Photonic architectures offer a promising avenue for optimizing the performance of various optoelectronic devices. Nevertheless, the future of optoelectronic devices hinges on cost-effective and large-scale manufacturing techniques that can reduce expenses and boost production efficiency. To harness the remarkable attributes of photonic structures for enhancing these devices, they must undergo a processing approach akin to that of the devices themselves.

In our research group, we employ soft nanoimprinting lithography, a versatile, rapid, and cost-efficient method for crafting nanostructures from a diverse variety of materials. In soft nanoimprinting lithography, we make use of pre-patterned soft elastomeric stamps to corrugate materials such as resists, biopolymers, colloids and nanomaterials in general. In all cases, the resulting photonic architectures can exhibit a resolution below 100 nm while covering areas up to 1 cm². During this presentation, I will demonstrate our utilization of pre-patterned stamps to induce the long-range alignment of different colloids, including gold colloids and perovskite nanocrystals, to attain distinct optical properties, such as lattice resonances with high Q-factors. Moreover, I will showcase how elastomeric molds pre-patterned with chiral motifs can lead to chiral metasurfaces, exhibiting strong circular dichroism and intense circularly polarized photoluminescence (CPL), making them seamlessly applicable in various practical applications.

Chiral plasmonic nanostructures have attracted much interest in recent years as new platforms for modulating the polarization of light. Other than photonic devices, chiral plasmonic nanostructures facilitate the ultrasensitive detection of chiral biological materials, e.g. DNA and proteins. To synthesize chiral plasmonic nanostructures, the most well-established method is chirality transfer from biological materials to inorganic nanostructures. Alternatively, circularly polarized light can be used to control the formation of the plasmonic enantiomeric nanostructures, where the chirality of the product solely defined by the polarization of the light. This synthetic method may be more cost-effective while allowing the wavelength and intensity of light to be used as tunable parameters. Here we investigate the use of circularly polarized light to create chiral gold nano bipyramids. Hot charge carriers can be generated with light and consumed to fabricate chiral plasmonic structures. Scanning electron microscopy (SEM) coupled with cathodoluminescence measurements comparing the nano bipyramids before and after circularly polarized light illumination is used for characterization. We will show that both dielectric or plasmonic chiral additions can be deposited depending on the identity of reagents. Circular dichroism (CD) spectra of the chiral plasmonic nanostructures further demonstrate the polarization dependent light-matter interactions of the materials synthesized.

Can we generate and transfer chirality?

Chirality is an important structural property: right-handed and left-handed chiral materials have identical chemical composition and connectivity, but they are related by mirror transformation, forming a couple of enantiomers.

Chiral materials show unique features: the intrinsic non-centrosymmetry leads to optical rotation, circular dichroism (CD), second-harmonic generation (SHG), piezoelectricity, pyroelectricity, ferroelectricity, and topological quantum properties.

In this talk, I will discuss the intriguing interplay between chirality and physical properties in innovative materials, ranging from twisted bilayers (non-magnetic as well magnetic) to chiral hybrid organic-inorganic perovskites.

About Figure 1: Generation of chirality by twisting in bilayer graphene and corresponding spin-
texture switching.

References: 
[ 1 ] K. Yananose, G. Cantele, P. Lucignano, S.-W. Cheong, J. Yu, and A. Stroppa, Phys. Rev. B. 104
(2021) 075407.
[ 2 ] K. Yananose, P. G. Radaelli, M. Cuoco, J. Yu, and A. Stroppa, Phys. Rev. B. 106 (2022) 184408.

Hybrid semiconductor materials are predicted to lock chirality into place and encode asymmetry into their electronic states, while the softness of their crystal lattice accommodates lattice strain and maintains high crystal quality with the low defect densities necessary for high luminescence yields. The realization of chiral bulk emitters with bright circularly-polarized luminescence from such materials is desired for the design of chiroptical photonic and opto-spintronic applications. Here, we report the fabrication of novel chiral layered hybrid lead-halide perovskites with high photoluminescence quantum efficiencies and large degrees of circularly polarized photoluminescence at room temperature.[1] Using state-of-the-art transient chiroptical spectroscopy, we rationalize the excellent photoluminescence yields from suppression of non-radiative loss channels and very high rates of radiative recombination. We further find that photo-excitations sustain polarization lifetimes that exceed the timescales of radiative decays, which rationalize the high degrees of polarized luminescence. We postulate that the superior optoelectronic properties of the layered hybrid perovskites arise from their special tolerance to crystal structure chirality, which we carefully designed by cation engineering. We demonstrate that our materials can be used in electroluminescent devices and polarized-light detectors. Our findings pave the way towards high-performance solution-processed photonic systems for chiroptical applications and chiral-spintronic logic at room temperature.

The significant growth, development, and evolution of technologies such as optoelectronics and spintronics have always been accompanied by access to materials with specific and extraordinary properties. Among these materials, recently, chiral trigonal tellurium (Te) stands out since it exhibits electrical magneto-chiral anisotropy and spin polarization,[1-3] electrical conductivity anisotropy and intrinsic polarized photoresponse,[3-5 tunable Rashba spin-orbit coupling,[6] optical activity[7,8] and bulk photovoltaic effect (BPVE)[9]. However, since its properties depend on its crystal structure, for its successful integration into devices and the development of new applications, it is key to determine its crystallographic orientation and handedness and its interaction with light. In this work, using bulk single crystals, we show how the response of Te to polarized light depends on the crystal orientation which has implications for optical and electrical transport studies. By linearly polarized Raman spectroscopy we identify different crystal faces (1 0 0), (1 1 0) and (0 0 1) and the orientation of the trigonal axis corresponding to the helical Te chains. Moreover, we correlate the angle-resolved experimental patterns derived from the data analysis with the symmetry of the crystal. Furthermore, by circularly polarized measurements, we highlight that only for incidence parallel to the trigonal axis, i.e. in the (0 0 1) face, is possible to determine the handedness. In this case, we observe different peaks shift for left- and right-handed crystals in the corresponding cross-helicity Raman spectra. We support our findings with X-ray diffraction and chirality- and orientation-sensitive chemical etching, providing robust insights for the analysis of chiral and low-dimensional materials.[10]

In recent years, photoactive chiral materials are attracting considerable interest owing to relevant applications in optoelectronics as well as high resolution imaging [1]. In these regards, hybrid materials are skyrocketing the field of material science for optoelectronics because they can tune the properties of soft and inorganic assemblies [2]. A recent interesting class of luminescent chiral materials is represented by chiral hybrid perovskites, since they are showing prominent circularly polarized emissions without any need of expensive ferromagnets or extremely low temperatures [3]. Indeed, the chiral source impacts specific non-covalent interactions occurring within the chiral scaffold, which in turn affect the efficiency of the chiral emissions [4]. Modern multiscale modeling and simulations nowadays have an unprecedented level of accuracy, enabling an efficient chiral design of luminescent materials. The chiral design concepts of low-dimensional perovskites herein discussed are based on enhanced sampling simulations and TD-DFT calculations [5] from the predicted free-energy basins. This simulation strategy enables to consider a variety of contributions including molecular rotations within the chiral framework, that may affect the generated chiroptical properties.

Chiral solution-processable semiconductors based, for example, on small molecules, polymers or halide perovskites offer an exciting new avenue to simultaneously control charge, spin and light using a single material. This could enable efficient spin-optoelectronic devices ranging from displays and holography to detectors, and even applications in quantum information technology.[1] In this talk, I will give an overview of our recent efforts to understand the underlying mechanisms by developing novel time-resolved chiroptical spectroscopy techniques.

By pushing broadband circular dichroism to diffraction-limited spatial and 15 fs time resolution, we create a spin cinematography technique to witness the ultrafast formation of spin domains in halide perovskite films due to local symmetry breaking and spin-momentum locking.[2]

In terms of circularly polarized photoluminescence (CPL), I will first explain the fundamentals and artifacts involved in measuring CPL reliably and introduce an open-access methodology and code to do so [3].

I will conclude by showing our most recent development of a transient broadband full Stokes vector polarimetry with unprecedented time and polarization resolution to track the emergence of chiral light emission [to be submitted].

[1] Nature Reviews Materials 8, 365 (2023)

[2] Nature Materials 22, 977 (2023)

[3] Advanced Materials 44, 2302279 (2023)

Magic-sized clusters (MSC) are identical inorganic cores that maintain a closed-shell stability, inhibiting conventional growth processes. Because MSCs are smaller than nanoparticles, they can mimic molecular-level processes, and because of their small size and high organic-ligand/core ratio, MSCs have “softer” inter-particle interactions, with access to a richer phase diagram beyond the classical close packed structures seen with larger particles. These MSCs display a surprising ability to self-organize into films with hierarchical assembly that spans over seven orders of magnitude in length scale. The films are optically active with g-factors among the highest reported for all semiconductor particles. In this talk I will highlight some remarkable behavior we have recently found in magic sized clusters, with a focus on the symmetry breaking and chirality transfer that occurs during their self-organization into thin films. And I will discuss our method for extracting the chiroptic-CD signal from the raw data that contains highly linear anisotropic effects, derived using Mueller matrix and Stokes vector conventions, and our extension of that expression using a more accurate third order Taylor series expansion to reveal "pairwise interference" contributions to CD spectra that, unlike LDLB contributions, cannot be averaged out of the signal.

Spin-crossover (SCO) complexes are promising building blocks for spintronic and high-density memory devices as they can be reversibly switched between two distinct spin states, low-spin (LS) and high-spin (HS) using a variety of external inputs (i.e. temperature, light, pressure, etc…). Additionally, the spin transition goes with change in other properties like volume, conductivity, or color. One step forward, chiral SCO compounds have been proposed as active components of magneto-optical devices.[1 Interestingly, when electrons are injected through chiral molecules, one electronic spin is preferably transmitted, working as spin filters at room temperature. This fascinating outcome is called Chiral Induced Spin Selectivity (CISS) effect.[2] Few years ago, we observed a CISS enhancement when adding paramagnetic centers in chiral self-assembled monolayers (SAMs).[3] Now, we want to push the field further by taking benefit from SCO bistability to modulate the magnitude of CISS effect. In this scenario, we pursue using chiral SCO compounds self-assembled on surfaces to further elucidate the role of the metallic center in the CISS performance. To reach this final goal, we have synthetized chiral SCO complexes and prepared self-assembled monolayers (SAMs) by a solution approach (preliminary step to have enhanced CISS effect). Specifically, in this project we have prepared chiral SCO complexes with tetradentate chiral Schiff base ligands (L) of formula [Fe(L)(NCX)₂] (X = S, Se).

In this presentation I will discuss our studies of controlling the spin currents using chiral metal-halide hybrid semiconductors thru the chiral induced spin-selectivity (CISS) effect. Chirality is introduced through an enantiomerically pure organic ammine as the A-site in layered 2D Ruddlesden-Popper type structures. The inorganic component make up the layers and are either lead or tin halide. Chirality is introduced into the inorganic component thru structural distortions and chiral arrangement of the organic components. We have studied this chiral transfer in the 2D layered systems, 0D dimer inorganic structures, and in metal-halide semiconductor nanocrystals with chiral ligands attached.

These systems exhibit CISS whereby only one spin sense can transport across the chiral layer and the other spin sense is blocked for one handedness of the chiral perovskite layer. We show that chiral perovskite layers are able to achieve > 80% spin-current polarization using magnetic conductive probe AFM. We have demonstrated the CISS effect in a half spin-valve where only one of the contacts is a ferromagnet and these results agree with the magnetic conductive probe AFM measurements.  However, to enable a broader range of opto-electronic functionality achieving spin accumulation in traditional semiconductor structures at room temperature and without magnetic fields is key.  Current efforts that employ ferromagnet/semiconductor interfaces is limited due to the conductivity mismatch.  We have demonstrated spin injection across chiral halide perovskite/III-V interfaces achieving spin accumulation in a standard semiconductor III-V [MH1] (AlxGa1-x)0.5In0.5P multiple quantum well (MQW) light emitting diode (LED). The MQW is recieved as a working LED and it's internal structure was not modified.  The spin accumulation in the MQW is detected via emission of circularly polarized light with a degree of polarization of up to ~15%. Characterization of the chiral perovskite/III-V interface demonstrates a clean conformal semiconductor/semiconductor electrical contact where the fermi-level can equilibrate. Our findings demonstrate chiral perovskite semiconductors can transform well-developed semiconductor platforms to ones that can also control spin at room temperature.