Presentation
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 5.6m, x0.05 10m, x0.1 16m, x0.2 250 (b) half-cavity full cavity: Rc=25m 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)
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