Microfibres for
Quantum Optics
Dr Síle Nic Chormaic
Quantum Optics Group
Motivation
Strong need to engineer atoms and photons for the development of new
technologies… “quantum technologies”
Future advances in nanotechnology, communications, cryptography,
information processing, computing all rely on these developments.
Explore techniques to control and manipulate internal and external
properties of particles (atoms, photons,…) in a manner that would have
been impossible 10-20 years ago.
Quantum Optics Group
What is Quantum Optics?
A field of research in physics, dealing with applying quantum mechanics
to phenomena involving light and its interactions with matter.
Research Activities
z
Design and testing of novel microcavity light sources from UV to near
IR, including lasing devices at 1560 nm.
z
Development of atom optics elements for cold atoms (diffraction
gratings, waveguides,…) to manipulate and control the centre-of-mass
motion using light and/or magnetic fields → enables atom+light
interaction studies.
The microfibre is the common element in all our work…
Research Laboratory
Based in Tyndall National Institute
Microfibres
125 µm
< 1 µm
16 µm
Taper
region: Cladding becomes
Single mode
fibre
core, vacuum becomes cladding
Evanescent field component in
tapered region extends beyond
fibre surface: very high intensities
achieved
1 micron fibre versus
100 micron hair
Evanescent field
100
Power (%)
80
1/e penetration (µm)
Core
Evanescent
60
40
20
0
0
0.2
0.4
0.6
Fibre Radius (µm)
0.8
1
a = 0.5 µm, Ev = 5%
a = 0.3 µm, Ev = 20%
a = 0.1 µm, Ev = 100%, but unguided!!
r = 0.15 µm
X-axis
Y-axis
Average
0.4
0.3
0.2
0.1
0
0.1
0.2
0.3
Radius ( µ m)
0.4
0.5
Decay length ~ 100 nm
r = 0.25 µm
Variation in the power distribution of
the evanescent field for two fibres of
different radii.
Microfibres for Microcavity Light Sources
z
Characterisation of light emissions from solid laser glass spheres,
diameter ~50-80 micron.
z
Use erbium-doped novel fluorides (ZBNA, ZBLAN, ZBLALiP) and
erbium/ytterbium co-doped commercial phosphate (IOG2) glass.
Images of our Er:ZBNA microspheres
z
Microspheres fabricated in ENSSAT (France) by dropping glass powder
through a microwave plasma torch. Surface tension produces spheres with
~10-3 eccentricity.
Mode Matching for Light Coupling
z
Couple 980 nm pump light into sphere using tapered optical
fibre with waist ~ 1 micron → evanescent field coupling.
980 nm
980 nm, 1.55 µm
Sphere
Whispering gallery modes. Strong green emissions
at 520 nm and 540 nm are characteristic of erbium
doping due to upconversion of 980 nm pump.
Whispering Gallery Modes
Light travelling inside sphere, strikes glass-air interface. Total internal reflection occurs.
If sphere is of good quality light undergoes multiple reflections … leads to long photon storage
lifetimes, high Q factor and low mode volume
l - │m│ = 1
polar mode
n=1
radial mode
r
Resonance
condition
2πr = βλs
Equator
Example: Temperature Sensing
Ratio of emissions between 520 nm and 540 nm lets us estimate temperature of
environment.
Equivalently, increasing intensity of pump light coupled into sphere, changes its
temperature and changes its size (monitor green ratio to see effect).
By changing the size, can tune the cavity to be resonant with any wavelength
that we need (e.g. atomic transitions)!!!!
Example: Quantum Dot Devices
Micro-cylinder resonators with quantum dots embedded in GaAs
structures. Ground state emission at 1.28 µm.
Microfibres for Cold Rubidium Atoms
Tapered micro fibre
Avalanche
Photo Diode
Intensity
Cold atomic cloud
Probe beam
ωo
When probe beam resonant with atoms, light is absorbed and a dip in the transmitted
intensity is noted. Note: only about 10 atoms interact with photons in evanescent field!!
Trapped atom
Fblue
Fred
Blue detuning (shorter λ): Repulsive force
Red detuning (longer λ): Attractive force
Tapered optical
fibre
colour of probe beam
Combination: Optical trapping of atoms ???
Cold Atom Source – the MOT
Temperature of atoms related to their velocity:
For rubidium atoms:
T = 300 K, v = 250 m/second (room temperature)
T = 3 K, v = 25 m/second (background radiation)
T = 50 µK, v = 10 cm/second (laser cooled atoms)
Cool atoms by bombarding
with a laser beam tuned to
an atomic transition.
Magneto-Optical Trapping
A magnetic field added to laser arrangement to form a zero field region in the
centre – the atom trap. Atoms pushed towards centre since they are “low-field
seekers”.
780 nm
Rb
Laser cooling performed
in Ultra High Vacuum
(similar to outer space!!!)
to reduce collisions.
Locking electronics for repumping
laser
Setup (experimental)
Locking electronics for cooling laser
Flip M2
Saturated absorption spectroscopy
for repump and cooling laser
Wave
Meter
Repumping Laser
Temp
Isolator
λ
Flip M1
Piezo
PI-100-AS
Heating
Circuit
Error Sig
Trig
λ
4
B
1 eam
Ch 2
Rb Cell
Oscilloscope
+ 12 V
Output
B.Pass
2 kHz
Ref
Current &
PBS
λ
4
Be
am
#3
AOM Driver
2
PBS
PBS
Iris
Iris
λ/22
50:50
BS
Pilot
Controller
Tiger Laser
Isolator
150 mm
AOM
Sat abs
AOM
150 mm
MOT
200 mm
+1 order
Shutter
50 mm
λ/22
Rf
Signal
Trig
Ch 2
Ch 1
+1 order
λ
4
PBS
Oscilloscope
Sync
Error Signal
PD Signal
50 mm
150 mm
250 mm
λ/2
PI-100-AS
PD
#1
150 mm
Temp
Piezo
B.Stop
2 kHz
Rb Cell
Signal
Generator
Attenuator
∫
#2
200 mm
TTL
∫
4
- 12 V
Lock-in Amp
Input
#1
4
#2
Iris
50:50
BS
PD 2
Cooling Laser
4
Re tro
λ
PD 1
Ch 1
λ
Re tro
15 mm
Balanced
receiver
Sync
PD Signal
Anti-Helmholtz
coil
Beam
3
Sanyo
Laser
Current &
Perspex
slab
VDC
28V
DC PSU
AOM Driver
17V
AOM Driver
DAQ Card
Setup (theoretical)
Setup (experimental)
Ion pump
Vacuum chamber
Intensity
Typical parameters achieved:
Base pressure : 10-10 mbar
Atom temperature: ~100 µK
Number of atoms: ~ 107
Cloud radius: ~ 1 mm
ωo
Projects…
Possible 4th year projects:
• Optical Tweezers for
Manipulating and Trapping
Particles (biophysics interest)
• Microdisk Resonators with
Quantum Dot Coatings
(photonics interest)
• Narrow Linewidth Laser for Rb
Spectroscopy and Atom Cooling
(quantum, atom, optics interest)
If curious, office 215d
www.physics.ucc.ie/nicchormaic
For podcasts on quantum engineering: http://physics.nist.gov/Mp4s/
Group Members
Funding SFI, IRCSET, RIA