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
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The Photonic Nanomaterials Group, Department of Materials
Jason Smith
Sub-femtolitre tunable microcavity arrays
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
Optically Detected Magnetic
Resonance of single spins (300K)
Modified emission spectra and transition rates
Characterisation of single
colour centres in diamond
Microwave frequency (GHz)
Nanocrystal synthesis, characterisation and modeling
http://www-png.materials.ox.ac.uk
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

g  ξ.d

ξ
V
Strong coupling:

g
is the coupling strength
is the field per photon
2 g
g   , 
Energy
output
time
Weak coupling:
  g , 
Energy
output
time
Fermi’s Golden Rule:
Can either
2
 '
 f Hˆ '  i

2
f
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)
3Q / n 
FP 
4 2V
3
Popular microcavity designs
From E. L. Hu, (then) UCSB
From K Vahala, Caltech
From J P Reithmayer, Wurzburg.
Planar-concave ‘half-symmetric’ cavities
High quality dielectric mirrors
• Fully tunable
• Efficient coupling
• Access to field maximum
Stability criterion
Trupke et al APL 2005, PRL 2007
Steinmetz et al APL 2006
Muller et al APL 2009
Cui et al Optics Express 2006
High Q open access microcavities with femtoliter mode volumes
SEM of arrayed concave surfaces by ion beam milling
Sub – nm surface roughness for high reflectivity mirrors
  4  2 
  0.9998
Rmax  exp  

    


P R Dolan et al, Femtoliter tunable optical cavity arrays, Optics Letters 35, p.3556 (2010).
White light transmission spectra
L  3m
L  12m
FSR 
2
2L
Hermite-Gauss mode structure
0,3
1,2
2,1
3,0
TEMx,y
3
2
0,2
1,1
2,0
0,1
1,0
0,0
1
0
Laser Transmission
Imaging of mode
structure
Quality factors
Q = 5 x104 achieved
Q ~ 106 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.
14 103048 (2012).
Fluorescence from CdSe/ZnS colloidal quantum dots coupled to cavity modes
http://users.ox.ac.uk/~png
Purcell effect at room temperature
Best aligned
quantum dots
𝑉 > 2𝜇𝑚3
Worst aligned
quantum dots
3Q / n 
FP 
4 2V
3
Q
res
QD  cav
 40
𝑉 = 0.53𝜇𝑚3
“Bad emitter” regime
FDTD calculations
F = FP +1
(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…
Nitrogen-vacancy centres in diamond
N
V
…awaiting first low T results!
Wavelength /nm
How small can open access cavities be made (with decent Q)?
•
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
Mode volume ~3
NB this is about as good as an L3 photonic crystal cavity (Chalcraft APL 90, 241117 2007)
Summary of cavity specifications
Current
Possible
Mirror reflectivity
99.9%
>99.995%
Q factor
5 x 104
>106
Mode volume
0.5 µm3
0.1 µm3
Field per photon
~1.8 kV cm-1
~6 kV cm-1
Purcell factor *
~70
~10000
Leakage rate 
~60 GHz
< 5 GHz
Applications
• Cavity QED/ quantum information science
• Sensing & spectroscopy
• Tunable lasers
Acknowledgments
Phil Dolan
Ziyun 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