Excitonics.
Light plays an increasingly important role in modern technology. It carries information in many forms, from the emerging field of photonic integrated circuits to well-established markets such as displays for electronic devices or imaging and sensing technology for medical, industrial, military, and scientific applications.
In our attempts to improve and innovate, we build on our ability to manipulate light in novel ways, using new materials such as colloidal quantum dots, perovskite semiconductors, and two-dimensional materials. Our projects span a broad range from fundamental to applied science. We use light to probe the charges and excitonic states of a material, and study the decay processes or other interactions. By understanding the physics of these materials, we can design devices with enhanced optical properties and invent novel devices such as low threshold lasers, all-optical switches, and super-bright LEDs.
Current Projects
Perovskite Microcavity Exciton-Polaritons.
Laitz et al., Uncovering Temperature-Dependent Exciton-Polariton Relaxation Mechanisms in Perovskites, arXiv:2203.13816 (2022)
The fundamental challenge in realizing all-optical transistors is that light is weakly interacting. While it is difficult to have one photon influence the behavior of another, it is possible to make interacting quasi-particles called polaritons that have characteristics of both photons and excitons – both light and matter. Polaritons are formed in optical microcavities in the strong coupling regime between bound excitons and cavity photons. This quantum superposition results in a half-light, half-matter bosonic quasi-particle. Polaritons can be tuned to adjust the fraction of photonic or excitonic features, so that, even when mostly photonic, polaritons have a finite interaction strength, resuling in the potential for engineering fast, low-loss, low-power all-optical transistors. Additionally, these properties establish opportunities for studying out of equilibrium Bose Einstein condensation, super-fluidity and quantum vortices for low-threshold polariton lasing.
Traditionally, polaritons have been formed in all-inorganic semiconducting materials (e.g. GaAs heterostructures) which require low operating temperatures (4-70 K) for polariton formation to ensure the exciton binding energy is above kT and the strong coupling interaction is faster than the exciton dissipation rate. The solution appears to lie in a material candidate that has been traditionally employed in photovoltaics. Hybrid perovskites have emerged as a leading active layer material in high efficiency single junction photovoltaics, now surpassing all other thin-film technologies in performance with a certified power conversion efficiency exceeding 25%. We have demonstrated room-temperature exciton-polariton formation in metallic cavities, probed by angle resolved reflectivity and PL measurements through a k-space imaging setup.
In the referenced work, we perform temperature-dependent measurements of polaritons in low-dimensional hybrid perovskite microcavities and demonstrate high light-matter coupling strengths with a Rabi splitting of 260 ± 5 meV. By embedding the perovskite active layer near the optical field antinode of a wedged microcavity, we are able to tune the Hopfield coefficients by moving the optical excitation along the wedge length and thus decouple the primary polariton relaxation mechanisms in this material for the first time. We observe the thermal activation of a bottleneck regime, and reveal that this effect can be overcome by harnessing intrinsic scattering mechanisms arising from the interplay between the different excitonic species, such as biexciton-assisted polariton relaxation pathways, and isoenergetic intracavity pumping. We demonstrate the dependence of the bottleneck suppression on cavity detuning, and are able to achieve efficient relaxation to k|| = 0 even at cryogenic temperatures. This new understanding contributes to the design of ultra-low-threshold BEC and condensate control by engineering polariton dispersions concomitant with efficient relaxation pathways, leveraging intrinsic material scattering mechanisms for next-generation polariton optoelectronics.
Stabilizing J-Aggregates.
Organic molecules, such as J-aggregated cynanine dyes are promising candidates for forming exciton-polaritons in high quality factor microcavities due to their unmatched high oscillator strengths, narrow emission linewidths at room temperature (FWHM ~ 12 nm), and small Stokes shift. Previously limited by poor stability, we recently demonstrated a marked improvement in the damage thresholds by suspending them in a hydrophobic trehalose/sucrose sugar matrix. In addition to having a wide range of molecules that form aggregates with different structural backbones, we can control the aggregation and electronic properties of these molecules through solution chemistry which can be preserved and directly transferred to the solid state.
Figure: Photoluminescence dispersion curve of lower polariton branch formed with J-aggregates showing preferential emission at higher energy and momentum possibly due to incompatible energy and momentum requirements for further relaxation through vibronic coupling.