基于拉曼晶体的多波长激光技术研究进展

利用受激拉曼散射效应,以拉曼晶体作为介质,可产生同轴输出的多波长激光信号,该种激光器具有结构紧凑、脉冲能量高和波长可调谐等特点,在全色激光成像与显示、光电对抗等领域有着重要的应用前景。本文介绍了受激拉曼散射基本原理和常用拉曼激光器结构,研究了国内外基于拉曼晶体的多波长激光技术的研究进展,总结了利用受激拉曼散射产生多波长激光存在的不足。针对目前受激拉曼散射高阶散射光较难生成,生成的多波长激光信号覆盖谱段较窄,输出功率较低,调谐方式单一等问题,提出了今后多波长激光技术发展方向。     

激光诱导等离子体对水团簇受激拉曼散射的影响

利用532 nm脉冲激光作用于水分子,研究其受激拉曼Stokes和anti-Stokes散射.实验表明:激光束经过聚焦后,在能量为4 mJ时,水分子产生等离子体;在泵浦激光能量由5 mJ增加到15 mJ的过程中,水分子OH键伸缩振动的受激拉曼Stokes散射光强逐渐增大、受激谱带宽度逐渐加宽,并且受激拉曼Stokes散射中心波长呈现蓝移趋势;当能量为15 mJ时,产生了OH键伸缩振动的受激拉曼anti-Stokes散射光.利用激光诱导等离子体增强水分子团簇的受激拉曼散射理论解释了以上现象,实验与理论符合地很好.     

BaWO4晶体的受激拉曼散射

受激拉曼散射(SRS，Stimulated Raman scattering)属三阶非线性光学效应，它利用非弹性散射使入射激光产生一定频移，从而获得新波长激光，是一种有效的激光变频方式。我们利用波长为532nm，频率为10Hz的皮秒脉冲激光作为抽运源，采用单次通过方法实现了BaWO4晶体的受激拉曼散射。     

纳米晶须SiC受激拉曼光散射性质研究

本文讨论了分布在SiO2中纳米碳化硅晶须的拉曼受激散射现象。在用非晶态SiO2包覆的纳米碳化硅材料中，随纳米碳化硅含量增高，激发阈值降低，激发强度增大。这是由于量子限域效应所致。随纳米晶须的半径降低，激发阈值降低；并且随激发波长的增长，蓝移减小。     

高功率激光单模光纤远距离传输实验研究

受激拉曼散射（SRS）效应是制约高功率激光单模光纤远距离传输应用的瓶颈因素。采用1550nm非偏振连续波单模光纤激光器作为光源, 通过实验研究了不同注入光功率条件下经过103km单模光纤传输所产生的受激拉曼散射现象,运用级联长周期光纤光栅带阻滤波器进行了受激拉曼散射抑制实验,并进行了相应的理论分析。提出了光纤受激拉曼散射抑制有效性判据,实验演示了受激拉曼散射现象,验证了级联长周期光纤光栅带阻滤波器对受激拉曼散射抑制的有效性。     

纳米SiC多晶膜的受激喇曼散射研究

讨论了无序结构的纳米SiC多晶膜中存在的喇曼受激辐射现象。研究结果表明,在连续波激发时,喇曼受激散具有空间均匀辐射或阈值低的特点。纳米晶无序结构强烈的散射作用形成的扩散－反馈机制和纳米晶大的喇曼散射截面发生了低阈值受激散射。     

用皮秒KrF激光脉冲测量气体中的拉曼散射阈值

我们用ＫｒＦ激光系统的２０ｐｓ脉冲测量了多种气体的受激拉曼散射阈值。用能量最高为２００μＪ的脉冲观察到在５ａｔｍ以下甲烷、氢、氨和六氟化硫的受激散射。同样条件下，在氧或氮中则观察不到受激散射。测量结果与瞬态受激拉曼散射的理论符合很好。     

包层抽运掺镱光纤激光器中受激拉曼散射和受激布里渊散射效应

高功率光纤激光器大多选用掺镱双包层光纤作为增益介质,由于光纤尺寸较小,极易在光纤谐振腔中产生受激布里渊散射、受激拉曼散射效应。包层掺镱双包层光纤激光器中一旦发生受激拉曼散射和受激布里渊散射效应,其产生高强度信号成为高功率光纤激光器的主要噪声来源,影响激光输出的特性和稳定性。对包层抽运掺镱光纤激光器中的受激布里渊散射和受激拉曼散射进行了实验研究,在单模双包层光纤中观察到受激布里渊散射和受激拉曼散射。实验结果表明,在光纤谐振腔中,抽运方式、谐振腔输出镜损耗、受激瑞利散射对受激布里渊散射的影响较大,尤其是受激瑞利散射为谐振腔提供了附加反馈,不仅压窄激光信号的线宽,而且使得受激布里渊散射的阈值迅速降低。     

脉冲激光退火纳米碳化硅薄膜的拉曼散射研究

采用XeCl准分子激光实现了碳化硅薄膜的脉冲激光晶化,对退火前后薄膜样品拉曼散射谱特征进行了分析,探讨了激光能量密度对纳米碳化硅薄膜结构和物相特性的影响.结果显示晶态纳米碳化硅薄膜的拉曼散射峰相对体材料的特征峰显著宽化和红移,并显示了伴随退火过程存在着硅和碳的物相分凝现象.随着激光能量密度的增大,薄膜的晶化度提高,晶化颗粒增大,而伴随的分凝程度逐渐减小.     

新型受激拉曼散射激活晶体光谱和固态受激拉曼散射激光器

拉曼散射发现后不久 ,在 G.Placzek建立的一般理论中 ,能找到拉曼散射受激过程的内涵。然而 ,光的受激拉曼散射直到激光辐射源出现以后才被发现。俄国科学家 N.G.Basov和 A.M.Prokhorov为激光的发明做出的贡献赢得了全世界的承认。 1 96 2年 ,E.J.Woodbury和 W.K.Ng用硝酸基苯克尔吸收池研究红宝石激光器 Q开关的时候 ,发现受激拉曼散射效应。在激光光谱中 ,他们观测到一个高强度的相对于激光频率频移1 34 5 cm- 1 的红外辐射成分 ,这种现象后来归结于受激拉曼散射效应。从那以后 ,这个令人着迷的非线性现象及其在激光光谱学和激光工…     

纳米4-H碳化硅薄膜的掺杂现象

对纳米晶SiC薄膜进行了P和B的掺杂,B掺杂效率比P高,其暗电导预前因子与激活能遵守Meyer-Neldel规则,并有反转Meyer-Neldel规则出现.掺杂效率比非晶态碳化硅薄膜高是纳米碳化硅薄膜的特点之一.非晶态中的隧穿和边界透射对输运有一定贡献.     

Synthesis and Lasing Properties of Highly Ordered CdS Nanowire Arrays

Highly ordered large‐area arrays of wurtzite CdS nanowires are synthesized on Cd‐foil substrates via a simple liquid reaction route using thiosemicarbazide and Cd foil as the starting materials. The CdS nanowires are single crystals growing along the [001] direction and are perpendicular to the surface of the substrate. The characteristic Raman peaks of CdS are red‐shifted and show asymmetric broadening, which is ascribed to phonon confinement effects arising from the nanoscale dimensions of the nanowires. Significantly, the uniform CdS nanowire arrays can act as laser cavities in the visible‐light range, leading to bandgap lasing at ca. 515 nm with obvious modes. The high density of nuclei and the preferential growth direction induce the formation of aligned CdS nanowires on the metal substrate.     

Method for Preparation of Semiconductor Quantum‐Rod Lasers in a Cylindrical Microcavity

An efficient method for preparation of semiconductor quantum rod films for robust lasing in a cylindrical microcavity is reported. A capillary tube, serving as the laser cavity, is filled with a solution of nanocrystals and irradiated with a series of intense nanosecond laser pulses to produce a nanocrystal film on the capillary surface. The films exhibit intense room‐temperature lasing in whispering‐gallery modes that develop at the film–capillary interface as corroborated from the spacing detected for the lasing modes. Good lasing stability is observed at moderate pump powers. The method was applied successfully to several quantum‐rod samples of various sizes.     

Photoluminescence origin of nanocrystalline SiC films

The nanocrystalline SiC films were prepared on Si then annealed at 800℃ and 1 000℃ for 30 minutes （111） substrates by rf magnetron sputtering and in a vacuum annealing system. The crystal structure and crystallization of as-annealed SiC films were determined by the Fourier transform infrared （FIR） absorption spectra and the X-ray diffraction （XRD） analysis. Measurement of photoluminescence （PL） of the nanocrystalline SiC （nc-SiC） films shows that the blue light with 473 nm and 477 nm wavelengths emitted at room temperature and that the PL peak shifts to shorter wavelength side and the PL intensity becomes stronger as the annealing temperature decreases. The time-resolved spectrum of the PL at 477 nm exhibits a bi-exponential decay process with lifetimes of 600 ps and 5 ns and a characteristic of the direct band gap. The strong blue light emission with short PL lifetimes suggests that the quantum confinement effect of the SiC nanocrystals resulted in the radiative recombination of the direct optical transitions.     

Laser-annealing-induced features of the Raman spectra of quartz/Si and glass/Si structures

The effect of laser radiation on the characteristics of amorphous silicon films on glassy or quartz substrates are studied by Raman spectroscopy. It is established that an increase in the laser-treatment power yields a phase transition from amorphous silicon to nanocrystalline silicon. The variation in the relation between the nanocrystalline and amorphous silicon fractions in the films is described in the context of the critical impact model.     

Lasing and stimulated emission in π-conjugated polymers

Recent studies of lasing and stimulated emission in luminescent π-conjugated polymers performed by our group are presented. Optical properties of cylindrical high-Q polymer microcavities are discussed. The emission spectra of plastic microring and microdisk lasers are measured and analyzed. Light-emitting polymer microdiodes are demonstrated as possible candidates for electrically driven plastic lasers. In addition, two unusual regimes of stimulated emission characterized by narrow laser-like spectral lines are demonstrated in thin waveguiding polymer films. These regimes may be associated with random optical feedback introduced by light scattering inside the polymer films and amplified Raman scattering, respectively     

Sol-gel ZnO-SiO/sub 2/ composite waveguide ultraviolet lasers

Random laser action with coherent feedback is realized in ZnO-SiO/sub 2/ composite films, which consists of ZnO clusters embedded in SiO/sub 2/ dielectric matrix prepared by sol-gel technique. The films are deposited on silicon substrate with a SiO/sub 2/ buffer layer to form a waveguide structure. Ultraviolet lasing at room temperature is observed from the composite films with ZnO : SiO/sub 2/ molar ratio varying between 1 : 5 and 1 : 30. The corresponding lasing wavelength and linewidth under 355-nm optical excitation are found to be /spl sim/388 nm and less than 0.6 nm, respectively. Our experiment has shown that the proper control of light confinement inside the random cavities leads to coherent random lasing.     

Threshold Reduction in Polymer Lasers Based on Poly(9,9‐dioctylfluorene) with Statistical Binaphthyl Units

We introduce novel statistical copolymers of poly(9,9‐dioctylfluorene), PFO, which contain various concentrations of 6,6′‐(2,2′‐octyloxy‐1,1′‐binaphthyl) spacer groups. We demonstrate that, owing to the large dihedral angle (> 60°) between neighboring naphthalene units, we could hinder the formation of the highly ordered β‐phase in thin films of the copolymers. In low‐temperature photoluminescence measurements, the typical signature of the PFO β‐phase at 442 nm is no longer observed for copolymers with a binaphthyl concentration of about 12 %. Moreover, the optical properties of the copolymers resembled those of the glassy α‐phase PFO. Second‐order distributed feedback (DFB) lasers based on thin films of the homopolymer PFO showed a minimum lasing threshold of 11.7 μJ cm^–2 (λ_max = 452 nm, excitation at λ = 337 nm with 500 ps pulses). With increasing binaphthyl concentration in the copolymer backbone, the lasing threshold steadily decreased to 3 μJ cm^–2 for a binaphthyl concentration of about 12 %. Therefore, our novel copolymers provide a vast improvement for PFO‐based optoelectronics.