene exhibits a significant optical Kerr nonlinearity, the optical thickness of the intermediate layer under irradiation deviates from the value multiple of ??₀/2; as a result, 100% transmission for ??₀ changes to almost 100% reflection at a certain radiation intensity. Thus, this structure behaves as a peculiar optical limiter.     

Electrical and luminescent properties of gallium phosphide with radiation defects

The energy spectrum of local levels and the effect of radiation damage on intensity and character of cathode luminescence lines in the visible region of the spectrum in gallium phosphide single crystals prepared from a solution-melt by the Chochralski method are examined. It is shown that as a result of irradiation by fast electrons with E_e=1 Mev and Co 60 -quanta, both donor and acceptor centers are introduced into the original crystals. New radiation centers producing lines in the spectral range studied (500–600 nm) were not observed after irradiation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 49–54, June, 1976.     

Donor binding energies and spin-orbit coupling in a spherical quantum dot

The ground state and a few excited state energies of a hydrogenic donor in a spherical quantum dot (GaAs in a GaAlAs matrix) are computed. While the 1s and the 2s-state energies behave normally for dots of all radii, the 2p₀ and 2p_± states are unbound for most of the radii of interest. It is predicted that a semiconductor quantum dot with a hydrogenic donor will exhibit photoconductivity for a low threshold wavelength ∼12 μm. The spin-orbit coupling gives a contribution of the order of 10⁻⁵ meV for both 2p₀ and 2p_± states.     

核爆炸光辐射探测中的大气传输性能研究

从核爆炸光辐射特点出发, 结合我国中纬度地区大气特点,研究了(0.2～1.2)μm波段大气传输性能。通过实际测量数据分析可知,大气对（0.2～1.2)μm光辐射起主要作用的成分有水蒸气、二氧化碳和臭氧的分子吸收和散射以及气溶胶大粒子的散射。综合考虑这些因素,改进了水平路径传输上（0.2～1.2）μm光辐射大气传输经验方程。计算机仿真结果表明：该方程能体现我国中纬度地区光辐射的传输情况,对（0.4～0.8）μm可见光波段能很好地吻合。将（0.4～0.8）μm波段光辐射大气传输定量计算结果直接应用到基于可见光辐射探测的核爆炸探测子系统中,模拟核爆炸探测,实验结果表明：与传统大气传输计算软件的定性结果相比,该方法的测量精度有明显的提高。     

Patterning of indium-tin oxide on glass with picosecond lasers

The results of patterning of the indium-tin oxide (ITO) film on the glass substrate with high repetition rate picosecond lasers at various wavelengths are presented. Laser radiation initiated the ablation of the material, forming grooves in ITO. Profile of the grooves was analyzed with a phase contrast optical microscope, a stylus type profiler, scanning electron microscope (SEM) and atomic force microscope (AFM). Clean removal of the ITO film was achieved with the 266 nm radiation when laser fluence was above the threshold at 0.20 J/cm², while for the 355 nm radiation, the threshold was higher, above 0.46 J/cm². The glass substrate was damaged in the area where the fluence was higher than 1.55 J/cm². The 532 nm radiation allowed getting well defined grooves, but a lot of residues in the form of dust were generated on the surface. UV radiation with the 266 nm wavelength provided the widest working window for ITO ablation without damage of the substrate. Use of UV laser radiation with fluences close to the ablation threshold made it possible to minimize surface contamination and the recast ridge formation during the process.     

Saturation of ionization edge absorption by donors in germanium

A study of saturation of the absorption and photoconductivity of Sb and P donors in Ge for radiation of 90 m wavelength, i.e., of energy very closely above their ionization edges is presented at T=9.3 K. Under these conditions negligible heating by the excess radiation energy is expected, which provides a convenient opportunity to study the kinetics of photoionization and recombination. From these measurements we have determined the donor capture cross section of electrons at 9.3 K to be _c=(1.2±0.7)×10^–12cm^–2, and the relaxation time from the 2s to the ground state as ₂₁=(5.8±1.0)×10^–10s. The saturation intensity of the absorption coefficient is around three orders of magnitude higher than the saturation intensity of the photoconductivity. We explain the nonlinear photoconductivity by the Debye-Conwell dependence of the mobility on the number of photoionized donors and compensating acceptors.     

Silicon donor and Stokes terahertz lasers

Two types of lasers based on hydrogen-like impurity-related transitions in bulk silicon operate at frequencies between 1 and 7 THz (wavelength range of 50-230 μm). These lasers operate under mid-infrared optical pumping of n-doped silicon crystals at low temperatures (<30 K). Dipole-allowed optical transitions between particular excited states of group-V substitutional donors are utilized in the first type of terahertz silicon lasers. These lasers have a gain ∼1-3 cm⁻¹ above the laser thresholds (>1 kW cm⁻²) and provide 10 ps-1 μs pulses with a few mW output power on discrete lines. Raman-type Stokes stimulated emission in the range 4.6-5.8 THz has been observed from silicon crystals doped by antimony and phosphorus donors when optically excited by radiation from a tunable infrared free electron laser. The scattering occurs on the 1s(E)→1s(A₁) donor electronic transition accompanied by an emission of the intervalley transverse acoustic g-phonon. The Stokes lasing has a peak power of a few tenths of a mW and a pulse width of a few ns. The Raman optical gain is about 7.4 cm GW⁻¹ and the optical threshold intensity is ∼100 kW cm⁻².     

石墨烯增强半导体态二氧化钒近场热辐射

本文基于涨落耗散定理和并矢格林函数求解麦克斯韦方程来研究两个半无限大平板的近场热辐射净热流,提出了两个半无限大块状二氧化钒组成的V/V结构、石墨烯覆盖两个半无限大块状二氧化钒组成的GV/GV结构和石墨烯覆盖VO2薄膜组成的GV0/GV0结构,深入研究了这三种结构中二氧化钒与石墨烯间的近场热辐射,并分析了真空间距、二氧化钒薄膜厚度和石墨烯化学势等物理参量变化对近场热辐射的影响.研究表明:三种结构的近场热辐射均随间距增大而减小;在真空间距为10 nm时,由石墨烯覆盖的GV/GV结构的近场辐射热流比无石墨烯覆盖的V/V结构增强35倍,耦合效果最好的是GV0/GV0结构,该结构的近场辐射热流比GV/GV结构增强8.6倍;在GV0/GV0结构中,当二氧化钒薄膜厚度为30 nm时,石墨烯化学势从0.1 eV增加到0.6 eV辐射热流会减小3.3倍.本文系统研究了二氧化钒与石墨烯间相互耦合的近场热辐射,对相关结构的近场热辐射实验和实际应用具有理论指导意义.     

Pulsed inductive discharge CO2 laser

A pulsed inductive discharge CO₂ laser with a wavelength of 10.6 μm has been created for the first time. The excitation system of a cylindrical pulsed inductive discharge (pulsed inductively coupled plasma) in the gas mixture of CO₂:N₂:He was developed. The temporal and energy parameters of the laser radiation were investigated. The maximum inductive discharge CO₂ laser radiation energy of 104 mJ was achieved. An average power of 3.2 W was obtained at laser generation energy of 65 mJ and pulse repetition rate of 50 Hz. In the cross-section, the laser radiation had the ring shape with an external diameter of 34 mm and thickness of 4-5 mm. The measured divergence of laser radiation was 12 mrad.     

The location and shape of the conduction band minima in cubic silicon carbide

We have measured the inter-bound state excitation spectrum of the N_C donor in cubic β-SiC through the ‘two-electron’ transition satellites observed in the luminescent recombination of excitons bound to neutral N donors. Transitions are seen to p as well as s-like donor states although the transition oscillator strength is derived from interaction with the impurity core since parity is conserved through inter-valley scattering by p-like X phonons. The Zeeman splitting of a luminescence line involving the 2p± donor state yield the electron mass parameter m_t = 0.24 ± 0.01 m₀. This and the directly measured energy separations of the 2p₀ and 2p± states yields m_t/m₁ = 0.36 ± 0.01 with the static dielectric constant K = 9.92 ± 0.1. Mutually consistent central cell corrections of 1.1 and 8.4 meV are observed for the 2s(A₁) and 1s(A₁) donor states, the latter being in agreement with a recent estimate from electronic Raman scattering by Gaubis and Colwell. The ionization donor energy of the N_C donor, 53.6 ± 0.5 meV is consistent with earlier, less accurate estimates from donor-acceptor pair and free to bound luminescence. There is no evidence for a ‘camel's back’ conduction band structure in cubic SiC, unlike GaP. The two-phonon sidebands of the N_C donor exciton luminescence spectrum in SiC can be constructed by X and Г phonons only.