Direct evidence of Rabi oscillations and antiresonance in a strongly coupled organic microcavity

Abstract

We report the direct observation of 30-fs period Rabi oscillations between excitons and cavity photons in a strongly coupled J-aggregate microcavity by means of time-resolved up-conversion and spectral interferometry measurements. The time structure of the transmitted electric field, measured by linear spectral interferometry, shows pronounced ultrafast beats. Its spectral phase reveals a distinct signature caused by destructive interference between the coherent drive and the field radiated by the exciton. This antiresonance selectively probes the uncoupled exciton excitation, and its observation uncovers the coherent and ultrafast exchange of energy between the optically excited cavity and the J-aggregate excitons, as confirmed by transfer matrix calculations. [7,8] have already been demonstrated. The microscopic origin of these phenomena lies in the coherent interaction between excitons and cavity photons, via coupling of the excitonic transition dipole moment to fluctuating (vacuum) electromagnetic fields stored within the cavity. This strong coupling results in the formation of new hybrid lower (LP) and upper (UP) excitonphoton polariton modes, separated in energy by normal mode splitting [9,10]. Polaritons are coherent superpositions of the bare exciton and cavity photon modes; when one of the pure components is driven, a periodic energy exchange between the excitonic and photonic components occurs in the form of Rabi oscillations. From a dynamical perspective, strong coupling is directly connected with a periodic flow of energy between excitons and cavity photons: The normal mode splitting reflects ultrafast Rabi oscillations between the excitonic and the photonic components of the system. Upon impulsive excitation, this coupling leads to characteristic interference beats in the light transmitted through the cavity. As such, one of the most direct ways to probe coupling dynamics is to study, in the time domain, the electric field emitted by the cavity.The optical properties of inorganic microcavities have been intensively studied, with experimental evidence of Rabi oscillations [11,12], nonlinearities in polariton splitting [13], polariton scattering, and relaxation effects [14,15]. In these systems, however, due to small normal mode splitting energies, most applications are confined to low temperatures. The large oscillator strength of organic semiconductors, on the other hand, allows the observation of a strong-coupling regime at room temperature and with easy fabrication techniques, thus * Email address: tvirgili@polimi.it opening up a wide range of applications for cavity polaritons [16][17][18][19]. Strongly coupled (5,6-dichloro-2-[[5,6-dichloro-1-ethyl-3-(4-sulphobutyl)benzimidazol-2 ylidene]propenyl]-1-ethyl-3-(4-sulphobutyl) benzimidazolium hydroxide, inner salt, sodium salt) (TDBC) cyanine dye J aggregates in a microcavity have therefore been studied intensely for their linear optical properties [20], temperature-dependent emission properties [21,22], and interactions with molecular vibrations [...

The Rise and Current Status of Polaritonic Photochemistry and Photophysics

“…The understanding of relaxation dynamics of polaritons is very important in order to use strong coupling phenomena in, for instance, polaritonic devices. Several femtosecond transient absorption spectroscopy (fs-TAS) studies have therefore been performed with this target in mind. ,− Worth noting here is that it is not always easy to distinguish between decay of states and cavity induced artifacts. It is therefore good to be aware of possible pitfalls before performing pump–probe spectroscopy on optical cavities .…”

“…Figure 15 shows a nonexhaustive collection of works reporting threshold values for organic polariton lasers after the first observation by Kena-Cohen and Forrest in 2010 using a single crystal of anthracence. 15 Since then, condensation has been observed in many other organic systems, including other single crystals, 15 , 457 , 468 , 469 polymers, 72 , 449 , 470 , 471 oligofluorenes, 128 , 163 , 448 , 472 − 476 dyes, 16 , 18 , 458 , 477 , 478 proteins, 479 , 480 and hybrid systems. 242 The value of the threshold for condensation has been reduced by more than 2 orders of magnitude, with the lowest threshold reported in the order of just a few μJ cm –2 .…”

“…Note that no distinction is made between incident and absorbed fluence, as these two quantities are used in the literature. Single crystal: (1)-, 481 (2)-, 457 (15)-, 469 (19)-; 15 Polymer: (5)-, 471 (13)-; 470 Oligofluorenes: (6)-, 482 (7)-, 128 (8)-, 483 (9)-, 476 (10)-, 475 and (11)-, 476 (14)-, 448 (16)-, 472 (23)-, 163 (24)-; 484 Dyes: (3)-, 485 (4)-, 486 (12)-, 18 (17)-, 478 (18)-, 487 (20)-, 16 (21)-; 477 Proteins: (22)-, 480 (25)-. 479 The red dashed line indicates the value of the threshold below which electric pumping would be able to overcome the losses associated with carrier injection.…”

Dual-colour pump-probe spectroscopy to observe the transition between polariton branches in an ultrastrongly coupled microcavity containing organic dye molecules

“…27,28) The ultrafast nonlinear dynamics of cavity polaritons have been investigated in various ways. [29][30][31][32][33][34][35] However, there are few reports on Rabi splitting, that is, the transition between the upper polariton (UP) and lower polariton (LP) branches. In the strong coupling regime in semiconductor quantum nanostructures, the vacuum Rabi splitting energy is often located in the terahertz region.…”

Lévy Defects in Matrix-Immobilized J Aggregates: Tracing Intra-and Intersegmental Exciton Relaxation

“…17 Immobilized TDBC J aggregates were prepared in a gel matrix at a TDBC concentration of about 4.5% weight. Details are found in ref 35.…”