Solid-state dye lasers based on polymethyl methacrylate doped with pyrromethene 650

Solid-state dye samples based on polymethyl methacrylate (PMMA) doped with different concentrations of the dye PM650 were prepared. With SHG of Q-switched Nd:YAG laser (532 nm, ∼15 ns) pumping the samples longitudinally, the fluorescence, lasing spectra and slope efficiencies of the samples were obtained. The highest slope efficiency 27.49% and laser output energy 22.0 mJ was achieved in the sample with a dye concentration of 2.0 × 10⁻⁴ mol/L. To our knowledge, the results are the best under the same condition so far. The maxima of output lasing spectrum is about 653.5 nm. Our results have shown that using solid-state samples doped with PM650 can extend the dye laser wavelength to the red.     

Collective spontaneous emission of dye molecules and lasing

The threshold pump power density for lasing in dye solutions is found to depend on the photon energy of pumping radiation. An increase in the pumping photon energy can significantly lower the threshold pump power of dye lasers. For an ethanol solution of rhodamine 6G with a concentration of 4×10¹⁸ cm^?3, the threshold power density for pumping radiation with a wavelength of 532 nm is 20-fold higher than for pumping radiation with a wavelength of 347 nm. This phenomenon is associated with the competition of collective spontaneous emission, which can lead to the efficient deactivation of excited molecules in femtosecond times, and the dephasing of excited molecules due to the intramolecular nonradiative processes of absorbed-energy conversion. An increase in the dephasing rate with the increasing energy of exciting photons lowers the efficiency of collective spontaneous emission and increases the concentration of dephased excited molecules responsible for lasing.     

Aligned growth of ZnO nanowires and lasing in single ZnO nanowire optical cavities

Ordered ZnO nanowire arrays have been fabricated in N₂ background gas by catalyst-free nanoparticle-assisted pulsed-laser deposition. A single ZnO nanowire was collected in an electrode gap by dielectrophoresis. Under the optical pumping above an exciting laser (λ= 355 nm) threshold of ∼ 334 kW/cm², ultraviolet lasing action in a single ZnO nanowire was observed at room temperature, indicating that the as-synthesized nanowires in pure N₂ background gas are of high quality. The crystalline facets of both ends of the nanowire acted to form an optical cavity. Therefore, the mode spacings corresponding to cavity lengths of the respective nanowires were observed in photoluminescence spectra. PACS 78.66.Hf; 81.07.Bc; 78.67.-n; 81.16.Mk     

Laser-diode-pumped Cr4+, Nd3+:YAG self-Q-switched laser with high repetition rate and high stability

A diode end-pumped self Q-switched Cr⁴⁺, Nd³⁺: YAG laser was established with 30-ns pulse width (FWHM) and 0.5-μJ pulse energy output. In normal pulse pumping operation, the lasing threshold changed greatly from 122 mJ to 2.4 mJ as the pump pulse frequency varied from 1 Hz to 500 Hz due to pumping-induced thermal effect. A pre-pumping method was proposed and the change of the lasing threshold was reduced; programmable Q-pulse output with maximum frequency of 16 kHz and high stability was achieved. Received: 16 January 2001 / Revised version: 21 May 2001 / Published online: 19 September 2001     

Plasma emission spectra of working media of KrC1 and XeC1 electric-discharge lasers

Results of investigations of survey spectra of plasma emission of the working media of inert-gas-chloride electric-discharge lasers in the 200–600 nm range are reported. In the UV spectra, FeI lines, which are of interest for lasing with optical pumping of iron vapors by excimer molecules, were observed. In the visible spectrum, a number of HeI, KrI, and XeII lines are suggested for determination of electron density and investigation of the discharge instability on XeCl^* and KrCl^* molecules. Uzhgorod State University, 46, Pidgirna St., Uzhgorod, 294000, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 4, pp. 523–527, July–August, 1997.     

Lasing with guided modes in ZnO nanorods and nanowires

A vertically arranged nearly parallel array of ZnO nanorods and randomly oriented nanowires has been grown by low pressure chemical vapor deposition (CVD) on silica substrates and on stainless steel gauze woven from a wire with a diameter of 40 μm, respectively. The quality of the produced material is high enough to act as a gain medium for stimulated emission in the ultraviolet spectral region. Multiple sharp lasing peaks were realized from single hexagonal nanorods and arrays of hexagonal ZnO nanorods. The lasing peaks display successive onset and saturation with increasing excitation power density and fit well the expected resonance spectrum of guided modes in hexagonal nanorods. The behavior of lasing spectra from shot to shot of pumping in randomly oriented nanowires along with the independence of the lasing threshold on the excitation spot area suggest that the emission spectrum results from the superposition of lasing modes in individual nanowires, rather than from random lasing due to photon coherent scattering.     

碟型量子阱微腔激光器的低阈值激射

利用反应离子刻蚀(RIE)和湿法腐蚀方法在InGaAs/InGaAsP多量子阱材料上研制出直径为8μm、4.5μm和2μm的碟型半导体微腔激光器。其中2μm直径的微碟在液氮温度下其光泵浦激射阈值仅为3μW左右。对高光功率密度下泵浦时出现的多模激射、跳模和激射光谱强度饱和现象进行了研究。并对微碟激光器的激射光谱线宽特性进行了初步的分析。     

Luminescence and lasing properties of neodymium- and uranium-activated inorganic laser liquids

An attempt was made to describe and show the possibilities of new inorganic neodynium- and uranium-activated laser liquids: SO₂-GaCl₃-NdCl₄; SO₂Cl₂-GaCl₃-NdCl₃-UO₂Cl₂; POCl₂-MCl_n-NdCl₃-UO₂Cl₂ for development and synthesis of direct nuclear reaction-excited lasers. Luminescence data presented in the work were used to calculate the luminescence parameters of the laser liquids such as oscillator strengths f, probability of spontaneous radiation A, intermultiplet luminescence branching coefficient β, cross-section for induced radiation σ, luminescence decay time τ, quantum yield η, and others. It is shown that the oscillator strengths of the normal absorption bands of Na³⁺, which play the main part in the pumping processes, exceed the oscillator strengths of Na³⁺ for aqueous and many other nonaqueous systems. In the luminescence excitation spectra of the Na³⁺ ion, bands are isolated in the range 400–1000 nm atλ _rec =1.06 μm. With excitation, luminescence occurs through the⁴F_3/2→⁴I_{9/2,11/2.13/2} channels. Luminescence spectral data are related to the lasing parameters. The threshold lasing energy is∼18 J/cm³. For a resonator with mirros h₁=100% and h₂=20, 40, 56, and 80%, the lasing energy is∼20–120 MJ/cm³ in the pumping energy range 18–180 J/cm³. The differential efficiency is ∼0.2% The substantial angular radiation divergence (θ∼4·10⁻² rad) and strong thermostatic distortions that occur in the active element (dn/dT≈−1.9·10⁻⁴K⁻¹) are a disadvantage of laser liquids. It is shown that operation of neodymium- and uranium-activated inorganic liquid lasers is stable under the present conditions. A. I. Gertsen Russian State Pedagogical University, Moika Embankment, 48, St. Petersburg, 191186, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 5, pp. 607–619, September–October, 1997.     

Hydrothermal fabrication of well-ordered ZnO nanowire arrays on Zn foil: room temperature ultraviolet nanolasers

Well-ordered nanowires of the hexagonal wurtzite ZnO having an average diameter of 80 nm, a typical length of 12 μm, and a mean packing density of 7.5 nanowires μm⁻² have been directly grown on Zn foil in a preferred [0001] direction by a hydrothermal process and employed for room temperature ultraviolet nanolasers. The lasing action of arrayed ZnO nanowires has been observed from 370 to 400 nm with threshold irradiance of 25 kW cm⁻². Photoluminescence decays biexponentially: the fast component is attributed to free-exciton decay, and the slow one is to bound-exciton decay. The amplitude of the fast component increases whereas its lifetime decreases with the increment of threshold irradiance, suggesting that ZnO nanowire arrays undergo a change in the lasing mechanism from exciton–exciton scattering to electron–hole plasma recombination.     

轴向隐失波激励的回音壁模式光纤激光器

采用轴向隐失波激励增益的方式,使激光增益区域局限在光纤回音壁模式的模场区域内,显著地降低了回音壁模式光纤激光辐射的抽运阈值,由此形成一种低阈值的回音壁模式光纤激光器.在微焦耳量级的低抽运能量条件下,用回音壁模式光纤激光器研究了激光染料的浓度效应.实验结果表明,随着激光染料浓度的增加,回音壁模式激光辐射的波长向长波方向移动,激光波长范围变宽.用回音壁模式染料激光的四能级模型得到激光上能级和所有能级上的分子数比值γ(λ)曲线后,很好地解释了实验结果.低抽运阈值的回音壁模式光纤激光器,为研究液体激光现象提供了极为便利的手段.     

Effective 2.5-μm ZnSe:Cr<Superscript>2+</Superscript> laser with transverse laser pumping

Effective lasing is obtained in the transversely pumped ZnSe single crystals that are doped with Cr²⁺ ions using diffusion methods. A Q-switched Er³⁺-doped glass laser with a radiation wavelength of 1.54 μm is used for pumping. The resulting laser energy is 150 μJ at an absorbed pump energy of 600 μJ, so that the efficiency is 25% and the slope efficiency is 29%. An increase in the gain (up to the superluminescence level) due to the application of the transverse pumping of the active element with a substantially non-uniform distribution of the dopant is discussed.     

Investigation of the pump wavelength influence on pulsed laser pumped Alexandrite lasers

Recent theoretical modelling and experimental results have shown that excess lattice phonon energy created dur ing the non-radiative energy transfer from the ⁴T₂ pump manifold to the ²E storage level in Alexandrite when pumped with wavelengths shorter than ∼645 nm causes chaotic lasing output. Shorter pump wavelengths have also been associated with increased non-radiative energy decay and reduced laser efficiency. We report studies of fluorescence emission spectra of Alexandrite illuminated at a range of wavelengths from green to red, which demonstrate reduced fluorescence yield for shorter pump wavelengths at elevated crystal temperatures. Investigations of pulsed laser pumping of Alexandrite over the same spectral range demonstrated reduced pump threshold energy for longer pump wavelengths. High repetition rate pulsed pumping of Alexandrite at 532, 578 and 671 nm showed stable and efficient laser performance was only achieved for red pumping at 671 nm. These results support the theoretical model and demonstrate the potential for scalable, red laser pumped, all-solid-state Alexandrite lasers.PACS 42.60.Lh; 42.60.Mi     

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     

Optical edge modes in photonic liquid crystals

An analytic theory of localized edge modes in chiral liquid crystals (CLCs) is developed. Equations determining the edge-mode frequencies are found and analytically solved in the case of low decaying modes and are solved numerically for the problem parameter values typical for the experiment. The discrete edge-mode frequencies specified by the integer numbers n are located close to the stop-band edge frequencies outside the band. The expressions for the spatial distribution of the n’s mode field in a CLC layer and for its temporal decay are presented. The possibilities of a reduction of the lasing threshold due to the anomalously strong absorption effect are theoretically investigated for a distributed feedback lasing in CLCs. It is shown that a minimum of the threshold pumping wave intensity may be reached, generally, for the pumping wave propagating at an angle to the helical axis. However, for lucky values of the related parameters, it may be reached for the pumping wave propagating along the helical axis. The lowest threshold pumping wave intensity occurs for the lasing at the first low-frequency band-edge lasing mode and the pumping wave propagating at an angle to the spiral axis corresponding to the first angular absorption maximum of the anomalously strong absorption effect at the high-frequency edge of the stop band. The study is performed in the case of the average dielectric constant of the liquid crystal coinciding with the dielectric constant of the ambient material. Numerical calculations of the distributed feedback lasing threshold at the edge-mode frequencies are performed for typical values of the relevant parameters.     

Dependence of cooperative emission spectra of complex organic molecules on exciting photon energy

The dependences of cooperative emission, superfluorescence, and lasing spectra on the wavelength of pumping radiation were studied for highly concentrated ethanol solutions of organic dye molecules. The relative intensity of cooperative emission was found to decrease, while the intensities of superfluorescence and lasing were found to increase with the energy of exciting photons. The increase in the dephasing rate of quantum states is supposed to be primarily responsible for the change in proportion between these intensities. On increasing the dephasing rate, with all other factors being the same, the concentration of the excited molecules with the phased quantum states decreases, while that with the dephased states increases. The ratio between the cooperative emission and dephasing rates determines the proportion between the intensities of cooperative emission, superfluorescence, and lasing. The onset of lasing at high concentrations (above >10¹⁹ cm^?3) in the case of pumping by high-energy photons occurs due to the high dephasing rates of quantum states resulting from a more rapid redistribution of vibrational energy.     

基于钠硼铝硅酸盐玻璃的PbSe量子点红外单模光纤激光

根据实验制备的钠硼铝硅酸盐PbSe量子点玻璃及其透射电子显微镜(TEM)图、吸收谱和发射谱,计算机数值模拟了以PbSe量子点作为激活增益介质的红外单模光纤激光。应用遗传算法,通过数值求解粒子数速率方程和激光谐振腔振荡方程,优化计算了量子点光纤激光器(QDFL)的最佳抽运波长、光纤长度、掺杂浓度及出射镜反射率。结果表明:饱和抽运功率为2 W,在1676nm激光波长处,QDFL最大输出功率可达1.36 W,抽运效率达68%。与通常的掺稀土离子(Yb3+、Er3+)的光纤激光器相比,QDFL具有抽运效率高、激励阈值低、掺杂密度可调、光纤饱和长度短等特点。由于量子点辐射波长的尺寸依赖特性,容易形成多波长激射或波长可调的新型激光器。     

ZnO粉末中无序激射现象时间分辨的研究

采用抽运-探测时间分辨方法实验研究了半导体材料ZnO纳米颗粒粉末中的无序激射现象.在2 67nm激光的抽运下，通过精确控制抽运光的能量和样品表面的抽运面积，获得了宽度小于1n m的单模无序激射光谱和多峰的多模无序激射光谱.时间分辨的抽运-探测结果显示，此时样 品的上能级寿命仅为几个皮秒，证明了ZnO粉末的单模无序光谱是受激辐射的结果. 关键词： 高散射介质 多重散射 抽运-探测 无序激射     

Amplified luminescence in InGaAs/AlGaAs laser diode arrays at high pump levels

The development of amplified luminescence fluxes in powerful InGaAs/AlGaAs laser diode arrays (lasing wavelength 940–960 nm) has been studied experimentally and theoretically at pump levels above the threshold value. Flux density values for amplified luminescence propagation along (1.88⋅10⁹ W/m²) and across (1.21⋅10⁹ W/m²) the laser diode array cavity axis have been evaluated for the threshold pump level at room temperature (293 K). The contribution of the recombination rate induced by the amplified luminescence to the threshold current generation of the laser diode array reaches 7%. It has been shown that the amplified luminescence flux density is increased by 49% as the pump level rises from one to three threshold values.     

A Compact Sol-Gel Laser with Dynamic Distributed Feedback

Lasing in dynamic-distributed-feedback (DDFB) laser on the basis of a modified sol-gel matrix activated with Rhodamine 4C is reported. The lasing is achieved in the range of 590 nm with a wavelength tuning of about 20 nm. The slope efficiency of the sol-gel DDFB laser excited by the second harmonic radiation of a YAG:Nd³⁺ laser with a pulse duration of 40 ns is 16.5% at a lasing wavelength of 593 nm. The DDFB laser is also characterized by a threshold pumping energy of 90 J and a pumping energy density up to 40 mJ/cm². The typical spectral width of the laser line is 0.025 nm.