Engineering the light matter interaction with ultra-small open access microcavities Jason M. Smith Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK

Photonics in Oxford Engineering Science Liquid crystals Optical wireless CMOS imagers/ detectors Microscopy Fibre/waveguide theory Metamaterials Acousto-optics Physics Quantum optics and control Quantum optics,fundamentals and processing Metrology Biophysics measurement CMOS imagers Telescope instrumentation Spectroscopy X ray generation Optical techniques in nano-technology Biophysics Photovoltaics Chemistry Cavity ringdown Spectroscopy Absorption spectroscopy Novel spectroscopic techniques Ultrafast spectroscopy Fluorescence imaging Molecular materials Synthetic organic chemistry Molecular electronics Organic chemistry Soft condensed matter Surface analysis Materials Nanocrystal quantum dots– synthesis, characterisation and modeling Biochemistry and Life sciences Advanced microscopy- Micron imaging centre Biochemistry Microscopy for single molecule Biochemistry Bionanotech, Biochemistry Correlative microscopy Wellcome trust centre for human genetics Cell imaging X-ray crystallography Diamond Imaging. Weatherall Inst. for Molecular Medicine. Radiation Oncology (Imaging) High speed imaging Processing of visual information. Exp. Psychology Cavity QED Photovoltaics – silicon and 3 rd Gen materials Carbon nano- materials – synthesis, characterisation and modeling Diamond photonics

The Photonic Nanomaterials Group, Department of Materials Jason Smith Characterisation of single colour centres in diamond Optically Detected Magnetic Resonance of single spins (300K) Microwave frequency (GHz) Engineering interfaces in quantum photonics / electronics / spintronics Novel optical microcavity arrays for enhanced light-matter interactions Engineering excitonic states in semiconductor nanocrystal quantum dots Photonics of diamond and its defects Modified emission spectra and transition rates Sub-femtolitre tunable microcavity arrays Nanocrystal synthesis, characterisation and modeling

Outline Optical microcavities – why small is beautiful Fabrication and characterisation of novel femtoliter open-access cavities Preliminary studies of light-matter coupling at room temperature

Introduction to optical microcavities Strong coupling: is the field per photon is the coupling strength Energy output time

Fermi’s Golden Rule: Energy output time Can either a)work out new matrix element with cavity vacuum field and ‘count’ photon states or b)use free space matrix element and work out change in the optical DoS (Purcell approach) Weak coupling:

From J P Reithmayer, Wurzburg. From K Vahala, Caltech From E. L. Hu, (then) UCSB Popular microcavity designs

Planar-concave ‘half-symmetric’ cavities Stability criterion High quality dielectric mirrors Fully tunable Efficient coupling Access to field maximum Trupke et al APL 2005, PRL 2007 Steinmetz et al APL 2006 Muller et al APL 2009 Cui et al Optics Express 2006

P R Dolan et al, Femtoliter tunable optical cavity arrays, Optics Letters 35, p.3556 (2010). High Q open access microcavities with femtoliter mode volumes Sub – nm surface roughness for high reflectivity mirrors SEM of arrayed concave surfaces by ion beam milling

White light transmission spectra

Hermite-Gauss mode structure TEM x,y 0,00,1 1,0 0,2 1,1 2,0 0,3 1,2 2,1 3,0 0123

Laser Transmission Imaging of mode structure

Quality factors Q = 5 x10 4 achieved Q ~ 10 6 anticipated

Photoluminescence measurements of solutions of intra-cavity quantum dots Z. Di, H. V. Jones, P. R. Dolan, S. M. Fairclough, M. B. Wincott, J. Fill, G. M. Hughes and J. M. Smith, Controlling the emission from semiconductor quantum dots using ultra-small tunable optical microcavities, New J. Phys (2012).

Fluorescence from CdSe/ZnS colloidal quantum dots coupled to cavity modes

Purcell effect at room temperature “Bad emitter” regime Best aligned quantum dots Worst aligned quantum dots

F = F P +1 FDTD calculations (assumes free space emission is unperturbed by cavity)

Suppression of leaky modes Purcell factor of resonant mode

Emission from a single quantum dot into a cavity Count rate ~ 100,000 s -1 into NA = 0.4. Compare ~50,000 s -1 with NA = 1.25 and no cavity.

Apparatus for cryogenic operation… …awaiting first low T results! Nitrogen-vacancy centres in diamond N V Wavelength /nm

Mode volume ~ 3 Mirrors: silica/titania (n=2.5) terminated with /4 titania. Above: planar mirror, 8 pairs Below: curved mirror, 10 pairs, β = 3 µm Mirror spacing = /2 (222 nm), n=1.44 Emitter = 640  8nm, dipole //x NB this is about as good as an L3 photonic crystal cavity (Chalcraft APL 90, ) How small can open access cavities be made (with decent Q)?

CurrentPossible Mirror reflectivity99.9%>99.995% Q factor5 x 10 4 >10 6 Mode volume0.5 µm µm 3 Field per photon~1.8 kV cm -1 ~6 kV cm -1 Purcell factor *~70~10000 Leakage rate  ~60 GHz< 5 GHz Summary of cavity specifications Applications Cavity QED/ quantum information science Sensing & spectroscopy Tunable lasers

Acknowledgments Phil DolanZiyun Di Helene Jones Gareth Hughes Postdoc position available soon Aurélien Trichet

Funding and support EPSRC The Leverhulme Trust The Royal Society Oxford Martin School The KC Wong Foundation Hewlett Packard Ltd