1. Wavelength tunability in whispering gallery mode resonators
Gerhard Schunk, Michael Förtsch, Josef Fürst,
Dmitry Strekalov, Florian Sedlmeir, Harald Schwefel,
Christoph Marquardt and Gerd Leuchs
Max Planck Institute for the Science of Light, Institute for Optics, Information and Photonics, University Erlangen-Nuremberg, Erlangen, Germany
Summer School on Quantum and Non-Linear Optics 2012

2. I. Whispering gallery mode (WGM) resonators
ACOUSTICS
OPTICS
ring down timesup to  > 100 s
bandwidth  < m
Savchenkov et al., Opt.Express, 15, 6768 (2007) 
controllable bandwidth via coupling
small mode volume
broad transparency window
B. Matsko and V. S. Ilchenko, IEEE J. Sel. Topics, vol. 12, no. 1, pp. 3–32 (2006)
efficient propagation of acoustic waves

3. APPLICATIONS IN GENERAL
I. Whispering gallery mode (WGM) resonators
APPLICATIONS IN GENERAL
narrowband photonic cavities (e.g. for filtering)
sensors (e.g., bio-sensing, temp., pressure, ...)
optical parametric oscillators (OPO)
...
APPLICATIONS IN ERLANGEN
Terahertz conversion, H. Schwefel group
bio-sensing, F. Vollmer group
three wave mixing in lithium niobate
naturally phase-matched OPO,
J. U. Fürst et al., PRL 104, (2010) and PRL 105, (2010)
single-mode and two-mode squeezing, J. U. Fürst et al., PRL 106, (2011)
single photon source, submitted 2012
Optical Resonator Biosensors: Molecular Diagnostic and Nanoparticle Detection on an Integrated Platform (M. Baaske, F. Vollmer), In ChemPhysChem, Wiley Online Library, volume 13, 2012. [bib] [pdf]

4. I. Whispering gallery mode (WGM) resonators
Maxwell equations yield eigenmodes with mode numbers q,l,m P. Debye, Ann. Physik, 30, 57 (1909)
[1]
spherical harmonics Ylm in angular directions
l-m = 0
l-m = 1
l = 10
spherical Bessel functions Jl in radial direction
M. Ornigotti, Phys. Rev. A 84, (2011)

5. II. Parametric down conversion in WGM resonators
energy conservation
phase matching
one pump photon generates one signal and one idler photon

6. OVERLAP INTEGRAL II. Parametric down conversion in WGM resonators
radial part
angular part
νs = ls –ms qi qs
νi = li –mi
Pth = 6.7 µW demonstrated
J. U. Fürst et al., PRL 105, (2010)
phase matching depends on spatial overlap
fundamental modes (q=1, l-m = 0) overlap best

7. heralded signal-signal corr.
II. Parametric down conversion in WGM resonators
SINGLE PHOTON SOURCE VIA SPDC
λidler= 1020 nm
λsignal = 1120 nm
heralded signal-signal corr.
submitted 2012
signal-idler corr.

8. III. Setup and experiment
532 nm
WGM resonator is a 4.8 % MgO-doped z-cut lithium niobate crystal
typ I phase matching (eoo) controlled via bias voltage and temperature
no active locking

9. wavelength tuning over 100 nm and mode-hop free tuning over 150 MHz!
III. Setup and experiment
first demonstation of tunability of our system:
submitted 2012
wavelength tuning over 100 nm and mode-hop free tuning over 150 MHz!

10. TEMPERATURE TUNING IV. Recent results T = 96,8°C λidler= 810 nm
WGR
λidler= 810 nm
1405 nm
1550 nm
OSA measurement

11. FURTHER TUNING PARAMETERS
IV. Recent results
FURTHER TUNING PARAMETERS
reducing disk size
changing Mg concentration
demonstration of coupling to a radius = 0.3 mm disk

12. V. Summary and Outlook
SUMMARY
PDC in WGM resonators is tunable over more than 300 nm
demonstration of PDC up to λsignal=1540 nm and λidler = 812 nm
fabrication and coupling to small discs (e.g. r = 0.3 mm)
THANK YOU
Outlook
investigate the whole parameter space of tuning
couple this single photon source to other quantum systems (quantum dots, atomic clouds, optomechanical resonators, etc.)