Selected Publications:

 


“Linear Wave Dynamics Explains Observations Attributed to Dark Solitons in a Polariton Quantum Fluid”

Phys. Rev. Lett. 113, 103901 – Published 3 September


We investigate the propagation and scattering of polaritons in a planar GaAs microcavity in the linear regime under resonant excitation. The propagation of the coherent polariton wave across an extended defect creates phase and intensity patterns with identical qualitative features previously attributed to dark and half-dark solitons of polaritons. We demonstrate that these features are observed for negligible nonlinearity (i.e., polariton-polariton interaction) and are, therefore, not sufficient to identify dark and half-dark solitons. A linear model based on the Maxwell equations is shown to reproduce the experimental observations.



 

“Polariton-mediated energy transfer between organic dyes in a strongly coupled optical microcavity”

Nature Materials Volume:13, Pages:712–719 (2014) DOI:doi:10.1038/nmat3950


Strongly coupled optical microcavities containing different exciton states permit the creation of hybrid-polariton modes that can be described in terms of a linear admixture of cavity-photon and the constituent excitons. Such hybrid states have been predicted to have optical properties that are different from their constituent parts, making them a test bed for the exploration of light–matter coupling. Here, we use strong coupling in an optical microcavity to mix the electronic transitions of two J-aggregated molecular dyes and use both non-resonant photoluminescence emission and photoluminescence excitation spectroscopy to show that hybrid-polariton states act as an efficient and ultrafast energy-transfer pathway between the two exciton states. We argue that this type of structure may act as a model system to study energy-transfer processes in biological light-harvesting complexes.


 

Polariton condensates: Going soft 


Nature Materials Volume:13, Pages:227–228 (2014) DOI:doi:10.1038/nmat3897

The experimental observation of polariton condensates at room temperature in soft organic materials makes the study of quantum condensed phases easily accessible and opens inroads to optoelectronic devices based on macroscopic quantum phenomena.








 

"Single-mode tunable laser emission in the single-exciton regime from colloidal nanocrystals"


Whispering-gallery-mode resonators have been extensively used in conjunction with different materials for the development of a variety of photonic devices. Among the latter, hybrid structures, consisting of dielectric microspheres and colloidal core/shell semiconductor nanocrystals as gain media, have attracted interest for the development of microlasers and studies of cavity quantum electrodynamic effects. Here we demonstrate single-exciton, single-mode, spectrally tuned lasing from ensembles of optical antenna-designed, colloidal core/shell CdSe/CdS quantum rods deposited on silica microspheres. We obtain single-exciton emission by capitalizing on the band structure of the specific core/shell architecture that strongly localizes holes in the core, and the two-dimensional quantum confinement of electrons across the elongated shell. This creates a type-II conduction band alignment driven by coulombic repulsion that eliminates non-radiative multi-exciton Auger recombination processes, thereby inducing a large exciton–bi-exciton energy shift. Their ultra-low thresholds and single-mode, single-exciton emission make these hybrid lasers appealing for various applications, including quantum information processing.


"Nonlinear Optical Spin Hall Effect and Long-Range Spin Transport in Polariton Lasers"
We report on the experimental observation of the nonlinear analogue of the optical spin Hall effect under highly nonresonant circularly polarized excitation of an exciton-polariton condensate in a GaAs/AlGaAs microcavity. The circularly polarized polariton condensates propagate over macroscopic distances, while the collective condensate spins coherently precess around an effective magnetic field in the sample plane performing up to four complete revolutions.



"Spontaneous Symmetry Breaking in a Polariton and Photon Laser"
We report on the simultaneous observation of spontaneous symmetry breaking and long-range spatial coherence both in the strong- and the weak-coupling regime in a semiconductor microcavity. Under pulsed excitation, the formation of a stochastic order parameter is observed in polariton and photon lasing regimes. Single-shot measurements of the Stokes vector of the emission exhibit the buildup of stochastic polarization. Below threshold, the polarization noise does not exceed 10%, while above threshold we observe a total polarization of up to 50% after each excitation pulse, while the polarization averaged over the ensemble of pulses remains nearly zero. In both polariton and photon lasing regimes, the stochastic polarization buildup is accompanied by the buildup of spatial coherence. We find that the Landau criterion of spontaneous symmetry breaking and Penrose-Onsager criterion of long-range order for Bose-Einstein condensation are met in both polariton and photon lasing regimes.

"Dependence of Resonance Energy Transfer on Exciton Dimensionality"
We investigate the dependence of resonance energy transfer from Wannier-Mott excitons to an organic overlayer on exciton dimensionality. We exploit the excitonic potential disorder in a single quantum well to tune the balance between localized and free excitons by scaling the Boltzmann distribution of excitons through temperature. Theoretical calculations predict the experimentally observed temperature dependence of resonance energy transfer and allow us to quantify the contribution of localized and free excitons. We show that free excitons can undergo resonance energy transfer with an order of magnitude higher rate compared to localized excitons, emphasizing the potential of hybrid optoelectronic devices utilizing resonance energy transfer as a means to overcome charge transfer related limitations.   
     


"Gauging the flexibility of fluorescent markers for the interpretation of fluorescence resonance energy transfer"

Journal of the American Chemical Society 133 (2), 279–285 (2011)

Abstract: Intramolecular distances in proteins and other biomolecules can be studied in the living cell by means of fluorescence resonance energy transfer (FRET) in steady state or pulsed-excitation experiments. The major uncertainty originates from the unknown orientation between the optical dipole moments of the fluorescent markers, especially when the molecule undergoes thermal fluctuations in physiological conditions. We introduce a statistical method for the interpretation of fluorescence decay dynamics in donor-acceptor FRET pairs that allows us to retrieve both the orientation and the extent of directional fluctuations of the involved dipole moments. We verify the method by applying it to donor-acceptor pairs controllably attached to DNA helices and find that common assumptions such as complete rotational freedom or fully hindered rotation of the dipoles fail a physical interpretation of the fluorescence decay dynamics. This methodology is applicable in single molecule and ensemble measurements of FRET to derive more accurate distance estimates from optical experiments, without the need for more complex and expensive NMR studies.



"Increased color conversion efficiency in hybrid light emitting diodes utilizing non-radiative energy transfer"

Advanced Materials 22, (5) 602 (2010)

Abstract: An efficient hybrid color-conversion light-emitting device consisting of colloidal nanocrystal quantum dots (NQDs) and a surface-patterned GaN-based LED is demonstrated (see figure). Excitation in a surface-patterned LED is efficiently transferred to NQD emitters via non-radiative energy transfer. A twofold enhancement of the NQD emission is achieved.









“Photocurrent enhancement in hybrid nanocrystal quantum dot/p-i-n photovoltaic devices”  

Physical Review Letters, 102, 077402 (2009) 
Abstract:We fabricate a hybrid nanocrystal quantum-dot patterned p-i-n structure that utilizes nonradiative energy transfer from highly absorbing colloidal nanocrystal quantum dots to a patterned semiconductor slab to demonstrate a sixfold increase of the photocurrent conversion efficiency compared to the bare p-i-n semiconductor device.


On the news: New Scientist, Physics World, Optics.org, ScienceDaily, university's press release   



"Efficient light harvesting in hybrid CdTe nanocrystal/bulk GaAs p-i-n photovoltaic devices"

Appl. Phys. Lett. 94, 233502 (2009)

Abstract:A hybrid colloidal CdTe nanocrystal/bulk GaAs p-i-n heterostructure is demonstrated to have potential for highly efficient light harvesting photovoltaic devices. An array of rectangular channels is fabricated on the surface of the GaAs heterostructure penetrating through its active layer and subsequently filled with water soluble CdTe nanocrystals emitting in the near infrared. Photogenerated carriers in the highly absorbing colloidal nanocrystals are efficiently transferred by means of nonradiative energy transfer to the patterned heterostructure possessing high carrier mobility and converted to electrical current. A threefold enhancement of both photocurrent and monochromatic power conversion efficiency has been achieved.


“Temperature Dependence of Exciton Transfer in Hybrid Quantum well/Nanocrystal Heterostructures” 
Applied Physics Letters 91, 092126 (2007)

Abstract: The authors investigate the temperature dependence of exciton transfer from a single InGaN quantum well (QW) donor to colloidal CdS nanocrystal quantum dot acceptors and obtain an optimum transfer efficiency of 65% at 60 K. Time and spectrally resolved measurements reveal that the transfer efficiency is dominated by the interplay between exciton localization and nonradiative recombination intrinsic to the QW.


“Room temperature exciton storage in elongated semiconductor nanocrystals”

Phys. Rev. Lett. 98, 017401 (2007)

Abstract:The excited state of colloidal nanoheterostructures consisting of a spherical CdSe nanocrystal with an epitaxially attached CdS rod can be perturbed effectively by electric fields. Field-induced fluorescence quenching coincides with a conversion of the excited state species from the bright exciton to a metastable trapped state (dark exciton) characterized by a power-law luminescence decay. The conversion is reversible so that up to 10% of quenched excitons recombine radiatively post turn-off of a 1  μs field pulse, increasing the delayed luminescence by a factor of 80. Excitons can be stored for up to 105 times the natural lifetime, opening up applications in optical memory elements.






“Room temperature polariton lasing in semiconductor microcavities”
Phys. Rev. Lett. 98, 126405 (2007)

Abstract:We observe a room-temperature low-threshold transition to a coherent polariton state in bulk GaN microcavities in the strong-coupling regime. Nonresonant pulsed optical pumping produces rapid thermalization and yields a clear emission threshold of 1 mW, corresponding to an absorbed energy density of 29 u.J cm.2, 1 order of magnitude smaller than the best optically pumped (In,Ga)N quantumwell surface-emitting lasers (VCSELs). Angular and spectrally resolved luminescence show that the polariton emission is beamed in the normal direction with an angular width of .10 deg. and spatial size around 5 u.m.


“Exciton accumulation in π-conjugated wires encapsulated by light-harvesting macrocycles”

Angewandte Chemie Int. Ed. 46, 3450 (2007)
















“Spin-conserving carrier recombination in conjugated polymers” 

Nature Materials 4, 340-346 (2005)

Abstract:The ultimate efficiency of polymer light-emitting diodes is limited by the fraction of charges recombining in the molecular singlet manifold. We address the question of whether this fraction can principally exceed the fundamental limit set down by spin statistics, which requires the possibility of spin changes during exciton formation. Sensitized phosphorescence at 4–300 K enables a direct quantification of spin conversion in coulombically bound electron–hole pairs, the precursors to exciton formation. These are stabilized in external electric fields over times relevant to carrier transport, capture and recombination in devices. No interconversion of exciton intermediates between singlet and triplet configurations is observed. Static magnetic fields are equally unable to induce spin mixing in electroluminescence. Our observations imply substantial exchange splitting at all times during carrier capture. Prior statements regarding increased singlet yields above 25% merely on the basis of higher singlet than triplet formation rates should therefore be re-examined. 



“Wavefunction engineering in elongated semiconductor quantum dots”

Nano Letters 5, 2044 (2005)

Abstract:We explore two routes to wave function engineering in elongated colloidal CdSe/CdS quantum dots, providing deep insight into the intrinsic physics of these low-dimensional heterostructures. Varying the aspect ratio of the nanoparticle allows control over the electron−hole overlap (radiative rate), and external electric fields manipulate the interaction between the delocalized electron and the localized hole. In agreement with theory, this leads to an exceptional size dependent quantum confined Stark effect with field induced intensity modulations, opening applications as electrically switchable single photon sources.