Reardon Christopher Paul, Rey Isabella H, Welna Karl, O'Faolain Liam, Krauss Thomas F
School of Physics & Astronomy, University of St Andrews, UK.
J Vis Exp. 2012 Nov 30(69):e50216. doi: 10.3791/50216.
Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view and for its considerable potential for practical applications. Slow light photonic crystal waveguides, in particular, have played a major part and have been successfully employed for delaying optical signals(1-4) and the enhancement of both linear(5-7) and nonlinear devices.(8-11) Photonic crystal cavities achieve similar effects to that of slow light waveguides, but over a reduced band-width. These cavities offer high Q-factor/volume ratio, for the realization of optically(12) and electrically(13) pumped ultra-low threshold lasers and the enhancement of nonlinear effects.(14-16) Furthermore, passive filters(17) and modulators(18-19) have been demonstrated, exhibiting ultra-narrow line-width, high free-spectral range and record values of low energy consumption. To attain these exciting results, a robust repeatable fabrication protocol must be developed. In this paper we take an in-depth look at our fabrication protocol which employs electron-beam lithography for the definition of photonic crystal patterns and uses wet and dry etching techniques. Our optimised fabrication recipe results in photonic crystals that do not suffer from vertical asymmetry and exhibit very good edge-wall roughness. We discuss the results of varying the etching parameters and the detrimental effects that they can have on a device, leading to a diagnostic route that can be taken to identify and eliminate similar issues. The key to evaluating slow light waveguides is the passive characterization of transmission and group index spectra. Various methods have been reported, most notably resolving the Fabry-Perot fringes of the transmission spectrum(20-21) and interferometric techniques.(22-25) Here, we describe a direct, broadband measurement technique combining spectral interferometry with Fourier transform analysis.(26) Our method stands out for its simplicity and power, as we can characterise a bare photonic crystal with access waveguides, without need for on-chip interference components, and the setup only consists of a Mach-Zehnder interferometer, with no need for moving parts and delay scans. When characterising photonic crystal cavities, techniques involving internal sources(21) or external waveguides directly coupled to the cavity(27) impact on the performance of the cavity itself, thereby distorting the measurement. Here, we describe a novel and non-intrusive technique that makes use of a cross-polarised probe beam and is known as resonant scattering (RS), where the probe is coupled out-of plane into the cavity through an objective. The technique was first demonstrated by McCutcheon et al.(28) and further developed by Galli et al.(29).
在过去十年中，慢光一直是光子学领域的热门话题之一，无论是从基础研究的角度还是其巨大的实际应用潜力，都引发了人们极大的兴趣。特别是慢光光子晶体波导发挥了重要作用，并已成功用于延迟光信号（1-4）以及增强线性（5-7）和非线性器件（8-11）的性能。光子晶体腔与慢光波导具有相似的效果，但带宽较窄。这些腔具有高的品质因数/体积比，可用于实现光泵浦（12）和电泵浦（13）的超低阈值激光器以及增强非线性效应（14-16）。此外，还展示了具有超窄线宽、高自由光谱范围和创纪录的低能耗值的无源滤波器（17）和调制器（18-19）。为了获得这些令人兴奋的结果，必须开发一种稳健且可重复的制造工艺。在本文中，我们深入研究了我们的制造工艺，该工艺采用电子束光刻来定义光子晶体图案，并使用湿法和干法蚀刻技术。我们优化的制造工艺可得到不存在垂直不对称且具有非常好的边缘壁粗糙度的光子晶体。我们讨论了改变蚀刻参数的结果以及它们对器件可能产生的有害影响，从而得出一条可用于识别和消除类似问题的诊断途径。评估慢光波导的关键是对传输和群折射率谱进行无源表征。已经报道了各种方法，最显著的是解析传输谱的法布里 - 珀罗条纹（20-21）和干涉测量技术（22-25）。在这里，我们描述了一种将光谱干涉测量与傅里叶变换分析相结合的直接宽带测量技术（26）。我们的方法因其简单性和强大功能而脱颖而出，因为我们可以对带有接入波导的裸光子晶体进行表征，无需片上干涉组件，并且该装置仅由一个马赫 - 曾德尔干涉仪组成，无需移动部件和延迟扫描。在表征光子晶体腔时，涉及内部光源（21）或直接耦合到腔的外部波导（27）的技术会影响腔本身的性能，从而使测量结果失真。在这里，我们描述了一种新颖的非侵入性技术，该技术利用交叉偏振探测光束，称为共振散射（RS），其中探测光束通过物镜从平面外耦合到腔中。该技术最早由麦卡琴等人（28）展示，并由加利等人（29）进一步发展。
