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

Light emitters in 2D heterostructures coupled to photonic devices

Polariton physics

TMDC/graphene LEDs

2D PHOTONICS

Photonic crystal nano-cavities

Tunable microcavities

2D

het

ero

stru

ctu

res

Schwarz Nano Lett (2014)

Withers Nat Materials (2015)

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)

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

Talk outline

Light emitters in optical cavities: ‘weak coupling’

600 700 800 9000.0

0.2

0.4

0.6

0.8

1.0

stop-band

Re

fle

ctivity

Wavelength (nm)

cavity

mode

Modification of emission spectrum and directionality

V

Q

nFP

3

24

3

Purcell enhancement of spontaneous emission rate dependent on the cavity quality factor (Q) and mode volume (V)

Weak coupling regime: Cavity and emitter dissipation rates exceed cavity-emitter coupling rate

Role of optical cavities: Control of light emission properties and light-matter interaction

Example of III-V planar microcavity

Wu et al, Nature (2015)

Lasing in WSe2 films coupled to a photonic crystal nano-cavity

660 670 680 690 700 710

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half-cavity

full cavity with Rc

5.6m, x0.05

10m, x0.1

16m, x0.2

Ph

oto

lum

ine

sce

nce

in

ten

sity (

co

un

ts/s

/sq

.mic

ron

)

Wavelength (nm)

(a) half-cavity

full cavity:

Rc=25m

(b)

Schwarz et al, Nano Lett. (2014)

2D materials in the weak coupling regime

Purcell enhancement for GaSe and MoS2 in tunable cavities

WSe2 LEDs in monolithic microcavities: electroluminescence into cavity modes

1.50 1.55 1.60 1.65 1.70 1.750.0

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1.0 no top mirror

full cavity

No

rma

lised

ele

ctr

olu

min

escen

ce in

ten

sity

Photon energy (eV)

Strong coupling regime

Cavity-emitter coupling rate exceeds the cavity and emitter dissipation rates

-5 0 5-5

0

5

-5 0 50.0

0.5

1.0

EX

Ec

LPB

En

erg

y (

meV

)Detuning (meV)

Wrabi

UPB

(b)

|X|2

Detuning (meV)

|C|2

(a)

Composite bosons which inherit nonlinear component from excitons and allow direct access to polarisation and dispersion via out-coupling through cavity mirrors

224 phXRabi g

Formation of part-light-part-matter exciton-polaritons

LPB

Exciton-polaritons in planar microcavities

Previous work: GaAs, organic microcavities with J-aggregates, GaN, CdTe, ZnO

Exciton

Photon

Upper Polariton

Lower polariton

Polariton condensate, superfluidity, polariton laser

Kasprzak Nature (2006)

k||

E

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)

Z

Y

X

Z

Y

X

Tilt

Tilt

Z

XY

TopPiezoNanoposi oners

Bo omPiezoNanoposi oners

Bo omDBRmirror/sample

0.55NAAsphericLens

Adjustableheightlenstube

TopDBRmirror

Titaniumsampleholder

Titaniumsampleholder

Bottom XYZ stage and tilt-stages

Top XYZ stage

Bottom DBR

Free space optical access

Sample holder

Top mirror

Objective lens

- 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

‘Open access’ cavity system

Optical mode

SiO2/TiO2 DBR

2D sample

- 2D film (or 2D heterostructure) placed on the flat 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

2D layers in open microcavities

Double and single ‘quantum well’ structure

Double QW region

Single QW region

Samples made by F. Withers

hBN

MoSe2

hBN

MoSe2

hBN

MoSe2

Single QW

Rabi splitting is expected to increase for multiple QWs:

QWRabi N

Double QW

Structure placed at the bottom planar DBR at an E-field antinode

Typical PL spectrum consists of neutral and charged exciton peaks with narrow linewidths of 11 meV and 15meV

X0

X-

Photoluminescence of MoSe2 heterostructures

1.55 1.60 1.65 1.700

50000

100000

150000

200000

250000

300000

X0

7.5 meV

PL In

ten

sity (

cts

/sec)

Photon energy (eV)

T=6K, PL

4 QW device

X-

16.2 meV

4 QW device surprisingly shows even narrower PL peaks with linewidth of 7.5 meV for X0

Ross, Nat Comms (2013), Li PRL (2014)

LPB

UPB

-Clear anticrossing observed in PL between cavity mode resonances and neutral exciton

-Spectrum at resonance shows well resolved polariton eigenstates

-Rabi splitting is 20 meV corresponding to exciton life-time of 0.4 ps (linewidth 1.6 meV)

Anticrossing

Strong exciton-photon coupling in a single QW

See also Liu et al, Nat Photonics (2015)

Linewidth broadening when in resonance with X-

Intermediate coupling with X-: polariton modes unresolved

Splitting of ~9 meV observed for X- in reflectivity

Single QW region

X-

Broadening Anticrossing

LPB UPB

‘Intermediate’ coupling regime for X-

Anticrossing shows an increased Rabi splitting of 29 meV in the double QW Consistent with the expected scaling with QW number:

Key stepping stone to observe room temperature strong coupling

Strong coupling for double-QW

QWRabi N

Glass substrate Glass substrate

Bottom DBR

SiO2/NbO2 DBR

Glass substrate

Bottom DBR

Glass substrate

Bottom DBR

Top DBR 2D

structure DBR

Fabrication of monolithic microcavities

Electroluminescence device (‘EL device’) Graphene h-BN WSe2 h-BN Graphene h-BN

-Single monolayer WSe2 flakes

-BN-WSe2-BN ‘sandwiches’

-WSe2 ‘EL devices’

Samples by Freddie Withers (Manchester)

EL device on DBR

DBR

WSe2 LED heterostructure in microcavity

1.50 1.55 1.60 1.65 1.70 1.750.0

0.2

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1.0 no top mirror

full cavity

N

orm

alis

ed

ele

ctr

olu

min

esce

nce

in

ten

sity

Photon energy (eV)

LEDs in monolithic microcavities

EL T=200 K

Strong electroluminescence coupled in the cavity mode(s)

1.56 1.60 1.64 1.680

200

400

600

0o

3o

6o

9o

12o

15o

18o

1.5 1.6 1.70.0

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escence inte

nsity (

cts

/s)

21o

PL

PL

EL

EL

full

cavity, 0o

full

cavity, 0o

no top

DBR

no top

DBR

Norm

aliz

ed P

L/E

L inte

nsity

Photon energy (eV)

T=300 K

Complex mode structure of ‘photonic dots’ formed due to finite thickness of the LED device

Summary

http://ldsd.group.shef.ac.uk/research/2d-materials/

- Versatile microcavity technology for 2D materials research and applications demonstrated

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)

-Strong exciton-photon coupling leading to formation of exciton-polaritons

- First electrically pumped 2D microcavity devices: prototype for vertically emitting lasers