嘉文博译
留学文书20类:详解与范例

Research Proposal

留学文书20类:详解与范例


留学文书的种类:嘉文博译Research Proposal范例


A.   Topic:
This research proposal suggests research on micro-laser on disorder nitride semiconductors material.

B.   Review of Literature:
In 1968, the pioneer work of Lethokov [1] predicted that laser action could be realized in randomly distributed scattering media. In the 1980s, Markushev [2] et al. observed lasing in Nd-doped laser crystal powder. They found a single particle, with size much larger than the optical wavelength, served as a laser resonator. Since then, there has been much work on powder lasers [3]. In 1994, Lawandy [4] applied 530nm laser pulse to pump the colloidal solution consisting of TiO2 particles suspended in Rodanmine methanol solution. Once the gain approached and surpassed the threshold value,laser action could be observed over the surface of the liquid, which triggered [4-5] many experimental and theoretical studies on light amplification in diffusive media. The term “random laser” appeared. However, this effect isn’t the result of resonant feedback due to localized modes, and the linewidth narrowing of the emission spectrum was interpreted via nonresonant feedback of spontaneous emission amplified.

In order to realize laser with resonant feedback in random solid media, Cao et al. [6] switched to a solid luminescence semiconductor. When the pump beam (the fourth harmonic =266nm Nd: YAG laser) is focused on the ZnO polycrystalline thin film surface and its intensity exceeds a threshold value, narrow discrete sharp peaks emerges in the emission spectrum, and simultaneously, a couple of bright micrometer-sized (about 0.3) spots randomly flashes in the image of the emitted light distribution, which are coherent, localized laser spots. Since this lasing action is independent of the shape and surface of ZnO powder clusters or polycrystalline film, a conclusion can be easily obtained that the confinement of light are caused by multiple scattering inside the random medium, not by reflection at the surface of the cluster [7].

For this novel lasing action, our work has revealed its mechanism [8]: EM wave (electromagnetic wave) multiple scattering in disorder medium induces a localized coherent eigen-mode, which can trap the light wave in a local region. When the gain surpasses a threshold, it should drive system to select this eigen-mode to emit light wave, namely, forming random laser. Different from the conventional laser, random laser’s resonator is made up of ZnO particles. Our research has extracted a core principle for this novel laser behavior that the wave multiple scattering not only induces localized mode, but bonds localization with coherence. What’s more important, our theory clearly reveals that the perfect coherence and localization of wave can greatly suppress the energy loss. It means that if the strong localization mode in disordered medium is obtained, it should extremely reduce the threshold of random laser as well as suppress the energy loss, which provide a new hint to obtain laser with lower threshold.

As micro-laser has important applications to integrated photonic circuits, several types of micro-lasers have been developed [9]. The key issue is to localize light in a small volume with a dimension on the order of optical wavelength. The light inside two distributed Bragg reflectors are confined by the vertical cavity surface-emitting laser. And the microdisk laser utilizes total internal reflection at the edge of a high index disk to form whispering gallery modes. In GaAs two-dimensional photonic band-gap defect mode laser achieves lateral confinement of light by Bragg scattering in a two-dimensional periodic structure. The fabrication of these micro-lasers requires expensive state-of-the-art semiconductor growth and microfabrication facilities. However, the random laser is made up of a disorder active medium, and the laser light is confined in micrometer-sized volume (0.3 and 0.7), which makes the fabrication of such a micro-laser much easier and cheaper than that of most micro-lasers.

As mentioned above, since wave localization always entangles with coherence, EM wave strong localized mode shortens its localization length, simultaneously, reduce the gain length and suppress energy loss. Thus, a stronger multiple scattering and localization becomes a critical precondition to achieve smaller random micro-laser with lower threshold. For the random micro-laser that is composed of the active crystallite, there are several approaches to overcome the difficulties in fabricating strong localization mode: 1. Utilize new etching technology to fabricate smaller active crystallite with nanometer-sized diameter (the conventional is 50-100 nanometers.); 2. Choose more excellent luminescence semiconductor to construct the micro-cavity of random laser.

Nitride semiconductor, such as GaN, is an III-V semiconductor with properties similar to ZnO. Like ZnO it grows with a hexagonal crystal structure, displays a piezoelectric effect, and has a band gap in the near UV range (3.4 eV) at room temperature. These similarities make it possible for us to attain random micro-laser on this nitride semiconductor. Meanwhile, GaN’s poor match of lattice with the foreign substrates and the thermal expansion coefficients provide the wonderful preconditions to prepare random polycrystalline thin film. Importantly, high refractive index (>2.5) of GaN and its disordered structure will lead to stronger multiple scattering in active polycrystalline film and make it easier to form smaller random micro-laser. Furthermore, GaN alloyed with InN and AlN allows the bandgap energies to vary anywhere between 1.9 eV and 6.2 eV, which extends the fascinating stage to fabricate different nitride-based micro-lasers from visible light to ultraviolet.

Based on these findings, as well as the similar luminescence between GaN and ZnO, it is possible for us to obtain a perfect random micro-laser with lower threshold and smaller size. Additionally, new chemical etching technology and the mechanism of random laser greatly enhances the opportunity to achieve random micro-laser with lower threshold. Continued research may open the door for micro-laser with electric pump.

C.   Key technology:
How to fabricate micro-laser with nitride semiconductors? If it is completed, how to achieve nitride-based micro-laser with lower threshold and smaller size?

D.   Hypothesis: 
When the pump intensity introduced to the random nitride semiconductors film surpasses the threshold, it is truly possible to achieve the micrometer-sized laser.


E.    Method:
(1)   Theory
In my theory on random laser, we have showed the mechanism of random laser that disorder-induced scattering forms the localized mode that entangles with coherence. Once the gain surpasses a threshold, the localized mode should serve as regular mode to emit localized coherent light beam, namely, random laser. Furthermore, stronger localized mode of EM wave not only has ability to confine the wave in a smaller local region and shorten the localization length, but also reduces the gain length, which open a valid approach to fabricate microlaser. The feasible way to get it is to utilize the smaller active medium particles to induce stronger coherent localization. And then, the smaller localization length and smaller medium particles leads into smaller micro-resonator. Meanwhile, new developed coherent phase diagram will provide a precise prediction on laser modes, behavior of micro-resonator and energy distribution. The detailed calculations on the model for random laser will provide useful guidance for devise fabrication and design.


(2)   Practice
•   Sample Fabrication:
In research on luminescence of porous silicon, researchers have successfully used HF solution to etch silica slice and attained the random porous silica substrate. The random distribution of pores provides a good disordered structure for random laser. Recently, our group has developed a new chemical etching, an ultrasonic pulsed etching technology, to construct perfect disorder porous Si substrate with the nanometer-sized pore (Its diameter is about 10 nanometers, however, the conventional is 100 nanometers. Appendix II). Through adjusting the concentration of solution and the time of etching, we can carefully tune the size and density of pores in porous silicon substrate, and then produce various amorphous silicon substrates.

In order to fabricate nitride semiconductors polycrystalline thin film, we will use a pulsed excimer laser to ablate a hot pressed nitride semiconductors target in an ultrahigh vacuum chamber to deposit nitride semiconductors over the random silicon substrate. Furthermore, the poor match between substrate and nitride semiconductors also enhances the disorder of polycrystalline film. (Another feasible approach is to use ultrasonic HF electrochemical etching technology to etch nitride semiconductor film to form microstructure.)

When the third harmonics or the fourth harmonic of a pulsed mode-locked Nd: YAG laser is used to pump the polycrystalline film, disorder of the polycrystalline film and the smaller crystal grain will greatly reduce size of micro-resonator (At a rough estimate, it should be 0.05) and its threshold. Combining the different nitride semiconductor and porous silicon substrate, we can change frequency of laser and forming resonator with different size.


•   Measurement
a.   Scanning Electron Microscope (SEM) can be used to observe the image of the porous silica substrate and the random structure of nitride semiconductors polycrystalline thin film.
When gain approaches the threshold, the measured emission spectrum should become narrow. Once the threshold is exceeded, some sharp peaks emerge in the emission spectrum. When random laser occurs, we should:

b.   Measure the single-shot emission spectrum in the disordered nitride semiconductors polycrystalline;

c.   Measure the threshold of pump pulse energy of random laser;

d.   Measure the localized electric fields of random micro-laser, and then reveal the relation between localized mode and pump.


F.   Conclusion:
The excellent luminescence of GaN and mechanism of random micro-laser, make it highly possible to achieve random micro-laser. Based on ultrasonic chemical etching and high refractive index (>2.5) of GaN, we could easily fabricate the perfect GaN random micro-laser, and greatly reduce its size as well as threshold. In addition, by doping InN and AlN to adjust GaN’s bandgap, it is possible for us to obtain different micro random laser from visible range to ultraviolet. Moreover, one valid theory on random laser will guide us to design new random laser devises and open a convenient approach to achieve micro-laser.


G.   Reference:
1.   R. V. Ambartsumian, N. G. Basov, P. G. Kryukov, and V. S. Letokhov, “Lasers with nonresonant feedback,” IEEE J.
      Quantum Electron
,Vol. QE-2, pp. 442–446, 1966.V. S. Letokhov, Generation of light by a scattering medium with
      negative resonance absorption,” Sov. Phys.—JETP, Vol. 26, pp. 835–840, 1968.


2.   V. M. Markushev, V. F. Zolin, and C. M. Briskina, “Luminescence and stimulated emission of neodymium in sodium lanthanum
      molybdate powders,” Sov. J. Quantum Electron., vol. 16, pp. 281–283, 1986. V. M. Markushev, N. E. Ter-Gabrielyan, C. M. Briskina,
      V. R. Belan, and V. F. Zolin, “Stimulated emission kinetics of neodymium powder lasers,” Sov. J. Quantum Electron., vol. 20, pp.
      773–777, 1990.


3.   C. Gouedard, D. Husson, C. Sauteret, F. Auzel, and A. Migus, “Generation of spatially incoherent short pulses in laser-pumped
      neodymium stoichiometric crystals and powders,” J. Opt. Soc. Amer. B, vol. 10, pp 2358–2363, 1993; M. A. Noginov, N. E. Noginova,
      H. J. Caulfield, P. Venkateswarlu, T. Thompson, M. Mahdi, and V. Ostroumov, “Short-pulsed stimulated emission in the powders
     of NdAl3 (BO3)4, NdSc3(BO3)4, and Nd:Sr5(PO4)3F laser crystals,” J. Opt. Soc. Amer. B, vol. 13, pp 2024–2033, 1996; M. A.
      Noginov, S. U. Egarievwe, N. E. Noginova, H. J. Caulfield, and J. C. Wang, “Interferometric studies of coherence in a
      powder laser,” Opt. Mater., vol. 12,pp. 127–134, 1999.


4.   N. M. Lawandy, R.M. Balachandran, A. S. L. Gomes, and E. Sauvain, “Laser action in strongly scattering media,” Nature, vol.
      368, pp 436–438, 1994; W. Sha, C-H. Liu, and R. Alfano, “Spectral and temporal measurements of laser action of rhodamine
      640 dye in stronglyscattering media,” J.Opt. Soc. Amer. B, vol. 19, pp. 1922–1924, 1994; G. van Soest, M. Tomita, and A. Lagendijk,
      “Amplifying volume in scattering media,” Opt. Lett., vol. 24, pp. 306–308, 1999; G. van Soest, F. J. Poelwijk, R. Sprik, and A.
      Lagendijk, “Dynamics of a random laser above threshold,” Phys. Rev. Lett., vol. 86, pp. 1522–1525, 2001.

5.   N. M. Lawandy, R. M. Balachandran, A. S. L. Gomes and E. Sauvain, et al, Nature 368, 436 (1994), D. S. Wiersma and A.
      Lagendijk, Phys. Rev. E 54, 4256 (1996).

6.   H. Cao, Y. G. Zhao, H. C. Ong, S. T. Ho, J. Y. Dai, J. Y. Wu, and R. P. H. Chang, “Ultraviolet lasing in resonators formed by
      scattering insemiconductor polycrystalline films,” Appl. Phys. Lett., vol. 73, pp 3656–3658, 1998; H. Cao, Y. Zhao, S. T. Ho, E.W.
      Seelig, Q. H.Wang, and R. P. H. Chang, “Random laser action in semiconductor powder”, Phys. Rev. Lett., vol. 82, pp.
      2278–2281, 1999; S. V. Frolov, Z. V. Vardeny, K. Yoshino, A. Zakhidov, and R. H. Baughman, “Stimulated emission in high-gain
      organic media,” Phys. Rev. B, vol. 59, pp. R5284–5287, 1999


7.   H. Cao, J. Y. Xu, E. W. Seelig, and R. P. H. Chang, “Microlasers made of disordered media,” Appl. Phys. Lett., vol. 76, pp.
      2997–2999, 2000; H. Cao, J. Y. Xu, S.-H. Chang, and S. T. Ho, “Transition from amplified spontaneous emission to laser action
      in strongly scatteringmedia,” Phys Rev. E., vol. 61, pp. 1985–1989, 2000; Y. Ling, H. Cao, A. L. Burin, M. A. Ratner, X. Liu, and
      R. P. H. Chang, “Investigation ofrandom lasers with resonant feedback,” Phys. Rev. A. vol. 64, pp. 63 808–63 815, 2000; J. X.
      Zhu, D. J. Pine, and D. A. Weitz, “Internal reflection of diffusive light in random media,” Phys. Rev. A, vol. 44, pp. 3948–3959,
      1991; H. Cao, J. Y. Xu, D. Z. Zhang, S.-H. Chang, S. T.Ho, E. W. Seelig, X. Liu, and R. P. H. Chang, “Spatial confinement of laser
      light in active random media,” Phys. Rev. Lett., vol. 84, pp. 5584–5587, 2000; C. M. Soukoulis, X. Jiang, J. Y. Xu, and H. Cao,
     “Dynamic response and relaxation oscillation in random lasers,” Phys. Rev. B, vol. 65, pp R41103–41 106, 2002; H. Cao, Y. Ling,
      J. Y. Xu, C. Q. Cao, and P. Kumar, “Photon statisticsof random lasers with resonant feedback,” Phys. Rev. Lett., vol. 86, pp
      4524–4527, 2001


8.   SZhen Ye, Sheng Li and Xin Sun, Phys. Rev. E 66, 045602 (2002). C. Vanneste and P. Sebbah “Selective Excitation of
      Localized Modes in Active Random Media” Phys. Rev. Lett., vol. 87, pp. 183903–183907, 2001; A. L. Burin, M. A. Ratner, H. Cao,
      and R. P. H. Chang,“Model for a random laser,” Phys. Rev. Lett., vol. 87, pp. 215 503–215 506, 2001; Sheng Li, Zi-Jun Wang,
      Liang-shan Chen, Thomas George, and Xin Sun, “Disorder-induced micro-resonator of random laser and its coherence”,
      submitted to Phys. Rev. Lett ( Abstractis in Appendix I ).


9.    J. L. Jewell, J. P. Harbison, A. Scherer, Y. H. Lee, and L. T. Florez, “Vertical-cavity surface-emitting lasers: Design,
      growth, fabrication,characterization,” IEEE J. Quantum Electron., vol. 27, pp. 1332–1346, June 1991; S. L. McCall, A. F. J. Levi,
      R. E. Slusher, S. J. Pearton, and R. A. Logan, “Whispering-gallery mode microdisk lasers,” Appl. Phys. Lett., vol. 60 pp. 289–291,
      1992. O. Painter, R. K. Lee, A. Scherer, A. Yariv, J. D. O’Brien, P. D. Dapkus, and I. Kim, “Two-dimensional photonic band-gap
      defect mode laser,” Science, vol. 284, pp. 1819–1821, 1999.


H.    Appendix:









Sample Etching Technology Current Density (mA/) Etching Time(s) Average diameter of pore (nm)
a

Direct current etching

50 180 33
b Pulsed anodic etching 50 360 26

c

Ultrasonic etching 50 180 20






嘉文博译郑重声明:

(1)     本网站所有案例及留学文书作品(包括“个人陈述”Personal Statement,“目的陈述”Statement of Purpose, “动机
          函”Motivation Letter,“推荐信”Recommendations / Referemces “, (小)短文”Essays,“学习计划”Study Plan,“研究计
          划”(Research Proposal),“签证文书”Visa Application Documents 及“签证申诉信”Appeal Letter等等),版权均为嘉文
          博译所拥有。未经许可,不得私自转载,违者自负法律责任。


(2)      本网站所有案例及留学文书作品(包括“个人陈述”Personal Statement,“目的陈述”Statement of Purpose, “动机
          函”Motivation Letter,“推荐信”Recommendations / Referemces “, (小)短文”Essays,“学习计划”Study Plan,“研究计
          划”(Research Proposal),“签证文书”Visa Application Documents 及“签证申诉信”Appeal Letter等等),版权均为嘉文
          博译所拥有。未经许可,不得私自转载,违者自负法律责任。仅供留学申请者在学习参考,不作其他任何用途。任何整句整段
          的抄袭, 均有可能与其他访问本网站者当年递交的申请材料构成雷同,而遭到国外院校录取委员会“雷同探测器”软件的检测。
          一经发现,后果 严重,导致申请失败。本网站对此概不负责。

北京市海淀区上地三街9号金隅嘉华大厦A座808B

电话:(010)-62968808 / (010)-13910795348

钱老师咨询邮箱:qian@proftrans.com   24小时工作热线:13910795348

版权所有 北京嘉文博译教育科技有限责任公司 嘉文博译翻译分公司 备案序号:京ICP备05038804号