Metallic and dielectric nanostructures provide distinct and unique means for shaping the electromagnetic near field, and for channelling radiation from the far field to the nanoscale. The associated electromagnetic field hot spots can be exploited for the enhancement of interactions between light and matter, most prominently for surface-enhanced spectroscopy and sensing, the boosting of non-linear interactions, and also for nanoscale spatial control over chemical reactions.

In my lecture I will approach plasmonic and dielectric nanoantennas from the viewpoint of being a means for energy conversion at the nanoscale. With example materials systems such as gold and silver (plasmonic), gallium phosphide (dielectric), and silicon carbide (polar dielectric), I will highlight applications such as non-linear optics, photon-phonon interactions for the launching of acoustic surface waves, and the plasmon-assisted triggering of redox reactions.

Control over the interplay between electromagnetic radiation and matter is central to optimize the performance of energy conversion devices such as solar cells or LEDs. In this talk, the fundamental photonic concepts behind efficiency enhancement in energy conversion devices, as well as a variety of approaches to prepare and integrate photonic materials in such devices, will be presented. Photonic crystals, aperiodic media, plasmonic nanostructures, corrugated surfaces and optically disordered media are among the different kinds of materials that will be discussed.[1-5] For each case, a specific example of integration in a device and its effect on the related energy conversion process (namely, luminous to electric, electric to luminous, luminous to thermal, luminous to luminous) will be presented.

Near infrared photodetectors are key components in many disciplines, from astronomy and material sciences all the way to medical sciences. Current technologies are now striving to include new aspects in this technology such as wearability, flexibility and tuneability. Organic photodetectors easily offer many of those advantages but their relatively high bandgaps hinder NIR operation. In this work, we demonstrate solution processed organic photodetectors with improved NIR response thanks to a nanostructured active layer in the shape of a photonic crystal. The latter strongly increases the charge transfer state absorption, which is normally weak but broadband, increasing the optical path of light, resulting in remarkable photoresponse significantly below the band gap of the blend.[1-4] We show responsibilities up to 50 mA W-1 at 900 nm for PBTTT:PC70BM based photodetectors. Furthermore, by varying the lattice parameter of the photonic crystal structure, the spectral response of the photodetectors can be easily tuned beyond 1000 nm. Furthermore, our photonic structure that can be easily implemented in the device in a single nanoimprinting step, with minimal disruption of the fabrication process, which makes this approach very promising for upscaling.

Regarding low-cost infrared photodetection, colloidal quantum dots (CQDs), thanks to their large tunability, appear to be a new interesting building-block.[1] However, due to hopping transport, the diffusion length of the carriers in CQD film is short (typically few 10-100 nm). The absorption depth of the light is much larger (few µm). As a result, there is a trade-off between transport and optical absorption: usually thin films are used then, and only few % of the incident light is absorbed.[2] Light-matter coupling based on sub-wavelength resonators are used to tackle this issue.

Our device relies on guided mode resonators (GMR) and is made of a slab of CQDs (waveguide) onto a gold grating. The latter has two roles: it focuses the light into the nanocrystal film increasing its absorption, and it plays the role of electrode. The device is designed to induce a resonance and to achieve 100% of absorption at the targeted wavelength, for one of the polarization.[3] This particular design also enables photoconductive gain to occur. Both those effects generate a boost of responsivity of few orders of magnitude. This method is versatile and can be applied at different wavelengths (1.55 µm SWIR and 2.5 µm extended SWIR) with different materials (HgTe, PbS and a mix of perovskite/PbS).[3,4]

The introduction of nano-resonators not only generates a responsivity enhancement but enable spectral shaping oh this responsivity. First, it is possible to tune the position peak (of few hundreds of nm) by changing geometrical parameters such as the period of the grating.[3] Secondly, polarized devices can be made by inducing unmatched resonances in TE and TM polarizations. Then it is possible to achieve broadband absorption by introducing multi-resonances.

The strength of electron-hole-pair Coulomb interactions in organic and two-dimensional (2D) semiconductors strongly affects the performance of such excitonic materials in optoelectronic devices. An important target for these materials is the tuning of the exciton binding energy independent of the electronic band gap. By incorporating donor-acceptor interactions into the organic sublattice of a layered lead iodide perovskite, we observe a reduction in exciton binding energy of almost 50%, due to enhanced electrostatic screening of the exciton with greater polarizability in the organic lattice. We present temperature dependent absorption and photoluminescence measurements to investigate the optical effects of this structural modification. The synthetic strategy developed here for 2D hybrid layered perovskites enables highly modular tuning of exciton binding energies with negligible modification of the inorganic structure.The strength of electron-hole-pair Coulomb interactions in organic and two-dimensional (2D) semiconductors strongly affects the performance of such excitonic materials in optoelectronic devices. An important target for these materials is the tuning of the exciton binding energy independent of the electronic band gap. By incorporating donor-acceptor interactions into the organic sublattice of a layered lead iodide perovskite, we observe a reduction in exciton binding energy of almost 50%, due to enhanced electrostatic screening of the exciton with greater polarizability in the organic lattice. We present temperature dependent absorption and photoluminescence measurements to investigate the optical effects of this structural modification. The synthetic strategy developed here for 2D hybrid layered perovskites enables highly modular tuning of exciton binding energies with negligible modification of the inorganic structure.

Plasmonic resonances can decay by internal damping mechanisms that create hot-electrons. Although these excited charge carriers typically relax on the femtosecond to nanosecond timescale, they can alter both chemical reaction rates and selectivity, which may prove useful for solar fuels. There is an ongoing and vigorous debate over the mechanism(s) causing the observed reactivity perturbations. Several possibilities proposed include: direct plasmon energy transfer to specific molecular adsorbate states, hot-electron (either thermal or non-thermal) transfer to molecular adsorbates, elevated local temperature and increased local electric field. This talk will discuss recent results in our group using single particle spectroscopy and finite element simulations to try and understand and control hot-electron driven chemistry, while looking towards further possibilities for the future.

Upconversion, the process of turning two low energy photons into one high energy photon, has the potential to revolutionize a number of fields, from optogenetics and bioimaging to anti-counterfeit and displays. Significant hurdles, however, stand between its current implementation, most often in non-polar solvents in air-free vials, and applications in the real world. In this talk, we will detail our attempts to identify and solve these hurdles and show the benefits to various technologies that result when these hurdles can be overcome.

First, we synthesize a series of diketopyrrolopyrrole (DPP) derivatives. These materials are straightforward to synthesize, stable, and easily tunable. By adjusting the pendant groups, we show that the emissive properties can be tuned in ~20 nm steps across the orange-red portion of the spectrum. Further, these materials are stable to air and moisture, opening up real world applications. We show that these materials upconvert with efficiencies on the same order as rubrene. 

Next, we turn to in situ implementation. Any interaction of upconversion with biological materials must be done in an aqueous environment, yet the most efficient molecules are all hydrophobic. This has been addressed via the introduction of upconversion into micelles or nano-encapsulants, yet these materials typically suffer from either low efficiency or low optical clarity, scattering the input excitation beam and limiting applications. We demonstrate a facile micellular synthesis that maintains both high optical clarity and high upconversion efficiency. By driving a high boiling point solvent into the core of the micelle, we maintain strong solvation of the materials and thus efficient upconversion. At the same time, we keep the size of the micelles low, allowing for optical clarity across 10 cm of solution. We demonstrate that this straightforward technique works for five different upconversion systems spanning the visible regime.

Finally, we demonstrate initial efforts into turning these foundational improvements into real world applications. We show that the application of upconversion to photochemistry allows reactions to be performed using only near infrared light, opening up these reactions to in vivo applications.