Enhancing optical gains in Si nanocrystals via hydrogenation and cerium ion doping

Dong-Chen Wang

Supervisor：Ming Lu

An efficient Si light source is of vital importance for Si optoelectronics. Si nanocrystals embedded in SiO₂, or Si-NC:SiO₂, is a promising material for designing Si light sources due to its stable light emission, robust structure and optical gain feature. Many efforts have been paid to enhancing the brightness, i.e., photoluminescence (PL) and electroluminescence (EL), of Si-NCs, and a number of practical and efficient approaches have been developed, including hydrogenation, excess Si content modulation, impurity doping for energy transfer or nucleation, structure modification, and field-effect approach. However, few reports on the enhancement of optical gain in Si-NCs have been found. Si-based laser, whether photo-pumped or electro-excited, has been pursued for years as an ideal Si light source, unfortunately, its realization still remains an open problem. Intensive investigations on the optical gain enhancement in Si-NCs are necessary for developing Si-based lasers. In this work, we study the optical gain enhancement in Si-NCs from Si-NC:SiO₂ that have been hydrogenated or/and Ce³⁺ ion doped. Variable stripe length (VSL) technique is used to acquire the net gains.

At 0.3 W/cm² pumping power density of pulsed laser, net gains were observed together with gain enhancements after hydrogenation or/and Ce³⁺ ion doping; gains after loss corrections were between 89.52 and 341.95 cm^-1; and the photoluminescence (PL) lifetime was found to decrease with the increasing gain enhancement. At 0.04 W/cm² power density, however, no net gain was found and the PL lifetime increased with the increasing PL enhancement. The results were discussed according to stimulated and spontaneous excitation and de-excitation mechanisms of Si-NCs.

References

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2μm Thulium-Doped Fiber Laser Mode-Locked through Nonlinear Polarization Rotation

Qiyao Liu

Supervisor: Deyuan Shen

Thulium-doped fiber lasers (TDFLs) open a new wavelength range of stable, efficient emission around 2µm, which is part of the eye-safe wavelength region. Laser systems operated in this region have exceptional advantages for free space applications, as well as for medical applications due to the strong water absorption. Particularly, passively mode-locked TDFLs are currently attracting much attention as short-pulse infrared sources，because the broad (~100 nm) and relatively smooth fluorescence spectrum of TDFLs make it possible to generate very short pulses which could be the basis of frequency combs.

Various passively mode-locking techniques, such as nonlinear polarization rotation (NPR), nonlinear loop nonlinear method, and the semiconductor saturable absorber method, have been used to mode-lock the lasers. Among them, NPR tecnique is employed to obtain nonlinear transform-limited pulses called solitons by using the effect of nonlinear fiber birefringence, which is equivalent to realistic saturable absorber (SA). This kind of artificial SAs have been extensively employed and investigated to generate and explore optical solitons in last decades. However, most reported works are based on single-cladding Tm-doped and Tm-Ho-doped fiber with complex configuration. In order to simplified our experiment system and improve the pump absorption efficiency, we used double-cladding Tm-doped fiber as our gain medium to make the setup more reliable and compact pumped by a 793nm laser diode.

We have oberved stable optical solitons with pulse duration of several picoseconds in our all-fiber system operated around 2042nm. To the best of our knowledge, it is the first diode-cladding-pumped NPR mode-locked fiber laser operating around 2μm. Our results demonstrate that the NPR technique is wavelength independent and the whole system shows the reliability of long lifetime operation though NPR is considered to be environmental sensitive. The experiment setup and the results will be shown in this report, as well as some basic operation regimes and mechanisms.

Reference

[1]  R. Kadel and B. R. Washburn, Applied Optics, 51(27), 2012, 6465

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[4]  K. Kieu, and F.W. Wise, Opt. Express 16(15), 2008, 11453.

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Design of Centering System with Two Channels and  Error Analysis

Weisen Liu

Supervisor: Min Xu

The size and the focal length of camera objectives (e.g. cell phones or digital cameras) are becoming smaller and smaller. At the same time the quality requirements are increasing. Besides surface accuracy, the imaging quality of the complete optics is mainly influenced by the alignment errors of the single elements. TRIOPTICS has developed a new technology called MultiLens® in order to measure the centering errors of all single surfaces within an objective lens with up to 40 surfaces or more. ^[1-4]

During the measurement the deviation of each center or curvature with respect to a reference axis is measured. These data are further processed in order to provide the shift and tilt of an individual lens or group of lenses in respect to a given reference axis , we can assemble the lens according to the measurement result so that assembling efficiency is promoted.

A centering system by using collimation can satisfy demands of a wide range, low sensitivity, low accuracy ,and interference centering system has a character of small range ,high sensitivity ,high accuracy .^[6]we adopt the collimation method so that we can get a wide range and high accuracy compared to mechanical centering system .we design the centering system with two channel , the accumulative error is reduced and we can reset work rapidly centering efficiency is improved remarkably

There are many factors resulting in measurement error, the installation error of a work piece is an important factor of influencing measurement accuracy .Owing to the existence of installation error, measuring the same lens after each installation may result in difference.^[7] In order to reduce this installation error, we introduce the concept of the fitting of light axle. Thanks to the supply of the fitting of light axle, measurement accuracy of the optical system is promoted.

References

[1]Heinisch J, Dumitrescu E, Krey S. Novel technique for measurement of centration errors of complex completely mounted multi-element objective lenses[C] //SPIE Optical Metrology.

[2]Heinisch J, Langehanenberg P, Pannhoff H. Complete characterization of assembled optics with respect to centering error and lens distances[C]//SPIE Optical Metrology.

[3]Langehanenberg P, Dumitrescu E, Heinisch J, et al. Automated measurement of centering errors and relative surface distances for the optimized assembly of micro-optics[C] //SPIE Optical Metrology.

[4] research on measurement of lense centering error based on imaging process[C]SPIE,2007,6834

[5]patent no :US 876066B2;date of patent :Jun.24，2014

[6] Chao F, Yang X. Design of Centering System by Using Collimation and Interference with Two Channels[J]. Acta Photonica Sinica, 2012, 10: 009.

[7] Di H. Optical Axis Fitting Based on Matlab in Decentration Measurement of Lens[J]. Electronics Optics & Control, 2010, 4: 026.

[8] Keller H E. Objective lenses for confocal microscopy[M]//Handbook of biological confocal microscopy. Springer US, 2006: 145-161.

Time:  6:30 pm, Thursday, 2014.12.4

Location: Optical Building. Room 525