Dispersion of Third‐Harmonic Generation in Organic Cavity Polaritons

Abstract

exchange of energy between excitons and photons is faster than any loss process. The momentum-energy dispersions of cavity polaritons are characterized by anticrossing of the exciton and cavity dispersions consisting of an upper-polariton (UP) branch (UPB) and a lower-polariton (LP) branch (LPB), with the minimal energy separation between UPB and LPB at the anticrossing point referred as the vacuum Rabi splitting energy (ℏΩ R ). [1] Due to the large oscillator strength and binding energy of organic materials, organic cavity polaritons exhibit large ℏΩ R even at room temperature. [2][3][4] When ℏΩ R is comparable (20%) to the uncoupled exciton energy (E Ex ), the system is characterized by the ultrastrong coupling (USC) regime, where the rotating wave approximation (RWA) is no longer applicable, and the antiresonant exciton-photon coupling terms can significantly modify cavity-polariton properties. [5][6][7][8] The hybrid light-matter characteristics of both single and multiple cavity polaritons are of significant theoretical and practical importance, such as organic light-emitting diodes (LEDs), [9,10] roomtemperature polariton condensation and lasing, [11][12][13] polaritonmediated energy transfer, [14][15][16] conductivity enhancement, [17,18] and superfluidity. [19] However, nonlinear optical processes, such as the optical Kerr effect and optical harmonic generation, arising from cavity polaritons in organic materials are now beginning to receive attention. Importantly, optical harmonic spectral dispersion measurements, which are widely used to study the excited states of organic molecular materials, [20,21] can be utilized to study the intrinsic nonlinearity of the hybrid light-matter system. Recently, the enhancement of resonant second-harmonic generation (SHG) of the lower polariton is reported from strongly coupled organic crystalline nanofiber microcavities, where the SHG wavelength is resonant with the lower polariton. [22] Also, the enhanced third-harmonic generation (THG) was observed from cavity polaritons in the ultrastrong coupling regime when the pump wavelength is resonant with the lower polariton. [23] Here, we experimentally and theoretically demonstrate the dispersion of the upper-polariton-enhanced THG using anglevarying reflection configuration in the ultrastrong coupling regime, and observe the THG intensity is largest when the THG wavelength is resonant with the upper polariton, in contrast to Organic cavity polaritons are bosonic quasiparticles arising from the strong interaction between organic molecular excitons and photons within microcavities. The spectral dispersion of third-harmonic generation near resonance with cavity polaritons is studied experimentally via angle-resolved reflected third-harmonic generation measurements with several pump wavelengths. Moreover, a three-step nonlinear optical transfer matrix model is used to simulate the third-harmonic generation using the sum-overstates dispersive nonlinear coefficients, which include polariton states. The angle-dependent experimen...

Enhanced and Polarization-Dependent Coupling for Photoaligned Liquid Crystalline Conjugated Polymer Microcavities

“…To our knowledge, this last value is the first Rabi splitting for a solid-state system to reach a value comparable to photons in the visible spectrum (∼ 689 nm), exceeding by more than 60% the previous record. 14,15 The corresponding coupling ratio g ∼ 65% is also slightly higher than that reported in Ref 25, a notable result given that the g-value reported here was obtained for an exciton lying some ∼ 1.6 eV higher in energy. The results for TM polarization for the aligned polymer cavities are shown in Figure 4 (b), (c) and (d).…”

“…7 Their large intrinsic oscillator strengths 8 combined with the small mode volumes V m of metallic microcavities 9 have enabled Ω R ≥ 1 eV, [10][11][12][13] with values up to Ω R = 1.12 eV. 14,15 This splitting is directly comparable to the exciton transition energy ω ex and yields normalized coupling ratios g = Ω R ωex ≥ 20%, thereby crossing into ultrastrong coupling (USC), an interaction space that has received great recent attention, with attractive research perspectives and multiple emerging applications. 16,17 Experimental realizations of increasingly higher coupling ratios have also been reported for inorganic semiconductor based intersubband polaritons, 18 and other physical systems, including superconducting circuits, 19 Landau polaritons 20 and plasmonic picocavities interacting with vibrational degrees of freedom of individual molecules.…”

Large optical nonlinearity enhancement under electronic strong coupling

“…For instance, strongly coupled organic molecules could enhance the conductivity 15,16 , the rate of energy transfer 17 of molecules, and furthermore, modify the work function 18 and chemical reactions of molecules 19,20 . Besides, recent studies also showed that second-harmonic generation and third harmonic generation could be enhanced in the presence of polaritonic states [21][22][23][24] . However, the measurements in these works did not characterize the intrinsic nonlinear optical parameters such as n 2 and β, which are necessary to evaluate the true potential of strong coupling for all nonlinear optical processes.…”

Chemistry under Vibrational Strong Coupling

“…intersystem crossing,[22][23][24] non-linear optics[25][26][27][28] in addition to chemical reactivity 7,9,[29][30]. Many…”