Performance analysis of near-field thermophotovoltaic devices considering absorption distribution

This paper elucidates the energy transfer and conversion processes in near-field thermophotovoltaic (TPV) systems, considering local radiation absorption and photocurrent generation in the TPV cell. Radiation heat transfer in a multilayered structure is modeled using the fluctuation-dissipation theorem, and the electric current generation is evaluated based on the photogeneration and recombination of electron-hole pairs in different regions of the TPV cell. The effects of near-field radiation on the photon penetration depth, photocurrent generation, and quantum efficiency are examined in the spectral region of interest. The detailed analysis performed in the present work demonstrates that, while the near-field operation can enhance the power throughput, the conversion efficiency is not much improved and may even be reduced. Subsequently, a modified design of near-field TPV systems is proposed to improve the efficiency.     

Fabrication of In-rich AgInS<Subscript>2</Subscript> nanoplates and nanotubes by a facile low-temperature co-precipitation strategy and their excellent visible-light photocatalytic mineralization performance

Visible-light-driven In-rich AgInS₂ nanoplates and nanotubes were successfully prepared by a convenient co-precipitation strategy at low temperature. The effect of different In/Ag molar ratio in the raw materials on the physicochemical properties and photocatalytic activity of AgInS₂ was investigated. The In/Ag molar ratio has an obvious effect on the morphology of AgInS₂, and the physicochemical properties and photocatalytic activity of AgInS₂ are also dependent on the In/Ag molar ratio. When the molar ratio of In/Ag is 9, the photoluminescence intensity of AgInS₂ reaches a minimum value, while its photocurrent density is maximum (0.011 mA/cm²), indicating the most efficient separation of electron-hole pairs. The AgInS₂ with the In/Ag molar ratio of 9 exhibits the highest visible-light photocatalytic activities with almost complete degradation of 2-nitrophenol, which is attributed to the narrowest band gap and the most efficient separation of electron-hole pairs. Moreover, In-rich AgInS₂ exhibits excellent regeneration ability.     

ZnO/P-Si异质结的光电特性研究

利用脉冲激光沉积方法在P-Si(100)衬底上生长ZnO薄膜,制备ZnO/P-Si异质结,研究衬底温度对异质结光电特性的影响.结果表明,在400℃,500℃,550℃和600℃下生长ZnO制备的异质结都有一定的整流特性,反向暗电流随着衬底温度的升高略有增加,在550℃下制备的样品具有最明显的光电效应.ZnO/P-Si异质结对可见光和紫外光呈现出不同的响应性.在可见光照射下,光电流随反向偏压急剧增大,偏压增大到某一值时,光电流增速变小,而在紫外光下,光电流有逐渐增大的趋势.根据ZnO的透射谱认为,可见光和紫外光是异质结不同的耗尽区诱导电子-空穴对产生光电流的.     

TiO_2-Loaded WO_3 Composite Films for Enhancement of Photocurrent Density

Titanium dioxide(TiO_2) loaded tungsten trioxide(WO_3) composite films are prepared by an E-beam vapor system. Associated with the existence of a heterojunction at the interface of TiO_2 and WO_3, the prepared TiO_2-WO_3 composite film shows enhanced photocurrent density; four times than the pure WO_3 film illuminated under xenon lamp, and higher incident-photon-to-current conversion efficiency. By varying the initial TiO_2 film thickness, such composite structures could be optimized to obtain the highest photocurrent density. We believe that thin TiO_2 films improve the light response and increase the surface roughness of WO_3 films. Furthermore,the existence of the heterojunction results in the efficient charge carriers' separation, transfer process, and a lower recombination of electron-hole pairs, which is beneficial for the enhancement of photocurrent density.     

Kinetics of resonance luminescence of a single quantum dot at room temperature

We have developed a theory of transient resonance luminescence of a single quantum dot from the lowest energy states of electron-hole pairs. We consider a process in which laser pulses directly excite photonemitting states of electron-hole pairs of the quantum dot at room temperature. For definiteness, the model under the development takes into account two states of electron-hole pairs that contribute to luminescence. We have analyzed the dependence of the secondary emission process on the energy gap between these states, the value of which is determined by the quantum dot size. In terms of the Pauli master kinetic equation, an analytical expression for the time-dependent signal of the resonance luminescence has been obtained. We show that, as the spectral width of the exciting laser pulse tends to zero, this expression yields the signal of stationary luminescence.     

Net negative charge of electron-hole drops in germanium

The giant fluctuating photocurrent has been observed in highly excited germanium samples with ohmic contacts at 1.6 K. From the observation of this fluctuating photocurrent spike with varying static electric field, it is found that the electron-hole drop is negatively charge as a whole.     

双金属/TiO2纳米管复合结构中增强的光电流

将传统半导体材料与金属微纳结构相结合,利用其表面等离激元共振效应,可有效地增强复合结构的光电转换效率,使其广泛地被用于光电化学和光电探测等领域.本文以氧化铝纳米管为模板,采用原子层沉积技术制备出高有序的TiO2纳米管,并通过电子束热蒸发技术在大孔径的纳米管薄膜中分别负载金、铝和双金属金/铝纳米颗粒,形成金属纳米颗粒/T...     

Wannier excitons signalling strong Coulomb coupling in graphene

The Wannier equation for quasiparticles with a linear dispersion is investigated. It is shown that Coulomb bound compounds of mass less electron-hole pairs can only exist if the effective coupling strength exceeds the critical value of approximately 0.46. At the critical value, a second-order transition to a weak Coulomb regime is found. It is shown that the existence of bound excitons indicates an instability of the noninteracting ground state.     

Kinetics of thermalized luminescence of a single quantum dot at room temperature

We have developed a theory of transient secondary emission of a single quantum dot from the lowest energy states of electron-hole pairs. We consider a process in which laser pulses excite a certain highenergy state of electron-hole pairs of a quantum dot at room temperature, with the electronic subsystem then relaxing to low-energy states and photons being emitted. Therefore, the investigated secondary emission process is thermalized luminescence. For definiteness, the developed model takes into account two states of electron-hole pairs that contribute to the luminescence. We have analyzed the dependence of the secondary emission signal on the energy gap between these states, the value of which is determined by the quantum dot size. In terms of the Pauli master kinetic equation, an analytical expression for the time dependent signal of the thermalized luminescence has been obtained. We show that, as the spectral width of the exciting laser pulse tends to zero, this expression yields the signal of stationary luminescence.     

Effect of electron-hole spatial correlation on spin relaxation dynamics in InAs submonolayer

Using time-resolved photoluminescence and time-resolved Kerr rotation spectroscopy, we explore the unique electron spin behavior in an InAs submonolayer sandwiched in a GaAs matrix, which shows very different spin characteristics under resonant and non-resonant excitations. While a very long spin relaxation lifetime of a few nanoseconds at low temperature is observed under non-resonant excitation, it decreases dramatically under resonant excitation. These interesting results are attributed to the difference in electron-hole interactions caused by non-geminate or geminate capture of photo-generated electron-hole pairs in the two excitation cases, and provide a direct verification of the electron-hole spatial correlation effect on electron spin relaxation.     

Exciton-spin memory with a semiconductor quantum dot molecule

We report on a single photon and spin storage device based on a semiconductor quantum dot molecule. Optically excited single electron-hole pairs are trapped within the molecule, and their recombination rate is electrically controlled over 3 orders of magnitude. Single photons are stored up to 1 μs and read out on a subnanosecond time scale. By using resonant excitation, the circular polarization of individual photons is transferred into the spin state of electron-hole pairs with a fidelity above 80%, which does not degrade for storage times up to the 12.5 ns repetition period of the experiment.     

Low-temperature photoluminescence of MBE-grown GaAs subject to an electric field

Low-temperature photoluminescence of GaAs has been investigated in MBE-grown Al _x Ga_1–x As-GaAs single heterojunctions subject to an electric field. No peak energy shift is observed in the emission lines due to free excitons and excitons bound to isolated centers when the electric field is applied. In contrast, the excitonic lines arising from the previously described defect-induced bound exciton (DIBX) transitions exhibit a prominent low-energy shift when the electric field is increased. We attribute these lines to excitons bound to acceptor pairs. The excitons bound to distant pairs have smaller binding energies than those bound to closer pairs. They are, therefore, easily dissociated in a weak electric field. The electrons and holes thus dissociated may again be trapped by closer pairs, which results in a low-energy shift of the overall spectrum. The photocurrent measured as a function of the electric field supports Dingle's rule for the valence bandedge discontinuity.     

Exciton states and excitonic absorption spectra in a cylindrical quantum wire under transverse electric field

The temperature dependence of the resistance in trilayer graphene is observed under different applied gate voltages. At small gate voltages the resistance decreases with increasing temperature due to the increase in carrier concentration resulting from thermal excitation of electron-hole pairs, characteristic of a semimetal. At large gate voltages excitation of electron-hole pairs is suppressed, and the resistance increases with increasing temperature because of the enhanced electron-phonon scattering, characteristic of a metal. We find that the simple model with overlapping conduction and valence bands, each with quadratic dispersion relations, is unsatisfactory. Instead, we conclude that impurities in the substrate that create local puddles of higher electron or hole densities are responsible for the residual conductivity at low temperatures. The best fit is obtained using a continuous distribution of puddles. From the fit the average of the electron and hole effective masses can be determined.     

Optical studies of fast plasma transport in Si

We present two new optical techniques for the investigation of the transport properties of ambipolar plasmas in semiconductors. The first method is a time-of-flight technique: Using surface doping with shallow impurities which provide a characteristic bound exciton emission we introduce optically active spatial markers into the thin Si wafers investigated. Carrier pairs are excited at the undoped surface of the wafer by short laser pulses. From time-resolved studies of the bound exciton emission we obtain average velocity values for the ambipolar transport through the sample. The most attractive feature of the time-of-flight method compared to other spatially resolved measurements is the combination of very high spatial resolution (submicron range) with a high sensitivity. The second method used to study the plasma transport is based on a time-resolved investigation of the Mott transition between the electron-hole plasma and free excitons in Si. Using well-established values for the Mott transition and the temporal evolution of the plasma and the free exciton emissions we obtain values for the velocity with which the plasma expands to the Mott density as functions of the excitation power and the temperature. The advantage of this method is the lack of any need for spatial resolution. Possible extensions of both methods are discussed.     

Raman scattering from optically pumped electron-hole plasma in insulating GaAs

We measured the single particle Raman Scattering from an optically pumped electron-hole plasma in bulk insulating GaAs, using IR radiation of 1.06 μm to both excite and probe the plasma. The relevant theory is derived in terms of a Landau generalized quasi-particle picture and we show how many-body effects are discernible in the spectrum. The results of the experiment lend independent support to the model of two-photon absorption in GaAs and allowed us to place a lower bound of 1 × 10¹⁶ cm^?3 for the threshold of saturation effects in the density of photo-generated electron-hole pairs in bulk GaAs.     

Many body effects and internal transitions of confined excitons in GaAs and CdTe quantum wells

Many body effects contribute significantly to the energy states of electron-hole pairs confined in quantum wells in the presence of excess electrons. We present results of optically detected resonance spectroscopy of the internal transitions of photo-excited electron-hole pairs in the presence of excess electrons for GaAs QWs and CdTe QWs. Compared to the case of isolated negatively charged excitons, excess electrons produce a large blue shift of the internal transitions in modulation-doped GaAs quantum wells (QWs) for filling factor <2, and similar effects are found in CdTe QWs. For filling factor >2 no internal transitions are observed. These measurements demonstrate the strong effects of electron-electron correlations on the internal transitions of charged excitons in these quasi-2D systems and the importance of magnetic translation invariance. In the presence of excess electrons, the observed internal transitions are those of a magnetoplasmon bound to a mobile valence band hole.     

半导体中电子-空穴对的相干态、复合辐射和噪声压缩

本文讨论了在光的相干激发下,半导体中电子-空穴对的玻色近似和SU(2)相干态的产生,讨论了两种情形下电子-空穴对噪声特性和复合辐射特性,给出了电子-空穴对的二阶相干函数和分布函数,说明了玻色近似相当于SU(2)群到谐振子群的收缩。 关键词：     

Unconventional quasiparticle lifetime in graphene

We address the question of how large can the lifetime of electronic states be at low energies in graphene, below the scale of the optical phonon modes. For this purpose, we study the many-body effects at the K point of the spectrum, which induce a strong coupling between electron-hole pairs and out-of-plane phonons. We show the existence of a soft branch of hybrid states below the electron-hole continuum when graphene is close to the charge neutrality point, leading to an inverse lifetime proportional to the cube of the quasiparticle energy. This implies that a crossover should be observed in transport properties, from such a slow decay rate to the lower bound given at very low energies by the decay into acoustic phonons.     

Transient anisotropy effects in the absorption saturation of GaAs

The absorption of polarized light imparts a transient anisotropy on GaAs due to the preferential orientational excitation of electron-hole pairs. We present a series of experiments designed to measure the relaxation of the wavevector orientation of these photoexcited electron-hole pairs.     

Transient photoconductivity associated with the formation of electron-hole droplets in optically pumped germanium

Transient decay properties of photoconductivity of highly excited germanium are investigated at 1.6–4.2 K by use of a pulsed laser with the width of 5 nsec. An anomalous profile is observed in the decay curve of the photocurrent, which is interpreted in terms of the electron-hole droplets model.