Colloidal Quantum Dots (QDs) are heavily investigated for their applications in light emission such as light emitting diodes and, more challenging, lasers due to their appealing processing conditions, compared to e.g. epitaxy, lowering cost and enabling patterning, and tunable optical properties. Using quantum confined Cd-based QDs, several groups have shown light amplification and ensuing lasing action in the red part of the spectrum. Although impressive milestones were achieved, there is to date no single material that can provide the demanding combination of gain metrics to be truly competitive with existing epitaxial growth approaches. In this work, we take a look at CdS/Se nanocrystals in the regime of vanishing quantum confinement, so-called ‘bulk nanocrystals’. We show that these unique materials display disruptive optical gain metrics in the green optical region. Indeed, while showing similar gain thresholds compared to state-of-the-art QD materials, the gain window (440-600 nm, … ), amplitude (up to 50.000/cm) and gain lifetime (up to 3 ns) vastly outpace other QD materials. Using these novel gain materials, we demonstrate lasing in the highly demanded green spectral region (480 – 530 nm) and in the red (650 – 740 nm) both with pulsed and quasi-CW optical excitation. These lasers are made using a Photonic Crystal Surface Emitting Laser (PCSEL) type cavity. As a final step, we attempt to further optimize the lasing properties, be it either narrow linewidth lasers, or high-power output, based on in-depth understanding of the hybrid QD-PCSEL laser system.
Colloidal quantum dots (QDs) are heavily investigated for their applications in light emission such as light emitting diodes and, more challenging, lasers. This is due to their appealing processing conditions, compared to e.g. epitaxy, resulting in lowering cost. They can also be patterned and their optical properties can be tuned. Using quantum confined Cd-based QDs, several groups have shown light amplification and ensuing lasing action in the red part of the spectrum. Although impressive milestones were achieved, there is to date no single material that can provide the demanding combination of gain metrics to be truly competitive with existing epitaxial growth approaches. In this talk, we take a look at material properties of CdS/Se nanocrystals in the regime of vanishing quantum confinement, so-called ‘bulk nanocrystals’. We show that these unique materials display disruptive optical gain metrics in the green optical region. Indeed, while showing similar gain thresholds compared to state-of-the-art QD materials, the gain window (440-600 nm, 640-750 nm), amplitude (up to 50.000/cm) and gain lifetime (up to 3 ns) vastly outpace other solution processible materials. These results, while very impressive, are also puzzling. In the solution processible community a material system without quantum confinement does not exactly inspire confidence to have good emission metrics. We attempt to explain the physics behind these huge gain coefficients, by using a bulk semiconductor model which includes a strong band-gap renormalization effect, and argue why going to a bulk semiconductor can be advantageous compared to confined systems for making integrated lasers.
Efficient energy transport is highly desirable for organic semiconductor (OSC) devices such as photovoltaics, photodetectors, and photocatalytic systems. However, photo-generated excitons in OSC films mostly occupy highly localized states over their lifetime. Energy transport is hence thought to be mainly mediated by the site-to-site hopping of localized excitons, limiting exciton diffusion coefficients to below ~10-2 cm2/s with corresponding diffusion lengths below ~50 nm. Here, using ultrafast optical microscopy combined with non-adiabatic molecular dynamics simulations, we present evidence for a new highly-efficient energy transport regime: transient exciton delocalization, where energy exchange with vibrational modes allows excitons to temporarily re-access spatially extended states under equilibrium conditions. In films of highlyordered poly(3-hexylthiophene) nanofibers, prepared using living crystallization-driven self-assembly, we show that this enables exciton diffusion constants up to 1.1 ± 0.1 cm2/s and diffusion lengths of 300 ± 50 nm. Our results reveal the dynamic interplay between localized and delocalized exciton configurations at equilibrium conditions, calling for a re-evaluation of the basic picture of exciton dynamics. This establishes new design rules to engineer efficient energy transport in OSC films, which will enable new devices architectures not based on restrictive bulk heterojunctions.
We have developed a new method, transient grating photoluminescence spectroscopy (TGPLS), allowing the collection
of broadband ultrafast photoluminescence spectroscopy with low photoluminescence background. In TGPLS, two
ultrafast laser pulses generate a multiplexed transient grating (TG) by the optical Kerr effect. The gated signal is
diffracted by the TG and spatially separated from background fluorescence. This high performance nonlinear optical gate
delivers time resolution less than 200 fs, spectral bandwidth covering the entire visible region with extremely low
fluorescence background. Here we present two applications of TGPLS that provide deeper insight into ultrafast energy
transfer in multi-chromophore perylene arrays and ultrafast structural relaxation in oligothiophenes.
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