The angular momentum of a light beam in the paraxial limit can be split into spin and orbital components. Only recently, optical processes involving a conversion of angular momentum from one form to another and related phenomena were conceived and experimentally demonstrated. I will more specifically focus on the effects generated by optical devices named q-plates and their ensuing generalizations, which have proved to be extremely convenient tools for controlling the phase and polarization structure of light beams. Several applications of these devices have been demonstrated in classical photonics and in quantum optics during the last years. In this presentation, after introducing the main concepts of spin-orbit optical couplings and the underlying physics, I will review a selection of recent applications.

An optically addressable light valve is used for high repetition rate dynamic laser beam shaping used in a unique metal additive manufacturing process [1]. The resulting Area Printing™ delivers high power pulses - each with an individually controlled shape - to a metal powder bed that locally sinters and melts to consolidate into a fully dense metal part. This technology and device enable scaling cheaper additive manufacturing with high spatial resolution, while capable of printing part features beyond reach of conventional manufacturing, and with greater efficiency and minimal spatter defects. We address here the unique optoelectronic properties needed from the optically addressable photoresistor that controls the dynamic beam shaping for high density, high-resolution laser 3D printing. Further description is presented of the device-level thermomechanical analysis from parasitic absorption of the laser at kW to MW power levels. We thus highlight key areas of semiconductor properties challenging the performance capabilities in optically addressed light valves used in high power switching applications. [1] https://www.seurat.com/area-printing

Organic light-emitting diodes (OLEDs) are widely used for displays and promising for applications lighting, sensing and medicine. Thermally assisted delayed fluorescence (TADF) is an attractive way of harvesting triplets, without the need for heavy metals such as iridium. However, many TADF materials suffer severe efficiency roll-off i.e. as the light output of the device increases, the efficiency decreases. We will show how efficiency roll-off compares between phosphorescent, fluorescent and TADF OLED materials. The TADF literature suggests that efficiency roll-off should be addressed by increasing the rate of reverse intersystem crossing. We show this incomplete and propose an improved approach. Furthermore we introduce a measurement using variable repetition rate to measure key photophysical parameters to understand efficiency roll-off. Our results and analysis suggest how the serious problem of efficiency roll-off in TADF materials can be reduced.

This conference presentation was prepared for SPIE Optics + Optoelectronics, 2023.

The development of cost-competitive materials capable of producing fuels or electricity directly from the energy harvested from sunlight offers a desirable approach to fulfilling the need for clean, sustainable, and secure energy. Semiconductor metal oxides (e.g., TiO2, Fe2O3, or BiVO4) are abundant, photoactive, stable and cheap; and thus they have been among the most widely adopted materials for the conversion of solar energy into storable and transportable chemical energy such as e.g. hydrogen (H2). However, despite a huge scientific effort, their overall efficiency for solar-driven applications remains rather low due to several crucial limitations such as particularly fast recombination of photo-generated charges (electron-hole pairs) and sluggish kinetics of the redox surface reactions that hinder the practical application in this field. Defect engineering has become an attractive research direction for improving the optical and electronic properties of semiconductor photocatalysts towards boosting their photo(electro)chemical performance. For example, so-called non-stoichiometric black TiO2 has demonstrated unexpectedly enhanced photo(electro)chemical activity, which has been attributed to the co-catalytic effect of unsaturated Ti3+ ion at the titania surface due to the presence of oxygen vacancies (VO). Additionally, these unsaturated defects/sites provide a strong affinity to tightly bond or anchor various species such as transition metals single atoms, or carbon dots that can be used as even more effective co-catalysts. In this contribution, recent advancements regarding defect engineering toward significantly enhanced photocatalytic activity of oxide semiconductors will be summarized.

This conference presentation was prepared for SPIE Optics + Optoelectronics, 2023.

This conference presentation was prepared for SPIE Optics + Optoelectronics, 2023.

Small metallic and dielectric particles behave as dipolar resonators with a resonant frequency determined by their size. When these resonators are placed in a periodic array, they can couple with diffraction orders forming collective modes, which are called surface lattice resonances (SLRs). SLRs lead to large field enhancements over extended areas, forming an ideal system for collective strong light-matter coupling and for optoelectronic applications. SLRs can also lead to Bound States in the Continuum (BICs), with full suppression of the radiation leakage and divergent Q-factors. In this presentation, we describe in detail SLRs and BICs in metallic and dielectric arrays, and the associated phenomena emerging from their coupling to quantum emitters and excitons in organic semiconductors. These phenomena include improved light emission and extraction from quantum wells and low threshold polariton lasing.

Despite fluorescent sensing is a reference method for the detection of a plethora of different compounds, the exploitation of this class of sensors is still limited to a few application scenarios as a result of the restricted availability of miniaturized, portable, and user-friendly devices. Here, the smart combination of an organic photodiode (OPD), a Distributed Bragg Filter (DBR), and an organic light-emitting diode (OLED) is proven to provide a stacked device architecture capable of detecting fluorescent signals for a wide range of concentrations of “Rhodamine 700” ranging from 10-3 M to 10-5 M.

Light-responsive materials capable of undergoing photo-induced molecular transformation are excellent candidates for energy storage. Herein we report a promising new liquid crystalline terpolymer (contains p-methoxyazobenzene) that is capable of trapping the absorbed photon energy in the smectic phase (75 °C) upon exposure to UV light through trans  cis isomerization and molecular aggregation. In the dark, the recovery process shows an increase in absorbance of the trans isomer at room temperature (glass transition) beyond the equilibrium level that can be maintained at the same level (monitored for 20 days). The stored energy was released by changing to the smectic phase, showing the suitability of this system as a solar-thermal fuel.

The nonlinear optical properties of heliconical cholesteric liquid crystals will be presented showing that the peculiar conical structure of these materials offers new opportunities for optoelectronics. Due to the bend distortion of the molecular director light-induced reorientation can be easily obtained with consequent redshift of the Bragg resonance. Using a pump-probe configuration this leads to optical control of light transmission and polarization, while the nonlinear optical propagation of a single beam may lead to self-oscillations and chaotic behavior in the transmitted light, depending on the applied static field, light intensity and wavelength.

Plasmonic particles like gold nanorods have aroused interest as contrast agents and labels for applications in biomedical optics and photonic sensing. Here, we review our recent work on complementary use cases of gold nanorods: A concept of cellular vehiculation with tumor-tropic cells, such as immune system cells or stem cells for efficient delivery to the tumor microenvironment; Ways to improve the biosensors of the types in use to detect COVID-19, and in particular, to make the so-called molecular tests much faster with plasmonic PCR (Polymerase Chain Reaction), and the rapid antigen tests much more sensitive by multiplexing and machine learning.

The continuous development of smart materials and photomechanical actuators has given novel possibilities to micro robotics. In addition to traditional walking, jumping and swimming capacities, locomotion in air through photomechanical control of aerodynamics is the frontier yet to be explored. Possible applications are foreseen in human rescue, artificial pollination, remote sensing and so on. The challenges lie in the difficulty of realizing light weight structure and stimuli-responsive element that can effectively influence the aerodynamics. Here, we demonstrate several nature-inspired passive structures that can “fly” in the air assisted by the wind flow. We will show a super light-weight hairy structure that can disperse and glides in the air. A photomechanical soft actuator made of liquid crystal elastomer is used to induced reversible geometry change. We will show the data of light induced shape-morphing in the polymer structure, and consequent results for tuning the terminal speed and dispersal/gliding behavior. The result provides a new method for light controlled mini-object passively flying in the sky.

Two-dimensional (2D) materials represent a fascinating material class for optoelectronics. While proof-of-concept devices with outstanding performance has been reported in literature, they often rely on micrometer-scale 2D materials and are thus of limited practical use. Overcoming the bottleneck to real-world applications requires both scalable materials and scalable device architectures. We report on wafer-scale 2D materials grown by MOCVD and their implementation in scalable optoelectronic devices. Light emitting devices realized by embedding WS2 monolayers in a vertical device design emit large area red electroluminescence with a turn-on voltage as low as 2.5 V on both, rigid as well as flexible substrates. Direct growth of 2D material heterostructures on a sapphire substrate enables the fabrication of photodetectors without involving any transfer process. We demonstrate an enhancement of the responsivity by more than 5 orders of magnitude in a WS2-MoS2 heterostructure device as compared to a single layer reference. In photosensors that combine a MOCVD-grown WS2 monolayer as light sensitizer with CVD-grown graphene as a conductive channel, we have been able to shed light on the widely varying values of responsivity reported in literature by disentangling adsorbate effects and intrinsic photoresponse.

Multi-photon photopolymerization finds a broad area of applications spanning from microelectronics to medicine. Polymerization process requires the use of photoinitiator molecules that produce free radicals triggering the cross-linking reactions in pre-polymer. In this work, we explore the differences in photoinitiation behaviour of commercial Irgacure369, Irgacure651, TPO and thioxanthone photoinitiators. The TA spectra of Irgacure369, Irgacure651 and TPO feature two broad induced absorption (IA) bands with different relaxation characteristics. IA of Irgacure369 reveals more complex structural dynamics than Irgacure651. Nano-microsecond transient absorption (Flash photolysis) measurements allow us to identify the spectral features of resulting photoactive species to facilitate their further dynamics and roles in polymerization process.

This conference presentation was prepared for SPIE Optics + Optoelectronics, 2023.

This conference presentation was prepared for SPIE Optics + Optoelectronics, 2023.