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1.
We report on polaron decay in InAs/GaAs self-assembled quantum dots. The polarons are probed by pump–probe spectroscopy through their optical intersublevel absorption around 62 meV (20 μm wavelength). A T1 polaron lifetime of the order of tens of picosecond is deduced from the low-temperature pump–probe measurements. We show that a long-lived component can be additionally observed on the pump–probe measurements. The spectral dependence of this long-lived component is, however, not correlated to the polaron absorption. It is thus not a signature of polaron relaxation quenching. The origin of this long-lived component is attributed to the two-phonon absorption of the bulk GaAs substrate.  相似文献   

2.
The excited state absorption dynamics of the fast mode-locking dye No. 5 for Nd-glass lasers is investigated. The excited state absorption cross-section σex for 1.053 μm picosecond pulse excitation is determined from energy transmission measurements. The excited state absorption populates a higher lying singlet state S4. The population lifetime τ4 of this state is determined from two-step excited fluorescence quantum yield measurements. The obtained values are σex= (5.5±1)×10-17 cm2 and τ4=60±20 fs.  相似文献   

3.
Fluorescence spectra in the near infrared are observed of the dye Eastman 9860 and of new, faster mode-locking dyes. The quantum efficiencies are determined to be small with η≃10−3. The short fluorescence lifetimes are deduced from optical gain measurements. The fluorescence time-constants are found to have the same value as the corresponding absorption recovery times. Even laser action is observed in these fast saturable absorbers previously belieyed to be non-fluorescent.  相似文献   

4.
LiF crystals with F2 - centers contain a whole series of F-based colour centers with spectrally overlapping absorption spectra. On the short-wavelength side the absorption spectrum of the F2 - centers is overlapping with the absorption spectra of other colour centers and is therefore not directly accessible by transmission measurements. Fluorescence excitation spectroscopy is employed here to gain information on the absorption spectrum of F2 - centers in spectral regions where the absorption by other centers is present. The fluorescence signal emitted beyond 1200 nm is detected as a function of the excitation wavelength in the range from 350 nm to 1100 nm. The F2 - center absorption cross-section spectrum in the wavelength range from 690 nm to 1100 nm and from 350 nm to 410 nm is determined from the fluorescence excitation quantum distribution and the transmission spectrum. Influences of fluorescence re-absorption and Förster-type energy transfer on the fluorescence excitation quantum distribution and the subsequent F2 - center absorption cross-section determination are discussed. PACS 61.72.Ji; 78.40.Ha; 78.55.Fv  相似文献   

5.
The so-called renormalization group (RG) method is applied to derive kinetic and transport equations from the respective microscopic equations. The derived equations include the Boltzmann equation in classical mechanics, the Fokker-Planck equation, and a rate equation in a quantum field theoretical model. Utilizing the formulation of the RG method which elucidates the important role played by the choice of the initial conditions, the general structure and the underlying assumptions in the derivation of kinetic equations in the RG method are clarified. It is shown that the present formulation naturally leads to the choice for the initial value of the microscopic distribution function at arbitrary time t0 to be on the averaged distribution function to be determined. The averaged distribution function may be thought of as an integral constant of the solution of the microscopic evolution equation; the RG equation gives the slow dynamics of the would-be initial constant, which is actually the kinetic equation governing the averaged distribution function. It is further shown that the averaging as given above gives rise to a coarse-graining of the time-derivative which is expressed with the initial time t0, and thereby leads to time-irreversible equations even from a time-reversible equation. It is shown that a further reduction of the Boltzmann equation to fluid dynamical equations and the adiabatic elimination of fast variables in the Fokker-Planck equation are also performed in a unified way in the present method.  相似文献   

6.
The spectral absorption cross section of the Swings band of C3 was determined from measurements behind incident shock waves that heated a gas mixture of argon and acetylene. These measurements spanned the spectral region between 300 and 540 nm, and were obtained at temperatures between 3200 and 4000°K. An electronic oscillator strength of 0.033 was deduced from the measurements.  相似文献   

7.
The absorption coefficient K of a quasi two dimensional (2D) hot free electron gas is calculated for the first time as a function of the lattice temperature T, the photon angular frequency w, the carrier density Ns as well as the electron temperature Te when the carriers are scattered by ionized impurities, acoustic phonons and polar optical phonons. Analytical expressions are derived in the limiting cases of non-degeneracy and degeneracy of the electron system. In the quantum limit ħw/kBTe ≳ 1 where the interaction between the electron and the photon is inelastic K sensitively depends on the limiting scattering mechanism showing that the electron motion is completely controlled by the photon field. In the classical limit ħw/kBTe ⪡ 1 the absorption decreases proportional to w1 independent of the limiting scattering mechanism in agreement with the experimental data deduced from far-infrared absorptivity measurements on GaAs heterolayers.  相似文献   

8.
The renormalization group (RG) method as a powerful tool for reduction of evolution equations is formulated in terms of the notion of invariant manifolds. We start with derivation of an exact RG equation which is analogous to the Wilsonian RG equations in statistical physics and quantum field theory. It is clarified that the perturbative RG method constructs invariant manifolds successively as the initial value of evolution equations, thereby the meaning to set t0=t is naturally understood where t0 is the arbitrary initial time. We show that the integral constants in the unperturbative solution constitutes natural coordinates of the invariant manifold when the linear operator A in the evolution equation is semi-simple, i.e., diagonalizable; when A is not semi-simple and has a Jordan cell, a slight modification is necessary because the dimension of the invariant manifold is increased by the perturbation. The RG equation determines the slow motion of the would-be integral constants in the unperturbative solution on the invariant manifold. We present the mechanical procedure to construct the perturbative solutions hence the initial values with which the RG equation gives meaningful results. The underlying structure of the reduction by the RG method as formulated in the present work turns out to completely fit to the universal one elucidated by Kuramoto some years ago. We indicate that the reduction procedure of evolution equations has a good correspondence with the renormalization procedure in quantum field theory; the counter part of the universal structure of reduction elucidated by Kuramoto may be Polchinski's theorem for renormalizable field theories. We apply the method to interface dynamics such as kink–anti-kink and soliton–soliton interactions in the latter of which a linear operator having a Jordan-cell structure appears.  相似文献   

9.
宋淑芳  赵德威  徐征  徐叙瑢 《物理学报》2007,56(6):3499-3503
采用多源有机分子气相沉积系统制备了不同类型的有机多量子阱结构,利用电化学循环伏安法和吸收光谱、荧光光谱研究了量子阱的类型、光致发光的特性.电化学循环伏安法和吸收光谱的测量结果表明,PBD/8-羟基喹啉铝(Alq3)有机量子阱为Ⅰ型量子阱结构,NPB/Alq3和BCP/Alq3有机量子阱为Ⅱ型量子阱结构.荧光光谱的研究结果表明,PBD/Alq3和BCP/Alq3量子阱结构可以实现PBD,BCP向Alq3能量完全转移,而NPB/Alq3量子阱结构,NPB和Alq3之间只是部分能量转移.文中对影响能量转移的因素进行了讨论. 关键词: 有机量子阱 能量转移  相似文献   

10.
Reflectivity and transmission spectra of a new semimagnetic compound Pb1−xGdxTe with x=0.01 in the 300-77K temperature range are reported.From reflectivity data the high dielectric constant ϵ(∞) is deduced. Lyddane-Sachs-Teller relation gives the static dielectric constant ϵ(0). The conductivity effective mass m1c measured from the plasma frequency agrees well with the one deduced from absorption measurements α vs the wavelength in the short wavelength range.The energy gap as well as its temperature dependence is given by absorption data. The introduction of low concentration magnetic ion Gd does not affect drastically the optical properties of the material comparatively to the optical PbTe ones.  相似文献   

11.
Room temperature measurements of self-, air-, and oxygen-broadening coefficients are reported for 101 pure rotational absorption lines for 16O3 covering a range of 7≤J″≤34 and 3≤K−1″≤11 in the spectral region 50 to 90 cm. In addition, the temperature dependence of the coefficients has been determined for most of these lines. A total of 14 high-resolution Fourier transform far-IR spectra (0.004 cm resolution) of ozone/air/dioxygen mixtures were recorded at various temperatures (212, 252, and 296 K). The broadening coefficients and corresponding temperature exponents were deduced by analyzing all of the 14 spectra simultaneously for each absorption line using a nonlinear least-squares fitting technique. Several sources of systematic errors were taken into account: the values of the partial pressures of the species involved, the value of the total optical path length, the sample temperature, thermal emission from sources other than the spectrometer source (e.g., sample emission), the effect of convolution with the instrument function, and intensity contributions from weak and neighbouring absorption lines. The variation of the determined broadening coefficients and temperature exponents with the rotational quantum numbers J″ and K−1″ has been examined. Furthermore the determined pressure-broadening parameters have been compared to the data reported in the 2000 edition of the HITRAN compilation.  相似文献   

12.
The rapid internal conversion dynamics at room temperature is determined by using the femtosecond time-resolved fluorescence depletion measurements of a complex solvated molecule of LD 700 (rhodamine 700) combined with steady-state absorption and fluorescence spectroscopy, as well as quantum chemical calculation. The molecule is excited by a 50fs laser pulse at 400nm which directly populated the highly excited singlet state, the rapid internal conversions (ICs) are observed, which leads to the directional changes of the emission transition moment following photoexcitation to the highly excited singlet state S5 of LD 700.  相似文献   

13.
Cyclotron resonance and photoluminescence measurements have been performed on two types of modulation-doped field-effect transistor heterostructures having their bidimensional channel based, respectively, on an InxGa1  xAs quantum well and an InAs–GaAs short-period superlattice. A linear dependence of the electron effective mass as a function of indium content of the channel was obtained from cyclotron resonance measurements. For a given average value of the indium content, the effective mass in the InAs–GaAs short-period superlattice channel is found to be systematically higher than that obtained in structures with an alloy-based channel. This is attributed to larger nonparabolic effects in the former case. In our theoretical model, the electron and heavy hole energy levels and the electron wavefunction are determined self-consistently and used to estimate the nonparabolic corrections that apply to the effective mass deduced from cyclotron resonance measurements.  相似文献   

14.
In the present study we have synthesized CdS semiconducting quantum dots by reverse micelle method using dodecanthiol (C12H26S) as the capping agent. The synthetic medium consists of a quaternary water/Sodium dodecyl sulfate (SDS)/buta-1-ol/hexane microemulsion. The size of the particles was controlled by changing the molar ratio W=[H2O]/[SdS], where [H2O] and [SdS] are the molar concentrations in hexane of water and SdS respectively. The CdS nanoparticles were embedded in a new PPV derivate named Fluorinated Bisphenol A P-phenylenevinylene (BPAF-PPV). Fourier transform infrared spectroscopy (FT-IR) showed a strong interaction of thiol groups with CdS nanoparticles. Blue shift of the optical absorption onset is observed due to quantum size effect. The band gap and particle sizes of the nanoparticles were deduced from optical absorption spectra and the use of an effective mass approximation (EMA) model. Photoluminescence spectroscopy evidenced a charge transfer process via the interface between BPAF-PPV/CdS nanoparticles.  相似文献   

15.
The vibrational–rotational absorption spectrum of D2O in the range from 10 120 to 10 450 cm–1 is recorded on a Fourier transform spectrometer with a resolution of 0.05 cm–1. The measurements were performed using a multipass White cell with an optical path length of 24 m. A light-emitting diode with brightness higher than that of other devices was used as a radiation source. The signal-to-noise ratio was about 104. The spectrum is interpreted as consisting of lines of more than 400 transitions. The spectral characteristics of lines (centers, intensities, and half widths) are determined by fitting the Voigt profile parameters to experimental data by the least-squares method. The intensities of lines and the experimental rotational energy levels of the (301) vibrational state of the D2 16O molecule with high rotational quantum numbers are determined for the first time.  相似文献   

16.
宋淑芳  赵德威  徐征  徐叙瑢 《物理学报》2007,56(5):2910-2914
采用多源有机分子气相沉积系统(OMBD)制备了Alq3,PBD/Alq3,PBD/Alq3/PBD单层、双层以及量子阱结构,利用电化学循环伏安法和吸收光谱、荧光光谱研究了量子阱的类型和样品的光致发光特性.电化学循环伏安法和吸收光谱的测量结果表明,PBD/Alq3有机量子阱为Ⅰ型量子阱结构.荧光光谱的研究结果表明,单层Alq3的光致发光峰不随Alq3厚度变化而变化;但是双层PBD/Alq3结构光致发光峰随Alq3厚度的减小而发生蓝移;同样对于PBD/Alq3/PBD量子阱结构光致发光峰随Alq3厚度的减小而发生蓝移.对引起光谱蓝移的原因进行了讨论. 关键词: 有机量子阱 光谱蓝移  相似文献   

17.
18.
In this study, we demonstrate how electroreflectance (ER) measurements as a function of bias, and of angle of incidence (θ0), together with bias dependent photocurrent (PC) measurements, can be used to provide understanding of the complex electric field profile and carrier transport effects in a GaAs/Al0.3Ga0.7As multiple quantum well (MQW), grown inside n+ contact layers. The PC measurements exhibit split excitonic features, the components of which change in strength with the applied bias. The effect is explained by absorption in the front of the MQW stack, with the back of the stack acting as detector. We examine the θ0-dependence of the ER lineshape, to determine the depth of the layers responsible for each feature. The ER and PC lineshapes and their bias dependence are explained by the unusual electric field profile across the stack. The field profile appears to be determined by tunnelling of the dark current.  相似文献   

19.
The present paper demonstrates the preparation and characterization of SnO2 semiconductor quantum dots. Extremely small ∼1.1 and ∼1.4 nm SnO2 samples were prepared by microwave assisted technique with a frequency of 2450 MHz. Based on XRD analysis, the phase, crystal structure and purity of the SnO2 samples are determined. UV-vis measurements showed that, for the both size of SnO2 samples, excitonic peaks are obtained at ∼238 and ∼245 nm corresponding to ∼1.1 nm (sample 1) and ∼1.4 nm (sample 2) sizes, respectively. STM analysis showed that, the quantum dots are spherical shaped and highly monodispersed. At first, the linear absorption coefficients for two different sizes of SnO2 quantum dots were measured by employing a CW He-Ne laser at 632.8 nm and were obtained about 1.385 and 4.175 cm−1, respectively. Furthermore, the nonlinear refractive index, n2, and nonlinear absorption coefficient, β, were measured using close and open aperture Z-scan respectively using the same laser. As quantum dots have strong absorption coefficient to obtain purely effective n2, we divided the closed aperture transmittance by the corresponding open aperture in the same incident beam intensity. The nonlinear refraction indices of these quantum dots were measured in order of 10−7 (cm2/W) with negative sign and the nonlinear absorption coefficients were obtained for both in order of 10−3 (cm/W) with positive sign.  相似文献   

20.
The organic light emitting diode (OLED) hole transport molecules N,N,N’,N’-tetraphenylbenzidine (TPB, triphenylamine dimer TAD or TPD) and N,N’-bis(2-naphtalenyl)-N,N’-bis(phenylbenzidine) (β-NPB, naphtyl-diphenylamine dimer β-NPD), dissolved in tetrahydrofuran (THF) and as neat film, are characterized by optical absorption and emission spectroscopy. The absorption and stimulated emission cross-section spectra, the fluorescence quantum distributions, fluorescence quantum yields, degrees of fluorescence polarization, and fluorescence lifetimes are determined. The lasing behaviour is studied by picosecond laser pulse excitation (excitation wavelength 347.15 nm, pulse duration 35 ps). The excited-state absorption at the pump laser wavelength is determined by saturable absorption measurement. Low-Q laser oscillation of TPB in THF is achieved by transverse pumping of the dye in a cell. The excited-state absorption of TPB in THF at the laser wavelength is extracted from the laser threshold. In TPB neat films, wave-guided travelling-wave lasing was obtained. No laser action was achieved for β-NPB because of small S1-S0 stimulated emission cross-section, and the presence of excited-state absorption in the fluorescence wavelength region. The TPB and β-NPB results are compared with the corresponding spectroscopic and lasing behaviour of the related methyl-substituted triphenylamine dimers, 3-methyl-TPD and 4-methyl-TPD, which are well established OLED hole transport materials.  相似文献   

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