Negative dispersion mirrors in Ta2O5/: SiO2 for femtosecond Ti : Sapphire lasers by using gires-tournois interferometers

We design and experimentally demonstrate some negative dispersion mirrors with optimized Gires-Tournois interferometers. The mirror structure is composed of 38 alternating Ta2O5 and SiO2 layers and could be regarded as two sections: high-reflectivity section consisting of a series of quarter-wavelength optical thickness stacks and negative-dispersion section consisting of only 13 layers. The designed mirrors exhibit the expected performance. These mirrors were fabricated by using ion beam sputtering. By adopting such mirrors, dispersion of a mode-locked femtosecond Ti:sapphire laser has been compensated for mostly. With two series of the mirrors, 32 fs and 15 fs pulses have been obtained respectively.

Laser-induced damage of Ta2O5/S1O2 narrow-band interference filters under different 1064 nm Nd : YAG laser modes

The laser-induced damage (LID) behavior of narrow-band interference filters was investigated with a Nd:YAG laser at 1064 nm under single-pulse mode and free-running mode. The absorption measurement of such coatings was performed with surface thermal lensing (STL) technique. The damage morphologies under the two different laser modes were also studied in detail. It was found that all the filters exhibited a pass-band-center-dependent absorption and laser-induced damage threshold (LIDT) behavior, but the damage morphologies were diverse. The explanation was given with the analysis of the electric field distribution and the operational behavior of the irradiation laser. (c) 2005 Elsevier B.V. All rights reserved.

Ta2O5／SiO2硬膜双腔滤光片激光损伤特性研究

研究了Ta2O5／SiO2硬膜双腔干涉滤光片带内、带边及带外的吸收和激光损伤特性。实验发现，对于作用激光，带通滤光片的驻波场分布、吸收率和损伤阈值在带内、带边和带外的响应特性对作用激光波长均呈现出明显的选择性。根据实验结果，结合滤光片的驻波场分析，给出了带通滤光片的损伤机理。

Ta2O5膜在532, 800 和 1064 nm波长的n-on-1体系下的激光损伤阈值研究

Ta2O5膜采用传统的电子束蒸发方法制备，并在氧气中673 K的条件下进行了退火12 h处理。首先在1-on-1体系下对Ta2O5膜进行了532和1064 nm波长下的激光损伤阈值（LIDT） 研究，然后在n-on-1体系下分别对532，800和1064 nm三种波长下的激光损伤阈值进行了研究。结果表明在n-on-1体系下Ta2O5膜在532和1064 nm波长下的阈值均高于1-on-1体系下的阈值。此外，在n-on-1体系下，薄膜的阈值随波长增加而增大。同时发现，在氧气中进行退火对Ta2O5膜的光学性能

SiO2保护层和退火对Ta2O5薄膜激光损伤阈值影响的对比研究

Ta2O5薄膜采用传统的电子束蒸发方法沉积在BK7基底上。文中对SiO2保护层和退火对Ta2O5薄膜的激光损伤阈值的不同影响进行了研究。结果表明，SiO2保护层不会影响薄膜内的电场分布，薄膜微结构和微缺陷密度，但是会使薄膜的吸收稍微增大；而退火对降低薄膜的微缺陷密度和吸收较有效。SiO2保护层和退火都有利于提高Ta2O5薄膜的抗激光损伤能力，并且退火对提高阈值的影响更为明显。此外，采用SiO2保护层和退火结合的方法，获得了具有最大激光损伤阈值的薄膜。

Influence of different substrates on laser induced damage thresholds at 1064 nm of Ta2O5 films

Ta2O5 films are prepared on Si, BK7, fused silica, antireflection (AR) and high reflector (HR) substrates by electron beam evaporation method, respectively. Both the optical property and laser induced damage thresholds (LIDTs) at 1064 nm of Ta2O5 films on different substrates are investigated before and after annealing at 673 K for 12h. It is shown that annealing increases the refractive index and decreases the extinction index, and improves the O/Ta ratio of the Ta2O5 films from 2.42 to 2.50. Moreover, the results show that the LIDTs of the Ta2O5 films are mainly correlated with three parameters: substrate property, substoichiometry defect in the films and impurity defect at the interface between the substrate and the films. Details of the laser induced damage models in different cases are discussed.

Laser induced damage threshold at 355 and 1064 nm of Ta2O5 films of different phases

Ta2O5 films are deposited on fused silica substrates by conventional electron beam evaporation method. By annealing at different temperatures, Ta2O5 films of amorphous, hexagonal and orthorhombic phases are obtained and confirmed by x-ray diffractometer ( XRD) results. X-ray photoelectron spectroscopy ( XPS) analysis shows that chemical composition of all the films is stoichiometry. It is found that the amorphous Ta2O5 film achieves the highest laser induced damage threshold ( LIDT) either at 355 or 1064 nm, followed by hexagonal phase and finally orthorhombic phase. The damage morphologies at 355 and 1064 nm are different as the former shows a uniform fused area while the latter is centred on one or more defect points, which is induced by different damage mechanisms. The decrease of the LIDT at 1064nm is attributed to the increasing structural defect, while at 355nm is due to the combination effect of the increasing structural defect and decreasing band gap energy.

Optical properties and microstructure of Ta2O5 biaxial film

This study investigates the optical properties and microstructure of Ta2O5 film deposited with the glancing angle deposition technique. The tilted nanocolumn microstructure, examined with scanning electron microscopy, induces the optical anisotropy of thin film. The optical properties of thin film are characterized with an inverse synthesis method. Based on the Cauchy model, the dispersion equations of optical constants of film are determined from the transmittance spectra measured at normal and oblique incidence over 400-800 nm. The starting values derived with an envelope method quicken the optimization process greatly. The dispersion of the principal indices N-1, N-2, and N-3 and the thickness d of thin film are presented statistically. A good agreement between the measured optical properties and theoretical calculation is obtained, which validates the model established for thin film produced by glancing angle deposition. (C) 2008 Optical Society of America

Investigation of annealing effects on the laser-induced damage threshold of amorphous Ta2O5 films

Ta2O5 films were deposited using the conventional electron beam evaporation method and then annealed at temperatures in the range 373-673 K. Chemical composition, scattering and absorption were examined by X-ray photoelectron spectroscopy (XPS), total integrated scattering (TIS) measurement and the surface thermal lensing (m) technique, respectively. The laser-induced damage threshold (LIDT) was assessed using the output from an Nd:YAG laser with a pulse length of 12 ns. The results showed that the improvement of the LIDT after annealing was due to the reduced substoichiometric and structural defects present in the film. The LIDT increased slightly below 573K and then increased significantly with increase in annealing temperature, which could be attributed to different dominant defects. Moreover, the root mean square (RMS) roughness and scattering had little effect on the LIDT, while the absorption and the LIDT were in accord with a general relation. (c) 2008 Elsevier Ltd. All rights reserved.

Effect of the work function of gate electrode on hysteresis characteristics of organic thin-film transistors with Ta2O5/polymer as gate insulator

Organic thin-film transistors (OTFTs) using high dielectric constant material tantalum pentoxide (Ta2O5) and benzocyclobutenone (BCBO) derivatives as double-layer insulator were fabricated. Three metals with different work function, including Al (4.3 eV), Cr (4.5 eV) and Au (5.1 eV), were employed as gate electrodes to study the correlation between work function of gate metals and hysteresis characteristics of OTFTs. The devices with low work function metal Al or Cr as gate electrode exhibited high hysteresis (about 2.5 V threshold voltage shift). However, low hysteresis (about 0.7 V threshold voltage shift) OTFTs were attained based on high work function metal Au as gate electrode.

Bismuth-activated luminescent materials for broadband optical amplifier in WDM system

New broadband near infrared luminescence covering the whole work windows (1260-1625 nm) of the current wavelength division multiplexing (WDM) system was found from bismuth-activated M2O-Al2O3-SiO2 (M = Li, Na) and Li2O-Ta2O5-SiO2 glasses at room temperature in the case of 808 nm-laser excitation. But the near infrared luminescence mechanism of the bismuth-activated glasses is not well understood up to now. The figure-of-merits of bandwidth and gain of the glasses are better than those of Er3+-doped silicate glasses and Ti3+ doped sapphire, implying they are the promising gain-medium candidates for the broadband amplifiers and the widely tunable laser sources. (c) 2007 Elsevier B.V. All rights reserved.

飞秒脉冲激光器中色散补偿膜的设计

阐述了用光学薄膜进行色散补偿的基本原理，介绍了设计的基本过程．根据Ti：Sapphire飞秒激光器中腔内色散补偿的要求，设定了色散补偿目标，通过计算机优化，得到了一种40层的Ta2O5／SiO2介质膜系．该膜系能在720～870am范围获得大于99．5％的反射率，在510～550nm获得大于90％的透射率，在740～850nm提供较平滑的-40fs^2的群延迟色散．这样的结果经过7次反射后，可以补偿5-mm Ti：sapphire晶体产生的绝大部分群延迟色散。