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Tapered optical fibres as a probe from MOT characteristics

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<strong>Tapered</strong> <strong>optical</strong> nano-<strong>fibres</strong> <strong>as</strong> a<strong>probe</strong> for <strong>MOT</strong> <strong>characteristics</strong>Michael Morrissey, K.De<strong>as</strong>y, , D. Gleeson, S.Nic ChormaicQuantum Optics GroupCork Institute of Technology&Tyndall National InstituteQuantum Optics Group


Outline Motivation <strong>Tapered</strong> <strong>fibres</strong> in ultra-high vacuum Me<strong>as</strong>uring <strong>MOT</strong> fluorescence Loading and loss of <strong>MOT</strong> <strong>MOT</strong> density and shape Lifetime of <strong>MOT</strong> Outlook Conclusion


MotivationSingle atoms on an <strong>optical</strong> nanofibre: Hakuta, JapanSingle atoms trapped on the nanofibreafter <strong>MOT</strong> is irradiated with UVl<strong>as</strong>erLight induced dipole force and surface inrteraction: Rauschenbeutel, Germany• Observe and anenhancement of thespontaneous emission rate• broadening of absorptionprofileNonlinear Optical Interactions with <strong>Tapered</strong> Optical Nanofiber Rubidium Vapour• Non linear absorption in vapourcell•Observation of EIT at low pumppower


<strong>MOT</strong>Correct• Beam beam power I > I sat• Beam size diam > 15 mm•Beam detuning Δ = 12 MHz• Beam polarisation σ + / σ -• Selected transitions stable• 3 pump vacuum system• Octagonal <strong>MOT</strong> chamber• 6 orthogonally intersectingbeams• magnetic gradientT: < 100 μKN: ~ 10 8Diam: 2 mm


<strong>Tapered</strong> Optical Nano-Fibres (TONFs(TONFs) 90%transmission can be produced using anoxy-butane flame <strong>as</strong> a heat sourceThe size and shape of the TONFcan be tailored by setting hotzonesize and pull lengthSEM Image0.6 μmFrom SEM TONFs are smooth,uniform with no deformations


<strong>MOT</strong> load & loss• The steady state number of atoms is determined by an equilibriumbetween the loading rate, R load,and the loss rate, R lossof the <strong>MOT</strong>.• Only atoms with a velocity below the capture velocity, υ c,∼20m/s,can be slowed sufficiently by the l<strong>as</strong>er beam to be trapped.• The rate at which the <strong>MOT</strong> load, R load, is given by:Rcapute=nRb2/3 4V υ ⎛2 2 ⎟ ⎞cm⎜⎝ kBT⎠3/ 21. Collisions losses between background species in the vacuum system.For our system this can be neglected.2. Collision losses due to background Rb vapour3. Collision losses due to inter-<strong>MOT</strong> collisions causing and incre<strong>as</strong>e in K.E.γ = n σ υ + β( I)lossRbRbRbncloud


Steady State atoms in <strong>MOT</strong>• The number of trapped atoms in a <strong>MOT</strong> <strong>as</strong> it loads is given byRload N( t)=γtγ[ 1−exp( − )]The number of atoms, N, in a <strong>MOT</strong> is proportional to the intensity offluorescence emitted <strong>from</strong> the trapped atoms. Thus to estimate thenumber of atom, the fluorescence is focused onto a photodiode.N=ε ⋅4 ⋅ πP ⋅R sc⋅ΩRsc=1+IIIISS⋅ π ⋅ Γ+ Δ4 ( ) 2Γ


Loading results<strong>MOT</strong>SPCMPDCounterMethod 1: <strong>MOT</strong> fluorescencefocused onto a photodiodeMethod 2: Spontaneousemission <strong>from</strong> the <strong>MOT</strong> iscoupled into the TONFLoading time decre<strong>as</strong>es <strong>as</strong> backgroundvapour density incre<strong>as</strong>es. Bothmethods achieve the same resultThe photon counter is also sensitive tothe <strong>MOT</strong> density <strong>as</strong> shown by thevariation in amplitudes of the signal


<strong>MOT</strong> Shape (CCD camera)Filtered image of <strong>MOT</strong> Intensity Profile of <strong>MOT</strong> Gaussian fit to cross section<strong>MOT</strong>SPCMNDFCCDCounterPCThe shape of the <strong>MOT</strong>can be determined byfluorescence imaging.• Magnetic coil moves <strong>MOT</strong> across TONF• Video allows us convert <strong>from</strong> time – mm• Signal depends on local density of <strong>MOT</strong>• Both results agree


<strong>MOT</strong> Lifetime results• <strong>MOT</strong> lifetime is a me<strong>as</strong>ure of the time a<strong>MOT</strong> can exist without any backgroundvapour to load the <strong>MOT</strong> and atoms collidewith atoms of different species• Effect is independent of getter current.• Difference in lifetime due to P.D.sensitivity to background fluorescenceThis effect is not seen when thelifetime is me<strong>as</strong>ured using aTONFThe fibre in sensitive to lownumbers of atoms that remaintrapped for long period of time.For higher getter currents thedensity of the <strong>MOT</strong> incre<strong>as</strong>es thustaking longer to decay.


Outlook: Evanescent field


Conclusion<strong>MOT</strong> experimentTONF & vacuum chamberFluorescence couple <strong>from</strong> <strong>MOT</strong> to TONF<strong>Tapered</strong> fibre can characterise <strong>MOT</strong>– Shape– Density– Loading time– Decay timeAlways beware of Murphy’s LawOutlook– evanescent field enhancement– Atoms interacting with evanescent field


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