1. No category

Exciton-polaritons in van der Waals
heterostructures in tunable microcavities
Alexander Tartakovskii
Department of Physics and Astronomy
University of Sheffield, UK
Inorganic
Semiconductors
Group in Sheffield
http://ldsd.group.shef.ac.uk
Microcavity polaritons, quantum dot
physics, III-V photonics (photonic
crystals, waveguides), III-V device
technology including crystal growth
Photonic crystal nano-cavities
Light emitters in 2D
heterostructures coupled
to photonic devices
Schwarz Nano Lett (2014)
Tunable microcavities
2D heterostructures
2D PHOTONICS
TMDC/graphene LEDs
Withers Nat Materials (2015)
Polariton physics
Dufferwiel, submitted
Contributions
‘Open access cavity’ experiments with 2D materials in Sheffield:
S Dufferwiel, S Schwarz, F Li, P Walker, M Sich, D Krizhanovskii, M Skolnick
2D materials and LED optics experiments in Sheffield:
O Del Pozo Zamudio, R Schofield, O Skrypka, T Godde
2D heterostructures:
F Withers & K Novoselov (Manchester)
Concave mirror design & fabrication:
A Trichet & J Smith (Oxford)
Dielectric layer deposition:
C Clark (Helia Photonics)
Talk outline
Microcavities
- Strong vs weak coupling regimes
- Open tunable microcavities
MoSe2/hBN heterostructures
- Single and double ‘QWs’
Strong light-matter coupling in MoSe2/hBN QWs
- Neutral vs charged exciton-polaritons
Monolithic microcavity LEDs
- WSe2/hBN/graphene LED heterostructures
Light emitters in optical cavities: ‘weak coupling’
Role of optical cavities: Control of light
emission properties and light-matter
interaction
Weak coupling regime: Cavity and emitter
dissipation rates exceed cavity-emitter
coupling rate
stop-band
Reflectivity
1.0
Modification of
emission spectrum
and directionality
0.8
0.6
cavity
mode
0.4
0.2
0.0
600
700
800
Wavelength (nm)
3  
FP 

2 
4  n 
3
900
Q
 
V 
Example of III-V
planar microcavity
Purcell enhancement of spontaneous
emission rate dependent on the cavity
quality factor (Q) and mode volume (V)
2D materials in the weak coupling regime
Lasing in WSe2 films coupled to a
photonic crystal nano-cavity
1000
300
(a)
half-cavity
full cavity with Rc
5.6m, x0.05
10m, x0.1
16m, x0.2
250
(b)
half-cavity
full cavity:
Rc=25m
800
200
600
150
400
100
Purcell enhancement for GaSe and
MoS2 in tunable cavities
Schwarz et al, Nano Lett. (2014)
200
50
0
660
670
680
690
700
710 600
610
620
0
630
Wavelength (nm)
WSe2 LEDs in monolithic microcavities:
electroluminescence into cavity modes
Normalised electroluminescence intensity
Photoluminescence intensity (counts/s/sq.micron)
Wu et al, Nature (2015)
1.0
no top mirror
full cavity
0.8
0.6
0.4
0.2
0.0
1.50
1.55
1.60
1.65
1.70
Photon energy (eV)
1.75
Strong coupling regime
Energy (meV)
Cavity-emitter coupling
rate exceeds the cavity
and emitter dissipation
rates
(b)
(a) 5
1.0
LPB
Ec
UPB
W rabi
0
2
|X|
EX
0.5
LPB
2
|C|
-5
0.0
-5
0
Detuning (meV)
5
-5
0
Detuning (meV)
Formation of part-light-partmatter exciton-polaritons
Rabi  4 g 2   X   ph 
2
Composite bosons which inherit nonlinear component from
excitons and allow direct access to polarisation and dispersion
via out-coupling through cavity mirrors
5
Exciton-polaritons in planar microcavities
Photon
Exciton
E
Upper
Polariton
Kasprzak Nature (2006)

k||
Lower polariton
Polariton condensate,
superfluidity, polariton laser
Previous work: GaAs, organic
microcavities with J-aggregates,
GaN, CdTe, ZnO
TMDCs: properties relevant for polariton physics
- Transition from indirect to direct band-gap semiconductor
- Very large exciton binding energy (100s of meV)
- Very large oscillator strength (~40 times larger than in GaAs)
Kin Fai Mak et al, PRL (2010)
Chernikov et al, PRL (2014)
‘Open access’ cavity system
Free space
optical access
Adjustable height lens tube
Z
Top XYZ
stage
Top Piezo Nanoposi oners
Y
Titanium sample holder
X
Objective lens
Top mirror
0.55NA Aspheric Lens
Top DBR mirror
Bottom DBR
Bo om DBR mirror/sample
Sample holder
Titanium sample holder
Tilt
Bottom XYZ
stage and
tilt-stages
Bo om Piezo Nanoposi oners
Tilt
X
Y
Z
Z
Y
X
- Microcavity formed by two
independent dielectric distributed
Bragg reflectors (DBRs)
-Top mirror - array of concave DBRs
- Spatial and spectral tuning at low
and room temperature
2D layers in open microcavities
- 2D film (or 2D heterostructure)
placed on the flat DBR
Optical mode
2D
sample
SiO2/TiO2 DBR
- Any spot on 2D film can be
selectively excited
- Size of the cavity mode on the
bottom DBR ~1
- Mode tuned spectrally by
adjusting the cavity length
Schwarz et al Nano Letters (2014)
Dufferwiel et al APL (2014)
Besga et al arXiv:1312.0819
Greuter al arXiv:1408.1357
Double and single ‘quantum well’ structure
Single QW
MoSe2
Single QW
region
hBN
Double QW
MoSe2
hBN
MoSe2
hBN
Double QW
region
Structure placed at the bottom planar
DBR at an E-field antinode
Rabi splitting is expected to increase
for multiple QWs:
Rabi  NQW
Samples made by F. Withers
Photoluminescence of MoSe2 heterostructures
X0
X-
Typical PL spectrum consists of
neutral and charged exciton
peaks with narrow linewidths
of 11 meV and 15meV
300000
4 QW device surprisingly shows
even narrower PL peaks with
linewidth of 7.5 meV for X0
PL Intensity (cts/sec)
Ross, Nat Comms (2013),
Li PRL (2014)
250000
T=6K, PL
4 QW device
0
X
7.5 meV
200000
150000
100000
50000
X
16.2 meV
-
0
1.55
1.60
1.65
Photon energy (eV)
1.70
Strong exciton-photon coupling in a single QW
LPB
UPB
-Clear anticrossing observed in PL
between cavity mode resonances
and neutral exciton
-Spectrum at resonance shows well
resolved polariton eigenstates
Anticrossing
See also Liu et al, Nat Photonics (2015)
-Rabi splitting is 20 meV
corresponding to exciton life-time
of 0.4 ps (linewidth 1.6 meV)
‘Intermediate’ coupling regime for XSingle QW region
LPB
UPB X-
Linewidth broadening when in
resonance with XIntermediate coupling with X-:
polariton modes unresolved
Broadening
Anticrossing
Splitting of ~9 meV observed
for X- in reflectivity
Strong coupling for double-QW
Anticrossing shows an increased Rabi
splitting of 29 meV in the double QW
Consistent with the expected scaling
with QW number:
Rabi  NQW
Key stepping stone to observe room
temperature strong coupling
Fabrication of monolithic microcavities
SiO2/NbO2 DBR
Bottom DBR
Glass substrate
Glass substrate
Top DBR
Bottom DBR
Glass substrate
DBR
Bottom DBR
2D
structure
Glass substrate
WSe2 LED heterostructure in microcavity
-Single monolayer WSe2
flakes
EL device
on DBR
-BN-WSe2-BN ‘sandwiches’
-WSe2 ‘EL devices’
Samples by Freddie Withers
(Manchester)
Electroluminescence device (‘EL device’)
DBR
Graphene
h-BN
WSe2
h-BN
Graphene
h-BN
1.0
no top mirror
full cavity
Strong electroluminescence
coupled in the cavity mode(s)
0.8
0.6
EL
T=200 K
0.4
3.5
T=300 K
0.2
600
o
21
3.0
no top
DBR
1.50
1.55
1.60
1.65
1.70
1.75
Photon energy (eV)
Complex mode structure of
‘photonic dots’ formed due to
finite thickness of the LED device
Normalized PL/EL intensity
0.0
o
PL
2.5
18
o
15
2.0
full
o
cavity, 0
PL
1.5
400
o
12
o
9
full
o
cavity, 0
EL
1.0
200
o
6
o
3
no top
DBR
0.5
EL
0.0
1.5
1.6
1.7
o
0
1.56
0
1.60
Photon energy (eV)
1.64
1.68
Electroluminescence intensity (cts/s)
Normalised electroluminescence intensity
LEDs in monolithic microcavities
Summary
- Versatile microcavity technology for 2D materials
research and applications demonstrated
-Strong exciton-photon coupling
leading to formation of excitonpolaritons
- First electrically pumped 2D
microcavity devices: prototype
for vertically emitting lasers
http://ldsd.group.shef.ac.uk/research/2d-materials/
Papers: Sercombe et al, Scientific Reports (2013); Schwarz et al, Nano
Letters (2014); Del Pozo Zamudio et al, arxiv (2015); Withers et al, Nature
Materials (2015)