Rydberg & plasma physics using ultra-cold strontium James Millen Supervisor: Dr. M.P.A. Jones Rydberg & plasma physics using ultra-cold strontium

Outline The strontium experiment Introduction and motivation The strontium MOT Rydberg & plasma physics using ultra-cold strontium

Rydberg physics A Rydberg state is one of high principle quantum number n Rydberg atoms can be very large (orbital radius scales as n 2 ) Very strong Rydberg-Rydberg interactions (van-der-Waals interaction scales as n 11 ) This can lead to “frozen” Rydberg gases, where the interaction energy is much greater than the thermal energy. Johannes Rydberg Rydberg & plasma physics using ultra-cold strontium Introduction

Ultra-cold plasma physics Most plasmas are hot, dense and dominated by their kinetic energy The behaviour of ultra-cold neutral plasmas is governed by Coulomb interactions Other “strongly coupled” plasmas are not accessible in the lab Killian, Science Rydberg & plasma physics using ultra-cold strontium Introduction

Ultra-cold plasma physics Plasmas can be formed from cold atoms by optically exciting above the ionisation threshold Some electrons leave, leading to the system being bound Frozen Rydberg gasses spontaneously evolve into plasmas and visa versa (T. F. Gallagher, P. Pillet, D. A. Tate et al. Phys. Rev. A (2004) S.L. Rolston et al. Phys. Rev. Lett. 86, 17 (2001) ) Killian, Science Rydberg & plasma physics using ultra-cold strontium Introduction

Rydberg & plasma physics using ultra-cold strontium Introduction Long term goals of our project (!) Create a cold Rydberg gas / neutral plasma in a 1D lattice Lattice spacing can be on the order of the size of the Rydberg atoms Narrow linewidth transitions (7kHz) could lead to single site addressability.

Introduction to Strontium Atomic Number: 38 An alkaline earth metal (Group II) Four naturally occurring isotopes: 88 Sr (82.6%), 87 Sr (7.0%), 86 Sr (9.9%) & 84 Sr (0.6%) 88,86,84 Sr have no hyperfine structure (Bosonic I=0), 87 Sr has I=9/2 (Fermionic) Negligible vapour pressure at room temperature (1 mTorr at 1000K) Rydberg & plasma physics using ultra-cold strontium Introduction

88 Sr energy level diagram F. Sorrentino, G. Ferrari, N. Poli, R. Drullingerand G. M. Tino arXiv:physics/ v1arXiv:physics/ v nm Rydberg & plasma physics using ultra-cold strontium Introduction

Why strontium? Singlet-triplet mixing leads to narrow intercombination lines, allowing cooling to <μK (spin forbidden 1 S P 1 red MOT ~800nK Katori et al. Phys. Rev. Lett. 82 (6) (1998)) This also allows high spectroscopic resolution (Same transition as above 7.6kHz) 1 S 0 ground state can make spectroscopy more simple (no optical pumping required) Singly charged ion Sr + has several transitions in the visible, allowing spatially resolved diagnostics (5s 2 S 1/2 → 5p 2 P 1/2 transition is at 422nm) Rydberg & plasma physics using ultra-cold strontium Introduction

Rydberg & plasma physics using ultra-cold strontium Experimental apparatus Rydberg & plasma physics using ultra-cold strontium Experimental apparatus Vacuum system Chamber internals Electrodes Zeeman slower Detection systems Laser system Strontium vapour cell

The vacuum system Rydberg & plasma physics using ultra-cold strontium Experimental apparatus

The vacuum system The oven is heated by thermocoax heater wire to ~600°C, and the strontium beam is collimated with a nozzle. The oven can be isolated from the chamber with a gate valve and there is good differential pumping. Rydberg & plasma physics using ultra-cold strontium Experimental apparatus

Internals Coils wound from 1mm Kapton insulated copper wire Can produce a field gradient of 30Gcm -1 at 2.5A Mounted directly on top flange so can directly “plug” into the chamber No electrical connections in any optical path Rydberg & plasma physics using ultra-cold strontium Experimental apparatus

The electrodes Split ring geometry mounted onto MOT coil formers Blocks no optical access 8 independently controllable electrodes Can produce reasonably flat fields and also gradients Rydberg & plasma physics using ultra-cold strontium Experimental apparatus

Field calculations Field changes by <1% in central 4mm cube Rydberg & plasma physics using ultra-cold strontium Experimental apparatus

The Zeeman slower 6mm mild steel “yoke” Copper former Heatsink block Vacuum pipe Extraction coil 27cm Rydberg & plasma physics using ultra-cold strontium Experimental apparatus

Field (Tesla) Data with shield Data without shield Simulation With Shield Without Shield The Zeeman slower Rydberg & plasma physics using ultra-cold strontium Experimental apparatus

Detection systems A home built photodiode for temporal fluorescence/absorption measurements A pixelfly qe CCD camera for taking images (controlled by LabView) A Hamamatsu micro-channel plate for detecting charges Rydberg & plasma physics using ultra-cold strontium Experimental apparatus

Laser System -240MHz Toptica frequency doubled laser system at 461nm Spectroscopy: Locking our laser using modified PolSpec Double pass at +120MHz → 0 MHz (All frequencies quoted relative to the 5s 2 1 S 0 → 5s5p 1 P 1 transition in 88 Sr) Imaging: For absorption imaging Double pass at +120MHz → 0 MHz Zeeman Slower Double pass at -136MHz → +512 MHz MOT beams Single pass at +200MHz → -40 MHz Rydberg & plasma physics using ultra-cold strontium Experimental apparatus

Strontium vapour cell Strontium must be heated, and hot strontium reacts with glass and copper. We have built a cell based on strontium dispensers that we use for spectroscopy and locking our 461nm laser A vapor cell based on dispensers for laser spectroscopy E. M. Bridge, J. Millen, C. S. Adams, M. P. A. Jones arXiv: v2 arXiv: v2 Second generation design has 100% optical thickness Rydberg & plasma physics using ultra-cold strontium Experimental apparatus

A magneto-optical trap for strontium Rydberg & plasma physics using ultra-cold strontium Strontium MOT Our strontium MOT Our very first strontium MOT August 22nd 2008 (Friday, 17:30!) Our much improved strontium MOT October 2008

Theory Rydberg & plasma physics using ultra-cold strontium Strontium MOT 689nm 7.6kHz Up to 13mins* *Yasuda, Katori Phys. Rev. Lett. 92, (2004) (5s 2 ) 1 S 0 461nm 32MHz (5s5p) 1 P 1 620Hz (5s4d) 1 D (105Hz) (5s5p) 3 P (213Hz) 3P13P1 679nm (1.4MHz) 707nm (6.4MHz) (5s6s) 3 S 0 3P03P0 7.6kHz

Experimental sequence Rydberg & plasma physics using ultra-cold strontium Strontium MOT Controlled by LabView via FPGA card MOT beams always on A)B-field & slowing light off B) Slowing light on C) B-field and slowing light on D) B-field on, slowing light off E) B-field & slowing light off

Some preliminary results – MOT lifetime Rydberg & plasma physics using ultra-cold strontium Strontium MOT Black line I = I peak Blue line I = I average Green line I = αI peak

Some preliminary results – MOT atom number Rydberg & plasma physics using ultra-cold strontium Strontium MOT

Some preliminary results – MOT atom number Rydberg & plasma physics using ultra-cold strontium Strontium MOT

Conclusion Rydberg & plasma physics using ultra-cold strontium We have a functioning magneto-optical trap for strontium, trapping on the primary transition at 461nm Preliminary number and lifetime measurements have been performed, and the apparatus is under computer control We are ready to take temperature and density measurements. Now we just need to decide on our first experiment...!

Rydberg & plasma physics using ultra-cold strontium Dr. Matt Jones Graham Lochead Clémentine Javaux (Ecole Superiure d'Optique ) Elizabeth Bridge (Durham, now Oxford/NPL) Sarah Mauger (Ecole Superiure d'Optique ) Benjamin Pasquiou (Ecole Superiure d'Optique )