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

Rydberg & plasma physics using ultra-cold strontium

James Millen

Supervisor: Dr. M.P.A. Jones


Outline

• The strontium experiment

• Introduction and motivation

• The strontium MOT


Rydberg physics

• A Rydberg state is one of high principle quantum number n

• Rydberg atoms can be very large (orbital radius scales as n2)

• Very strong Rydberg-Rydberg interactions (van-der-Waals interaction scales as n11)

• This can lead to “frozen” Rydberg gases, where the interaction energy is much greater than the thermal energy.

Johannes Rydberg

1854-1919

Rydberg & plasma physics using ultra-cold strontiumIntroduction

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 316 705-708


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 70 042713 (2004)S.L. Rolston et al. Phys. Rev. Lett. 86, 17 (2001) )

Killian, Science 316 705-708



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: 88Sr (82.6%), 87Sr (7.0%), 86Sr (9.9%) & 84Sr (0.6%)

• 88,86,84Sr have no hyperfine structure (Bosonic I=0), 87Sr has I=9/2 (Fermionic)

• Negligible vapour pressure at room temperature(1 mTorr at 1000K)


88Sr energy level diagram

F. Sorrentino, G. Ferrari, N. Poli, R. Drullingerand G. M. Tino arXiv:physics/0609133v1

412.7nm


Why strontium?

• Singlet-triplet mixing leads to narrow intercombination lines, allowing cooling to <μK(spin forbidden 1S0-3P1 red MOT ~800nK Katori et al. Phys. Rev. Lett. 82 (6) (1998))

• This also allows high spectroscopic resolution(Same transition as above 7.6kHz)

• 1S0 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 2S1/2 → 5p 2P1/2 transition is at 422nm)Rydberg & plasma physics using ultra-cold strontium

Introduction


Experimental apparatus

Rydberg & plasma physics using ultra-cold strontiumExperimental apparatus

• Vacuum system

• Chamber internals

• Electrodes

• Zeeman slower

• Detection systems

• Laser system

• Strontium vapour cell

The vacuum system


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.


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


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


Field calculations

• Field changes by <1% in central 4mm cube


The Zeeman slower

6mm mild steel “yoke”

Copper former Heatsink block

Vacuum pipe

Extractioncoil

27cm


Field

(Tesla)

Data with shield

Data without shieldSimulation

With Shield

Without Shield

The Zeeman slower


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


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

5s2 1S0 → 5s5p 1P1

transition in 88Sr)

Imaging:

For absorption

imaging

Double pass at +120MHz

→ 0 MHz

Zeeman SlowerDouble pass at -136MHz

→ +512 MHz

MOT beamsSingle pass at

+200MHz

→ -40 MHzRydberg & plasma physics using ultra-cold strontium

Experimental apparatus

Strontium vapour cell

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

-1,50E-01 -1,00E-01 -5,00E-02 0,00E+00 5,00E-02 1,00E-01 1,50E-01

Frequency (arb.)

Tra

ns

mis

sio

n

11.0 A

12.2 A

13.0 A

14.0 A

15.0 A

15.6 A

15.8 A

• 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 laserA vapor cell based on dispensers for laser spectroscopy E. M. Bridge, J. Millen, C. S. Adams, M. P. A. Jones arXiv:0710.1245v2

• Second generation design has 100% optical thickness


A magneto-optical trap for strontium

Rydberg & plasma physics using ultra-cold strontiumStrontium MOT

Our strontium MOT

Our very first strontium MOT

August 22nd 2008 (Friday, 17:30!)

Our much improved strontium MOT

October 2008

Theory


689nm

7.6kHz

Up to

13mins*

*Yasuda, Katori

Phys. Rev. Lett. 92, 153004 (2004)

(5s2) 1S0

461nm

32MHz

(5s5p) 1P1

620Hz

(5s4d) 1D2

0.33

(105Hz)

(5s5p) 3P2

0.67

(213Hz)

3P1

679nm

(1.4MHz)707nm

(6.4MHz)

(5s6s) 3S0

3P07.6kHz

Experimental sequence


Controlled by LabView via FPGA cardMOT 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


Black line I = Ipeak Blue line I = Iaverage Green line I = αIpeak

Some preliminary results – MOT atom number


Conclusion


• 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...!


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 )http://massey.dur.ac.uk/research/strontium/

strontium.html

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

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