The effect of post‐growth thermal annealing on the emission spectra of GaAs/AlGaAs quantum dots grown by droplet epitaxy

We fabricated GaAs/AlGaAs quantum dots by droplet epitaxy, and obtained the geometries of the dots by scanning transmission electron microscopy. Post‐growth thermal annealing is essential for the optical activation of quantum dots grown by droplet epitaxy. We measured the emission energy shifts of the dots and the underlying superlattice by post‐ growth thermal annealing, and specified the emission from dots by selectively etching the structure down to a low layer of quantum dots. We studied the influence of the degree of annealing on the optical properties of the dots from the peak shifts of the superlattice, since the superlattice has a uniform and well‐defined geometry. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Giant two‐photon absorption in monolayer MoS2

Strong two‐photon absorption (TPA) in monolayer MoS₂ is demonstrated in contrast to saturable absorption (SA) in multilayer MoS₂ under the excitation of femtosecond laser pulses in the near‐infrared region. MoS₂ in the forms of monolayer single crystal and multilayer triangular islands are grown on either quartz or SiO₂/Si by employing the seeding method through chemical vapor deposition. The nonlinear transmission measurements reveal that monolayer MoS₂ possesses a nonsaturation TPA coefficient as high as ∼(7.62 ±0.15) ×10³ cm/GW, larger than that of conventional semiconductors by a factor of 10³. As a result of TPA, two‐photon pumped frequency upconverted luminescence is observed directly in the monolayer MoS₂. For the multilayer MoS₂, the SA response is demonstrated with the ratio of the excited‐state absorption cross section to ground‐state cross section of ∼0.18. In addition, the laser damage threshold of the monolayer MoS₂ is ∼97 GW/cm², larger than that of the multilayer MoS₂ of ∼78 GW/cm².

     

水溶性CdTe:Mn量子点的双光子吸收

采用激发波长800 nm、脉宽50 fs、重复频率1 kHz的Ti:sapphire放大飞秒激光器作为激发光源,利用开孔Z扫描技术研究了不同粒径的CdTe:Mn量子点的非线性吸收性质。理论计算结果表明,同一生长时间CdTe:Mn量子点的双光子吸收系数是CdTe量子点的1.1倍,其双光子吸收系数随量子点尺寸的减小而增大,这是由于CdTe:Mn量子点非线性吸收属于反饱和吸收,掺杂了Mn元素,减小了表面缺陷浓度,表明掺杂量子点具有很好的双光子吸收现象。     

An analytical study of the various current components of an AlGaAs/GaAs multiple quantum well solar cell

An analytical study has been carried out on the photo-generated current of an AlGaAs/GaAs multiple quantum well solar cell. The expressions for the various current components and the total current of the cell have been obtained. Suitable modifications have been incorporated in the theoretical work reported earlier by several researchers in this topic. Based on this analysis the temperature dependence of the various current components and the total current of the cell have been investigated. It has been observed that the output current is a strong function of the temperature of the device.     

InAs/GaAs量子点1.3 μm单光子发射特性

采用双层耦合量子点的分子束外延生长技术生长了InAs/GaAs量子点样品,把量子点的发光波长成功地拓展到1.3 μm.采用光刻的工艺制备了直径为3 μm的柱状微腔,提高了量子点荧光的提取效率.在低温5 K下,测量得到量子点激子的荧光寿命约为1 ns;单量子点荧光二阶关联函数为0.015,显示单量子点荧光具有非常好的单光子特性;利用迈克耳孙干涉装置测量得到单光子的相干时间为22 ps,对应的谱线半高全宽度为30 μeV,且荧光谱线的线型为非均匀展宽的高斯线型.     

Two‐photon absorption induced anti‐Stokes emission in single InGaN/GAN quantum‐dot‐like objects

We observed crossed transitions and anti‐Stokes emissions in single quantum‐dot‐like objects embedded in the active layer of InGaN/GaN quantum disks by two‐photon absorption techniques. We proposed a phenomenological model based on the interplay between Auger effect and crossed transitions to explain the origin of anti‐Stokes emissions and the preferential excitation of 0D objects at the expense of their surroundings. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Self-assembled GaAs/AlGaAs quantum dots by molecular beam epitaxy and in situ AsBr3 etching

We report on a new method to produce self-assembled, unstrained, GaAs/AlGaAs quantum dots (QDs) with large confinement energy. First we create nanoholes on a GaAs surface by growing InAs islands on GaAs(0 0 1), overgrowing them with GaAs and etching the surface in situ with AsBr₃ gas. Then we transfer the holes to an AlGaAs surface, fill them with GaAs and overgrow them with AlGaAs. In this way we obtain GaAs inclusions in an AlGaAs matrix. We investigate the optical properties of such QDs by photoluminescence spectroscopy and their morphology by atomic force microscopy. We show that the QD size can be tuned and emission with inhomogeneous broadening down to 8.9 meV can be achieved.     

Investigation of various optical transitions in GaAs/Al0.3Ga0.7As double quantum ring grown by droplet epitaxy

This work examines the optical transitions of a GaAs double quantum ring (DQR) embedded in Al_0.3Ga_0.7As matrix by photoreflectance spectroscopy (PR). The GaAs DQR was grown by droplet epitaxy (DE). The optical properties of the DQR were investigated by excitation‐intensity and temperature‐dependent PR. The various optical transitions were observed in PR spectra, whereas the photoluminescence (PL) spectrum shows only the DQR and GaAs band emissions. The various optical transitions were identified for the GaAs near‐band‐edge transition, surface confined state (SCS), DQR confined state, wetting layer (WL), spin–orbital split (E_GaAs + Δ_o), and AlGaAs band transition. PR spectroscopy can identify various optical transitions that are invisible in PL. The PR results show that the GaAs/AlGaAs DQR has complex electronic structures due to the various interfaces resulting from DE.     

Type II GaSb/GaAs quantum dot/ring stacks with extended photoresponse for efficient solar cells

We report on the fabrication of GaAs based p–i–n solar cells containing 5 and 10 layers of type II GaSb quantum rings grown by molecular beam epitaxy. Solar cells containing quantum rings show improved efficiency at longer wavelengths into the near-IR extending up to 1500 nm and show enhanced short-circuit current under 1 sun illumination compared to a GaAs control cell. A reduction in the open-circuit voltage is observed due to the build-up of internal strain. The MBE growth, formation and photoluminescence of single and stacked layers of GaSb/GaAs quantum rings are also presented.     

PbS colloidal quantum dot/ZnO‐based bulk‐heterojunction solar cells with high stability under continuous light soaking

PbS colloidal quantum dot (CQD)‐based depleted bulk‐heterojunction solar cells were constructed, using the 1.2 μm thick nanowire array infiltrated with PbS QDs bearing Br ligands. The long‐term stability tests were performed on the solar cells without encapsulation in air under continuous light soaking using a Xe lamp with an AM1.5G filter (100 mW cm^?2). Time course of solar cell performances during the tests showed two time periods with distinct behavior, that is, the initial transient time period and the relatively stable region following it. The power conversion efficiency was found to keep approximately 90% of the initial value at the end of the 3000 h light soaking test. The stability tests suggest that the PbS surface modification or passivation reactions play an important role in achieving such a high stability, and demonstrate that PbS CQD/ZnO nanowire array‐based depleted bulk‐heterojunction solar cells are highly stable. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Analysis of three-wave mixing in GaAs/AlGaAs multiple quantum wells via biexciton coherence

A cascade-type three-level system of GaAs/AlGaAs multiple quantum wells (MQWs) is constructed by using biexciton coherence in the transient optical response. Then, we theoretically and numerically study the system with Maxwell–Schrödinger equations to obtain the generated three-wave mixing (TWM) field. By means of appropriately adjusting the strength of the control filed and the propagation length in the MQWs sample, the maximum efficiency of the generated TWM field can be achieved at η ≈ 80%.     

Resonant magnetopolaron effects in GaAs/AlGaAs multiple quantum well structures

A detailed experimental study of electron cyclotron resonance (CR) has been carried out at 4.2 K in three modulation-doped GaAs/Al_0.3Ga_0.7As multiple quantum well samples in fields up to 30 T. A strong avoided-level-crossing splitting of the CR energies due to resonant magnetopolaron effects is observed for all samples near the GaAs reststrahlen region. Resonant splittings in the region of AlAs-like interface phonon modes of the barriers are observed in two samples with narrower well width and smaller doping concentration. The interaction between electrons and the AlAs interface optical phonon modes has been calculated for our specific sample structures in the framework of the memory-function formalism. The calculated results are in good agreement with the experimental results, which confirms our assignment of the observed splitting near the AlAs-like phonon region is due to the resonant magnetopolaron interaction of electrons in the wells with AlAs-like interface phonons.     

Theoretical investigation of two‐photon absorption and fluorescence properties of cypridina luciferin‐based derivatives: 2,3,5‐trisubstituted pyrazine compounds

Two‐photon fluorescent probe materials are significant for achieving observation of living phenomena in entire organs and tissues. To explore new materials with high fluorescence and large two‐photon absorption (TPA) cross section, a series of 2,3,5‐trisubstituted pyrazine derivatives were designed. Their equilibrium geometries, one‐photon absorption, TPA, and luminescence properties have been studied by using density functional theory (DFT), time‐dependent DFT, and Zerner's intermediate neglect of differential overlap program. The results show that the introduction of styrene groups to 2,3,5‐trisubstituted pyrazine derivatives can efficiently increase the conjugated effect and enhance the TPA activity. Moreover, the luminescence properties of 2,3,5‐trisubstituted pyrazine derivatives were compared, and the effect of three substituents on the fluorescence of trisubstituted pyrazine derivatives was analyzed by means of different contribution of the basis functions localized on pyrazine fragment into the highest occupied molecular orbital and lowest unoccupied molecular orbital. The oscillator strengths in the excited state (?_em) for the pyrazine derivatives substituted by styryl are larger than that of other derivatives with acetylamino and indole groups substituted at 2‐site and 5‐site of the pyrazine core, and the ?_em of 3‐indolyl pyrazine derivatives is larger than that of 3‐styrene pyrazine derivatives. It suggests that the styrene group has a great influence on the luminescence property. In addition, the indole group substituted at 3‐site of the pyrazine derivatives can also promote the fluorescence property. Copyright © 2013 John Wiley & Sons, Ltd.     

脉冲软X光辐射三种材料的喷射冲量实验研究

在强光一号脉冲加速器上进行了国内首次的实验室软X光辐射三种材料的喷射冲量研究。结果表明，在能量为(0.2～0.33)keV、平均脉宽为39ns左右的X光辐射下，对灰漆、白漆和硬铝，在能注量分别为(92～152)J/cm2、(115～136)J/cm2和(163～192)J/cm2时，它们的冲量耦合系数分别为(0.61～0.80)Pa·s/(J·cm-2)、(0.58～0.97)Pa·s/(J·cm-2)和(0.61～0.84)Pa·s/(J·cm-2)。     

Free standing modulation doped core–shell GaAs/AlGaAs hetero‐nanowires

Modulation doped AlGaAs/GaAs core–shell nanowire structures were grown by molecular beam epitaxy. A Si delta‐doping was introduced in the AlGaAs shell around the {110} facets of the GaAs core. The wires are typically highly resistive at low temperatures. However, they show a pronounced persistent photoconductivity effect indicating activation of free carriers from the delta‐doped shell to the GaAs core. The n‐type character of the channel is demonstrated by applying a back‐gate voltage. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

脉冲激光原位辐照对InAs/GaAs(001)量子点生长的影响

在InAs/GaAs(001)量子点生长过程中, 当InAs沉积量为0.9 ML时, 利用紫外纳秒脉冲激光辐照浸润层表面, 由于高温下In原子的不稳定性, 激光诱导的原子脱附效应被放大, 样品表面出现了原子层移除和纳米孔. 原子力显微镜测试表明纳米孔呈现以[110]方向为长轴(尺寸: 20-50 nm)、[110]方向为短轴(尺寸: 15-40 nm)的表面椭圆开口形状, 孔的深度为0.5-3 nm. 纳米孔的密度与脉冲激光的能量密度正相关. 脉冲激光的辐照对量子点生长产生了显著的影响: 一方面由于纳米孔的表面自由能低, 沉积的InAs优先迁移到孔内, 纳米孔成为量子点优先成核的位置; 另一方面, 孔外的区域因为In原子的脱附, 量子点的成核被抑制. 由于带有纳米孔的浸润层表面具有类似于传统微纳加工技术制备的图形衬底对量子点选择性生长的功能, 该研究为量子点的可控生长提供了一种新的思路.     

GaAs pyramidal quantum dot coupled to wetting layer in an AlGaAs matrix: A strain-free system

In this contribution, the electronic and linear and nonlinear optical properties of pyramid-shaped GaAs quantum dots (QDs) coupled to wetting layer (WL) in an Al_0.3Ga_0.7As matrix have been investigated. This nanostructure is relaxed from strain effects due to very small lattice-mismatching. Three transitions of P-to-S, WL-to-P, and WL-to-S were considered and the corresponding transition dipole moments, oscillator strengths, and linear and nonlinear optical properties regarding to these transitions were investigated as a function of the QD height. The results showed that for P-to-S transition, which is a purely in-plane-polarized transition, the dependence of electronic and optical properties on the size is moderate and can be neglected. But for WL-to-P and WL-to-S transitions, which are in-plane- and z-polarized transitions, respectively, the electronic as well as optical properties are strongly size-dependent. Furthermore, a competition between WL-to-S and WL-to-P transitions was observed when the QD size changed.     

Exciton behavior in GaAs/AlGaAs coupled double quantum wells with interface disorder

In this work, we present a detailed study on the optical properties of two GaAs/Al_0.35Ga_0.65As coupled double quantum wells (CDQWs) with inter-well barriers of different thicknesses, by using photoluminescence (PL) spectroscopy. The two CDQWs were grown in a single sample, assuring very similar experimental conditions for measurements of both. The PL spectrum of each CDQW exhibits two recombination channels which can be accurately identified as the excitonic e₁-hh₁ transitions originated from CDQWs of different effective dimensions. The PL spectra characteristics and the behavior of the emissions as a function of temperature and excitation power are interpreted in the scenario of the bimodal interface roughness model, taking into account the exciton migration between the two regions considered in this model and the difference in the potential fluctuation levels between those two regions. The details of the PL spectra behavior as a function of excitation power are explained in terms of the competition between the band gap renormalization (BGR) and the potential fluctuation effects. The results obtained for the two CDQWs, which have different degrees of potential fluctuation, are also compared and discussed.     

Photoluminescence studies of confined states in AlGaAs/GaAs asymmetric quantum well

In the recombination spectra of AlGaAs/GaAs heterostructures, a peculiar and asymmetric photoluminescence (PL) band F has previously been reported [Aloulou et al., Mater. Sci. Eng. B 96 (2002) 14] to be due to recombinations of confined electrons from the two-dimensional electron gas (2DEG) formed at AlGaAs/GaAs interface in asymmetric quantum well (AQW). Detailed experiments are reported here on GaAs/Al_0.31Ga_0.69As/GaAs:δSi/Al_0.31Ga_0.69As/GaAs samples with different spacer layer thicknesses. We show that the band F is the superposition of two PL bands F′ and F″ associated, respectively, to AQW and a symmetric quantum well (SQW). In the low excitation regime, the F′ band present a blue shift (4.4 meV) followed by important red shift (16.5 meV) when increasing optical excitation intensity. The blue shift in energy is interpreted in terms of optical control of the 2DEG density in the AQW while the red shift is due to the narrowing of the band gaps caused by the local heating of the sample and band bending modification for relatively high-optical excitation intensity. Calculation performed using self-consistent resolution of the coupled Schrödinger–Poisson equations are included to support the interpretation of the experimental data.