Highly Unidirectional Emission and Ultralow-Threshold Lasing from On-Chip Ultrahigh-Q Microcavities
Highly Unidirectional Emission and Ultralow-Threshold Lasing from On-Chip Ultrahigh-Q Microcavities
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来自片上超高 Q 微腔的高度单向发射和超低阈值激光
DOI：
10.1002/adma.201201229
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发表时间：
2012-09-11
影响因子：
29.4
通讯作者：
Gong, Qihuang
中科院分区：
文献类型：
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作者：
Jiang, Xue-Feng;Xiao, Yun-Feng;Gong, Qihuang
十多年来，利用微腔限制和操纵光子引起了人们的强烈研究兴趣。[1]最突出的例子是回音廊模式(WGM)微腔[2，3]，它通过沿着弯曲而光滑的表面连续的全内反射来限制光子。长的光子寿命(用高Q因子描述)、强场约束和面内发射特性使它们有望成为新型光源[4-9]和生化传感器的候选材料，能够检测到很少甚至单个的纳米颗粒。圆形WGM微腔的主要缺点是其旋转对称性导致其固有的各向同性发射。除了由两个或多个理想球形微腔组成的光子结构外，一个重要的解决方案是通过打破旋转对称性来使用变形微腔，[13-16]它不仅可以提供定向发射，而且可以通过自由空间光束高效而稳健地激发WGMS。[17-20]在芯片上制造变形微腔特别适合于高密度光电集成，但在实验中它们存在Q值低的问题。Q因子一般在10,000左右，甚至小于10,000[21-27]，受非自愿表面粗糙度的大的散射损失的限制。高Q因子在基础研究和片上光子应用中具有重要意义。在这里，通过图案转移技术和回流工艺确保了接近原子尺寸的微腔表面，我们在实验上展示了芯片上非掺杂二氧化硅变形微腔，它支持近单向发射和超过1亿的超高Q因子。通过掺铒和方便的自由空间激发，在这样一个Q因子约为300万的微腔中实现了低阈值的单向微激光。变形的微腔是由硅片上2μm厚的二氧化硅层制成的。首次采用两步干法刻蚀工艺和激光回流工艺相结合的方法转移图形，获得了具有设计形状和超光滑腔面的微腔。流程细节如图1a所示。首先，我们有目的地从期望的变形环形边界R(ϕ)出发，通过在所有极角的半径中增加额外的15μm来设计掩模图案。通过光学光刻和缓冲HF刻蚀，在硅片上形成变形的二氧化硅圆盘，很好地继承了掩模图形。随后，将得到的二氧化硅盘暴露在2.7Torr的XeF2气体中，以大约15μm的速度刻蚀下面的硅。在此过程中，二氧化硅盘保持其原始形状，并用作下面硅的蚀刻掩模；因此，在每个盘的下面形成硅柱。由于硅的各向同性干法刻蚀，下面的硅柱获得了所需的图案。然后，用CO2脉冲激光照射使硅片回流，使硅片沿硅柱外围熔化，并使硅片塌陷成具有超光滑表面的环状。[28]最重要的是，所需形状如预期的那样从硅柱完美地转移回二氧化硅环状。最后，采用第二次XeF2干法刻蚀工艺，将硅柱的顶径缩小到小于10μm。与硅环重叠较小的小硅柱是降低硅硅损耗的关键。目前研究的变形微腔的边界定义在极坐标(R，ϕ)中为
Confinement and manipulation of photons using microcavities have triggered intense research interest for more than a decade.[1] Prominent examples are whispering gallery mode (WGM) microcavities,[2, 3] which confine photons by means of continuous total internal reflection along a curved and smooth surface. The long photon lifetime (described by high Q factors), strong field confinement, and in-plane emission characteristics make them promising candidates for novel light sources [4–9] and biochemical sensors with the ability of detecting few or even single nanoparticles.[10, 11] The principal disadvantage of circular WGM microcavities is their intrinsic isotropy of emission due to their rotational symmetry. In addition to the photonic structures consisting of two or more perfectly spherical microcavities,[12] one of vital solutions is to use deformed microcavities by breaking the rotational symmetry,[13–16] which can provide not only the directional emission but also the efficient and robust excitation of WGMs by a free-space optical beam.[17–20] Deformed microcavities fabricated on a chip are particularly desired for high-density optoelectronic integration, but they suffer from low Q factors in experiments. The Q factors are typically around or even smaller than ten thousand [21–27] limited by the large scattering losses from the involuntary surface roughness. The high Q factor is of great importance in fundamental studies and on-chip photonic applications. Here, with a pattern transfer technique and a reflow process ensuring a nearly atomic-scale microcavity surface, we demonstrate experimentally on-chip undoped silica deformed microcavities which support both nearly unidirectional emission and ultrahigh Q factors exceeding 100 million. Consequently, low-threshold, unidirectional microlasing in such a microcavity with Q factor about 3 million is realized by erbium doping and a convenient free-space excitation.The deformed microcavities are fabricated from a 2-μmthick layer of silicon dioxide on a silicon wafer. Combining a two-step dry etching process and a laser reflow process is employed for the first time to transfer patterns and achieve microcavities with designed shapes and ultra-smooth cavity surface. The process details are depicted in Figure 1a. First, we purposely design the mask patterns with minor modifications from the desired deformed toroidal boundaries R (ϕ) by adding an extra 15 μm in the radius at all polar angles. Through optical lithography followed by buffered HF etching, deformed silica disks are created on the silicon wafer, which inherit the mask patterns well. Subsequently, the resulting silica disks are exposed to XeF 2 gas at 2.7 torr to etch the underneath silicon by about 15 μm. In this process, the silica disks keep their original shapes and also serve as etching masks for the silicon underneath; consequently a silicon pillar is formed under each disk. Owing to the isotropic dry etching of silicon, the underlying silicon pillars obtain the desired patterns. Then, the silica disks are reflowed by a CO 2 pulse laser irradiation, which melts the silica disks along the peripheries of the silicon pillars and causes the disks to collapse into toroids with ultra-smooth surfaces.[28] Most importantly, the desired shapes are perfectly transferred from the silicon pillars back to the silica toroids as expected. Finally, a second XeF 2 dry etching process is applied to shrink the top diameter of the silicon pillars smaller than 10 μm. The small silicon pillar having a small overlap with the silica toroid is crucial to reduce the silica-to-silicon loss. The deformed microcavity studied at present has the boundary defined in the polar coordinates (R, ϕ) as