Tyrosyl Fluorescence Decays and Rotational Dynamics in Tyrosine Monomers and in Dipeptides

Picosecond time-correlated single-photon counting was used to measure fluorescence lifetimes and fluorescence anisotropy decays of tyrosine and the tyrosine–alanine and tyrosine–leucine dipeptides. After excitation of tyrosine at 287 nm two emitting species were observed, one at 303 nm with a lifetime of 3.3 ns and another at 340 nm with a lifetime of 360 ps. The rotational correlation time of tyrosine at 303 nm is 38 ps in water at pH 7 and depends linearly on viscosity with a slope of 44 ps/cP, consistent with Stokes–Einstein–Debye theory. We calculated a value of 45 ns for the radiative lifetime of tyrosine, yielding a fluorescence quantum yield of 0.07. The dipeptides Tyr–Ala and Tyr–Leu exhibit two- or three-exponential decays. The amplitudes of the decay components for three-exponential fits correlate closely with the populations of rotamers in these peptides as determined by NMR. The quenching of dipeptide fluorescence is shown to depend on the solvent polarity, strongly supporting the hypothesis that tyrosyl fluorescence in peptides is quenched by charge transfer. The rotational correlation times of tyrosine, Tyr–Ala, and Tyr–Leu increase linearly with the van der Waals volumes. However, rotational relaxation is somewhat faster than expected from Stokes–Einstein–Debye theory with stick boundary conditions.     

Resonantly driven exciton Rabi oscillation in single quantum dots emitting at 1300 nm

We report on the resonance fluorescence(RF) from single In As quantum dots(QDs) emitting at the telecom band of 1300 nm. The InAs/GaAs QDs are embedded in a planar optical microcavity and the RF is measured by an orthogonal excitation-detection geometry for deeply suppressing the residual laser scattering. An ultra-weak He–Ne laser is necessary to be used as a gate laser for obtaining RF. Rabi oscillation with more than one period is observed through the picosecond(ps) pulsed laser excitation. The resonant control of exciton opens up new possibilities for realizing the on-demand single photon emission and quantum manipulation of solid-state qubits at telecom band.     

Spectroscopic and lasing characterisation of a dicarbazovinylene-MEH-benzene dye

The dye 1,4-bis(9-ethyl-3-carbazovinylene)-2-methoxy-5-(2′-ethyl-hexyloxy)-benzene (abbreviated 2CzV-MEH-B) dissolved in tetrahydrofuran (THF) and as neat film is characterised by optical absorption and emission spectroscopy. The absorption and stimulated emission cross-section spectra, the fluorescence quantum distributions, fluorescence quantum yields, degrees of fluorescence polarisation, and fluorescence lifetimes are determined. A lasing characterisation is carried out by pumping with single second harmonic pulses of a mode-locked ruby laser (wavelength 347.15 nm, pulse duration 35 ps). The excited-state absorption at the pump laser wavelength is determined by saturable absorption measurements. Laser oscillation of the dye in THF in a rectangular cell is achieved by transverse pumping using the uncoated cell windows for light feedback. From the emission behaviour around threshold the excited-state absorption cross-section spectrum in the laser active spectral region is extracted. The wave-guided travelling-wave lasing behaviour of the dye as neat film is studied by analysis of the amplification of the transverse pumped spontaneous emission. Surface emitting distributed-feedback lasing was achieved with a neat film on corrugated second-order periodic gratings.     

Near-field mechanism of photoluminescence excitation in quantum well heterostructures

We report a study into the process of energy transfer between quantum wells divided by 30-nm-thick opaque barriers. It was experimentally observed that the intensity of a photoluminescence signal from a quantum well increased by 15% under resonant excitation of exciton transition in the adjacent quantum well. The quantum wells were 30 nm apart. A radiative mechanism of energy transfer in the near-field region of emitting exciton is proposed. Within this theoretical model, the efficiency of the energy transfer decreases by a power law with greater distance between the quantum wells. The theory is found to be in qualitative agreement with the experimental results.     

Radiative life time of an exciton confined in a strained GaN/Ga1-xAlxN cylindrical dot: built-in electric field effects

The binding energy of an exciton in a wurtzite GaN/GaAlN strained cylindrical quantum dot is investigated theoretically.The strong built-in electric field due to the spontaneous and piezoelectric polarizations of a GaN/GaAlN quantum dot is included.Numerical calculations are performed using a variational procedure within the single band effective mass approximation.Valence-band anisotropy is included in our theoretical model by using different hole masses in different spatial directions.The exciton oscillator strength and the exciton lifetime for radiative recombination each as a function of dot radius have been computed.The result elucidates that the strong built-in electric field influences the oscillator strength and the recombination life time of the exciton.It is observed that the ground state exciton binding energy and the interband emission energy increase when the cylindrical quantum dot height or radius is decreased,and that the exciton binding energy,the oscillator strength and the radiative lifetime each as a function of structural parameters (height and radius) sensitively depend on the strong built-in electric field.The obtained results are useful for the design of some opto-photoelectronic devices.     

ZnSe/SiO2半导体量子点玻璃的光谱特性

对采用溶胶凝胶法制备的ZnSe/SiO2半导体量子点玻璃的光谱性质进行了测试分析.UV-Vis透射光谱中观察到光吸收边相对于体相半导体有明显蓝移.稳态发射光谱(PL)中观察到ZnSe纳米晶体的位于蓝区的基本呈高斯分布的弱的最低激子发射峰、强而宽的表面态发光带以及对应杂质能级的三个锐峰发光.时间分辨荧光光谱(TRPL)中观察到发光效率高的最低激子发射峰,并测量其荧光衰减寿命,经尾部拟合为28.5 ps.同时,结合有效质量近似(EMA)模型,估计ZnSe纳米晶体的平均粒径介于2.45~3.60 nm之间,尺寸分布基本呈高斯型.     

Influence of Mg Composition on Optical Properties of Exciton Confinement in Strained Wurtzite ZnO/MgxZn1-xO Cylindrical Quantum Dots

Based on the effective-mass approximation and variational approach, excitonic optical properties are investigated theoretically in strained wurtzite (WZ) ZnO/Mg_xZn_1-xO cylindrical quantum dots (QDs) for four different Mg compositions: x=0.08, 0.14, 0.25, and 0.33, with considering a three-dimensional carrier confinement in QDs and a strong built-in electric field effect due to the piezoelectricity and spontaneous polarization. The ground-state exciton binding energy, the interband emission wavelength, and the radiative lifetime as functions of the QD structural parameters (height and radius) are calculated in detail. The computations are performed in the case of finite band offset. Numerical results elucidate that Mg composition has a significant influence on the exciton states and optical properties of ZnO/Mg_xZn_1-xO QDs. The ground-state exciton binding energy increases with increasing Mg composition and the increment tendency is more prominent for small height QDs. As Mg composition increases, the interband emission wavelength has a blue-shift if the dot height L<3.5 nm, but the interband emission wavelength has a red-shift when L>3.5 nm. Furthermore, the radiative lifetime increases rapidly with increasing Mg composition if the dot height L>3 nm and the increment tendency is more prominent for large height QDs. The physical reason has been analyzed in depth.     

Influence of Mg Composition on Optical Properties of Exciton Confinement in Strained Wurtzite ZnO/Mg_x Zn_(1-x) O Cylindrical Quantum Dots

Based on the effective-mass approximation and variational approach, excitonic optical properties are investigated theoretically in strained wurtzite (WZ) ZnO/Mg x Zn 1-x O cylindrical quantum dots (QDs) for four different Mg compositions: x = 0.08, 0.14, 0.25, and 0.33, with considering a three-dimensional carrier confinement in QDs and a strong built-in electric field effect due to the piezoelectricity and spontaneous polarization. The ground-state exciton binding energy, the interband emission wavelength, and the radiative lifetime as functions of the QD structural parameters (height and radius) are calculated in detail. The computations are performed in the case of finite band offset. Numerical results elucidate that Mg composition has a significant influence on the exciton states and optical properties of ZnO/Mg x Zn 1 x O QDs. The ground-state exciton binding energy increases with increasing Mg composition and the increment tendency is more prominent for small height QDs. As Mg composition increases, the interband emission wavelength has a blue-shift if the dot height L 3.5 nm, but the interband emission wavelength has a red-shift when L 3.5 nm. Furthermore, the radiative lifetime increases rapidly with increasing Mg composition if the dot height L 3 nm and the increment tendency is more prominent for large height QDs. The physical reason has been analyzed in depth.     

Non-radiative exciton recombination through excitation energy transfer in quantum dot clusters

Excitation energy transfer (EET) processes in CdSe/CdZnS quantum dot (QD) clusters have been investigated in this study by measuring their time-resolved and spectrally resolved fluorescence intensities. The contributions of radiative and non-radiative exciton recombination through EET are evaluated, where the latter is expected to occur in a large class of QD ensembles because of the presence of nonluminescent QDs. It appears that the fluorescence decay in larger QDs serving as acceptor does not show an initial rise, in addition the lifetime of the acceptor QD is independent of the excitation wavelength, suggesting that an EET is followed mostly by non-radiative recombination.     

Spectroscopic and travelling-wave lasing characterisation of tetraphenylbenzidine and di-naphtalenyl-diphenylbenzidine

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.     

Temperature dependent time-resolved exciton luminescence in GaAs/AlGaAs quantum wires and dots

Transient photoluminescence of GaAs/AlGaAs quantum wires and quantum dots formed by strain confinement is studied as a function of temperature. At low temperature, luminescent decay times of the wires and dots correspond to the radiative decay times of localized excitons. The radiative decay time can be either longer or shorter than that of the host quantum well, depending on the size of the wires and dots. For small wires and dots (∼ 100 nm stressor), the exciton radiative recombination rate increases due to lateral confinement. Exciton localization due to the fluctuation of quantum well thickness plays an important role in the temperature dependence of luminescent decay time and exciton transfer in quantum wire and dot structures up to at least ∼ 80 K. Lateral exciton transfer in quantum wire and dot structures formed by laterally patterning quantum wells strongly affects the dynamics of wire and dot luminescence. The relaxation time of hot excitons increases with the depth of strain confinement, but we find no convincing evidence that it is significantly slower in quasi 1-D or 0-D systems than in quantum wells.     

Lateral carrier tunnelling in stacked In(Ga)As/GaAs quantum rings

Exciton recombination dynamics in vertical stacks of InGaAs quantum rings have been studied by means of continuous wave and time resolved photoluminescence under low excitation density conditions. We have paid special attention to the effect of the carrier coupling on the exciton radiative lifetime: weak (14 nm spacer sample), intermediate (4.5 nm spacer sample), where the size filtering effects (towards small rings) compensate partially that arising from carrier coupling (towards lower energies), and strong electron and hole coupling (1.5 nm spacer sample) between layers. Experimental decay times in the latter two cases have been compared to the times simulated with a multi-quantum well based model, which accounts for the observed change of carrier coupling regime. The most important effect is observed when the hole wave function overlap along the growth direction becomes important (1.5 nm spacer sample). This situation makes important the lateral tunneling of excitons between rings, given their large lateral size, which is characterized by times around 5 ns at the emission peak energy (rings with the most probable size of the distribution).     

Abnormal Energy Dependence of Photoluminescence Decay Time in InGaN Epilayer

We employ photoluminescence (PL) and time-resolved PL to study exciton localization effect in InGaN epilayers.By measuring the exciton decay time as a function of the monitored emission energy at different temperatures,we have found unusual behaviour of the energy dependence in the PL decay process. At low temperature, the measured PL decay time increases with the emission energy. It decreases with the emission energy at 200K, and remains nearly constant at the intermediate temperature of 12OK. We have studied the dot size effect on the radiative recombination time by calculating the temperature dependence of the exciton recombination lifetime in quantum dots, and have found that the observed behaviour can be well correlated to the exciton localization in quantum dots. This suggestion is further supported by steady state PL results.     

Donor binding energy and radiative life time of exciton in a strained InGaN/GaN quantum wire

Donor binding energies of positively and negatively charged impurities in a strained InGaN/GaN cylindrical quantum wire are investigated. The interband optical transition with and without the exciton is computed as a function of wire radius. The exciton oscillator strength and the exciton lifetime for radiative recombination as a function of wire radius have been computed.     

利用N型半导体纳米材料抑制单量子点的荧光闪烁特性

利用N型半导体纳米材料氧化铟锡(ITO)作为单CdSe/ZnS量子点的基质来抑制单量子点的荧光闪烁特性. 实验采用激光扫描共聚焦显微成像系统测量了单量子点荧光的亮、暗态持续时间的概率密度分布的指数截止的幂律特性, 并与直接吸附在SiO₂玻片上的单CdSe/ZnS量子点的荧光特性进行比较. 研究发现处于ITO中的单量子点比SiO₂玻片上的单量子点荧光亮态持续时间提高两个数量级, 掺杂于ITO中的单量子点的荧光寿命约减小为SiO₂玻片上的单量子点的荧光寿命的41%, 并且寿命分布宽度变小50%.     

Size‐Dependent Structural and Optical Characteristics of Glucose‐Derived Graphene Quantum Dots

It is of scientific importance to obtain graphene quantum dots (GQDs) with narrow‐size distribution in order to unveil their size‐dependent structural and optical properties, thereby further to explore the energy band diagram of GQDs. Here, a soft‐template microwave‐assisted hydrothermal method to prepare GQDs with diameters less than 5 nm ± 0.55 nm is reported. The size‐dependent photoluminescence (PL) quantum yield (QY) decay lifetime and electron energy loss spectroscopy (EELS) of the GQDs are studied systematically. The QY of the GQDs with an average diameter of 2 nm is the highest (15%) among all the samples investigated and the QY decreases with increasing diameter of the GQDs. The size‐dependence of the PL decay lifetime is also observed. The result suggests that spatial confinement effects related to radiative relaxation play an important role in the size‐dependent decay lifetime. A realistic energy band diagram of the GQDs is deduced from the experimental results.     

Spin-flip limited exciton dephasing in CdSe/ZnS colloidal quantum dots

The dephasing time of the lowest bright exciton in CdSe/ZnS wurtzite quantum dots is measured from 5 to 170 K and compared with density dynamics within the exciton fine structure using a sensitive three-beam four-wave-mixing technique unaffected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing of 109 ps at 5 K, much faster than the ~10 ns exciton radiative lifetime. The zero-phonon line dephasing is explained by phonon-assisted spin flip from the lowest bright state to dark-exciton states. This is confirmed by the temperature dependence of the exciton lifetime and by direct measurements of the bright-dark-exciton relaxation. Our results give an unambiguous evidence of the physical origin of the exciton dephasing in these nanocrystals.     

Nonradiative and radiative Förster energy transfer between quantum dots

We theoretically study nonradiative and radiative energy transfer between two localized quantum emitters, a donor (initially excited) and an acceptor (receiving the excitation). The rates of nonradiative and radiative processes are calculated depending on the spatial and spectral separation between the donor and acceptor states and for different donor and acceptor lifetimes for typical parameters of semiconductor quantum dots. We find that the donor lifetime can be significantly modified only due to the nonradiative Förster energy transfer process at donor–acceptor separations of approximately 10 nm (depending on the acceptor radiative lifetime) and for the energy detuning not larger than 1–2 meV. The efficiency of the nonradiative Förster energy transfer process under these conditions is close to unity and decreases rapidly with an increase in the donor–acceptor distance or energy detuning. At large donor–acceptor separations greater than 40 nm, the radiative corrections to the donor lifetime are comparable with nonradiative ones but are relatively weak.     

阳离子对Er3+掺杂硼酸盐玻璃结构和发光特性的影响

针对Si/Al掺杂量增大使BaO-(1-x)SiO₂-xAl₂O₃-B₂O₃-0.005Er³⁺(x=0,0.5,1,摩尔分数)玻璃样品的1 540 nm红外发光和540 nm上转换发光强度呈现相反的变化趋势,讨论了阳离子掺杂对稀土离子周边对称性和电负性变化与荧光行为的内在关系。红外傅里叶透射光谱显示,不同掺杂浓度下样品的最大声子能量没有明显变化,说明光谱随掺杂浓度的变化与玻璃体系的最大声子能量无关。根据Judd-Ofelt理论计算了⁴I_13/2和⁴S_3/2能级的光学参数,通过比较两能级的跃迁几率(A)、寿命(τ)、荧光分支比(β)和受激发射截面(σ)等光学参数,发现⁴I_13/2能级和⁴S_3/2能级的相关参数呈现相反的趋势。最后测试了样品⁴I_13/2能级下1 540 nm的寿命,进一步从电负性角度考虑了样品的发光效率。理论计算和实验结果相符。     

Exciton emission dynamics in single InAs/GaAs quantum dots due to the existence of plasmon-field-induced metastable states in the wetting layer

A very long lifetime exciton emission with non-single exponential decay characteristics has been reported for single InA-s/GaAs quantum dot(QD) samples,in which there exists a long-lived metastable state in the wetting layer(WL)through radiative field coupling between the exciton emissions in the WL and the dipole field of metal islands.In this article we have proposed a new three-level model to simulate the exciton emission decay curve.In this model,assuming that the excitons in a metastable state will diffuse and be trapped by QDs,and then emit fluorescence in QDs,a stretchedlike exponential decay formula is derived as I(t)=At~(β-1)e~(-(rt)β),which can describe well the long lifetime decay curve with an analytical expression of average lifetime