Infrared energy transfer in Tm3+:LiNbO3

The infrared optical properties of LiNbO₃ crystals doped with variable thulium concentrations have been studied. In particular, the infrared emissions that appear after direct pumping to the ³H₄ multiplet (795 nm) have been analyzed. The emission spectra, as well as the lifetimes, show that an efficient energy transfer between ³F₄ and ³H₄ levels takes place. The concentration dependence of the lifetimes has been studied and the transfer probability has been characterized.     

Modelling of Tm3+-doped LiNbO3 Waveguide Lasers

In this work, CW laser operation of Tm³⁺-doped LiNbO₃ channel waveguides has been modelled. The model is based on time dependent laser rate equations coupled with the laser signal and pump photon flux equations. Steady state solutions for the population densities, pump and signal powers are obtained by using finite difference discretization of the active volume. The effects of spectroscopic parameters such as concentration dependent cross-relaxation and excess waveguide loss have been analyzed. We demonstrate good agreement with experimental data previously reported in Zn-diffused LiNbO₃:Tm³⁺ channel waveguide lasers. It is shown also that laser performance can be substantially improved by optimizing the cavity length.     

Polarized emission and absorption cross-section calculation in LiNbO3:Tm3+

Absorption and emission cross-sections are essential spectroscopic parameters that determine the performance of rare-earth solid state lasers and amplifiers. A precise characterization of such parameters is of great interest in order to proceed with the necessary optimization stage which will assure the development of photonic devices with high efficiency. In particular, in order to model Tm³⁺ infrared lasers under 800 nm pumping, the absorption and emission cross-sections associated with optical transitions involving the ³H₄ and ³F₄ manifolds are needed. In this work, the classical McCumber theory is applied to evaluate the polarized absorption and emission cross-sections of Tm³⁺ ions in LiNbO₃.     

Excitation energy transfer between Er3+ and Tm3+ in LiNbO3

Conclusions Nonradiative energy transfer between similar and different rare-earth ions in LiNbO₃ has been evidenced for the first time. The contribution of nonradiative energy transfer to the decay of the¹G₄ and³H₄ levels of thulium and the⁴S_3/2 and⁴I_13/2 levels of erbium is significant for activator concentrations of the order of several thousand ppm in LiNbO₃. The excitation of higher-lying levels of both activators is transferred faster in codoped crystal to the lowest luminescent levels, from which the luminescence in the 1–2-μm spectral region occurs. This accelerated relaxation is advantageous for broadband optical pumping since the contribution of radiative transitions competing with the³F₄−³H₆ laser transition of thulium is reduced. On the other hand, it appears that erbium ions will influence adversely the efficiency of a laser-diode-pumped LiNbO₃∶Er, Tm laser operating near 1.8 μm since the part of the³H₄ excitation transferred to erbium ions is emitted from the long-lived⁴I_11/2 level of Er³⁺. Furthermore, the⁴I_13/2 excitation is not transferred fully to thulium ions and is lost in⁴I_13/2−⁴I_15/2 emission. Lithium niobate singly doped with thulium is more promising since the cross-relaxation process that transfers the excitation from the³H₄ level to the³F₄ level appears to be efficient in this host. Published in Zhurnal Prikladnoi Spektroskopii, Vol. 62, No. 5, pp. 125–133, September–October, 1995.     

Upconversion white-light emission in Ho3+/Yb3+/Tm3+ codoped LiNbO3 polycrystals

Ho³⁺/Yb³⁺/Tm³⁺ codoped LiNbO₃ polycrystals exhibiting upconversion white-light under 980 nm excitation have been successfully fabricated by the high temperature solid-state reaction method. CIE coordinate of the Ho³⁺/Yb³⁺/Tm³⁺/LiNbO₃ polycrystal is (0.34, 0.35), which is very close to the standard equal energy white-light illuminate (0.33, 0.33). Efficient green, red, and blue upconversion emissions have been observed. The luminescent decay dynamics are studied, and rate equations for the blue, green, and red emissions are set up to analyze the upconversion luminescence mechanism. The present results demonstrate that the competition between the linear decay and the upconversion process for the depletion of the intermediate excited states plays an important role in upconversion mechanism. The LiNbO₃ with upconversion white-light will be a promising luminous material.     

Eu3+掺杂LiNbO3晶体的变温发射光谱特征

在488nFn波长光的激发下，研究了用坩埚下降法生长的掺杂Eu^3 离子的下部与上部LiNbO3晶体从77到600K的变温发射光谱特征。实验结果表明，对于生长后期的上部晶体，随着温度的升高，其发光强度总体增强；然而对于生长初期的下部晶体，随着温度的升高，其发光强度首先增强，到540K时达最大值，然后随着温度的升高，其发光强度反而下降。用热激活效应、声子辅助吸收效应与热猝灭效应解释了晶体上部与下部的发光特征。     

Tm3+离子掺杂氟铝酸盐玻璃红外及上转换光谱性质

制备了Tm^3 掺杂氟铝酸盐玻璃,以实测的吸收光谱使用Judd-Ofelt理论详细计算了Tm^3 在氟铝酸盐玻璃中的光谱参数,在此基础上研究了Tm^3 离子掺杂氟铝酸盐玻璃的上转换光谱性质和红外光谱性质。研究表明：在800nm激光二级管激发下,1.45μm（^3F4→^3H4)荧光浓度猝灭要明显强于1.77μm（^3H4→^3H6)荧光;在655nm激发下上转换荧光主要是由单个Tm^3 离子的步进二光子吸收过程所致。     

Tm3+掺杂Ge-Ga-Se玻璃中红外发光特性与多声子弛豫分析

用熔融急冷法制备了系列不同Tm3+掺杂浓度的Ge30Ga5Se65玻璃样品,测试了样品折射率、拉曼光谱、吸收光谱以及800nm激光泵浦下的近红外及中红外波段荧光光谱和荧光寿命。用Judd-Ofelt理论计算了Tm3+在Ge30Ga5Se65玻璃中的强度参数Ωi(i=2,4,6)、自发辐射跃迁概率A、荧光分支比β和辐射寿命τrad等光谱参数。讨论了800nm激光泵浦下掺杂1Wt%Tm3+样品在近红外1.23,1.48和1.8μm处的发光特性及各量子效率,研究了800nm激光泵浦下的样品中红外荧光特性与掺杂浓度之间的关系,计算了常温下该基质玻璃中Tm3+:3H5→3F4跃迁对于3.8μm处的多声子弛豫速率Wmp和Ge30Ga5Se65玻璃基质中多声子弛豫常数W(0)和电子-声子结合常数α值。结果表明Ge30Ga5Se65硒基玻璃较低的声子能量可以大大降低稀土离子能级间跃迁的多声子弛豫概率,从而提高中红外荧光发光效率,因此硒基玻璃作为稀土离子的掺杂基质材料对实现中红外荧光输出是非常有利的。     

YLiF4: Er3+, Tm3+, Yb3+中Tm3+浓度对上转换发光的影响

利用水热法合成了YLiF4: Er3 , Tm3 , Yb3 , 其中Er3 和Yb3 的浓度保持固定不变, 分别为1 mol%和1.5 mol%, Tm3 浓度变化范围是2 mol%～8 mol%. 在这种共掺杂体系中, 同时观察到了Er3 , Tm3 和Yb3 的吸收, 且Tm3 的吸收随着其浓度的增强而增强. 在980 nm光的激发下, 当Tm3 浓度很小时, 这种材料的上转换发光为白光. 其中蓝光主要来源于Tm3 的激发态1G4到基态3H6的跃迁, 绿光来源于Er3 的4S3/2和2H11/2到基态4I15/2的跃迁, 红光既来源于Tm3 的1G4→3F4的跃迁, 也来源于Er3 的4F9/2→4I15/2的跃迁. 并且这种上转换发光强度随着Tm3 浓度的增强而降低, 但对应不同能级跃迁的发光强度降低的幅度不同, 这是因为Er3 和Tm3 之间的相互作用.     

Spectroscopic properties and exicted-state relaxation dynamics of Er3+ in LiNbO3

Nonequivalent Er³⁺ sites in LiNbO₃ crystals have been evidenced by low-temperature optical absorption measurements. The radiative transition rates for the ⁴ S _3/2, ⁴ F _9/2, ⁴ I _9/2, ⁴ I _11/2, and ⁴ I _13/2 levels have been evaluated by the Judd-Ofelt method, and the contribution of multiphonon relaxation to the decay of excited states has been estimated. The quantum efficiency of the ⁴ I _11/2 level has been estimated to be 6% only, thus the excitation of levels lower than ⁴ S _3/2 is expected to populate efficiently the ⁴ I _13/2 level.     

Low-threshold continuous-wave Tm3+:YAlO3 laser

We describe a continuous-wave, low-threshold Tm:YAlO₃ (Tm:YAP) laser operating at 1945 nm with incident threshold pump powers in the 10–20 mW range. The z-cavity containing a 2-mm-long Tm:YAP crystal with 4 at.% Tm³⁺ concentration was end pumped by a continuous-wave Ti:sapphire laser at 795 nm. Tight focusing of the pump and the laser beams enabled low-threshold operation. The power performance of two different cavity configurations with 5-cm radius (R = 5 cm cavity) and 10-cm radius (R = 10 cm cavity) curved mirrors was tested. The best performance was obtained with the R = 10 cm configuration, where, the incident threshold pump power could be lowered to 10 mW after optimizing the polarization direction of the pump beam and by employing double pumping. Theoretical estimation of the threshold power was in good agreement with the experimental observations. The laser could be further tuned from 1842 to 1994 nm.     

Sr3Gd(PO4)3:Tm3+的真空紫外光谱特性

研究了Sr3Gd(PO4)3 : Tm3 和GdPO4 : Tm3 样品的结构特性、光谱特性.GdPO4 : Tm3 为单斜晶系,基质掺入铥离子后结构没有明显变化.GdPO4:Tm3 在164和210 nm附近有强烈的吸收峰.位于164 nm附近的强烈的吸收峰是归因于基质的吸收引起,210 nm附近的吸收峰则归因于Gd 的8S7/2-6GJ的能级跃迁.在164 nm真空紫外光激发下,样品于453及363 nm处有较强的发射峰,发射主峰位于453nm,属于Tm3 的1D2→3H4(22,123 cm-1)跃迁的典型发射.由于阳离子质量的不同,Sr3Gd(PO4)3:Tm3 在166 nm附近的激发峰高于GdPO4: Tm3 的同位置的激发峰,其在363 nm处的发射有明显减弱,而在453 nm处的蓝色发射有显著的增强.     

Tm3+/Yb3+共掺碲酸盐玻璃的近红外发光及能量传递机理

采用高温熔融法制备了组分为TeO2-ZnO-Na2O的Tm3+离子单掺和Tm3+/Yb3+共掺碲酸盐玻璃,应用Judd-Ofelt理论计算分析了玻璃样品的强度参量Ωt(t=2,4,6),自发辐射跃迁几率A,荧光分支比β和荧光辐射寿命τrad等光谱参量,测量得到了不同Yb3+离子掺杂浓度下玻璃样品的Tm3+离子上转换发光谱.结果显示,在980nm泵浦光激励下玻璃样品发射出强烈的近红外上转换荧光.对Tm3+离子上转换发光分析表明,强烈的Tm3+离子近红外上转换发光主要来自于Yb3+/Yb3+离子间的共振能量传递以及基于单声子和双声子辅助的Yb3+/Tm3+离子间的非共振能量传递过程,并进一步计算得到了声子贡献比和能量传递系数.最后,计算分析了Tm3+∶3 F4→3 H6能级间跃迁的1.8μm波段吸收截面、受激发射截面和增益系数.研究表明,Yb3+/Tm3+共掺TeO2-ZnO-Na2O玻璃可以作为近红外波段固体激光器的潜在增益基质.     

Tm:YLF laser-pumped periodically poled MgO-doped congruent LiNbO3 crystal optical parametric oscillators

At room temperature, highly efficient, doubly resonant periodically poled MgO-doped congruent LiNbO3 crystal optical parametric oscillators (OPO) pumped at 1.9 μm have been realized for the first time to our knowledge. A 2.9 W, 2 kHz acousto-optic Q-switched homemade Tm:YLF laser served as a pump source. A 900 mW mid-infrared emission generated from an OPO is illustrated. The overall mid-infrared generation conversion efficiency reached 31%, whereas the slope efficiency was up to 70%.     

Energy transfer and mid-infrared luminescence properties of Tm3+ /Dy3+ codoped chalcohalide glasses

A series of chalcohalide glasses based on the composition 0.9 (Ge25 Ga5 S70)-0.1CsI doped with the different Tm3+ / Dy3+ ions ratio were synthesized by melt-quenching technique. The absorption spectra, and mid-infrared fluorescence of different glass samples under 800 nm laser excitation were measured. The results prove that, Tm3+ is an efficient sensitizer, which can enhance the Dy3+ : 2.9 microm fluorescence intensity significantly. A decrease in the intensity of 1.8 microm fluorescence and lifetimes of the Tm3+ : (3)F4 level occurred with increasing the concentration of Dy3+ ions from 0 to 1 Wt% where Tm3+ concentration was fixed to 0.5 Wt%. Also a wide spectral overlap between Tm+ : 1.8 microm emission and the absorption of Dy3+ : 6 H(15/2) --> (6)H(11/2) showed that the effective energy transfer between the two rare-earth ions was mainly attributed to the resonance energy from Tm3+: (3)F4 to Dy(3)+ : (6)H(11/2) level.     

Spectroscopic characteristics of Tm3+ in Tm and Tm,Nd, Yb:Sc2O3 ceramic

The spectroscopic investigation of Tm³⁺ or (Nd, Yb, Tm)-doped Sc₂O₃ transparent ceramics, as laser-active media for visible and IR emission, was performed. The spectra are dominated by Tm³⁺ ions in sites of C₂ symmetry and an energy level scheme and other spectral parameters were determined. The strong concentration quenching of ³H₄ level emission in Tm:Sc₂O₃ at 300 K is discussed by considering the resonant cross-relaxation (³H₄→³F₄)?(³H₆→³F₄) process and multipolar interactions of various orders. The main energy-transfer processes leading to the blue upconversion emission from the ¹G₄ Tm³⁺ level in (Tm, Nd, Yb):Sc₂O₃ ceramic, under pulsed pumping at 808.3 nm were evidenced.     

Emission properties of a Tm30.1mm+:GdVO4 microchip laser at 1.9 μm

4 as a host for thulium has several advantages for diode pumping in comparison with other crystals. The absorption cross section of thulium in GdVO₄ is considerably stronger and broader than in YAG and YLF and the spectrum is shifted closer to the emission wavelength of commercially available AlGaAs laser diodes. In our paper, we report on the temporal and spectral emission properties of a monolithical Tm^30.01mm+([%at.]6.9):GdVO₄ microchip laser at 1.9 μm. The laser can be adjusted to emit either cw or in an oscillating regime. Slope efficiencies up to 47% are achieved. This value exceeds the Stokes limit of 42%. Received: 31 March 1998/Revised version: 02 June 1998