diff --git a/content/zettels/quadrant_photodiodes.md b/content/zettels/quadrant_photodiodes.md
index c5828cc..db8b1ac 100644
--- a/content/zettels/quadrant_photodiodes.md
+++ b/content/zettels/quadrant_photodiodes.md
@@ -147,6 +147,9 @@ We now that the maximum translation of the beam is \\(\Delta z = 1\\,mm\\) and t
 
 </div>
 
+
+## Bibliography {#bibliography}
+
 <style>.csl-entry{text-indent: -1.5em; margin-left: 1.5em;}</style><div class="csl-bib-body">
   <div class="csl-entry"><a id="citeproc_bib_item_1"></a>Lee, Eun Joong, Youngok Park, Chul Sung Kim, and Taejoon Kouh. 2010. “Detection Sensitivity of the Optical Beam Deflection Method Characterized with the Optical Spot Size on the Detector.” <i>Current Applied Physics</i> 10 (3): 834–37. doi:<a href="https://doi.org/10.1016/j.cap.2009.10.003">10.1016/j.cap.2009.10.003</a>.</div>
   <div class="csl-entry"><a id="citeproc_bib_item_2"></a>Li, Qing, Shaoxiong Xu, Jiawei Yu, Lingjie Yan, and Yongmei Huang. 2019. “An Improved Method for the Position Detection of a Quadrant Detector for Free Space Optical Communication.” <i>Sensors</i> 19 (1): 175. doi:<a href="https://doi.org/10.3390/s19010175">10.3390/s19010175</a>.</div>