Organic molecular materials can produce excited states localized on an isolated molecule after absorbing photons (defined as Frenkel excitons), which show large binding energy, high stability at room temperature, and strong spatial confinement. In our previous works, we found that Frenkel excitons can strongly couple with photons to form a new quantum state called exciton polariton (EP) (J. Am. Chem. Soc. 2011; Acc. Chem. Res. 2016). Based on the special features of EP, we demonstrated a series of new photonic elements.

 

1. Novel photonic behavior based on EPs

Unlike electrons, photons are bosons without charge and rest mass, therefore their transport behavior is difficult to be manipulated in general. How to break the time reversal symmetry of photon transmission and realize nonreciprocal optical components for on-chip integration have always been a great challenge in the field of integrated optoelectronics. Utilizing the special properties of EP, we broke the symmetry of light transmission in a single organic microwire through an electrostatic field at room temperature for the first time, and demonstrated an electric field controlled optical switch. This switch showed very fast response to the electric field (<3 ns) (Sci. Adv. 2018).

Bose-Einstein condensates for controllable coherent light output (Nat. Commun. 2021, 12, 3265)

Recently, we have realized Bose-Einstein condensation (BEC, Adv. Mater. 2021) in a single organic microbelt at room temperature, where the microbelt served as optical resonator and provided photogenerated excitons. The arrangement of organic molecules in the crystals led to a large coupling strength of excitons and photons, resulting in mass production of EPs in the microbelt without an external mirror. EPs relax to the ground state through scattering with the aid of the vibrational energy level of organic molecules, and thus form BEC (Nat. Commun. 2021).

 

2. Photonic elements based on exciton conversion in organic crystals

The generation of EP allows us to control the transmission and coupling behavior of photons in organic microstructures through exciton transfer and conversion processes. Self-regulation effect of excitons in the propagation process was realized by controlling the singlet-singlet energy transfer (S-SET) and triplet-triplet energy transfer (T-TET), thereby generating a stable white light output at the micro/nanoscale (Adv. Mater. 2011). This feature of energy transfer was also applied to realize unidirectional transmission behavior in an isolated heterostructure for an optical diode (Adv. Mater. 2012).

Photonic routers based on energy transfer in organic heterostructures (J. Am. Chem. Soc. 2012, 134, 2880)

One-dimensional organic branched heterostructures, fabricated through controlled epitaxial growth, were used to realize photonic routers based on varied exciton conversion efficiency (J. Am. Chem. Soc. 2012). Related researches show that the energy transfer process effectively establish connections between the control and the input units for photonic transistors with optical amplification characteristics (Adv. Mater. 2013). Nonlinear amplification of acceptor emission was achieved during the nonlinear transition of donor from spontaneous to stimulated emission as a result of synergistic energy transfer processes. Therefore, significant nonlinear amplification of the output signal from such a photonic transistor was achieved at a low excitation power (J. Am. Chem. Soc. 2018).

 

3. Photon manipulation through exciton-SPP coupling in organic/metal composite system

SPP is a special form of electromagnetic field on the surface of metal. It can confine and propagate photons below the diffraction limit with distinct polarization state, mode volume, and dispersion characteristics, which are conducive to photonic integration at subwavelength scale. We have designed a series of organic/metal composite systems, which effectively combined the excitons of organic materials with the surface plasma of metals to simultaneously solve the problems of optical diffraction limit and SPP transmission loss (Adv. Mater. 2012; Adv. Mater. 2013).

Full-color subwavelength output of organic flexible laser (J. Am. Chem. Soc. 2017, 139, 11329)

We designed organic/silver nanowire composites and successfully achieved subwavelength output of laser modes by the coupling between light and SPP (J. Am. Chem. Soc. 2016). We found that the SPP-involved laser mode output can be further regulated by pump mode (J. Am. Chem. Soc. 2017).