Er, Yb fiber laser resonantly pumped

Q-switched polycrystalline Er:YAG ceramic laser

Yong Wang

Supervisor: Heyuan Zhu

1.6 μm erbium lasers have been widely used in numerous areas, for example, the laser radar deployed in unmanned aerial vehicle X-47B chose 1.645 μm Er:YAG laser as laser source, they are also important pump source for the generation of mid-infrared lasers. As a category of quasi-three-level lasers, erbium lasers have distinct properties.  The emission cross section is small and hence small gain coefficient [1]. Energy-transfer upconversion (ETU) among proximate first-excited-state Er³⁺ ions (i.e., ⁴I_13/2 manifold) reduces the population on upper laser level and increases waste heat generation in active material, laser performance decrease therefore [2-5].  Laser operation even halted for high doping concentration erbium gain materials (There is on report on erbium laser with >2 at.% doping concentration so far.).

Improvements of laser performance such as output power and slope efficiency need optimizing laser design in terms of doping concentration, length of active material and the geometry of laser resonator.  For continuous-wave operation, theory of quasi-three-level lasers has been suggested [6], laser oscillating threshold, slope efficiency were expressed as functions of parameters of active materials and cavity mode.  The effects of ETU on laser performance were also investigated and it was found that the slope efficiency was affected only slightly but the threshold was dramatically influenced [3,4].  For Q-switched pulsed Er:YAG lasers, there was few research on optimization design, it may partially be attributed to low output power and no urgent requirement of power scaling.  But to my opinion, even for low output power, optimization design is necessary since power scaling can be achieved with directly improving pumping power.  On the other hand, the principles and experiences can be extended to other quasi-three-level lasers.

Rare-earth doped transparent ceramics have drawn much attention as new laser gain media since first high-efficiency Nd:YAG ceramic laser was demonstrated in 1995 because they have some specific advantages, e.g., rapid and large scale fabrication, flexibility in producing ceramics with composite structures, possibility of fabricating laser materials difficult to be prepared with melt-growth process, such as sesquioxide ceramics (Sc₂O₃, Y₂O₃, Lu₂O₃), with optical quality. As fabrication technology advances, Nd- and Yb- doped YAG ceramics are now routinely available near 1 μm wavelength region with essentially the same laser performances as that of single crystals [6]. In recent years, ceramic lasers at ~1.6 μm have also been reported for Er³⁺-doped polycrystalline sesquioxide Sc₂O₃, Y₂O₃ and YAG ceramics ([7] and references therein). However, those sesquioxide ceramic erbium lasers have been demonstrated mostly in liquid nitrogen cooling condition. First room-temperature operation of erbium ceramic laser was demonstrated for a composite ceramic Er:YAG with 56.9% of slope efficiency with respect to the absorbed pump power. In 2013, more than 16 W of CW output power was reported for 1645 nm and 1617 nm Er:YAG ceramic laser resonantly pumped by an Er,Yb fiber laser. Recently, Q-switched operation of Er:YAG ceramic has been also demonstrated using multilayer graphene as saturable absorber with several μJ pulse energy and dozens of kHz repetition rate.

References

[1] S. A. Payne, L. L. Chase, L. K. Smith, W. L. Kway, and W. F. Krupke, “Infrared cross-section measurements for crystals doped with Er³⁺, Tm³⁺, and Ho³⁺,” IEEE J. Quantum Electron. 28(11), 2619 (1992).

[2] N. P. Barnes, B. M. Walsh, F. Amzajerdian, D. J. Reichle, G. E. Busch, and W. A. Carrion, “Up conversion measurements in Er:YAG; comparison with 1.6 μm laser performance,” Opt. Mater. Express 1(4), 678 (2011).

[3] J. W. Kim, J. I. Machenzie, and W. A. Clarkson, “Influence of energy-transfer-upconversion on threshold pump power in quasi-three-level solid-state lasers,” Opt. Express 17(14), 11935 (2009).

[4] J. O. White, M. Dubinskii, L. D. Merkle, I. Kudryashov, and D. Garbuzov, “Resonant pumping and upconversion in 1.6 μm Er³⁺ lasers,” J. Opt. Soc. Am. B 24(9), 2454 (2007).

[5] M. O. Iskandarov, A. A. Nikitichev, and A. I. Stepanov, “Quasi-two-level Er3+:Y3Al5O12 laser for the 1.6-μm range,” J. Opt. Technol. 68(12), 885 (2001).

[6] A. Ikesue, Y. L. Aung, and V. Lupei, Ceramic Lasers (Cambridge University Press, 2013).

[7] Y. Wang, T. Zhao, D. Y. Shen, J. Zhang, and D. Y. Tang, “Resonantly pumped Q-switched Er:YAG ceramic laser at 1645 nm,” Opt. Express, in publication.

Fabrication and Characterizations of Needle-like NiC_x Nanowires by Pulsed Laser Deposition Accompanied by N₂ Annealing

Hui Li

Supervisor: Jiada Wu/Ning Xu

In recent years, carbon nano-materials such as carbon nanotubes (CNTs), carbon nanofibers (CNFs) and carbon nanowires (CNWs) have been a hot topic and garnered much attention because of their unique properties and potential applications in nanotechnology [1, 2]. For instance, CNTs can find their applications in electrodes for biosensing or solar cells [1, 2]. For growing carbon nano-materials, the commonly used techniques are plasma-enhanced chemical vapor deposition for the growth of CNWs [3], pulsed laser evaporation for the growth of CNTs [4], pulsed-laser deposition for the growth of CNTs [5] and pyrolysis for the growth of CNFs [6].

Besides pure carbon materials, transition metal carbides are traditionally promising materials because of their excellent mechanical properties, high resistance to chemical attack and oxidation, remarkable thermal and electronic conductivity as well as superconductivity [7]. For example, nickel-carbide materials can be used as catalyst for ammonia synthesis and decomposition, hydrogenolysis and hydroprocessing [8]. Among them, NiCx is calculated to be a good and stable magnetic material from the first-principle [9], but there are only a few literatures about it, it has been discussed briefly [10]. In reference 10, during deposition, carbon atoms diffused into the Ni crystal to form the NiCx structure. After saturation, CNTs were formed along the [110] direction of NiCx crystal.

In this work, the transparent needle-like nickel carbide nanowire (NCNW) films are successfully fabricated on quartz substrates using the method of pulsed laser deposition (PLD) accompanied by nitrogen annealing. The PLD method is simple and efficient to deposit refractory material (like carbon) films. After the PLD of carbon/nickel films, the samples were annealed in N₂ at the substrate temperature of 1000-1250 °C for the growth of the needle-like NCNWs. To the best of our knowledge, this is the first time to prepare nickel-carbon nanowires ever since. The morphology, structure and composition of the as-grown NCNW films were characterized in detail. The light transmittance of the as-grown NCNW films was studied for their potential use as transparent electrodes in solar cells.

References

[1] J. Li, H.T. Ng, A. Cassell, W. Fan, H. Chen, Q. Ye, J. Koehne, J. Han, M. Meyyappan, Nano Lett. 3 (2003) 597.

[2] C. Klinger, Y. Patel, H.W.C. Postma, Plos One 7 (2012) e37806.

[3] K.H. Liao, J.M. Ting, Carbon 42 (2012) 509.

[4] A.P. del Pino, E. Gyorgy, L. Cabana, B. Ballesteros, G. Tobias, Carbon 50 (2012) 4450.

[5] M. Gaillard, C. Boulmer-Leborgne, N. Semmar, E. Millon, A. Peti, Appl. Surf. Sci. 258 (2012) 9237.

[6] H. Ogihara, S. Takenaka, I. Yamanaka, E. Tanabe, A. Genseki, J. Koki, K. Otsuka, Chem. Lett. 37 (2008) 868.

[7] S.T. Oyama, Catal. Today 15 (1992) 179.

[8] B. Ghosh, H. Dutto, S.K. Pradhan, J. Alloy Compd. 479 (2009) 193.

[9] C.M. Fang, M.H.F. Sluiter, M.A. van Huis, H.W. Zandbergen, Phys. Rev. B 86 (2012) 134114.

[10] H.L. Wang, Y.L. Yuan, Diam. Relat. Mater. 22 (2012) 124.

Preparation of positive charged and negative charged surface-modified TiO₂ nanoparticles : Dispersibility and interaction with cancer cells

Jin Xie

Supervisor: Lan Mi

Semiconductor titanium dioxide (TiO₂) was regarded as a potential photosensitizer in the field of photodynamic therapy (PDT) due to its low toxicity, high chemical stability, good photoreactivity and excellent biocompatibility^[1-4]. But it has poor solubility and tends to aggregate under neutral pH conditions, which hinders its application and development in PDT.

To improve the solubility of titania in aqueous solution, both amino-modified TiO₂ (TiO₂-NH₂) and carboxyl-modified (TiO₂-COOH) were prepared. The present work used 3-phosphonopropionic acid and 3-aminopropyltriethoxysilane (APTES) to modify TiO₂ under different conditions and the samples were characterized by measuring Zeta potentials to find the optimum conditions. The Zeta-potential result demonstrated that TiO₂-NH₂ charged positively and TiO₂-COOH charged negatively and they both greatly enhanced the dispersibility in water. TEM result showed that the average size of TiO₂ conjugates were around 40 nm.

Interactions between cancer cells and TiO₂ composites were studied. Firstly,  both TiO₂-COOH and TiO₂-NH₂ showed no significant toxicity after 24 hour incubation with cancer cells. Secondly, according to the observation by laser scanning confocal microscopy (LSCM), the cellular uptake of TiO₂-COOH and TiO₂-NH₂ was more effectively than that of bare TiO₂. TiO₂-NH₂ showed a more rapidly uptake by cells compared to TiO₂-COOH. Moreover, we traced TiO₂-NH₂ in its uptake process by HeLa and found most of it finally incorporated in lysosome. The final destination of TiO₂-COOH is yet to be researched.

Reference

[1] Fujishima A, Rao T N, Tryk D A. Titanium dioxide photocatalysis [J]. J Photoch Photobio C, 2000, 1(1): 1-21.

[2] Hashimoto K, Irie H, Fujishima A. TiO₂ photocatalysis: a historical overview and future prospects [J]. Jpn J Appl Phys 1, 2005, 44(12): 8269-8285.

[3] Fabian E, Landsiedel R, Ma-Hock L, Wiench K, Wohlleben W, van Ravenzwaay B. Tissue distribution and toxicity of intravenously administered titanium dioxide nanoparticles in rats [J]. Arch Toxicol, 2008, 82(3): 151-157.

[4] Gupta S, Tripathi M. A review of TiO₂ nanoparticles [J]. Chinese Sci Bull, 2011, 56(16): 1639-1657.

Time:  6:30 pm, Thursday, 2014.10.16

Location: Optical Building. Room 525