A mechanisms study of electroluminescence and field-effect enhanced electroluminescence in silicon nanocrystals

Jiarong Chen

supervisor：Ming Lu

Si light emissions mainly refer to photoluminescence and electroluminescence of Si-nc. As compared to the PL of Si-nc, additional processes of carrier injection and transport exist for the EL, which make the EL process more complicated than the PL one. In fact, the origin of the EL of Si-nc has not been fully understood yet. Three mechanisms are usually applied at present. One mechanism states that the observed EL or its high energy component is due to the defects within SiO2 . The other two are the model of band-filling under bipolar tunneling conditions, and that of Si-nc size selection by the injected electron energy, respectively.

Spectral shift in peak position of electroluminescence spectrum of Si-nc with respect to its photoluminescence counterpart has been often observed. Explanations for the spectral difference are different for different EL mechanisms adopted. I will introduce three EL mechanisms that are mainly applied nowadays, i.e., the model of defect light-emission, that of band-filling, and that of Si-nc size selection by the carrier energy. Different Si-nc samples and working conditions are designed and their EL and PL emissions monitored according to the predictions of the three models.

Meanwhile，practical Si light-emitting devices are indispensably important in the field of Si photonics. However, how to efficiently increase the brightness of Si LED, still remains a great challenge. The bottleneck problem lies basically in the poor charge transport within the device. A number of approaches have been proposed to improve the situation, such as increasing Si-NC density, preparing surface nanostructures, and designing novel charge transport channels. However, most of the current approaches focus on the optimization of charge tunneling through the dielectric barriers of the active layer, but how to facilitate the charge transport in the charge injection region has not received sufficient attention so far. Since the charge injection region makes up a part of the whole charge transport route, the enhancement of charge transport there should be equally important for achieving high brightness Si-NC LEDs. we propose an approach of field effect on the enhancement of charge transport in the charge injection regions within Si-NC LEDs. The fields are built at the interface between the active layer and the introduced i-type Si layer, and that between the p-type Si substrate and the introduced Al2O3 layer, for enhancements of electron and hole transports, respectively. The interfacial fields are so managed that their directions are the same as that of the forwardly biased electric field.

参考文献:

[1] G. R. Lin, Y. H. Pai, C. T. Lin, C. C.Chen, Appl. Phys. Lett. 2010, 96, 263514.

[2] K. Y.Cheng, R.Anthony, U. R.Kortshagen, R. J. Holmes, Nano Lett. 2011, 11, 1952.

[3] L. Ding, M. B.Yu, X. G.Tu, G. Q.Lo, S.Tripathy, T. P. Chen, Opt. Express 2011, 19, 2729.

[4] R. J.Anthony, D. J.Rowe, M.Stein, J.Yang, U. Kortshagen, Adv. Func. Mater. 2011, 21, 4042.

[5] B. G. Lee, D. Hiller, J. W. Luo, O. E. Semonin, M. C. Beard, M. Zacharias, P. Stradins, Adv. Func. Mater. 2012, 22, 3223.

[6] D. Li, Y. B. Chen, Y. Ren, J.Zhu, Y. Y.Zhao, M. Lu, Nanoscale Res. Lett. 2012, 7, 200.

[7] D. Li, Y. B. Chen, M. Lu, Mater. Lett. 2012, 89, 11.

[8] D. C.Wand, J. R.Chen, J. Zhu, C. T. Lu, M. Lu, J. Nanopart. Res. (in press) DOI: 10.1007/s11051-013-2063-x.

A brief introduction to wavefront shaping and its applications with a Spatial Light Modulator (SLM)

Yuchen Xu

Supervisor: Hao Zhang

Generally speaking, wavefront shaping exists everywhere in optical experiments. For example, when one focuses a beam of parallel light using a convex lens, the wavefront of the incident light is shaped, from plane to spherical. However, this kind of shaping is called “uniform” or “highly symmetric”. Often is the case that when we speak of wavefront shaping we mean some non-uniform amplitude or phase manipulations on the incident light, which can be accomplished by a device providing spatially variant modulation for light like a spatial light modulator (SLM). An SLM is, in fact, a matrix of amplitude or phase modulating pixels. SLM manifests its flexibility on space and spectrum by controlling each pixel separately. Liquid crystal is usually used to produce those pixels due to its special photoelectric properties.

SLM can be applied to produce holographic optical tweezers, generate cylindrical vector beams, and even image through a random scattering media. These are all based on wavefront shaping. In addition, we can combine cylindrical vector beams with holographic optical tweezers to acquire better performance for particle manipulation by using more than one SLMs.

In this report I will introduce the principle and operation of a pure phase modulating SLM. Based on that, a Gerchberg-Saxton algorithm gives the phase hologram for the production of holographic optical tweezers. The method of generating an arbitrary cylindrical vector beam using single SLM is introduced after that. At last, how to image through a random scattering media is introduced which can be attributed to reverse thinking on the spatial flexibility of SLM.

参考文献:

[1] 马百恒，全息光镊及相关技术的理论与实验研究，中国科学院研究生院博士论文，2012；

[2] X. L. Wang, J. P. Ding, W. J. Ni, C. S. Guo and H. T. Wang, Generation of arbitrary vector beams with a spatial light modulator and a common path interferometric arrangement, Opt. Lett., 32, 24, 2007;

[3] I. M. Vellekoop, A. Lagendijk, A. P. Mosk, Exploiting disorder for perfect focusing, Nat. Photon., 5, 320, 2010.

Resonantly pumped superfluorescence in Tm-doped short fiber with watt-level output

Nanyi Chen

Supervisor: Deyuan Shen

The amplified spontaneous emission (ASE) fiber source operating in the 2 μm spectral region plays an important role in the environmental and biomedical field [1] for its coincidence with the water absorption band [2], and is interested in many areas including spectroscopy, gas sensing, low coherence interferometry and medical imaging [3]. Compared with numbers of long fiber structure as the above cases, short geometry drew very few attention and so far only two papers employed fibers within one meter to set up a Tm: fiber ASE source [4] [5] but had poorer performance in terms of output power and slope efficiency, which is primarily due to the inadequate pump absorption in a short fiber. However short fibers can be favorable for avoiding bending losses and optical nonlinearity [6]. Moreover, the background loss of the fiber could be ignored for a short fiber length [7].

Here I report on a broadband superfluorescent in a Tm-doped fiber with a double-pass backward pumped arrangement. 1.56 W of single-ended ASE output was obtained, corresponding to a slope efficiency of 39.6% and the bandwidth (FWHM) at the maximum output level is 55 nm.

参考文献：

[1]   K. Oh, A. Kilian, L. Reinhart, Q. Zhang, T. F. Morse, and P. M. Weber, “Broadband superfluorescent emission of the 3H4→3H6 transition in a Tm-doped multicomponent silicate fiber,” Opt. Lett., vol. 19, no. 15, pp. 1131-1133, 1994.

[2]   J. A. Curcio and C. C. Petty, “The Near Infrared Absorption Spectrum of Liquid Water,” J. Opt. Soc. Am., vol. 41, no. 5, pp. 302-304, 1951.

[3]   B. E. Bouma, L. E. Nelson, G. J. Tearney, D. J. Jones, M. E. Brezinski, and J. G. Fujimoto, “Optical coherence tomographic imaging of human tissue at 1.55 μm and 1.81 μm using Er- and Tm-doped fiber sources,” J. Biomed. Opt., vol. 3, no. 1, pp. 76-79, 1998.

[4]   Y. H. Tsang, A. F. E. Sherif, and T. A. King, “Broadband amplified spontaneous emission fiber sources near 2 μm using resonant in-bank pumping,” J. Modern Opt., vol. 52, pp. 109-118, 2005.

[5]   P. W. Kuan, K. Li, G. Zhang, X. Wang, L. Zhang, G. Bai, Y. Tsang and L. Hu, “Compact broadband amplified spontaneous emission in Tm3+-doped tungsten tellurite glass double-cladding single-mode fiber,” Opt. Materials Express, vol. 3, no. 6, pp. 723-728, 2013.

[6]   Y. Tang, Y. Yang, X. Cheng, and J. Xu, “Short Tm3+-doped fiber lasers with watt-level output near 2 µm,” Chin. Opt. Lett., vol. 6, no. 1, pp. 44-46, 2008.

[7]   H. Wana, D. Zhanga, and X. Suna, “Stabilization of a superfluorescent fiber source with high performance erbium doped fibers,” Opt. Fiber Technol., vol. 19, no. 3, pp. 264–268, 2013.