Synthesis and optical characterization of cadmium sulfide (CdS) single crystals for laser cooling effect

• Main Author: Zhang, Chiyuan
• Other Authors: Xiong Qihua
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Microelectronics
• Online Access: https://hdl.handle.net/10356/62159
• Institution: Nanyang Technological University

In this thesis, the author will describe what he have done in the past two years of his studentship. He will mainly focus on the topic of laser cooling of cadmium sulfur (CdS) single crystals. The first part of the work is to characterize the laser cooling effect of CdS commercial single crystals. CdS has been proved very promising in laser cooling effect where nanobelts of CdS can be cooled down 40 K from room temperature and 20 K from 100K if excited by certain lasers1. But large scale is needed instead of nano size in order to make this material useful in real life, for example, optical refrigerator or self-cooling laser. Hence, the author first characterizes the commercial single crystals of CdS (1 cm2 square meters) to see the laser cooling character of bulk CdS single crystals. The author also analyzes two different commercial crystals from two industry companies, one from MTI China and one from Semiwafer Taiwan. By analyzing those crystals using optical spectroscopy, the author gets the Stokes spectroscopy and anti-Stokes spectroscopy as well as its dependence relationship figure between peak densities and exciting laser power. Through the analysis of the study, it shows that those commercial crystals cannot have the laser cooling effect because it has a very large defect which may be caused by anti-sites defect or sulfur vacancies. Those defects will prevent the crystals from having laser cooling effect because those defects may cause the emission of phonons with lower energy which is the opposite of up-conversion laser cooling process. In order to get a higher quality of single crystals, the author looks forward to the lab synthesis of single crystals. The second part of the work includes the synthesis and characterization of lab grown CdS single crystals. The author tried to grow his own crystals to remove the defects which he found in the commercial CdS single crystal. He carried the synthesis of CdS single crystal by physical vapor transport method using cadmium shots and sulfur pieces. Growing condition covers different temperatures and different ratio between Cadmium and Sulfur. From the analysis of optical spectroscopy on those crystals, he concludes that some of the conditions gives us a much better quality of crystals which may exist a higher potential to have laser cooling effect because the defect that he found in commercial crystals no longer exists in his crystals. In the laser cooling experiment he carried out later, laser cooling effect was unfortunately not observed. A thorough comparison is also given between the three crystals: two commercial single CdS crystals and the best lab grown crystals. Clear improvement of removing the defect can be observed through the comparison. The third part of the thesis includes an illustration of the device design for optical refrigerator. It’s an improvement and detailed explanation of what a device using CdS crystal to realize laser cooling looks like and how it works based on what we have got so far. It provides an idea to people who are looking forward to step further in semiconductor laser cooling field. All the three part of the work will be illustrated and explained in details in the following chapters. Moreover, a summary and future plan is clearly stated in the end of the thesis.