Use of short-wavelength synchrotron radiation to measure luminescence quantum yield and the luminescence excitation spectrum of polymers

We describe an optical diagnostics module and the instrumental and methodological features of ultrahigh vacuum experiments investigating the optical characteristics of condensed media in the short-wavelength (hv ~ 3.5–25 eV) range of the spectrum of probing synchrotron radiation. We give a brief presentation of the results of an experimental determination of the spectral dependence of the luminescence quantum yield and the luminescence excitation spectrum of ablatable polymer dielectrics on the Kurchatov synchrotron radiation source at values of the probing radiation power density (I ₀ ~ 10¹² photons/cm²∙sec) that are below threshold for extended surface vaporization and a surface temperature of the condensed targets equal to 77–300 K.     

Measurement of the luminescence decay times of PbS quantum dots in the near-IR spectral range

Methods for recording luminescence decay times of semiconductor PbS quantum dots (QDs) with optical transitions in the near-IR spectral range have been analyzed. A measuring complex for spectral and kinetic analysis in the near-IR range (0.8?C2.0 ??m) in the time interval from several tens of nanoseconds to several tens of microseconds is described. In this complex, a semiconductor picosecond laser is used as an excitation source; luminescence decay times are recorded by a fast InGaAs photodiode, a high-speed amplifier, and a high-frequency oscilloscope; and the measurement results are multiply averaged (up to a million times) by a program. The technical features of this method are discussed and compared with the characteristics of techniques based on photon counting or application of more powerful radiation sources, and the limitations on sensitivity are analyzed. The results of measuring the luminescence decay kinetics of PbS QDs 2.7?C7.6 nm in size prepared in the form of solutions and incorporated into matrices are reported.     

Resonant Raman scattering of optical phonons in self-assembled quantum dots

We have investigated the carrier relaxation mechanism in InGaAs/GaAs quantum dots by photoluminescence excitation (PLE) spectroscopy. Near-field scanning optical microscope successfully shows that a PLE resonance at a relaxation energy of 36 meV can be seen in all single-dot luminescence spectra, and thus can be attributed to resonant Raman scattering by a GaAs LO phonon to the excitonic ground state. In addition, a number of sharp resonances observed in single-dot PLE spectra can be identified as resonant Raman features due to localized phonons, which are observed in the conventional Raman spectrum. The results reveal the mechanism for the efficient relaxation of carriers observed in self-assembled quantum dots: the carriers can relax within the continuum states, and make transitions to the excitonic ground state by phonon emission.     

Strong Photoluminescence Enhancement from Colloidal Quantum Dot Near Silver Nano-Island Films

We present the fabrication and optical investigation of highly random self-assembled, nano-scale films, probing their influence on the luminescence properties of near surface CdSe/ZnS colloidal quantum dots. When compared to quantum dots distributed on unstructured quartz substrates, the average luminescence intensity is found to be enhanced by a factor of 160×. The silver nanoparticles are prepared using slow thermal evaporation on quartz substrates and post-deposition annealing to produce a randomly-arranged layer of smooth nano-islands. Clear polarization dependent hot spots are observed. Such hot spots deliver a maximal enhancement of the emission intensity of 240× and have a spatial density of (0.050±0.002) μm^− 2. The results show that silver nano-island films strongly enhance the optical efficiency of near quantum dots emitters.     

Tunable Infrared Luminescence and Optical Amplification in PbS Doped Glasses

PbS-doped glasses are prepared. Absorption and luminescence spectra show that both the absorption and infrared emission can be tuned widely by thermal treatment conditions. Optical amplification at 1300 nm is observed, and amplified spontaneous emission （ASE） spectrum is also measured to confirm the optical gain from PbS quantum dots.     

Spectrum of electron-hole states of the Si/Ge structure with Ge quantum dots

The lateral photoconductivity spectra of Si/Ge multilayer structures with Ge quantum dots of various sizes are investigated. We observed optical transition lines between the hole levels of quantum dots and electronic states of Si. This enabled us to construct a detailed energy level diagram of the electron-hole spectrum of the Si/Ge structures. It is shown that the hole levels of Ge quantum dots are successfully described by the “quantum box” model using the actual sizes of Ge islands. It I found that the position of the longwavelength photosensitivity boundary of Si/Ge structures with Ge quantum dots can be controlled by changing the growth parameters.     

Influence of the thickness of CdTe and ZnTe layers on cathodoluminescence spectra of strained CdTe/ZnTe superlattices with quantum-dot layers

The influence of the thickness of ZnTe barrier layers on the cathodoluminescence spectra of strained CdTe/ZnTe superlattices containing layers of quantum dots with an average lateral size of approximately 3 nm has been investigated. In samples with thick barrier layers (30, 15 nm), the cathodoluminescence spectra of quantum dots exhibit one band with a maximum at E = 2.03 eV. It has been revealed that, at a barrier layer thickness of ∼3 nm, the luminescence band is split. However, at a ZnTe layer thickness of 1.5 nm, the luminescence spectrum also contains one band. The experimental results have been interpreted with allowance made for the influence of elastic biaxial strains on the energy states of light and heavy holes in the CdTe and ZnTe layers. For the CdTe/ZnTe heterostructure with quantum dots in which the thickness of the deposited CdTe layer is 1.5 monolayers and the thickness of the barrier layer is 100 monolayers, the cathodoluminescence spectrum contains 2LO-phonon replicas. This effect has been explained by the resonance between two-phonon LO states and the difference between the energy states in the electronic spectrum of wetting layer fragments.     

Near field scanning optical spectroscopy of InP single quantum dots

We study InP quantum dots which are prepared by strain induced self-assembly on GaAs substrates with a GaInP buffer layer using a near field scanning optical microscope operating at near liquid He bath temperatures in the collection mode. Single quantum dots are identified spatially and spectrally due to their photoluminescence spectrum. Series of luminescence lines due to single dots of different sizes are discussed in terms of dot height and width fluctuations. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 7, 497–501 (10 October 1997)     

Pressure-induced modulation of the confinement in self-organized quantum dots produced and detected by a near-field optical probe

Near-field optical probing, or nanoprobing, achieves spatial resolution that surpasses the diffraction limit of light and makes possible the luminescence imaging and spectroscopy of single quantum dots in dense arrays of dots. We use optical nanoprobing to study self-organized InGaAs quantum dots grown on (3 1 1)B oriented GaAs substrates. Here, we emphasize a new feature of nanoprobing: pressure-induced strain modulation near the surface. Operating in near-field optical excitation–collection mode, the probe makes contact with the surface and exerts direct pressure whose main effect is a compressive uniaxial strain under the probe. By adjusting the applied pressure, we modulate the local strain environment in and around a quantum dot, but still preserve the capability to capture its near-field luminescence. Nanoprobe pressure effects modify the confinement potential and radiative emission of single quantum dots, and the coupling strength between dots. This opens new possibilities for the study and control of the optical and electronic properties of single- and coupled-quantum dots.     

Pyramidal quantum dots: High uniformity and narrow excitonic emission

We report on the optical properties of site-controlled InGaAs dots in GaAs barriers grown in pre-patterned, large pitch, pyramidal recesses by metalorganic vapour phase epitaxy. The inhomogeneous broadening of excitonic emission from an ensemble of quantum dots is found to be extremely narrow, with a standard deviation of 1.19 meV. A dramatic improvement in the spectral purity of emission lines from individual dots is also reported (18–30 μeV ) when compared to the state-of-the-art for site controlled quantum dots.     

Optical properties of InAs quantum dots in a Si matrix

We report on the optical properties of nanoscale InAs quantum dots in a Si matrix. At a growth temperature of 400°C, the deposition of 7 ML InAs leads to the formation of coherent islands with dimensions in the 2–4 nm range with a high sheet density. Samples with such InAs quantum dots show a luminescence band in the 1.3 μm region for temperatures up to 170 K. The PL shows a pronounced blue shift with increasing excitation density and decays with a time constant of 440 ns. The optical properties suggest an indirect type II transition for the InAs/Si quantum dots. The electronic structure of InAs/Si QDs is discussed in view of available band offset information.     

Optical pumping in a L-system by parametric luminescence light

We examine the optical pumping effect in an ensemble of three-level atoms with a Λ configuration of the energy sublevels excited by parametric luminescence light in the squeezed state. We derive quantum kinetic equations that describe the evolution of the density matrix of atoms irradiated by low-intensity squeezed light with a finite-width spectrum. In particular, we show that because of the quantum statistical properties of the squeezed light there can be a redistribution of atoms among the lower energy sublevels, despite the equality of the intensities of the spectral components of the light that resonantly excites optical transitions in the Λ-system. The relation of the optical pumping effect to the correlation and spatial-temporal spectral properties of squeezed light is discussed in detail. Finally, we show that the effects are closely linked to the finiteness of the width of the squeezed-light spectrum. Zh. éksp. Teor. Fiz. 112, 137–162 (July 1997)     

Spontaneous far-IR emission accompanying transitions of charge carriers between levels of quantum dots

The spontaneous emission of far-infrared radiation (λ≅10–20 μm) from diode structures with vertically coupled InGaAs/AlGaAs quantum dots is observed. This emission is due both to transitions of holes and electrons between size-quantization levels in quantum dots and to transitions from the continuum to a level in a quantum dot. It is observed only when accompanied by lasing at short wavelengths (λ≅0.94 μm) and, like the short-wavelength emission, it exhibits a current threshold. The spontaneous emission of long-wavelength radiation is also observed in InGaAs/GaAs quantum-well laser structures. This radiation is approximately an order of magnitude weaker than that from quantum-dot structures, and it has no current threshold. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 4, 256–260 (25 February 1998)     

可调谐地物光谱辐射源研究

介绍了一种以半导体发光二极管(LED)和溴钨灯为光源的光谱可调谐地面景物光谱辐射源,及其系统结构和控制系统设计。利用地物光谱辐射源模拟了大量不同地物光谱,在控制系统中建立了地物光谱数据库,引进了改进型遗传算法匹配出了光谱数据库中的500余种地物光谱。进而详细分析了几种典型地物的光谱匹配相似度,平均光谱匹配误差以及全波段,红,绿,蓝和近红外波段光谱匹配相似度。目标辐亮度为50W·(m2·sr)-1时,地物光谱辐射源平均光谱匹配误差在10%以内,最低可达6.0%;全波段光谱匹配相似度高于0.895,最高可达0.983;对于同一景物光谱红、绿、蓝和近红外波段(特别是绿光和近红外波段)光谱匹配相似度低于全波段,绿光和近红外波段光谱匹配相似度与红光和蓝光波段相比,最大低幅为50%。地物光谱辐射源能够用于光学遥感器的辐射定标,能有效解决光学遥感器的工作目标与定标光源光谱不匹配造成的定标误差。     

Dynamics of confined excitons and biexcitons of CuCl quantum dots

We investigated the dynamics of confined excitons and biexcitons in CuCl quantum dots in an NaCl matrix by using subpicosecond pump and probe absorption spectra, and time-resolved luminescence spectra. Then temporal changes of the absorption, optical gain, and luminescence are interpreted in relation to the exciton dynamics. The optical nonlinearity is discussed in terms of the exciton-exciton interaction in the quantum dot.     

Photonics of zinc complexes of 3,3′-bis(dipyrrolylmethenes)

We measure the spectral luminescence characteristics (absorption, fluorescence, lasing, phosphorescence, and luminescence excitation spectra; fluorescence and phototransformation quantum yields; lifetimes of long-lived radiation) and estimate the fluorescence radiation constants and lifetimes for new synthesized binuclear bihelical complexes of zinc(II) with 3,3′-bis(dipyrrolylmethenes) of different structure in liquid and solid solutions. We discuss possible applications of these compounds.     

Photoinduced improvement of the properties of light-emitting diodes

Results from studies of the effect of the action of optical radiation on the characteristics of light-emitting diodes (LEDs) produced using the binary heterostructure Ga_xAl_1−xAs (λ=0.88 μm) are presented. High sensitivity of the LED to the following parameters of the optical radiation is shown: flux density, quantum energy, and exposure dose. The action of optical radiation in the form of a band with a maximum at 255 nm on the LED heterostructures lowers the leakage current into the bulk, decreases the loss identified as surface leakage current by about an order of magnitude, increases the radiated power by 50–100% in the current region up to 10⁻³ A, and increases the overall light output of the diodes. Tomsk Polytechnic University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 109–114, August, 1997.     

Correlation between optical,structural, and sensor properties of nanostructures of nickel oxide on aluminum oxide substrates

We have established a correlation between diffuse reflection of nickel oxide-based composites in the red region of the spectrum and the nanostructural properties of the roughness profile of the composite. We present calculations of the absorption spectra for samples of nanostructured nickel oxide of thickness 0.25–0.35 μm on aluminum oxide substrates from diffuse reflection and transmission of the layers by multiple scattering methods, taking into account the luminescence of the samples in the weak absorption region. Using a special optical system, we have determined the selective sensitivity to carbon monoxide exposure of a nanostructured nickel oxide composite at room temperature. The result is of fundamental importance for development of optical selective gas sensor technology in the visible region of the spectrum.     

Physical and Optical Properties of Calcium Zinc Borophosphate Glasses Doped with Manganese Ions

This article reports on the physical and optical properties, absorption, and luminescence spectra in the visible region, of calcium zinc borophosphate glasses doped with manganese ions. The manganese composition was varied up to 10 mol%. The aim of this work was to investigate the effect of the luminescence properties when the glasses were doped with different compositions of manganese ions. X-ray diffraction profiles confirmed their glassy nature. The optical absorption spectrum showed bands characteristic of manganese ions in octahedral symmetry. Both excitation and emission spectra were recorded for these glasses to understand their optical performances. The emission spectrum showed a single broad band (green region) in octahedral symmetry at 582 nm as a result of transition from the upper ⁴T_1g state to the ⁶A_1g ground state of manganese ions. As the concentration of manganese ions increased, the emission band increased from 582 nm (green-light emission) to 650 nm (red-light emission). Apart from the spectral analysis, different physical properties of these glasses were also analyzed. Based on the physical and optical properties, we found the samples to be more promising for their use as novel luminescent optical materials.     

Dependence of the spectral characteristics of quartz on the fluence of neutrons

The influence of radiation on the change in the optical properties of a crystalline quartz is investigated. The results of the measurement of the optical spectra of luminescence, reflection, and scattering in specimens irradiated with different fluences of neutron radiation are presented. It is shown that the corresponding spectral characteristics experience substantial changes in the region of fluences that are associated with rearrangement of the crystalline structure. The results of approximation of some radiation kinetics are given, and the trend in the process of accumulation of disordered zones is established. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 4, pp. 543–545, July–August, 2000.