Unconventional Lasing Mechanisms in Organic Semiconductors

Transcription

1 Unconventional Lasing Mechanisms in Organic Semiconductors Stéphane Kéna-Cohen Postdoctoral Research Fellow Center for Plasmonics and Metamaterials t (Stefan Maier Group)

2 Outline of talk 1. Motivation 2. Crystalline Organic Microcavities (Michigan) 3. Organic Polariton Lasing (Michigan) 4. Random Lasing in Organic Films (Imperial) 5. Capabilities in our group (Imperial) Collaborators: S. R. Forrest (Princeton/Michigan) P. N. Stavrinou (Imperial) D.D.C. Bradley (Imperial)

3 Motivation Phenomena demonstrated in inorganic microcavities: Polariton Lasing, BEC and Superfluidity, Parametric amplification, Parametric Oscillation etc. Polariton lasing has been demonstrated at thresholds one order of magnitude lower than that required for conventional lasing 1 Route towards realizing electrically pumped organic lasing 2 Tunable from the UV to the IR Temperature independent Low cost fabrication on arbitrary substrates Major differences: Weak intermolecular l interaction ti Disorder (structural and energetic) Lower quality mirrors 1 Deng et al. PNAS (2003), Kasprzak et al, Nature (2006), Christopoulos et al. PRL (2007) 2 REVIEW: Kozlov et al. JAP 84, p. 4096

4 Frenkel exciton band structure * Most organic electronic devices (OLEDs, PVs, OTFTs) based on amorphous or nanocrystalline organic films. Excitons are localized and diffuse via Förster transfer. Organic crystals have been studied for more than 60 years! e.g. anthracene b a M L Davydov splitting Band structure calculation: M. Philpott, J. Chem Phys (1971)

5 Anthracene as an active layer material b a Viewed along [001]-plane E//b Well-studied >99% PL Yield Can be purified via zone refining Monoclinic crystal structure 2 Molecule Basis E//a 1 Morris and Sceats, Chem Phys 3, p. 164 (1974) 2 RWI de Boer et al., PSSA, 201, p (2004) (photo)

6 Controlled growth of Anthracene Single Thickness control Crystals Growth from the melt Crystal is grown in gap between two quartz slides Grains on the order of ~cm can be obtained Anthracene melted at 240 C N 2 atmosphere Cooled at 1 C/min Thickness 1-3m Thickness is controlled with <10nm Quartz/Si/etc accuracy by spacer ~100 nm Quartz/Si/etc SiO 2 or Au used for spacer Low-temperature sodium silicate bonding Au-Au cold-welding S. Kéna-Cohen et al., PRL, vol. 101, p

7 Controlled single crystal growth Optical 120 nm thick anthracene channels on quartz mm 0-2 Fluorescence 1mm (Dark stripes=anthracene)

8 Crystalline Organic Polariton Structure Quartz gold 12x SiO 2 /SiN x 140 nm anthracene gold channel area 1 mm 12x SiO 2 /SiN x Quartz 0 polarization-dependent PL p-polarized polarized angle-resolved reflectivity Q~800 LP b LP a

9 Dispersion Due to anisotropy, the dispersion depends on both polarization and the direction of propagation 1,2 =90 90 // a V V V E ph ) ( 4-body coupled harmonic oscillator Hamiltonian =0 E E V E V E V ex ex ex p =0 1 M. Litinskaya et al., Phys. Stat. Solidi (a), Vol. 201, 646 (2004) 2 H. Zoubi et al., PRB 71, (2005) // b

10 Measuring the Full Dispersion Resonant Rayleigh Scattering is used to visually probe the full dispersion x CCD Screen k y (cm -1 ) LP a ev x 10 5 k (cm -1 ) x x /2-0.5 BBO k y (cm -1 ) 0 Tunable IR laser LP b ev k x (cm -1 ) x S. Kéna-Cohen et al., PRB 78, (2008)

11 Dynamics from photoluminescence A peak in the LP b PL occurs at ~2.94 ev regardless of detuning

12 Dynamics from photoluminescence Peak in PL at ~2.94 ev regardless of detuning W ki-kf E E pump =3.8 ev Relaxation must be from the reservoir! Can model luminescence using semiclassical Boltzmann equation dn kf dt P k k n k n k kf W kf ki (1 n kf ) (1 n kf ) ki W ki kf n ki n k : Polariton density P k : Pump term k : Decay rate W k k i f 2 P kf H int P ki 2

13 Theoretical Models What H int is responsible for the peak? Non-radiative H ge B B ( b b ) ex phonon vib E vib Optical phonon energy g Exciton-phonon coupling constant B n Exciton annihilation operator Optical phonon annihilation operator b n We have calculated the transfer rate k nr ~(100ns) -1 n n n n n Radiative H exciton-photon = E strong weak S 1 S 0 Transfer rate on the order of k rad ~(5ns) -1 Litinskaya et al. JoL 110 p. 364 (2004) L. Mazza et al. PRB 80 p (2009)

14 Optimized Laser structure DBRs photon exciton k k z k // UP b MP2 b MP1 b (1) 0-0 LP b (2) b a Cavity thickness (120 nm) chosen such that E LP (k=0)~2.94 ev

15 Intensity dependence at 0 Pumped at E pump =3.45 ev with 4 th harmonic from OPAidl idler. OPA pumped by 1kHz Ti:Sapphire regen LP b P th =120 nj LP a

16 Linewidth collapse m=1 cavity m=2 cavity 200 m spot size 100 m spot size A reduction in linewidth is observed immediately upon reaching A reduction in linewidth is observed immediately upon reaching threshold

17 Photoluminescence Below threshold (P=880 pj) Above threshold (P=389 nj) 200 m 200 m Structure remains in the strong coupling regime beyond threshold Pump spot acquires Hermite-Gaussian structure and bright spots develop Kéna-Cohen et al., Nat. Phot. 4, 371 (2010)

18 Time dependence and thermalization Occupation per state Photoluminescence decay (0º) Polariton distribution function fits Emission lifetime collapses to <30 Boltzmann distribution ib ti function ps above threshold above threshold (T lattice =326K)

19 Compare to threshold for conventional lasing P pump =15.9 J/cm 2 g 1 ln 1 th L R PL Inten nsity (a.u.) Counts = cm Length (mm) Energy (ev) Lowest threshold possible for conventional photon lasing: 430 J/cm 2 Our threshold for polariton laser: 320 J/cm 2

20 Thermodynamic Limit N critical Brillouin zone k 2 / R e 1 E( k ) E(0) / k b T 1 1E14 Polariton Density (cm -3 ) 1E13 1E12 1E11 Polariton Laser Polariton LED Temperature (K) The critical density at 300K for condensation is ~2x10 13 /cm 3

21 Polariton Lasing Summary First demonstration of polariton lasing in an organic semiconductor Performance limited by bimolecular quenching. A lot of room for improvement! In the thermodynamic limit, electrical pumping would be readily achievable. How do we get there? Non-radiative scattering Exciton-exciton scattering (Coulomb vs. Kinematic) Low temperature properties and coherence

22 Random Lasing

23 Open Systems and Stimulated Emission * In an open system, the Hamiltonian is non-hermitian. Only quasimodes are present. 1 Amplified spontaneous emission (ASE) happens preferentially along the pump direction and at the peak of the gain spectrum. Emission In ntensity (a.u.) Also shows: 0.6 Threshold Polarized output 0.4 Some Narrowing Limited coherence DCM DCJTB PL ASE Landau and Lifshitz, Quantum Mechanics Wavelength (nm)

24 Random Lasing In a disordered system, lasing can occur when the gain overcomes the losses of the least lossy quasimode. Theory: Ge et al., PRA (2010) H. Türeci et al., Science (2008) Three regimes: Highly scattering (Anderson localization) li Weakly scattering Ballistic limit (our work) Random Lasing in ZnO Nanoparticles Cao et al. PRL (1999) Chaotic resonators Gmachl et al. Science (1998)

25 Random lasing in weakly scattering films Alq:DCM Em mission Inten nsity (a.u.) J/cm 2 80 J/cm J/cm Wavelength (nm) Peak Intensit ty (a.u.) J/cm I th =(955) J/cm Pump Intensity (J/cm 2 ) Well defined lasing modes are observed. The frequency/number of lasing modes is highly-dependent on the location of the excitation spot Power dependence of lasing mode from a 250 nm Alq:DCM neat film

26 Dependence of I th on excitation area Thre eshold fluenc ce density (m mj/cm2) Mean Fit y=a x p A= p= Pump Length (mm) At a fixed intensity, the lasing threshold is highly dependent on sample area. This can be related to the statistical probability of finding a low-loss loss (high Q) quasimode. Theory: Apalkov and Raikh, PRB (2005) Opposite to trend observed in: Turek et al, Nature Materials (2010)

27 Relevance to previous work Alq:DCM laser on plastic Kozlov et al. JAP Vol 84, p (1998) 3 mm-long P th =300 kw/cm 2 P random =12 kw/cm 2 The first coherent random laser? Alq:DCM VCSEL Bulovic et al., Science, p. 553 (1998) P 2 th =600 kw/cm P random =92 kw/cm 2 Exhibits unusually narrow linewidth and small divergence angle for a multimode VCSEL. Possible combination of conventional (out- of-plane) and random (in-plane) lasing?

28 Some of our spectroscopic capabilities Single particle confocal spectroscopy (VIS/NIR) -Extinction -Dark field -Lifetime FTIR Microscope for IR spectroscopy up 20 m VIS/NIR VASE ellipsometer Dual-tip SNOM for near-field imaging Variable stripe length ASE and gain measurements Home-built high-brightness single photon source and Brown-Twiss Interferometer

29 Thank you for your attention