An Ultraviolet Laser for Beryllium Photoionization Christian Michael Pluchar Advisor: Professor David A. Hanneke May 1, 2018 Submitted to the Department of Physics & Astronomy of Amherst College in partial fulfillment of the requirements for the degree of Bachelors of Arts with honors c 2018 Christian Michael Pluchar Abstract Trapped ions are attractive systems for quantum state manipulation and detection, and have been used for quantum information processing, precision measurement, and optical atomic clocks. The beryllium ion is a popular choice in these experiments, but a method is needed for efficiently ionizing neutral beryllium and loading it into a trap. This thesis describes the design and construction of an ultraviolet laser capable of ionizing neutral beryllium atoms. The UV light produced has a wavelength of 235 nm and is resonant with the 2s to 2p transition in neutral beryllium. A second 235 nm photon ionizes the atom once in the excited state. The resonant step of the two-photon process selectively ionizes beryllium atoms. Blue light with a wavelength of 470 nm from a commercial external cavity diode laser is passed through a nonlinear crystal (beta barium borate), which generates the second harmonic, producing 235 nm light. To increase UV pro- duction, a power enhancement cavity was built around the nonlinear crystal. The custom monolithic cavity provides additional isolation from vibration and environmental disturbances. To lock the cavity on resonance, a feedback loop using photodetectors and a FPGA servo was assembled. The system generates over 300 µWofultravioletlight,enoughtosaturate the 2s to 2p transition over a small area of beryllium atoms and ionize them at a modest rate. In the future, improving the coupling into the power enhancement cavity will increase the UV power output to 25 mW, which will produce ions + at a faster rate. The laser system will also be used to photodissociate O2 . Acknowledgments First and foremost, thank you to Professor Hanneke. I am so very grateful for the many hours you spent with me in lab, explaining concepts to me, and reading over drafts. You have a way of bringing out my best work, and I left every meeting feeling confident I could finish this work. From iLab to the last week of my thesis, some of my favorite and most influential parts of my Amherst Physics experience has been due to your guidance. Designing and manufacturing the monolithic cavity would not have been possible without the help of Jim Kubasek. Thank you for helping me learn SolidWorks, and develop a certain style of thinking about the assembly and components. These skills were invaluable for such a complex project. The hours you spent to get the parts precisely machined, as well as tackling the difficult angled cuts and exterior cuts for the mirror mounts is appreciated. Ryan Carollo was a steady presence in helping me throughout the project. Thank you for going out of your way to help me with the experimental work, lending eyes and ears to problems, most notably the cavity coupling, giving me advice and encouragement, as well as providing feedback and helping me with cavity design decisions. Thank you to Brian Creapeau, who helped me troubleshoot and repair the i FPGA servo which will become the brains for locking the cavity. Thank you to all my friends, especially those who listened to me talk about “lazers.” Thank you to my teammates past and present who supported me for the last two months. Thanks and congratulations to my fellow senior Physics Majors, who have been kind and supportive, and each of which has made a mark on my time here. Finally, thank you to my family for supporting me through college, encour- aging me, and always being there for me. This material is based upon work supported by the Amherst College Dean of the Faculty and by the National Science Foundation under CAREER grant number PHY-1255170. Thank you to the Thomases Family, Baines Family, and the Gregory S. Call Student Research Fund for helping support me and my work. ii Contents 1 Introduction 1 1.1 Versatile Uses for Trapped Ions ................. 1 1.1.1 Searching for New Physics ................ 3 1.2 Generating 235 nm Light ..................... 5 2 Theory of Second Harmonic Generation 10 2.1 Gaussian Beams .......................... 10 2.2 External Cavity Diode Lasers .................. 14 2.2.1 Semiconductor Laser Fundamentals ........... 14 2.2.2 External Cavity Diode Lasers .............. 19 2.3 Second Harmonic Generation .................. 22 2.4 Optimal Second Harmonic Generation in Beta Barium Borate 26 2.4.1 Birefringent Phase Matching in BBO .......... 27 2.4.2 Optimal Focusing and Walko↵ in BBO ......... 30 2.5 Optical Resonators ........................ 31 2.5.1 Power Enhancement ................... 35 2.5.2 Impedance Matching ................... 39 3 Building and Optimizing the Cavity 41 3.1 Monolithic Cavity Design in Solidworks ............. 41 3.2 Input Optics ............................ 43 3.3 Generating UV Light from a Single Pass ............ 48 3.4 Aligning the Cavity and Mode Matching ............ 50 4 Next Steps 53 4.1 Theoretical Rate for Photoionizing Beryllium with the Present Power Output ........................... 53 4.2 Improved Cavity Performance .................. 54 4.3 Locking the Cavity ........................ 56 4.4 Laser Frequency Stability .................... 61 iii 4.5 Conclusion ............................. 62 A Monolithic Cavity Design 64 iv List of Figures 1.1 Scheme for Photoionizing Beryllium ............... 2 1.2 Oxygen Photodissociation .................... 6 1.3 Trapped Beryllium Ions ..................... 7 2.1 Mode Hopping .......................... 21 2.2 Potentials ............................. 24 2.3 Symmetric Bowtie Cavity .................... 35 2.4 SHG Power Vs Input Power ................... 38 2.5 Impedance Matching ....................... 40 3.1 Cavity Design with Beam Path ................. 43 3.2 Beryllium Photoionization Laser System ............ 44 3.3 Beam Circularization ....................... 45 3.4 Oscilloscope Trace of Scanning Through Resonance ...... 51 3.5 Oscilloscope Trace of Blue and UV Buildup .......... 52 4.1 Spot size as a Function of Lens Separation ........... 56 4.2 Circulating Power versus UV Output Power .......... 57 4.3 Reflected Power versus Frequency ................ 58 4.4 ECDL Frequency Measurement ................. 62 A.1 SolidWorks Rendering of the Cavity ............... 64 A.2 SolidWorks Rending of the Cavity with Crystal Aligner Removed 65 A.3 SolidWorks Rendering of the Cavity’s Internal Design ..... 65 A.4 Machined Cavity ......................... 66 A.5 Crystal Mount and BBO Crystal ................ 67 A.6 Cavity Internal View ....................... 68 A.7 Top View of the Cavity ...................... 69 A.8 Cavity Exterior .......................... 70 A.9 Cavity and Input Optics ..................... 71 v Chapter 1 Introduction 1.1 Versatile Uses for Trapped Ions Precisely controlling individual atoms and molecules has enabled a new range of experiments to precisely measure the behavior of matter. Techniques for trapping and manipulating atomic and molecular ions have been at the fore- front of research for improving precision measurement techniques, and lead to sympathetic cooling of co-trapped atoms [1], advances in quantum informa- tion processing, quantum simulation, and progress towards a general purpose quantum computer [2], as well as building more precise atomic clocks operat- ing at optical frequencies [3]. Many of these advances have utilized beryllium ions because of its single valence electron when ionized and a closed cyclic transition which is ideal for laser cooling. This thesis describes the design and implementation of a laser to ionize neutral beryllium. The two-photon ion- ization scheme employed (Fig. 1.1)requiresultravioletlightatawavelength 1 where few commercial lasers are available. Instead, a power enhancement cav- ity around a nonlinear crystal is used to generate the second harmonic of a pump beam from an external cavity diode laser. This method promises to improve the speed and efficiency of the loading process over electron-impact ionization, the current technique used. Additionally, this method is selective, so only beryllium is ionized and loaded into the trap. Figure 1.1: Two-photon ionization scheme for beryllium. To photoionize neu- tral beryllium, one photon at the resonant transition frequency will move it between the 2s and 2p state. A second photon from the same laser will then move the electron to the continuum. The experimental apparatus is centered around a Paul (radiofrequency) Trap. Earnshaw’s Theorem prohibits the existence of a configuration of static electric fields to confine a particle. Instead, a Paul trap uses a combination of stationary and oscillating electric fields to confine charged particles to a limited area of space so they can be studied over a longer duration of time [4]. The contents of the trap are in vacuum, isolating the trap contents from external interactions. Paul traps are versatile compared with other commonly used 2 trapping techniques, such as magneto-optical traps. Multiple particle species, including molecules, can be trapped with identical trap parameters, and can even be trapped simultaneously. This enables an entire range of new techniques to be utilized and particle interactions to be studied. In our apparatus, co-trapping ions enables us to use beryllium ions to sym- pathetically cool diatomic oxygen ions. Laser cooling ions to their ground state places them