Spectroscopic ellipsometry applied to phase transitions in ferroelectric thin films

JinBo Zhang

Supervisor: Yuxiang Zheng

 Ferroelectric films are greatly important for various technological applications. One of the significant phenomena observed in ferroelectric thin films different from bulk is the size effect . It has been shown that phase-transition temperature changes noticeably with decreasing film thickness and the surface defects. Many experimental and theoretical studies have been devoted to this effect and result is that a surface layer may exist in the film, which is different from the interior due to the limitations of processing techniques and the interaction with substrate.

    To clarify the influence of surface imperfections on the properties of ferroelectric thin films, in the first place, spectroscopic ellipsometry was applied to phase transitions investigation in oxide thin films and crystals, along with the use of various parameters calculated from ellipsometric data (band gap energy Eg, refractive index n and surface roughness). The efficiency of spectroscopic ellipsometry on “surface” phase transition and its sensitivity to surface defects are also demonstrated. In the next place,a generalized Ginzburg–Landau–Devonshire (GLD) theory is used to study the properties of second-order phase transition in ferroelectric thin films sandwiched between two metal electrodes with a natural boundary condition. By taking into account the effect of the imperfect surface layer , the temperature and film-thickness dependence of the spontaneous polarization and the dielectric susceptibility are calculated.

It has been demonstrated that the depolarization field makes the polarization distribution more uniform. In addition, an asymmetric hysteresis loop can be obtained due to the existence of asymmetrical imperfect surface layers.

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Effects of crystallographic surface and co-sputtered atom on the growth of ion-sputter induced Si nanocone arrays

Shengchi Song

Supervisor: Ming Lu

Ion-sputtering induced self-organized surface nanostructures have attracted considerable attentions due to their easy fabrications and cost effectiveness. Although a great amount of theoretical and experimental efforts have been paid, the applications of the nanodot or nanowire arrays grown on semiconductors have not been available so far. One main reason is that the average aspect ratios of the nanostructures are low, which brings only slight changes in macroscopic physical properties. Recently, it is found that with the help of co-sputtering of metals at elevated sample temperatures, nanocone arrays with average aspect ratio well greater and the cone height of several hundred nanometers can be grown on Si. The Si sample with nanocone arrays turns out to be highly photo-absorptive in the wavelength range of 300-2000 nm as measured, and is a promising material in photoelectric and photovoltaic applications. However, how the nanocone arrays grow under Ar+ ion sputtering is still unclear, making their effective applications difficult. We study the effects of Si crystallographic surface and co-sputtered atoms on the growth of Si nanocone arrays. The purpose is to explore the growth mechanism of the cone arrays via observing their responses to all of these changing parameters.

In my work, self-organized Si nanocone arrays grown on different crystallographic surfaces with different co-sputtered foreign atoms by Ar+ ion sputtering are investigated. The surfaces are (100), (110), (111) and multi-crystalline ones; the foreign elements are Ta, Mo, Fe, Al and C, respectively. It is found that the growth of nanocone arrays is independent on the initial crystallographic surface, but sensitive to the atomic weight of foreign atom. At the same Ar+ ion influence and sample temperature, the heavier the foreign atom is, the higher the average cone height becomes. A model based on the surface-curvature dependence of sputtering yield and the preferential sputtering is proposed, which explains the observed results and the growth mechanism of Si nanocone arrays.

参考文献：

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[4] G. Ozaydin, A. S. Ozcan, Y. Wang, K. F. Ludwig, H. Zhou, R. L. Headrick, D. P. Siddons. Real-time x-ray studies of Mo-seeded Si nanodot formation during ion bombardment[J]. Applied Physics Letters, 2005, 87(16): 163104

[5] H. Hofsass, K. Zhang. Surfactant sputtering[J]. Applied Physics A: Matericals Science & Processing, 2008, 92(3): 517-524

[6] J. Zhou and M. Lu, Mechanism of Fe impurity motivated ion-nanopatterning of Si (100) surfaces, Physical Review B 82, 125404 (2010)

[7] Qiu Y, Hao HC, Zhou J, Lu M. A close to unity and all-solar-spectrum absorption by ion-sputtering induced Si nanocone arrays, Opt Express, 2012, 20(20):22087-94

Magneto-Optical Farady Effects Enhencement of BIG films Incorporating Gold Nanoparticles

 Gongyuan Situ

Supervisor: Bin Ma

       It is known that magneto-optic (Faraday and Kerr)effects are controlled by spin-orbit coupling, which depends on local electric fields. This suggests that the enhancement in local electric fields may lead to the enhancement of magneto-optic effects. Local electric fields can be optically enhanced by exciting plasmon resonances in metallic (silver or gold) nanoparticles in direct proximity to (or embedded in) magneto-optic media. Experimental observations of such plasmon resonance enhancement of magneto-optic effects have been recently reported.

       Here I reported the results of the experiment which we fabricated the BIG(Bi-Fe Garnet)films incorporating gold nanoparticles by PLD.We measured the transmission characteristics and XRD of the samples. We expecte to measure the Farady rotation of the samples, however, the related equipment is broken,and we have to wait for it to be fixed.But we have got some conclusions about the Farady rotation by the transmission picture and we are on the way to get close to the ultimate point.

       Approximately, we have concluded that the incorporating of gold nanoparicles actually enhanced the magneto-optical Farady effect by introducing Plasmon Resonance effect compared to those samples which we did not sputter gold nanoparticles on.It is a promising physical effect which is potential to be used in isolators integrated in photo-communication system.

参考文献：

[1]I. D. Mayergoyz, G. Lang, L. Hung, S. Tkachuk, C. Krafft, and O. Rabin,J. Appl. Phys. 107, 09A925 (2010).

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[5] H. Uchida, Y. Masuda, R. Fujikawa, A. V. Baryshev, and M. Inoue,

J. Magn. Magn. Mater. 321, 843 (2009).

Passively Mode-Locked Tm-Doped Fiber Lasers at 2.0μm wavelength

Xuliang Fan

Supervisor: Deyuan Shen

Passively mode-locked fiber lasers at ~1μm and ~1.5μm as simple and economic sources have been extensively investigated in last decades. Various mode-locking regimes and mode-locking mechanisms have been discovered, and various pulses have been demonstrated experimentally and theoretically in these regimes. Mode-locked thulium-doped fiber lasers have attracted a great deal of interest as novel laser sources, due to their potential applications in eye-safe radar, medicine, remote sensing and 3-5μm optical parameter oscillator.

The development of mode-locked Tm-doped fiber lasers at 2μm wavelength range are classified in this report, which includes the technical approach of the mode-locked pulse output and the characteristics of the passively mode-locked fiber laser output. Meanwhile, several main mode-locked techniques are introduced, such as nonlinear polarization rotation, and saturable absorber.

We have built a numerical model to investigate the characteristics of the pulse evolution in a passively mode-locked Tm-doped fiber laser by extending the conventional Ginzburg-Landau equation model through incorporating the cavity features. Numerical simulations on multiple-soliton generation are demonstrated in a soliton fiber laser passively mode locked by using the nonlinear polarization rotation technique. For a certain resonator configuration, by increasing the pump power, new solitons could be generated one by one in the cavity.

参考文献：

[1]Q. Wang, J. Geng, T. Luo, and S. Jiang, Optical Letters 34(23), (2009).

[2] D.Y. Tang, L.M. Zhao, B. Zhao, and A.Q. Liu, Phys. Rev.A72, 043816(2005).

[3]L.M. Zhao, D.Y. Tang, and J. Wu, Opt. Lett. 31, 1788(2006).

[4] K. Kieu, and F.W. Wise, Opt. Express 16(15), pp.11453-11458, July 21, 2008.

[5]Ying An, Deyuan Shen, Wei Zhao, and Jingyu Long, Optics Communications 285(2012), pp.1949-1953.

[6]B. Oktem, C. Ulgudur, F.O. Ilday, Nature Photonics 4(2010)307.

[7]W.H. Renninger, A, Chong, F.W. Wise,Journal of the Optical Society of America B 27(10)(2010)1978.

[8] K. Kien, F.W. Wise. Opt. Express, 2008 July 21; 16(15): 11453–11458.

[9]Liu, X. et al. Versatile multi-wavelength ultrafast fiber laser mode-locked by carbon nanotubes. Sci. Rep. 3, 2718(2013).

[10] Rajesh Kadel et al. Frontiers in optics 2013,Laser Science,2013.

[11] Andreas Wienke et al. Optics Letters Vol.7, No.(13), pp.2466-2468(2012).