그래핀 성장 및 소형 광도파로 펄스 레이저 발진용 포화 흡수체로의 응용
Graphene growth and application as efficient saturable absorber in compact pulsed channel waveguide lasers
- 주제(키워드) graphene , channel waveguide laser
- 발행기관 아주대학교
- 지도교수 이상민
- 발행년도 2016
- 학위수여년월 2016. 2
- 학위명 석사
- 학과 및 전공 일반대학원 에너지시스템학과
- 실제URI http://www.dcollection.net/handler/ajou/000000021989
- 본문언어 영어
- 저작권 아주대학교 논문은 저작권에 의해 보호받습니다.
초록/요약
Graphene, a one-atom thick layer of sp²-bonded carbon atoms arranged in a two-dimensional hexagonal lattice, is a special semimetal with zero bandgap and a linear energy-momentum dispersion relation near the Dirac point. Such a unique electronic structure of graphene allows it to exhibit remarkable properties including ultrabroad absorption band, large third-order nonlinearity, an ultrafast response to the photo-excitation, and high optical damage thresholds, which allow graphene to be utilized in optical devices. When used in saturable absorbers (SAs) of laser systems, monolayer graphene with optical absorption of 2.3% is more preferable than the multilayer graphene because lasers are very sensitive to optical losses. Among the methods of graphene growth, chemical vapor deposition (CVD) is the most suitable candidate for growing high-quality monolayer graphene as large as over a few centimeter scale. Unlike mechanically exfoliated or solution-processed graphene, CVD graphene can be transferred onto any substrate regardless of its size and location. The number of layers and the quality of the graphene synthesized using CVD are determined by the type and state of the catalytic metal substrate, the pressure and temperature of the CVD chamber, and the flow rate of hydrogen or methane gases. In this work, the conditions for the monolayer graphene growth on pre-cleaned copper foil are optimized by adjusting the total pressure and the flow rates of hydrogen and methane. The grown graphene is confirmed to be a high quality monolayer by optical microscopy, Raman spectrum spectroscopy, and linear transmission measurements. In addition, the grown graphene is found to have proper modulation depth and optical response time by nonlinear transmission and pump-probe measurements. In recent years, compact optical waveguide lasers consisting of a gain medium with a waveguide structure and laser cavities with a size of several mm have become the most promising integrated laser sources due to the light confinement inside a small volume, good overlap between the pump and the laser modes, and good thermal management. To date, semiconductor saturable absorber mirrors (SESAMs), carbon nanotubes (CNTs), and graphene oxides (GOs) have been used as SAs in waveguide laser. However, since most SESAMs are reflection-type SAs, it is difficult to form diverse laser cavities in compact waveguide lasers. Although CNTs and GOs can possibly be used as both transmission- and reflection-type SAs, they can damage the waveguide when covered onto its end facet or surface via spin-coating. In the present work, we demonstrate the three types of Q-switched Yb:YAG channel waveguide lasers that utilize the advantages of the CVD-grown graphene. The three types of laser configurations are as follows: a waveguide by itself as a resonant cavity, a waveguide with an attached incoupling mirror, and a waveguide with an incoupling mirror and an output coupler (OC) (denoted by Setup I, Setup II, and Setup III, respectively). The monolayer graphene is transferred onto the end facets of the waveguides in Setups I and II, and also onto the surface of the OC, a previously established form of SAs, in Setup III. The highest laser slope efficiency of 49% and the maximum pulse energy of 78 nJ are obtained for Setup I, and the shortest pulse duration of 79 ns is obtained for Setup III. Nevertheless, the most stable Q-switched operation is performed with Setup II with maximum average output power of 85 mW and highest repetition rate of 1.33 MHz. The pump power is varied depending on the repetition rate and the pulse duration for all laser configurations. Such a behavior is typically observed in lasers passively Q-switched by fast SAs. Therefore, diverse cavity configurations of the compact Yb:YAG waveguide lasers with graphene-based SAs exhibit stable Q-switched ns pulse generation. Moreover, we demonstrate, for the first time, a pulsed laser operation using only a waveguide by itself with monolayer graphene deposited onto its end facet without any mirror.
more목차
1. Introduction 1
References 4
2. Graphene applicable as saturable absorber: Growth, devices fabrication and characteristics 8
2.1 Properties of graphene 9
2.2 Growth of monolayer graphene using chemical vapor deposition technique 15
2.3 Fabrication of graphene-based saturable absorbers 22
2.4 Optical characteristics of the grown monolayer graphene 25
2.4.1 Optical microscopy 25
2.4.2 Linear transmission and absorption 28
2.4.3 Raman spectroscopy 30
2.4.4 Nonlinear optical characterization of graphene-based saturable absorbers 33
References 37
3. Optical channel waveguides as laser gain medium 44
3.1 Properties of channel waveguides 44
3.2 Fabrication of channel waveguides by the femtosecond-laser inscription method 46
References 48
4. Monolayer graphene Q-switched Yb:YAG channel waveguide lasers 50
4.1 Three different laser configurations with graphene as saturable absorbers. 51
4.2 Operational characteristics of the Yb:YAG channel waveguide lasers Q-switched by graphene-based saturable absorbers 55
References 63
5. Conclusions and prospects 65
국문 요약 69
