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 Design and testing of novel microcavity light sources from UV to near IR, including lasing devices at 1560 nm. 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 Single mode Taper fibre region: Cladding becomes 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 Power (%) 100 80 60 40 20 Core Evanescent 0 0 0.2 0.4 0.6 0.8 1 Fibre Radius (µm) a = 0.5 µm, Ev = 5% a = 0.3 µm, Ev = 20% a = 0.1 µm, Ev = 100%, but unguided!! 1/e penetration (µm) 0.4 0.3 0.2 0.1 X-axis Y-axis Average 0 0.1 0.2 0.3 0.4 0.5 Radius (µm) Decay length ~ 100 nm r = 0.15 µm r = 0.25 µm Variation in the power distribution of the evanescent field for two fibres of different radii.
Microfibres for Microcavity Light Sources Characterisation of light emissions from solid laser glass spheres, diameter ~50-80 micron. Use erbium-doped novel fluorides (ZBNA, ZBLAN, ZBLALiP) and erbium/ytterbium co-doped commercial phosphate (IOG2) glass. Images of our Er:ZBNA microspheres 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 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 r Resonance condition 2πr = βλs l - m = 1 polar mode n = 1 radial mode 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 Cold atomic cloud Avalanche Photo Diode Intensity Probe beam ω o 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 F red colour of probe beam F blue Tapered optical fibre Blue detuning (shorter λ): Repulsive force Red detuning (longer λ): Attractive force 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.
Setup (experimental) Wave Meter Flip M 2 Locking electronics for repumping laser Locking electronics for cooling laser Saturated absorption spectroscopy for repump and cooling laser Repumping Laser Current & Temp Piezo PI-100-AS PD Signal Sync Ch 1 Ch 2 Error Sig Trig Oscilloscope Cooling Laser Sanyo Laser PD 2 Balanced receiver PD 1 Isolator Heating Circuit + 12 V - 12 V Perspex slab Flip M 1 50:50 BS Rb Cell Anti-Helmholtz coil Re tro λ Re tro λ # 1 #2 4 4 15 mm Beam 1 λ 4 #1 PBS Beam 3 λ 4 #3 PBS λ 4 PBS #2 Beam 2 Iris AOM Driver Lock-in Amp 200 mm Input Output Ref TTL Signal Generator B.Pass 2 khz Rb Cell 50:50 BS Iris λ/2 2 Iris 150 mm 50 mm B.Stop 2 khz PD Attenuator Current & Temp Piezo PI-100-AS Pilot Controller Tiger Laser Sync Error Signal PD Signal Isolator Trig Ch 2 Ch 1 Shutter λ/2 Oscilloscope DC PSU 150 mm AOM Sat abs Rf Signal 28V AOM Driver 250 mm +1 order V DC 17V 150 mm 200 mm PBS 150 mm 50 mm AOM MOT AOM Driver +1 order λ/2 2 λ 4 DAQ Card
Setup (theoretical)
Setup (experimental) Ion pump Vacuum chamber Typical parameters achieved: Base pressure : 10-10 mbar Atom temperature: ~100 µk Number of atoms: ~ 10 7 Cloud radius: ~ 1 mm Intensity ω o
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