Fiber laser in-band pumped Ho:YAG ceramic lasers

Xiaofang Yang

Supervisor: Heyuan Zhu and Deyuan Shen

Solid-state lasers based on Ho³⁺ ions operating slightly above 2 μm have potential applications in medicine, spectroscopy, remote sensing and as a pump source for OPO’s. Gain materials generally take a critical role for developing high performance laser system. With the advent of 1.9 µm high power lasers such as thulium fiber lasers^[1], Tm:YLF solid state lasers^[2]and semiconductor InGaAsP/AsSb diode lasers^[3], resonantly pumped singly Ho³⁺-doped crystal lasers have obviated the energy-transfer loss between Tm³⁺, Ho³⁺ ions in Tm, Ho-codoped materials and demonstrated high efficiency and power scalability. In recent years, transparent polycrystalline ceramics are of interest for laser community, continuous-wave (cw) operations of Ho:YAG ceramics have exhibited nearly the same laser performance in terms of output power and slope efficiency as single-crystal materials^[4]. Q-switched and mode-locked pulsed laser outputs are also anticipated due to high gain cross-section and long lifetime of the upper energy level of 2.1 μm transition of these gain materials. However, only a few pulsed Ho³⁺ solid-state lasers have been reported because there is few appropriate saturable absorbers in this wavelength range^[5].

With a Dirac-type electronic states distribution near the Fermi energy, unlike the traditional saturable absorbers including Cr:YAG, semiconductor saturable absorber mirrors(SESAM) and GaAs,graphene possesses the optical saturable absorption properties within a ultrabroad wavelength range from visible to mid-IR. Since first demonstration of mode-locking near 1.5 μm in Er³⁺ fiber was reported by Bao et al. ^[6], However, to the best of our knowledge, there is no report of Q-switched or mode-locked Ho³⁺ lasers with graphene as the saturable absorber yet.

In this work, we will report on the first atomic-layer graphene passively Q-switched Ho:YAG ceramic laser. The laser operated with the slope efficiency of 33.8% for the cw mode without graphene in the cavity while the slope efficiency decreased to 16.5% under the modulation of the graphene. Stable operations of 2.6-9 μs of pulse durations with 28-64 kHz repetition rates were demonstrated with increasing the pump power of the diode pumped Tm fiber laser from 2 W to 3.3 W. Both repetition rates and pulse durations show smooth and monotonous tendency with the increase of the pump power. Average Q-switched output power up to 200 mW was generated and maximum pulse energy was 9.3 μJ at 64 kHz repetition rate.

In this report, we also introduced the actively acousto-optic Q-swiched ceramic Ho:YAG and Ho:LuAG lasers. The shortest pulse duration of 6.8 ns and 21 ns was achieved at 500 Hz PRF, respectively.

Reference

1. Fei Wang, Deyuan Shen, Dianyuan Fan, and Qisheng Lu, “Widely tunable dual-wavelength operation of a high-power Tm:fiber laser using volume Bragg gratings,” Opt. Lett. 35 (4), 2388-2390 (2010).
2. Ying-Jie Shen, Bao-Quan Yao, Xiao-Ming Duan, Guo-Li Zhu, Wei Wang, You-Lun Ju and Yue-Zhu Wang, “103 W in-band dual-end-pumped Ho:YAG laser,” Opt. Lett. 37(17), 3558-3560 (2012).

3.      K. Scholle, P. Fuhrberg. In-band pumping of high-power Ho:YAG lasers by laserdiodes at 1.9μm. OSA / CLEO/QELS, 2008: CTuAA1.

4.      Hao Chen, Deyuan Shen, Jian Zhang, Hao Yang, Dingyuan Tang, Ting Zhao, and Xiaofang Yang, “In-band pumped highly efficient Ho:YAG ceramic laser with 21 W output power at 2097 nm,”, Opt. Lett.2011, 36 (9): 1575-1577.

5.      Yuri Terekhov, Igor S. Moskalev, Dmitri V. Martyshkin, Vladimir V. Fedorov, and Sergey B. Mirov, “Cr-ZnSe Passively Q-switched fiber-bulk Ho:YAG hybrid laser,” SPIE 2009: 7578-54V.1,.

6. Qiaoliang Bao, Han Zhang, Yu Wang, ZhenHua Ni, Yongli Yan, Ze Xiang Shen, Kian Ping Loh, and Ding Yuan Tang, “Atomic-layer Graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 2009, 19: 3077-3083.

Amorphous Silicon Melting to Low Density Liquid

Bo Shen

Supervisor: Songyou Wang

Silicon has great potential to exhibit polymorphic transitions[1], since an open structure has been considered a key factor of such phenomena. It hadonce long been a common belief that the amorphous-liquid transition of silicon is a first-order melting transition[2], while recently reported experiments[3]and simulations[4]reinterpreted the transition as a former first-order liquid-liquid phase transition[3, 4](LLPT) from high-density liquid (HDL) to low-density liquid (LDL), along with a lattercontinuous glass transition[5]from LDL to low-density amorphous (LDA) upon cooling.

In this work, based on the classical Stillinger–Weber (SW) model[6]of silicon, a molecular dynamics simulation study on the heating and melting of LDA, and long-time annealing of the melt before transition to HDL is presented. By means of effective crystalline order characterization during annealing, structural and dynamical analysis are ensured unaffected by ambiguous crystallization. Dynamical comparison of the amorphous melt to normal liquids is performed. Phenomena such as hopping process[7], cage effect[8]and dynamical heterogeneity[9]are observed and discussed. Synthesizing of these results suggests the amorphous melt as a highly viscous liquid, namely LDL, rather than a solid, supporting liquid polymorphism[1]of silicon. The carefully calculated diffusivity of LDL excluding the non-equilibrium crystallization turns out one order smaller than previously reported[4]. The existence of LDA-LDL glass melting transitionat ~ 963 K is confirmed.

Reference

[1] P.H. Poole, T. Grande, C.A. Angell, P.F. McMillan, Polymorphic Phase Transitions in Liquids and Glasses, Science, 275 (1997) 322-323.

[2] P. Baeri, S.U. Campisano, M.G. Grimaldi, E. Rimini, Experimental investigation of the amorphous silicon melting temperature by fast heating processes, J. Appl. Phys., 53 (1982) 8730-8733.

[3] M. Beye, F. Sorgenfrei, W.F. Schlotter, W. Wurth, A. Föhlisch, The liquid-liquid phase transition in silicon revealed by snapshots of valence electrons, Proceedings of the National Academy of Sciences, 107 (2010) 16772-16776.

[4] S. Sastry, C. Austen Angell, Liquid-liquid phase transition in supercooled silicon, Nat Mater, 2 (2003) 739-743.

[5] A. Hedler, S.L. Klaumunzer, W. Wesch, Amorphous silicon exhibits a glass transition, Nat Mater, 3 (2004) 804-809.

[6] F.H. Stillinger, T.A. Weber, Computer simulation of local order in condensed phases of silicon, Physical Review B, 31 (1985) 5262-5271.

[7] M.C.C. Ribeiro, Translational and reorientational heterogeneity in the glass-forming liquid Ca0.4K0.6(NO3)1.4, Physical Chemistry Chemical Physics, 6 (2004) 771-774.

[8] W.J. R., Prog. React. Kinet., 27 (2002) 165.

[9] W. Kob, C. Donati, S.J. Plimpton, P.H. Poole, S.C. Glotzer, Dynamical Heterogeneities in a Supercooled Lennard-Jones Liquid, Physical Review Letters, 79 (1997) 2827-2830.

 Manufacture of Gold NanoMaterials in Organic Phase and the Fréedericksz Phase Transition Properties of Liquid Crystal/Gold Nanoparticle System

Qi Wang

Supervisor：Lei Xu

      In the last decade, the plasmonic field has become a rapidly expanding area for materials and device research ^[1-2]. As a result of large array of tools available for nanoscale fabrication and nanophotonics characterization and powerful electromagnetic simulation methods, the first application of the localized surface plasmon resonance (LSPR) phenomenon for sensing in 1980s has made great strikes both in terms of instrumentation development and application^[3-4].

       Due to the wide use of liquid crystal^[5] and gold nanoparticle’s localized surface plasmon resonance, gold nanoparticles doped nematic liquid crystal cell can enhance the effect of the outfield. With such a property, this reportstudies the magnetic-optical Fréedericksz transition^[6] and its threshold of vertical-aligned gold nanoparticles doped nematic liquid crystal cell^[7-8].We find that under additional light field, the Fréedericksz transition threshold of gold nanoparticles doped liquid crystal cell has descended obviously, and for 0.5%wt gold nanoparticles doped nematic liquid crystal cell, the Fréedericksz transition threshold descended to 8.3% of pure liquid crystal. Meanwhile, contract with additional magnetic field, it is found that for the same density of gold nanoparticle in liquid crystal cell, the Fréedericksz transition threshold descends more under additional light field than under additional magnetic field, the theory of localized surface plasmon resonance’s enhancement effect can explain this phenomenon as a result of the enhancement of light field around the gold nanoparticles.

References

[1] Zayats A V,Smolyaninov I, Journal of Optics A: Pure and Applied Optics,2003, 5, S16.

[2] Steinmann W, Phys. Rev. Lett. 1960, 5, 470.

[3] Hoheisel W,Jungmann K, Vollmer M, et al. Phys. Rev. Lett.1988,60,1649

[4] Rether H. Surface Plasmons[M], Spriner-verlag Berlin,1988

[5] P G de Gennes. 液晶物理学. 上海:上海翻译出版公司, 1990, 7.

[6] H. L. ONG, Phys. Rev. A, 1981, 33, 3550

[7] S. J. Barrow, A. M. Funston, D. E. Gomez, T. J .David and P. Mulvaney, Nano. Lett.2011,11, 4180.

[8] S. D. Durbin, S. M. Arakelian and Y. R. Shen, Phys. Rev.Lett. 1981, 47, 1411.

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Time:  6:30 pm, Friday, 2014.10.12

Location: Optical Building. Room 525