Mohammad H. Tahersima

Roadmap on optical energy conversion

For decades, progress in the field of optical (including solar) energy conversion was dominated by advances in the conventional concentrating optics and materials design. In recent years, however, conceptual and technological breakthroughs in the fields of nanophotonics and plasmonics combined with a better understanding of the thermodynamics of the photon energy-conversion processes reshaped the landscape of energy-conversion schemes and devices. Nanostructured devices and materials that make use of size quantization effects to manipulate photon density of states offer a way to overcome the conventional light absorption limits. Novel optical spectrum splitting and photon-recycling schemes reduce the entropy production in the optical energy-conversion platforms and boost their efficiencies. Optical design concepts are rapidly expanding into the infrared energy band, offering new approaches to harvest waste heat, to reduce the thermal emission losses, and to achieve noncontact radiative cooling of solar cells as well as of optical and electronic circuitries. Light–matter interaction enabled by nanophotonics and plasmonics underlie the performance of the third- and fourth-generation energy-conversion devices, including up- and down-conversion of photon energy, near-field radiative energy transfer, and hot electron generation and harvesting. Finally, the increased market penetration of alternative solar energy-conversion technologies amplifies the role of cost-driven and environmental considerations. This roadmap on optical energy conversion provides a snapshot of the state of the art in optical energy conversion, remaining challenges, and most promising approaches to address these challenges. Leading experts authored 19 focused short sections of the roadmap where they share their vision on a specific aspect of this burgeoning research field. The roadmap opens up with a tutorial section, which introduces major concepts and terminology. It is our hope that the roadmap will serve as an important resource for the scientific community, new generations of researchers, funding agencies, industry experts, and investors.

Two-Dimensional Material-Based Mode Confinement Engineering in Electro-Optic Modulators

The ability to modulate light using 2-dimensional (2D) materials is fundamentally challenged by their small optical cross-section leading to miniscule modal confinements in diffraction-limited photonics despite intrinsically high electro-optic absorption modulation (EAM) potential given by their strong exciton binding energies. However the inherent polarization anisotropy in 2D materials and device tradeoffs lead to additional requirements with respect to electric field directions and modal confinement. A detailed relationship between modal confinement factor and obtainable modulation strength including definitions on bounding limits are outstanding. Here, we show that the modal confinement factor is a key parameter determining both the modulation strength and the modulator extinction ratio-to-insertion loss metric. We show that the modal confinement and hence the modulation strength of a single-layer modulated 2D material in a plasmonically confined mode is able to improve by more than 10× compared to diffraction-limited modes. Combined with the strong-index modulation of graphene, the modulation strength can be more than 2-orders of magnitude higher compared to Silicon-based EAMs. Furthermore, modal confinement was found to be synergistic with performance optimization via enhanced light-matter-interactions. These results show that there is room for scaling 2D-material EAMs with respect to modal engineering toward realizing synergistic designs leading to high-performance modulators.

Enhanced photon absorption in spiral nanostructured solar cells using layered 2D materials

Recent investigations of semiconducting two-dimensional (2D) transition metal dichalcogenides have provided evidence for strong light absorption relative to its thickness attributed to high density of states. Stacking a combination of metallic, insulating, and semiconducting 2D materials enables functional devices with atomic thicknesses. While photovoltaic cells based on 2D materials have been demonstrated, the reported absorption is still just a few percent of the incident light due to their sub-wavelength thickness leading to low cell efficiencies. Here we show that taking advantage of the mechanical flexibility of 2D materials by rolling a molybdenum disulfide (MoS(2))/graphene (Gr)/hexagonal boron nitride stack to a spiral solar cell allows for optical absorption up to 90%. The optical absorption of a 1 μm long hetero-material spiral cell consisting of the aforementioned hetero stack is about 50% stronger compared to a planar MoS(2) cell of the same thickness; although the volumetric absorbing material ratio is only 6%. A core-shell structure exhibits enhanced absorption and pronounced absorption peaks with respect to a spiral structure without metallic contacts. We anticipate these results to provide guidance for photonic structures that take advantage of the unique properties of 2D materials in solar energy conversion applications.

Low-loss tunable 1D ITO-slot photonic crystal nanobeam cavity

Tunable optical material properties enable novel applications in both versatile metamaterials and photonic components including optical sources and modulators. Transparent conductive oxides (TCOs) are able to highly tune their optical properties with applied bias via altering their free carrier concentration and hence plasma dispersion. The TCO material indium tin oxide (ITO) exhibits unity-strong index change and epsilon-near-zero behavior. However, with such tuning the corresponding high optical losses, originating from the fundamental Kramers–Kronig relations, result in low cavity finesse. However, achieving efficient tuning in ITO-cavities without using light–matter interaction enhancement techniques such as polaritonic modes, which are inherently lossy, is a challenge. Here we discuss a novel one-dimensional photonic crystal nanobeam cavity to deliver a cavity system offering a wide range of resonance tuning range, while preserving physical compact footprints. We show that a vertical silicon-slot waveguide incorporating an actively gated-ITO layer delivers ~3.4 nm of tuning. By deploying distributed feedback, we are able to keep the Q-factor moderately high with tuning. Combining this with the sub-diffraction limited mode volume (0.1 (λ/2n)3) from the photonic (non-plasmonic) slot waveguide, facilitates a high Purcell factor exceeding 1000. This strong light–matter-interaction shows that reducing the mode volume of a cavity outweighs reducing the losses in diffraction limited modal cavities such as those from bulk Si3N4. These tunable cavities enable future modulators and optical sources such as tunable lasers.

Towards a 2D printer: a deterministic cross contamination-free transfer method for atomically layered materials (Conference Presentation)

Precision and chip contamination-free placement of two-dimensional (2D) materials is expected to accelerate both the study of fundamental properties and novel device functionality. Current transfer methods of 2D materials onto an arbitrary substrate deploy wet chemistry and viscoelastic stamping. However, these methods produce a) significant cross contamination of the substrate due to the lack of spatial selectivity b) may not be compatible with chemically sensitive device structures, and c) are challenged with respect to spatial alignment. Here, we demonstrate a novel method of transferring 2D materials resembling the functionality known from printing; utilizing a combination of a sharp micro-stamper and viscoelastic polymer, we show precise placement of individual 2D materials resulting in vanishing cross contamination to the substrate. Our 2D printer-method results in an aerial cross contamination improvement of two to three orders of magnitude relative to state-of-the-art dry and direct transfer methods. Moreover, we find that the 2D material quality is preserved in this transfer method. Testing this 2D material printer on taped-out integrated Silicon photonic chips, we find that the micro-stamper stamping transfer does not physically harm the underneath Silicon nanophotonic structures such as waveguides or micro-ring resonators receiving the 2D material. Such accurate and substrate-benign transfer method for 2D materials could be industrialized for rapid device prototyping due to its high time-reduction, accuracy, and contamination-free process.

Temperature dependence of a sub-wavelength compact graphene plasmon-slot modulator

We investigate a plasmonic electro-optic modulator with an extinction ratio exceeding 1 dB/μm by engineering the optical mode to be in-plane with the graphene layer, and show how lowering the operating temperature enables steeper switching.

Graphene-based plasmonic slot electro-optic modulator (Conference Presentation)

Graphene, as the first identified two dimensional material, has shown great electro-optic response via Pauli-blocking for near IR frequencies and modulating functionality. However, this ability to modulate light is fundamentally challenged by its small optical cross-section leading to miniscule modal confinement factors in diffraction-limited photonics despite intrinsically high electro-optic absorption modulation (EAM) potential given by its strong index change. Yet the inherent polarization anisotropy in graphene and device tradeoffs lead to additional requirements with respect to electric field directions and modal confinement. The extinction ratio of graphene based EAM has, so far, been limited due to the small light matter interaction given the monolayer structure nature. Here we report an ultra-compact graphene based EAM by integrating graphene with a plasmonic slot waveguide. We show that the modal confinement and hence the modulation strength of a single-layer modulated graphene in this plasmonically confined mode is able to improve by more than 10x compared to diffraction-limited modes. Combined with the strong-index modulation of graphene the modulation strength could achieve more than 1dB/um, which is more than 2-orders of magnitude higher compared to Silicon platform graphene modulators. Furthermore, the modal confinement was found to be synergistic with performance optimization via enhanced light-matter-interactions. These results show that there is room for scaling 2D material EAMs with respect to modal engineering towards realizing synergistic designs leading to high-performance modulators.

Physical Scaling Laws of Nanophotonics: Case Photon Conversion

We show that nanophotonic device performance scales non-monotonically with critical length and define the metric to assess the quality of photodetector and conversion processes. Furthermore, we discus an example of a 2D material-based solar cell.

2D materials for photon conversion and nanophotonics

The field of two-dimensional (2D) materials has the potential to enable unique applications across a wide range of the electromagnetic spectrum. While 2D-layered materials hold promise for next-generation photon-conversion intrinsic limitations and challenges exist that shall be overcome. Here we discuss the intrinsic limitations as well as application opportunities of this new class of materials, and is sponsored by the NSF program Designing Materials to Revolutionize and Engineer our Future (DMREF) program, which links to the President’s Materials Genome Initiative. We present general material-related details for photon conversion, and show that taking advantage of the mechanical flexibility of 2D materials by rolling MoS2/graphene/hexagonal boron nitride stack to a spiral solar cell allows for solar absorption up to 90%.

Strong Light-Matter-Interactions in Nanoscale and 2D Material Systems

We report on showing strong light-matter-interaction enhancements of waveguide-integrated and 2D material-based spiral nanocavities. Both exploit high Q/Vmode values outperforming diffraction-limited cavities due to high optical densities.