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群速度色散会限制太赫兹量子级联激光器频率梳的稳定以及频谱宽度. 对于太赫兹量子级联激光器频率梳, 其色散主要由器件增益、波导损耗、材料损耗引起. 研究基于4.2 THz量子级联激光器双面金属波导结构, 通过建立德鲁德模型, 利用有限元法计算了激光器的波导损耗; 器件未钳制的增益由费米黄金定则计算得到, 结合增益钳制效应, 计算了器件子带电子跃迁吸收以及镜面损耗, 得到了器件钳制后的增益; 利用Kramers-Kronig关系得到了器件的增益、波导损耗、材料损耗引起的色散, 结果表明器件的激射区域存在非常严重的色散(–8 × 105—8 × 105 fs2/mm). 同时, 计算了一种基于Gires-Tournois干涉仪结构的色散, 结果表明, 该结构的色散具有周期性, 可以用于太赫兹量子级联激光器的色散补偿.The frequency comb which is characterized by equally-spaced frequency lines with high mode coherence has received much attention since its first demonstration in near-infrared and optical frequency range. In the terahertz frequency range, the electrically-pumped terahertz quantum cascade laser (THz QCL) based on semiconductors is an ideal candidate for achieving frequency comb operation in a frequency range between 1 THz and 5 THz. The group velocity dispersion (GVD) is a key factor for the frequency comb. A higher GVD can pull the frequencies from their equidistant values and limit the comb bandwidth. Therefore the laser dispersion needs to be compensated for in order to make the total GVD sufficiently low and flat, such as using a Gires-Tournois interferometer (GTI) or the double chirped mirror (DCM). However, a successful design still depends on the knowledge of the total GVD in the laser. In this paper, we show how to calculate the GVD in metal-metal waveguide THz QCLs by taking into account the dispersions from the GaAs material, the waveguide, and the laser gain, which conduces to the understanding of the frequency comb behavior. The waveguide loss is modelled by the finite element method. The loss due to intersubband absorption is calculated by Fermi's gold rule. All the losses, i.e., waveguide loss, mirror loss, and intersubband absorption loss, are summed up to calculate the clamped gain. The material loss can be calculated by using the reststrahlen band model. Because of these losses and gain, the refractive index needs to be replaced by a complex refractive index. The real part of the complex refractive index is the refractive index, which can be calculated from the Kramers-Kronig relationship that connects the loss or gain with the refractive index. Then the GVD introduced by the material loss, waveguide loss, and clamped gain can be finally calculated. The results show that the total GVD of THz QCL is approximately –8 × 105~8 × 105 fs2/mm which is strongly determined by the clamped gain. Finally, the developed numerical model is employed to study the dispersion compensation effect of a GTI mirror which is coupled into a QCL gain cavity. The design of the THz QCL based on GTI structure is more flexible and feasible than that of the DCM. The result shows that by carefully designing the geometry of GTI, the dispersion of a THz QCL can be compensated for, thus achieving the broadband terahertz frequency combs.
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Keywords:
- terahertz /
- quantum cascade laser /
- frequency comb /
- dispersion
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图 1 由于GVD引起的频率偏移Δ与模式数目m的关系
Fig. 1. The relation between frequency offset Δ and mode numbers m.
图 3 (a)计算得到的波导损耗
${\alpha _{\rm{W}}}$ 与频率的关系; (b)等效折射率与频率的关系Fig. 3. (a) Simulated the relationship between waveguide loss
${\alpha _{\rm{W}}}$ and frequency; (b) the relation between the effective refractive index and frequency.
图 5 子带电子吸收随频率变化的关系
Fig. 5. The relation between intersubband absorption and frequency.
图 6 材料折射率与频率的关系
Fig. 6. The relation between the material refractive index and frequency.
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[1] Udem T, Holzwarth R, Hansch T W 2002 Nature 416 233
Google Scholar
[2] Faist J, Capasso F, Sivco D L, Sirtori C, Hutchinson A L, Cho A Y 1994 Science 264 553
Google Scholar
[3] Williams B S 2007 Nature Photon. 1 517
Google Scholar
[4] Diddams S A 2010 J. Opt. Soc. Am. B-Opt. Phys. 27 B51
Google Scholar
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[6] Villares G F F 2016 Ph. D. Dissertation (Zurich: Swiss Federal Institute of Technology Zurich)
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Google Scholar
[8] Vitiello M S, Scalari G, Williams B, De Natale P 2015 Opt. Express 23 5167
Google Scholar
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Google Scholar
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Google Scholar
[11] Treacy E B 1969 IEEE J. Quantum Electron. QE 5 454
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Google Scholar
[13] Fork R L, Martinez O E, Gordon J P 1984 Opt. Lett. 9 150
Google Scholar
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Google Scholar
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Google Scholar
[16] Tempea G, Krausz F, Spielmann C, Ferencz K 1998 IEEE J. Sel. Top. Quantum Electron. 4 193
Google Scholar
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Google Scholar
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Google Scholar
[19] Faist J, Villares G, Scalari G, Rösch M, Bonzon C, Hugi A, Beck M 2016 Nanophotonics 5 272
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Google Scholar
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Google Scholar
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Google Scholar
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Google Scholar
[24] Rösch M, Scalari G, Beck M, Faist J 2014 Nature Photon. 9 42
[25] Bidaux Y, Sergachev I, Wuester W, Maulini R, Gresch T, Bismuto A, Blaser S, Muller A, Faist J 2017 Opt. Lett. 42 1604
Google Scholar
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Google Scholar
Zhu Y H, Li H, Wan W J, Zhou T, Cao J C 2017 Acta Phys. Sin. 66 099501
Google Scholar
[27] Weber E R, Willardson R K, Liu H, Capasso F 1999 Intersubband Transitions in Quantum Wells: Physics and Device Applications (Vol. 62) (Beijing: Academic Press)
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Google Scholar
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[30] Gires F, Tournois P 1964 Comptes Rendus Hebdomadaires Des Seances De L Academie Des Sciences 258 6112
[31] Lu Q Y, Manna S, Wu D H, Slivken S, Razeghi M 2018 Appl. Phys. Lett. 112 141104
Google Scholar
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