基于石墨烯的Au纳米颗粒增强染料随机激光

石墨烯和纳米颗粒的复合材料具有新颖的光学和电学特性,被广泛应用于信息传感、光电转换、医学诊断等领域,具有十分广阔的发展前景.虽然石墨烯拥有优异的光电性能,可以实现对随机激光性质的调控,但目前实现特殊结构的石墨烯与金属纳米结构的复合过程复杂繁琐,利用石墨烯有效降低随机激光阈值仍存在挑战.本文利用便捷的化学还原及吸附法制备Au/石墨烯结构,以染料DCJTB为增益介质,使用旋涂法制备了均匀的薄膜样品;研究对比Au纳米颗粒和Au/石墨烯结构随机激光特性,分析了石墨烯的作用机理.研究结果表明,Au/石墨烯复合材料透射峰与增益介质的光致发光光谱峰最为接近,对于染料分子的能级迁跃起到了促进作用.在相同的增益介质中,石墨烯的加入不仅使得光子在无序介质中散射频次增加,促进了增益效果,而且增强了Au纳米颗粒表面的等离子体共振效应.二者相互协同,展现出了优异的随机激光特性,阈值降低至2.8μJ/mm~2;对样品重复测量可得样品的激射重复性较强、品质较高.本研究对促进随机激光应用、实现高性能的光电器件起到了一定的推动作用.     

染料掺杂光子晶体荧光带隙边缘的激射研究

利用激光全息光刻技术, 在重铬酸盐明胶 感光材料中制备了掺杂有机染料的层状光子晶体. 在532 nm纳秒脉冲激光激励下, 样品的荧光光谱表现出良好的带隙特征; 随着抽运能量的增加, 在荧光带隙带边位置获得了激射光, 并进一步研究了光子晶体的带边位置与染料荧光峰的匹配对激射的影响.带边位置越靠近染料的荧光峰, 激射阈值越低, 反之则不易产生激射.该研究为超低阈值光子晶体激光器的发展提供了思路和方法. 关键词： 全息光刻 光子晶体 荧光带隙 低阈值激射     

基于多形貌银纳米颗粒的多波长相干随机激光研究

采用溶剂热法分别制备了球形银纳米颗粒和多形貌银纳米颗粒,其中球形银纳米颗粒具有400 nm的窄带等离激元共振峰,而多形貌银纳米颗粒的共振区间在400～700 nm之间,将它们分别掺入R6G与PVP的混合溶液中,利用旋涂法在玻璃基板上制备银纳米颗粒嵌入染料掺杂聚合物薄膜随机激光器。采用纳秒脉冲激光进行随机激光泵浦实验,实验结果表明球形银纳米颗粒染料掺杂聚合物薄膜只有自发辐射峰,而多形貌银纳米颗粒染料掺杂聚合物薄膜具有线宽<0.8 nm的相干随机激光发射光谱,其阈值为1.9 mJ·cm-2, 这可能是由于银纳米颗粒的等离激元共振区间与R6G的发射光谱重叠,支持局域等离激元效应的形成,明显的局域场增强有效地改善了与附近分子的相互作用,从而激发了更多的辐射光子,促进了高增益的形成。进一步,利用多形貌银纳米颗粒在银纳米颗粒染料掺杂聚合物薄膜中随机分布的特性,通过改变泵浦位置,实现了20 nm范围内的随机激光输出波长的调控,具体输出范围为590.1～610.4 nm。认为这是由于多形貌银纳米颗粒在不同位置的组成和分布不同,改变了表面等离激元的相互作用和光子的散射能力,从而形成不同的增益效应和不同的封闭光振荡路径。此外,考虑到多形貌银纳米颗粒的共振波长较宽,探究了其用于输出其他颜色光的可能性。以与上述银纳米颗粒R6G染料掺杂聚合物薄膜相似的制备方法,制备了多形貌银纳米颗粒掺杂DCJTB染料聚合物薄膜,并且进行随机激光泵浦实验。结果表明,可以有效的产生波长为675 nm,半高宽<0.8 nm的相干红光随机激光,并且阈值仅为0.98 mJ·cm-2。研究结果在宽带可调谐随机激光器研究以及多色随机激光器研究领域具有重要的参考价值。     

光在随机增益介质中的放大

 结合环形腔理论，运用蒙特卡罗法模拟了光子在介质中的随机行走。研究了倍频Nd:YAG（脉宽6 ns，频率20 Hz）脉冲激光器作为泵浦光，在TiO₂ / 若丹明 B有机增益介质中，散射微粒的颗粒密度和泵浦光面积对随机激光器阈值强度的影响。模拟结果表明：随机激光阈值和光子在增益介质中的随机行走路程长度和光子通过边界返回增益区和非增益区的几率有关。随着泵浦光面积的增加，随机激光器阈值降低；增益介质中散射颗粒密度的增加降低了随机激光器的阈值。     

ZnO纳米颗粒受激发射的时间分辨特性

在飞秒脉冲激光激发下,观察到了均匀沉积法获得的ZnO纳米颗粒的受激辐射现象,并从频域和时域两方面研究了ZnO纳米多晶的室温激射特性。氧化锌纳米颗粒中出现激子-激子散射导致的激射阈值为7.2 GW·cm-2,激射模式类似于F-P谐振腔模式,时域谱则表现为寿命曲线中出现快速衰减成分。与荧光的时间衰减曲线不同,P带时间衰减具有对称结构,高斯拟合结果只有几个ps,接近条纹相机的时间分辨率极限。研究ZnO纳米颗粒的受激发射与激光特性对揭示ZnO晶体的内部结构和激子激发态的性质、激光产生的机理等有重要意义。     

CdTe QDs/Au NRs的SERS及光诱导电荷转移研究

控制和利用半导体基底的荧光背景是解决半导体纳米材料应用在表面增强拉曼(SERS)领域的关键问题。该研究通过将具有高荧光背景的碲化镉量子点(CdTe QDs)与贵金属金纳米棒(Au NRs)纳米材料复合,依赖于光激发下CdTe QDs与Au NRs之间的电荷转移,获得了兼具多功能特性(例如表面增强拉曼散射和光催化)的纳米复合材料。在该研究中,将染料分子亚甲基蓝(MB)作为探针分子,CdTe QDs/Au NRs纳米复合材料的之间的电荷转移降低了CdTe QDs的荧光背景,展现出良好的SERS增强性能。     

脉冲激光沉积法制备的ZnO薄膜的低阈值电抽运紫外随机激射

分别采用直流反应溅射法和脉冲激光沉积法在硅衬底上沉积ZnO薄膜, 用X射线衍射、扫描电镜、光致发光谱等手段对两种方法沉积的ZnO薄膜的结晶状态、 表面形貌和光致发光等进行了表征. 进一步对比研究了以上述两种方法制备的ZnO薄膜作为发光层的金属-绝缘体-半导体结构器件的电抽运紫外随机激射. 结果表明, 与以溅射法制备的ZnO薄膜作为发光层的器件相比, 以脉冲激光沉积法制备的ZnO薄膜为发光层的器件具有更低的紫外光随机激射阈值电流和更高的输出光功率. 这是由于脉冲激光沉积法制备的ZnO薄膜中的缺陷更少, 从而显著地减少了紫外光在光散射过程中的光损耗. 关键词： 随机激射 ZnO薄膜 脉冲激光沉积 溅射     

SU-8光栅结构中的液晶激光特性

设计制作了SU-8光栅结构的染料掺杂手性向列相液晶激光器件,在器件正面和侧面均实现了随机激光辐射。将激光染料PM597、手性剂S-811、向列性液晶TEB30A按一定比例均匀混合,注入反平行摩擦处理的液晶盒中,器件的下基板通过光掩模法刻蚀出周期为15μm的光栅。利用532 nm的Nd∶YAG固体脉冲激光器作为泵浦源,器件的侧面既在580~590 nm范围内出现了多个离散分立的随机激光辐射峰,FWHM约0.19 nm,又在579~585 nm范围内出现独立的两个激光辐射峰,FWHM约0.19 nm;在器件正面获得了584~590 nm范围的随机激光辐射谱,FWHM约0.17 nm。加热器件至61℃,液晶相变为各向同性态,器件侧面仍出现了波长约590.60 nm、FWHM约0.24 nm的激光辐射峰。分析得出,液晶盒中引入SU-8光栅结构后,光子同时在液晶分子间多重散射和SU-8光栅中布拉格反射获得反馈放大,两种机制相辅相成。器件侧面出现的独立激光辐射峰主要由SU-8光栅布拉格反射提供反馈放大形成,而器件侧面和正面的随机激光辐射峰主要由液晶分子间多重散射提供反馈放大形成。     

TiO2纳米颗粒对染料掺杂聚合物分散液晶的随机激光辐射特性的影响

随机激光的阈值与体系内的散射强度存在密切关系。高折射率二氧化钛(TiO_2)纳米颗粒与染料掺杂聚合物分散液晶(DDPDLC)均匀混合后,由于液晶微滴与TiO_2纳米颗粒之间的强散射作用,DDPPLC的随机激光具有更低的激光发射阈值,并且随TiO_2纳米颗粒的掺杂浓度而变化;在优化TiO_2纳米颗粒掺杂浓度的基础上,DDPDLC的发射阈值为270μJ/cm~2,线宽下降至0.08nm;温度实验证明了PDLC结构的散射是产生随机激光的主要工作机制,在TiO_2纳米颗粒掺杂后,DDPDLC样品依然保证了良好的可调性。     

金溶胶中邻羟基苯甲酸吸附行为的密度泛函理论与实验研究

研究了邻羟基苯甲酸(OHBA)的常规拉曼散射(NRS)光谱以及其吸附在Au纳米颗粒上的表面增强拉曼散射(SERS)光谱。以氯金酸为原料,柠檬酸三钠为还原剂,用化学还原法制备了球形的金纳米粒子溶胶,采用激光显微拉曼光谱仪(激发波长为785 nm),测定OHBA分子的NRS光谱及其吸附在Au纳米颗粒上的SERS光谱。同时,应用密度泛函理论(DFT),在B3LYP/6-31+G**(C,H,O)/LANL2DZ(Au)水平上,对OHBA分子进行了结构优化,在此基础上计算了OHBA分子的NRS光谱以及其吸附在Au纳米颗粒上两种不同吸附构型下的SERS光谱,并和实验值进行比较。结果表明,OHBA分子通过羧基吸附构型的计算值比通过羟基吸附构型的计算值与实验值符合的更好。最后,利用GaussView可视化软件对其振动模式进行了全面归属。通过对邻羟基苯甲酸分子拉曼谱峰的详细指认能够得出：Au溶胶中的邻羟基苯甲酸分子,是通过羧基倾斜地吸附在Au纳米颗粒表面的。可视化软件直观形象地展示出了该分子的结构特征和分子基团振动情况,对其振动峰位的归属提供了重要依据。本文工作对推进邻羟基苯甲酸在生物医药等领域进一步的应用具有重要作用。     

An efficient lasing action from pyrromethene 556 dye-doped organically modified silicates

Organically modified solid-state silicates (ORMOSILS) doped with a new laser dye 1,3,5,7,8-pentamethylpyrromethene-2,6-disulfonate-BF₂ complex (pyrromethene 556) have been synthesized by a sol-gel method and the compositional effects on pore characteristics, fluorescence and lasing properties have been investigated. It is found that the use of dimethylsulfoxide and γ-glycidoxypropyltrimethoxysilane could greatly change the structure properties of sol-gel derived ORMOSILS cage, and thus the fluorescence and lasing properties of the materials could improve significantly. A successful laser oscillation from this dye-doped ORMOSILS sample has been achieved upon pumping with a Q-switched frequency-doubled Nd:YAG laser at 532 nm. A slope efficiency of 54% with a useful lifetime greater than 10,000 shots has been demonstrated at a pump repetition rate of 1 Hz and a pump intensity of 1 J/cm² by using the new ORMOSILS cage on our newly designed laser system. Our results have shown that it is possible to obtain a high-efficiency with a long-lifetime for a compact new laser device by low cost dye-doped solid-state ORMOSILS.     

Nanocomposite ZnO/Au formation by pulsed laser irradiation

The ZnO/Au nanocomposite formation involves synthesis of Au and ZnO colloidal solutions by 532 nm pulse laser ablation of metal targets in deionized water followed by laser irradiation of the mixed colloidal solution. The transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) images show evolution of spherical particles into ZnO/Au nanonetworks with irradiation time. The formation mechanism of the nanonetwork can be explained on the basis of near resonance absorption of 532 nm irradiation by gold nanoparticles which can cause selective melting and fusion of gold nanoparticles to form network. The ZnO/Au nanocomposites show blue shift in the ZnO exciton absorption and red shift in the Au plasmon resonance absorption due to interfacial charge transfer.     

Rapid synthesize of gold nanoparticles by microwave irradiation method and its application as an optical limiting material

We present rapid synthesis of gold nanoparticles by microwave irradiation method. Sample with average particle size 7.7 nm is obtained from TEM. Linear and nonlinear optical studies of the prepared samples are discussed. Reverse saturable absorption (RSA) at longitudinal surface plasmon resonance (SPR) in gold nanoparticles (Au NPs) have been observed using Z-scan and transient absorption techniques with 532 nm laser pulses. Such RSA behavior makes Au NPs an ideal candidate for optical limiting applications.     

Random lasing in dye-doped polymer dispersed liquid crystal film

A dye-doped polymer-dispersed liquid crystal film was designed and fabricated,and random lasing action was studied.A mixture of laser dye,nematic liquid crystal,chiral dopant,and PVA was used to prepare the dye-doped polymer-dispersed liquid crystal film by means of microcapsules.Scanning electron microscopy analysis showed that most liquid crystal droplets in the polymer matrix ranged from 30 μm to 40 μm,the size of the liquid crystal droplets was small.Under frequency doubled 532 nm Nd:YAG laser-pumped optical excitation,a plurality of discrete and sharp random laser radiation peaks could be measured in the range of 575–590 nm.The line-width of the lasing peak was 0.2 nm and the threshold of the random lasing was 9 m J.Under heating,the emission peaks of random lasing disappeared.By detecting the emission light spot energy distribution,the mechanism of radiation was found to be random lasing.The random lasing radiation mechanism was then analyzed and discussed.Experimental results indicated that the size of the liquid crystal droplets is the decisive factor that influences the lasing mechanism.The surface anchor role can be ignored when the size of the liquid crystal droplets in the polymer matrix is small,which is beneficial to form multiple scattering.The transmission path of photons is similar to that in a ring cavity,providing feedback to obtain random lasing output.     

Two-photon pumping of random lasers by picosecond and nanosecond lasers

We experimentally demonstrated two-photon pumping of random lasers using picosecond and nanosecond pump lasers. The picosecond laser pumping experiment was performed with 400 ps laser pulses at 770 nm, and the gain media was a Coumarin 480D dye solution doped with TiO₂ nanoparticles. Onset of laser action was observed at a pump laser pulse energy below 500 μJ. The nanosecond laser pumping experiment was performed with 7 ns laser pulses at 1064 nm, and the gain media was a Rhodamine 640 dye solution doped with TiO₂ nanoparticles. Onset of laser action was observed at a pump laser energy ∼18 mJ. Our results suggest that there exists an optimal pulse duration of the pumping laser in two-photon pumped random lasing that leads to minimum photodamage of the gain media and still keeps a high pumping efficiency. PACS 33.50.Dq; 42.55.Mv; 42.55.Zz     

Laser-assisted generation of gold nanoparticles and nanostructures in liquid and their plasmonic luminescence

Nanoparticles (NPs) and surface nanostructures (NS) are produced via laser ablation of a bulk gold target in liquid using second harmonics of 10 ps Nd:YAG laser (532 nm) with repetition rate of 50 kHz. The morphology and plasmon photoluminescence (PL) properties of obtained nanoscale objects are described. Transmission electron microscopy and field emission scanning electron microscopy are used for morphology characterization of NPs and NS, respectively. Plasmon PL of both gold NPs and NS is experimentally studied using the third harmonics of the Nd:YAG picosecond laser (355 nm) as a pump. The wavelength of intensity maximum of PL of Au NPs colloidal solution virtually coincides with the position of Au NPs plasmon absorption peak. Real-time excitation of both plasmon PL and Raman scattering of surrounding liquid by picosecond laser pulses in aqueous colloidal solution is also investigated. The efficient cross section of plasmon PL of Au NPs colloid is evaluated using Raman scattering of water as a comparative parameter. The results are in good agreement with values obtained in previous works. Plasmon PL from self-organized NS on the Au surface produced via laser ablation is observed for the first time. Its spectrum is compared to PL spectra of both aqueous colloidal solutions of NPs and of NPs deposited on a Si wafer. The obtained experimental data are discussed with reference to the band structure of bulk Au.     

Dye-concentration-dependent lasing behaviors and spectral characteristics of cholesteric liquid crystals

The laser behavior and spectral changes occurring in cholesteric liquid crystals with varying dye-doped concentrations were investigated when pumped at 532 nm. It was found that the long-wavelength band edge and the laser line exhibit a blue shift over 21 nm with increasing dye concentration. The circularly polarized fluorescence spectra were examined, and the location of the sense reversion of circular polarization was determined to coincide well with the discrete lasing lines. The blue shift can be ascribed to the decrease in average refractive index and pitch of the dye-doped cholesteric liquid crystals. The dependence of the slope efficiency and threshold energy on the dye concentration can be attributed to the shift in photonic stopband and the change in penetration depth of excitation. The temperature and incident angle of pumping beam also have a significant impact on the lasing properties. The optimal dye concentration is found to be 0.5 wt% at 30.5 °C with an incident angle of 10°. The laser emission located at 601.4 nm with slope efficiency of 4 % was achieved above the threshold energy of 14.3 μJ.