双光子激发ZnSe自由载流子超快动力学研究

采用Z扫描和抽运-探测实验技术, 在波长为532 nm、脉冲宽度为41 fs的条件下测得ZnSe晶体的双光子吸收系数, 并获得了不同激发光强下的自由载流子吸收截面、电子-空穴带间复合时间和电子-声子耦合时间. 研究发现, 随着激发光强的增大, 自由载流子吸收截面减小, 复合时间变短. 当激发光强增大导致载流子浓度大于10¹⁸ cm^-3时, 抽运-探测信号出现明显改变, 原因归结为强光场激发导致样品在短时间内带隙变窄和电子-空穴等离子体的形成.     

Cu_2O/ZnO氧化物异质结太阳电池的研究进展

介绍了近年来低成本Cu_2O/ZnO氧化物异质结太阳电池方面的研究进展.应用于光伏器件的吸收层材料Cu_2O是直接带隙半导体材料,天然呈现p型;其原材料丰富,且对环境友好.Cu_2O/ZnO异质结太阳电池结构主要有平面结构和纳米线/纳米棒结构.纳米结构的Cu_2O太阳电池提高了器件的电荷收集作用;通过热氧化Cu片技术获得的具有大晶粒尺寸平面结构Cu_2O吸收层在Cu_2O/ZnO太阳电池应用中展现出了高质量特性.界面缓冲层(如i-ZnO,a-ZTO,Ga_2O_3等)和背表面电场(如p~+-Cu_2O层等)可有效地提高界面处能级匹配和增强载流子输运.10 nm厚度的Ga_2O_3提供了近理想的导带失配,减少了界面复合;Ga_2O_3非常适合作为界面层,其能够有效地提高Cu_2O基太阳电池的开路电压V_(oc)(可达到1.2 V)和光电转换效率.p~+-Cu_2O(如Cu_2O:N和Cu_2O:Na)能够减少器件中背接触电阻和形成电子反射的背表面电场(抑制电子在界面处复合).利用p型Na掺杂Cu_2O(Cu_2O:Na)作为吸收层和Zn_(1-x)Ge_x-O作为n型缓冲层,Cu_2O异质结太阳电池(器件结构:MgF_2/ZnO:Al/Zn_(0.38)Ge_(0.62)-O/Cu_2O:Na)光电转换效率达8.1%.氧化物异质结太阳电池在光伏领域展现出极大的发展潜力.     

退火温度对电子束蒸发沉积Cu_2O薄膜性能的影响

近年来,钙钛矿太阳电池(PSCs)得到了迅猛发展,而无机空穴传输材料(IHTMs)的使用可进一步降低电池的成本,提高电池的稳定性.本文通过电子束蒸发制备了Cu_2O薄膜,研究了空气中退火温度及时间对薄膜组成、结构及光电性能的影响,并构筑了p-i-n反型平面异质结钙钛矿太阳电池.研究发现:由于热解作用,直接通过电子束蒸发制备的薄膜为Cu_2O和Cu的混合物;而在空气中经过退火后,由于氧化作用,随着退火温度的升高,薄膜的组分由混合物转变为纯的Cu_2O,再转变成纯的CuO.通过控制退火温度制备的Cu_2O薄膜的光学带隙约为2.5 eV,载流子迁移率约为30 cm~2·V~(-1)·s~(-1).应用于PSCs,薄膜的最佳厚度为40 nm,但电池性能低于PEDOT:PSS基的PSCs.这主要是由于钙钛矿前驱液在Cu_2O薄膜的润湿性较差,吸收层中有大量微孔洞存在,致使漏电流增强,电池的性能降低.然而,当采用Cu_2O/PEDOT:PSS双HTMs设计时,由于PEDOT:PSS对Cu_2O具有较强的腐蚀作用,使电池性能恶化.     

Scanning the optical characteristics of lead-free cesium titanium bromide double perovskite nanocrystals

Cs₂TiBr₆ nanocrystals (NCs) are a type of promising optoelectronic materials, owing to their high photoelectric properties and non-toxicity. Here, we synthesize the colloidal Cs₂TiBr₆ NCs using a hot-injection approach. The temperature-dependent absorption data shows that its energy band changes about 30 meV with temperature, reflecting that its energy band structure is much stable. The excitation intensity-dependent transient absorption data confirms its linear absorption cross-sections and carrier recombination rate constants, involving monomolecular and bimolecular recombination, which are all superior to those of conventional perovskite bromide counterparts. In addition, its nonlinear absorption cross-sections are also measured based on femtosecond Z-scan. Our results suggest that Cs₂TiBr₆ NCs can be extensively applied in the field of optoelectronics, owing to its excellent carrier dynamics and nonlinear optical properties.     

Nonlinear optical properties of an azo-based dye irradiated by picosecond and nanosecond laser pulses

The nonlinear optical properties of an azo-based dye were investigated using Z-scan technique employing 38 ps pulses at 532 and 1064 nm, and 6 ns laser pulses at 532 nm. Large nonlinear absorption and nonlinear refraction were observed at both ps and ns 532 nm in the azoic dye. When excited at ps 1064 nm, this dye displayed a large two-photon absorption cross-section (σ₂=1810 GM). Meanwhile, the optical nonlinearity mechanism was discussed in terms of molecular structure, excitation wavelength, and pulse width.     

GaN晶体在飞秒紫外波段激发下的可变非线性吸收效应和光动力学过程研究

采用Z扫描和泵浦-探测技术研究了GaN薄膜在370 nm时的非线性光学效应和非线性光动力学过程。首先,基于GaN薄膜的透射光谱,结合线性光学理论分析得到了其在370 nm的线性折射率n₀、线性吸收系数α₀、光学带隙E_g等线性光学性质。采用飞秒激光Z扫描技术,得到了不同光强激发下的Z扫描实验响应结果,结合非线性光学理论提取出GaN薄膜可变的光学非线性吸收效应。在激发光子能量接近GaN带隙情况下,低光强时材料表现为饱和吸收而高光强时为反饱和吸收,这是因为低光强下单光子吸收占主导而高光强下以单光子感应自由载流子吸收为主。闭孔Z扫描测量得到了GaN薄膜的三阶非线性折射系数为n₂=-(1.0±0.1)×10-3 cm2·GW-1,它几乎比传统非线性介质的高出一个数量级。为了探究上述非线性过程的动力学弛豫时间以及进一步探究GaN薄膜非线性光动力学过程的深层物理机制,采用了交叉偏振飞秒退相泵浦探测技术观察GaN薄膜的光激发载流子动力学弛豫过程。实验结果表明,在低光强下,饱和吸收效应来源于瞬态单光子吸收,高光强下单光子感应自由载流子吸收为非瞬态光动力学过程,其自由载流子弛豫时间约为17 ps。该工作将为GaN薄膜在紫外非线性纳米器件应用以及GaN薄膜非线性过程的机制分析理解提供新的思路。     

Resonant Raman scattering near the 1S exciton level in cuprous oxide

Raman scattering experiments on the Γ₁₅⁻ LO₁ mode in Cu₂O crystals were performed using incident light near the resonance with the 1S exciton level of the ‘yellow serie’. The enhancement of the Raman efficiency observed in this experiment is attributed to the activation of the normally forbidden transition by stoechiometric defects in the Cu₂O crystals.     

Two- and three-photon excitation of Gd in CaAl12O19

We have employed two-photon excitation to study the higher energy levels of Gd³⁺ ions in CaAl₁₂O₁₉ and we compare the results with those obtained using conventional UV excitation techniques. Under two-photon excitation, the luminescence intensity exhibits an unusual temporal behavior, a very long build-up followed by a decrease by orders of magnitude, ascribed to a recombination-assisted luminescence excitation mechanism assuming photo-ionization of Gd³⁺ ions and trapping of free electrons on deep traps.

We also find that the two-photon excitation spectra contain an additional broadening contribution which can be attributed to homogeneous broadening of excitation levels caused by excited state absorption into the conduction band. We believe that this may be a general phenomenon whenever participating photons produce ionization of impurity ions from metastable excited states. The phenomenon can manifest itself also in two-photon ionization spectral hole burning and in up-conversion processes (in the latter case, the homogeneous broadening can be caused by an intra-ion excited-state absorption).     

Ga掺杂对Cu_3SbSe_4热电性能的影响

采用熔融-淬火方法制备了Cu_(2.95)Ga_xSb_(1-x)Se_4(x=0,0.01,0.02和0.04)样品,系统地研究了Ga在Sb位掺杂对Cu_3SbSe_4热电性能的影响.研究结果表明,少量的Ga掺杂(x=0.01)可以有效提高空穴浓度,抑制本征激发,改善样品的电输运性能.掺Ga样品在625 K时功率因子达到最大值10μW/cm·K~2,比未掺Ga的Cu_(2.95)SbSe_4样品提高了约一倍.但是随着Ga掺杂浓度的进一步提高,缺陷对载流子的散射增强,同时载流子有效质量增大,导致载流子迁移率急剧下降.因此Ga含量增加反而使样品的电性能恶化.在热输运方面,Ga掺杂可以有效降低双极扩散对热导率的贡献,同时掺杂引入的点缺陷对高频声子有较强的散射作用,因此高温区的热导率明显降低.最终该体系在664 K时获得最大ZT值0.53,比未掺Ga的样品提高了近50%.     

Optical nonlinearities and photo-excited carrier lifetime in CdS at 532 nm

Bound-electronic and free-carrier optical nonlinearities, and relaxation of photo-excited free carriers in CdS have been investigated by the use of a single-beam Z-scan technique at 532 nm. Under pulsed radiation of 35-ps duration with the input irradiances up to 4.8 GW/cm², the two-photon absorption coefficient, the bound-electron nonlinear refractive index, the free-carrier absorption cross-section, and the change in the refractive index per unit carrier density are determined to be 5.4±0.8 cm/GW, −(5.3±0.8)×10⁻¹³ cm²/W, (3.0±0.5)×10⁻¹⁷ cm² and −(0.8±0.1)×10⁻²¹ cm³, respectively. By using these values in the open-aperture Z-scans conducted with 7-ns laser pulses, the carrier recombination time is extracted to be 3.6±0.7 ns. The measured parameters are compared to theoretical calculations.     

Two-photon absorption of high-power picosecond pulses in PbWO<Subscript>4</Subscript>, ZnWO<Subscript>4</Subscript>, PbMoO<Subscript>4</Subscript>, and CaMoO<Subscript>4</Subscript> crystals

The nonlinear process of two-photon interband absorption is studied in tungstate and molybdate oxide crystals excited by a sequence of high-power picosecond pulses with a wavelength of 523.5 nm. The transmission of the crystals is measured for the excitation pulse intensity up to 100 GW/cm². The pulse intensity in the crystals initially transparent at a wavelength of 523.5 nm is strongly limited due to two-photon absorption (TPA), and the reciprocal transmission in PbWO₄ and ZnWO₄ crystals reaches 50–60. In all crystals, TPA induces long-lived one-photon absorption, which affects the nonlinear process dynamics and leads to a hysteresis in the dependence of the transmission on the laser excitation intensity. Absorption dichroism manifests itself in a significant difference in the transmission intensities when the principal orthogonal optical axes of the crystals are excited. The TPA coefficients are determined during the excitation of two optical axes of the crystals. TPA coefficients β for the crystals vary over a wide range, namely, from β = 2.4 cm/GW for PbMoO₄ to β = 0.14 cm/GW for CaMoO₄, and the values of β can differ almost threefold when different optical axes of a crystal are excited. Good agreement is achieved between the measured intensities limited by TPA and the estimates calculated from the measured nonlinear coefficients. Stimulated Raman scattering (SRS) upon excitation at a wavelength of 523.5 nm is only detected in two of the four crystals under study. The experimental results make it possible to explain the suppression of SRS by its competition with TPA, and the measured nonlinear coefficients are used to estimate this suppression.     

通过飞秒泵浦探测技术研究PbS纳米颗粒的激发态瞬态动力学过程

利用飞秒泵浦探测技术研究了PbS半导体纳米颗粒复合的SiO₂溶胶凝胶薄膜的瞬态动力学过程。通过改变激发探测波长和激发光强度,研究引起PbS半导体纳米颗粒的非线性吸收的两种机制。当激发探测波长选在激子吸收峰附近(620nm)时,由于激子的饱和吸收引起的光致漂白,当激发波长选在激子能态的低能侧(753,800nm),同时观察到激子的饱和吸收和双激子效应引起的光致吸收。研究了激子的饱和吸收和双激子效应引起的激发态吸收随激发态电子-空穴对浓度的变化关系,表明双激子效应与载流子浓度有很大关系。在高激发强度下,双激子效应引起的诱导吸收远远大于激子跃迁引起的光致漂白,双激子效应在非线性吸收中起着决定性作用。     

Cuprite, paramelaconite and tenorite films deposited by reactive magnetron sputtering

Copper oxides films (Cu₂O, Cu₄O₃ and CuO) have been deposited by magnetron sputtering of a copper target in various Ar–O₂ reactive mixtures. The films are characterized by X-ray diffraction, scanning electron microscopy, four-point probe method and UV-Vis spectrometry. The three defined compounds in the Cu---O binary system can be deposited by varying the oxygen flow rate introduced into the reactor. All the films are crystallized with a mean crystal size ranging from 10 to about 35 nm. They are highly resistive and present a direct optical band gap higher than 2 eV. The application of a bias voltage during the deposition phase modifies the texture of the Cu₂O films and also induces a preferential resputtering of oxygen from the Cu₄O₃ ones. This resputtering phenomenon leads firstly to the occurrence of the cuprite phase mixed with the paramelaconite one and secondly to the amorphisation of the films. Finally, the thermal stability in air of cuprite, paramelaconite and tenorite films has been investigated. The results show that the stability of Cu₂O and Cu₄O₃ films in air is influenced by the thickness and/or the texture of the films. Tenorite films with a low optical band gap (1.71 eV) can be formed after air annealing at 350 °C of an unbiased cuprite film.     

Hot photoluminescence of GaAs-AlGaAs multiple quantum well structures under high excitation by a single shot of 30 ps, 532 nm laser

Energy-resolved decay times and time-resolved spectra of hot photoluminescence of GaAs-AlGaAs multiple quantum well structures at 77 K under high excitation (50 MW/cm²) were measured by single shot experiments with the use of a 30 ps, 532 nm laser, a streak camera, and an optical multichannel analyzer. It was found that the transition between higher subbands (n=2 e-hh) appears and its intensity relaxes with a decay time of about 200 ps. The theoretical expression for carrier relaxation in a two-dimensional quantum well was obtained. We found that the calculated energy loss rate due to Lo-phonon scattering (10 meV/ps) is at least four times larger than the observed one (2.7 meV/ps).     

Phase equilibria at low oxygen partial pressure in the Y---Ba---Cu---O system

The phase equilibria around YBa₂Cu₃O_7−x (123) and YBa₂Cu₄O₈ (124) phases at low oxygen partial pressure (1 atm) were investigated by X-ray diffraction and thermal analysis. The coexistence of 123 and 124 phase was confirmed under 1 atm oxygen pressure. By using the high temperature X-ray diffraction method, the univariant reaction YBa₂Cu₃O_7−x+Cu₂OY₂BaCu₂O₂+O₂ was identified. The oxygen partial pressure dependence of several univariant reactions has been investigated and the existence of two invariant reactions of L+O₂YBa₂Cu₃O_7−x+ BaCuO₂+CuO+Cu₂O and L+Y₂BaCuO₅+O₂YBa₂Cu₃O_7−x+CuO+Cu₂O was deduced to occur at 1103 K under 0.0032 atm O₂ and at 1143 K under 0.0085 atm O₂, respectively.     

Ultrafast dynamics of intersubband excitations in a quasi-two-dimensional hole gas

We present the first study of ultrafast hole dynamics after resonant intersubband excitation in a quasi-two-dimensional semiconductor. p-type Si0.5Ge 0.5/Si multiple quantum wells are studied in pump-probe experiments with 150 fs midinfrared pulses. Intersubband scattering from the second heavy-hole back to the first heavy-hole subband occurs with a time constant of 250 fs, followed by intrasubband carrier heating within 1 ps. Such processes give rise to a strong reshaping of the intersubband absorption line, which is accounted for by calculations of the subband structure, optical spectra, and hole-phonon scattering rates.     

二硫化钼纳米点中的缺陷辅助载流子俘获超快动力学

二硫化钼纳米点正在成为有潜质的半导体材料用于光电设备的应用.然而,关于对其中激子动力学的研究却很少.本文利用飞秒瞬态吸收光谱学来研究二硫化钼纳米点的载流子动力学.结果显示,缺陷辅助的载流子再复合过程与观测到的动力学相符,通过俄歇散射对光激载流子进行俘获至少存在两种不同俘获速率的缺陷.四个过程参与了载流子驰豫,在受到光激发后,立即在~0.5 ps内载流子冷却,然后大部分载流子被缺陷快速俘获,随着泵浦能量的增加,该过程对应的时间从~4.9 ps增加到~9.2 ps,这可以用缺陷态的饱和来解释.接下来,拥有相对慢的载流子俘获速率的其它类型缺陷对小部分载流子进行俘获,该过程约65 ps.最后,剩余的少量载流子通过直接带间跃迁发生电子-空穴再复合,时间约为1 ns.研究结果可以深入了解二硫化钼纳米点中的载流子动力学基本原理,引导其更多的应用.     

Theoretical investigation on nonlinear absorption of multilevel organic molecular system in ns, ps and fs regime

The nonlinear absorption properties of organic materials in nanosecond (ns), picoseond (ps) and femtosecond (fs) regime were theoretically investigated by rate equations based on multilevel organic molecular system, respectively. The simulation results show that the RSA is mainly due to the absorption contribution of the triplet first excited state in the ns regime and that of the singlet first excited state in the ps regime, both of which are a third-order nonlinear phenomena. The RSA in fs regime is mainly due to the excited state absorption (ESA) induced by two-photon absorption (TPA), which is a fifth-order nonlinear phenomenon.     

Nonresonant and Resonant Nonlinear Absorption of CdSe-Based Nanoplatelets

We present a comprehensive understanding of the nonlinear absorption characteristics of CdSe-based nanoplatelets(NPLs) synthesized by the solution-phase method and the colloidal atomic layer deposition approach through Z-scan techniques at 532 nm with picosecond pulses. The CdSe NPLs exhibit strong two-photon induced free carrier absorption(effective three-photon absorption) upon the nonresonant excitation, resulting in a remarkable optical limiting behavior with the limiting threshold of approximately 75 GW/cm~2. A nonlinear optical switching from saturable absorption(SA) to reverse saturable absorption(RSA) with increasing the laser intensity is observed when coating CdSe NPLs with a monolayer of CdS shell to realize the resonant absorption. The SA behavior originates from the ground state bleaching and the RSA behavior is attributed to the free carrier absorption.These findings explicitly demonstrate the potential applications of CdSe-based NPLs in nonlinear optoelectronics such as optical limiting devices, optical pulse compressors and optical switching devices.     

In-plane oriented CH_3NH_3PbI_3 nanowire suppression of the interface electron transfer to PCBM

One-dimensional nanowire is an important candidate for lead-halide perovskite-based photonic detectors and solar cells. Its surface population, diameter, and growth direction, etc., are critical for device performance. In this research,we carried out a detailed study on electron transfer process at the interface of nanowire CH_3 NH_3 PbI_3(N-MAPbI_3)/Phenyl C61 butyric acid methyl-ester synonym(PCBM), as well as the interface of compact CH_3 NH_3 PbI_3(C-MAPbI_3)/PCBM by transient absorption spectroscopy. By comparing the carrier recombination dynamics of N-MAPbI_3, N-MAPbI_3/PCBM,C-MAPbI_3, and C-MAPbI_3/PCBM from picosecond(ps) to hundred nanosecond(ns) time scale, it is demonstrated that electron transfer at N-MAPbI_3/PCBM interface is less efficient than that at C-MAPbI_3/PCBM interface. In addition, electron transfer efficiency at C-MAPbI_3/PCBM interface was found to be excitation density-dependent, and it reduces with photo-generation carrier concentration increasing in a range from 1.0 × 1018 cm~(-3)–4.0 × 1018 cm~(-3). Hot electron transfer,which leads to acceleration of electron transfer between the interfaces, was also visualized as carrier concentration increases from 1.0 × 10~(18) cm~(-3)–2.2 × 10~(18) cm~(-3).