Propagation Of Exciton-polaritons Research Articles

Long-Range Ballistic Propagation of 80% Excitonic Fraction Polaritons in a Perovskite Metasurface at Room Temperature.

Exciton-polaritons, hybrid light-matter excitations arising from the strong coupling between excitons in semiconductors and photons in photonic nanostructures, are crucial for exploring the physics of quantum fluids of light and developing all-optical devices. Achieving room temperature propagation of polaritons with a large excitonic fraction is challenging but vital, e.g., for nonlinear light transport. We report on room temperature propagation of exciton-polaritons in a metasurface made from a subwavelength lattice of perovskite pillars. The large Rabi splitting, much greater than the optical phonon energy, decouples the lower polariton band from the phonon bath of the perovskite. These cooled polaritons, in combination with the high group velocity achieved through the metasurface design, enable long-range propagation, exceeding hundreds of micrometers even with an 80% excitonic component. Furthermore, the design of the metasurface introduces an original mechanism for unidirectional propagation through polarization control, suggesting a new avenue for the development of advanced polaritonic devices.

Unraveling the Ultrafast Coherent Dynamics of Exciton Polariton Propagation at Room Temperature.

Exciton polaritons are widely considered as promising platforms for developing room-temperature polaritonic devices, owing to the high-speed propagation and nonlinear interactions. However, it remains challenging to explore the dynamics of exciton polaritons specifically at room temperature, where the lifetime could be as small as a few picoseconds and the prevailing time-averaged measurement cannot give access to the true nature of it. Herein, by using the time-resolved photoluminescence, we have successfully traced the ultrafast coherent dynamics of a moving exciton polariton condensate in a one-dimensional perovskite microcavity. The propagation speed is directly measured to be ∼12.2 ± 0.8 μm/ps. Moreover, we have developed a time-resolved Michelson interferometry to quantify the time-dependent phase coherence, which reveals that the actual coherence time of exciton polaritons could be much longer (nearly 100%) than what was believed before. Our work sheds new light on the ultrafast coherent propagation of exciton polaritons at room temperature.

Long-Range Propagation of Exciton-Polaritons in Large-Area 2D Semiconductor Monolayers.

Atomically thin transition metal dichalcogenides (TMDs), a subclass of two-dimensional (2D) layered materials, have numerous fascinating properties that make them a promising platform for photonic and optoelectronic devices. In particular, excited state transport by TMDs is important in energy harvesting and photonic switching; however, long-range transport in TMDs is challenging due to the lack of availability of large area films. Whereas most previous studies have focused on small, exfoliated monolayer flakes, in this work we demonstrate metal-organic chemical vapor deposition grown centimeter-scale monolayers of WS2 that support polariton propagation lengths of up to 60 μm. The polaritons form through the strong coupling of excitons with Bloch surface waves (BSWs) supported by all-dielectric photonic structures. We observe that the propagation length increases with the number of dielectric pairs due to the increased quality factor of the supporting distributed Bragg reflector. Furthermore, a longer propagation length is observed as the guided or BSW content of the polariton is increased. Our results provide a practical approach for the systematic engineering of long-range energy transport mediated by exciton-polaritons in TMD layers. Along with the accessibility of large area TMDs, our work enables applications for practical TMD-based polaritonic devices that operate at room temperature.

Adiabatic theory of one-dimensional curved polariton waveguides

We construct a general theory of adiabatic propagation of spinor exciton polaritons in waveguides of arbitrary shape, accounting for the effects of TE-TM splitting in linear polarizations and Zeeman splitting in circular polarizations. The developed theory is applied for the description of waveguides of periodically curved shape. We show that in this geometry the periodic rotation of the effective in-plane magnetic field produced by TE-TM interaction results in a nontrivial band-gap structure, which can be additionally tuned by application of an external magnetic field. It is also demonstrated that spin-dependent interactions between polaritons lead to the formation of stable gap solitons.

Electrically switchable anisotropic polariton propagation in a ferroelectric van der Waals semiconductor.

Tailoring of the propagation dynamics of exciton-polaritons in two-dimensional quantum materials has shown extraordinary promise to enable nanoscale control of electromagnetic fields. Varying permittivities along crystal directions within layers of material systems, can lead to an in-plane anisotropic dispersion of polaritons. Exploiting this physics as a control strategy for manipulating the directional propagation of the polaritons is desired and remains elusive. Here we explore the in-plane anisotropic exciton-polariton propagation in SnSe, a group-IV monochalcogenide semiconductor that forms ferroelectric domains and shows room-temperature excitonic behaviour. Exciton-polaritons are launched in SnSe multilayer plates, and their propagation dynamics and dispersion are studied. This propagation of exciton-polaritons allows for nanoscale imaging of the in-plane ferroelectric domains. Finally, we demonstrate the electric switching of the exciton-polaritons in the ferroelectric domains of this complex van der Waals system. The study suggests that systems such as group-IV monochalcogenides could serve as excellent ferroic platforms for actively reconfigurable polaritonic optical devices.

Optical propagation of exciton polaritons in ultrathin van der Waals microcrystals down to few monolayers

The exciton polariton is a kind of quasiparticles and provides a promising opportunity to explore fundamental quantum phenomena for photonic applications. Transition-metal dichalcogenide (TMD) materials provide the platform of nanophotonics that supports the propagative exciton polaritons even at room-temperature. Previously, real space studies on thin flakes of TMDs by scattering-type scanning nearfield optical microscopy (s-SNOM) were limited to waveguide thickness down to 30 nm. In this work, we present the nano-optical imaging of ordinary transverse electric modes of exciton polaritons in MoS2 and WSe2 down to a few atomic layers, measured by atomic force microscope-based s-SNOM. Surprisingly, the interference fringe patterns can be observed clearly at the prepared ultrathin TMD flakes with thickness down to ~3 nm (4 ML) and ~8 nm (12 ML) for MoS2 and WSe2, respectively, which breaks greatly the previous measurement limitation. The wavevectors stay around 1.6k0−1.7k0 constantly when the thickness approaching to a few MLs, instead of 1k0 according to the theory. These modes are supported by the nearly-freestanding TMD microflakes in the form of three-layer symmetric waveguide to confine the exciton polaritons. Our results provide in-depth understanding and open new avenues to explore the polaritonic devices operating at the near infrared region based on ultrathin TMD materials.

Spin–orbit interaction in hybrid photonic crystals and clusters

The tailoring of resonant hybrid photonic crystals (RHPC) is performed starting from a symmetric elementary cell and increasing its complexity removing, step by step, some selected symmetry properties in order to obtain an hybrid minimum model for the elementary cell of the crystal that shows unidirectional exciton–polariton propagation and surface protected states. The topological properties of these non-magnetic hybrid system were discussed in a former paper [D. Schiumarini, A. D’Andrea, Eur. Phys. J. B 92, 92 (2019)] and the main optical parameter values, in order to observe unidirectional exciton–polariton propagation, were evaluated in the self-consistent semiclassical mesoscopic framework that takes into account light-hole exciton dipole polarization. In the present work also spin–orbit effect of the light-hole exciton is taken into account, and its effect on the unidirectional exciton-polariton propagation is discussed.

Optical valley Hall effect for highly valley-coherent exciton-polaritons in an atomically thin semiconductor.

Spin-orbit coupling is a fundamental mechanism that connects the spin of a charge carrier with its momentum. In the optical domain, an analogous synthetic spin-orbit coupling is accessible by engineering optical anisotropies in photonic materials. Both yield the possibility of creating devices that directly harness spin and polarization as information carriers. Atomically thin transition metal dichalcogenides promise intrinsic spin-valley Hall features for free carriers, excitons and photons. Here we demonstrate spin- and valley-selective propagation of exciton-polaritons in a monolayer of MoSe2 that is strongly coupled to a microcavity photon mode. In a wire-like device we trace the flow and helicity of exciton-polaritons expanding along its channel. By exciting a coherent superposition of K and K' tagged polaritons, we observe valley-selective expansion of the polariton cloud without either an external magnetic field or coherent Rayleigh scattering. The observed optical valley Hall effect occurs on a macroscopic scale, offering the potential for applications in spin-valley-locked photonic devices.

Unidirectional propagation in hybrid photonic crystals and clusters

In the present work, we will perform the tailoring of resonant hybrid photonic crystals (RHPC) and clusters, based on the elementary cell of a resonant hybrid minimum model, in order to study its optical chiral properties, due to the space inversion symmetry breakdown and optical rotation effect, that are able to preserve unidirectional exciton-polariton propagation and photonic band gap formation. The optical chiral properties of these non-magnetic hybrid system will be discussed and the main optical parameter values, in order to observe unidirectional exciton-polariton propagation, will be evaluated in the self-consistent mesoscopic framework for the YVO4 /GaAs hybrid system.

Light waveguiding in bioinspired peptide nanostructures.

Basic optical properties of bioinspired peptide nanostructures are deeply modified by thermally mediated refolding of peptide secondary structure from α-helical to β-sheet. This conformational transition is followed by the appearance in the β-sheet structures of a wideband optical absorption and fluorescence in the visible region. We demonstrate that a new biophotonic effect of optical waveguiding recently observed in peptide/protein nanoensembles is a structure-sensitive bimodal phenomenon. In the primary α-helical structure input, light propagates via optical transmission window demonstrating conventional passive waveguiding, based on classical optics. In the β-sheet structure, fluorescent (active) light waveguiding is revealed. The latter can be attributed to completely different physical mechanism of exciton-polariton propagation, characterized by high effective refractive index, and can be observed in nanoscale fibers below diffraction limit. It has been shown that peptide material requirements for passive and active waveguiding are dissimilar. Original biocompatibility and biodegradability indicate high potential future applications of these bioinspired waveguiding materials in precise photobiomedicine towards advanced highly selective bioimaging, photon diagnostics, and optogenetics.

An exciton-polariton bolometer for terahertz radiation detection

We experimentally investigate the feasibility of a bolometric device based on exciton-polaritons. Initial measurements presented in this work show that heating – via thermal expansion and bandgap renormalization – modifies the exciton-polariton propagation wavevector making exciton-polaritons propagation remarkably sensitive to thermal variations. By theoretical simulations we predict that using a single layer graphene absorbing layer, a THz bolometric sensor can be realized by a simple exciton-polariton ring interferometer device. The predicted sensitivity is comparable to presently existing THz bolometric devices with the convenience of being a device that inherently produces an optical signal output.

Unidirectional oblique propagation of exciton-polaritons in hybrid photonic crystals

In this work we study theoretically the axial spectral asymmetry of a 1D hybrid (isotropic and anisotropic) photonic crystal for photon energies close to the electronic energy gaps of semiconductors (excitons). The non-normal optical properties of these resonant non-magnetic photonic crystals will be computed in the framework of exciton-polariton, by self-consistent solutions of the Maxwell-Schrödinger equations in the effective mass approximation. The tailoring of the hybrid photonic crystals, performed by implementing a simple two-layer “minimum model”, and novel optical properties, due to the unidirectional polariton propagation, will be discussed.

Resonant Bragg quantum wells in hybrid photonic crystals: optical properties and applications

The exciton–polariton propagation in resonant hybrid periodic stacks of isotropic/anisotropic layers, with misaligned in-plane anisotropy and Bragg photon frequency in resonance with Wannier exciton of 2D quantum wells is studied by self-consistent theory and in the effective mass approximation. The optical tailoring of this new class of resonant Bragg reflectors, where the structural periodicity of a multi-layer drives the in-plane optical -axis orientation, is computed for symmetric and asymmetric elementary cells by conserving strong radiation–matter coupling and photonic band-gaps. The optical response computation, on a finite cluster of N-asymmetric elementary cells, shows anomalous exciton–polariton propagation and absorbance properties strongly dependent on the incident wave polarizations. Finally, the behaviour of the so-called intermediate dispersion curves, close to the unperturbed exciton resonance, and located between upper and lower branches of the first band gap, is studied as a function of the in-plane -axis orientation. This latter optical property is promising for storing exciton–polariton impulses in this kind of Bragg reflector.

Plasmon-controlled excitonic emission from vertically-tapered organic nanowires.

Organic molecular nanophotonics has emerged as an important avenue to harness molecular aggregation and crystallization on various functional platforms to obtain nano-optical devices. To this end, there is growing interest to combine organic molecular nanostructures with plasmonic surfaces and interfaces. Motivated by this, herein we introduce a unique geometry: vertically-tapered organic nanowires grown on a plasmonic thin film. A polarization-sensitive plasmon-polariton on a gold thin-film was harnessed to control the exciton-polariton propagation and subsequent photoluminescence from an organic nanowire made of diaminoanthraquinone (DAAQ) molecules. We show that the exciton-polariton emission from individual DAAQ nanowires can be modulated up to a factor of 6 by varying the excitation polarization state of surface plasmons. Our observations were corroborated with full-wave three-dimensional finite-difference time-domain calculations performed on vertically-tapered nanowire geometry. Our work introduces a new optical platform to study coupling between propagating plasmons and propagating excitons, and may have implications in emerging fields such as hybrid-polariton based light emitting devices and vertical-cavity nano-optomechanics.

Diffusive Propagation of Exciton-Polaritons through Thin Crystal Slabs.

If light beam propagates through matter containing point impurity centers, the amount of energy absorbed by the media is expected to be either independent of the impurity concentration N or proportional to N, corresponding to the intrinsic absorption or impurity absorption, respectively. Comparative studies of the resonant transmission of light in the vicinity of exciton resonances measured for 15 few-micron GaAs crystal slabs with different values of N, reveal a surprising tendency. While N spans almost five decimal orders of magnitude, the normalized spectrally-integrated absorption of light scales with the impurity concentration as N1/6. We show analytically that this dependence is a signature of the diffusive mechanism of propagation of exciton-polaritons in a semiconductor.

Electro-optic switch based on near-field-coupled quantum dots

The propagation of exciton polaritons in near-field-coupled quantum-dot (QD) chains is modeled by a density-matrix formalism. It is shown that at least for low-temperature operation it is possible using electronically controlled switching by the quantum-confined Stark effect in such QD chains to rival and outperform room-temperature CMOS electronics in footprint and switch energy, though not in speed.

Planar Active Organic Waveguide and Wavelength Filter: Self-Assembled meso-Tetratolylporphyrin Hexagonal Nanosheet

We have fabricated nearly monodispersed nanocrystalline sheet waveguides from a well-known red emitting meso-tetratolylporphyrin molecule (1) by following a bottom-up solvent assisted self-assembly technique. The nano-sheets thickness is in the range of 110-180 nm. Localized laser illumination showed excitation position dependent exciton polariton (653 and 719 nm) propagation behavior of the sheets. The spatially resolved fluorescence spectra of the sheets showed optical modes at the input and output points, indicating cavity effect. Additionally, because of the reabsorption of the 653 nm emission, the nanosheets also act as wave length filter by cutting off the 653 nm photons from reaching the output end.

Ultra-Compact Photonic Circuit Components based on Propagation of Exciton Polaritons in Organic Dye Nanofibers

We fabricated a micron-sized ring resonator channel drop filter by micromanipulation of nanofibers of thiacyanine dye that propagate exciton polaritons along the fibers. The device consisted of a microring with a radius of r ≈ 4 μm and a straight nanofiber that functioned as an I/O bus channel. The performance of the device was evaluated using spatially resolved fluorescence microscopy. The device exhibited an extinction ratio of re = 2-4 dB for the visible wavelength region of 490-540 nm. Our results demonstrate that thiacyanine nanofibers are promising building blocks for exciton polariton-based photonic circuits, which can be highly miniaturized compared with conventional waveguide-based photonic circuits.

Ultracompact Asymmetric Mach–Zehnder Interferometers with High Visibility Constructed from Exciton Polariton Waveguides of Organic Dye Nanofibers

AbstractManipulation of light using subwavelength waveguides is a key technology in the development of miniaturized photonic circuits, which possess various advantages over their electronic counterparts. The novel approach presented for such waveguiding involves the propagation of exciton polaritons (EPs), which are quasi‐particles formed by strong exciton–photon coupling, along organic dye nanofibers. A self‐assembled nanofiber of thiacyanine (TC) with a width of ≈200 nm propagates the EPs created by an optical excitation over a submillimeter‐scale distance and passes through a bend with a micrometer‐scale radius with low bending loss. To demonstrate the remarkable potential of EP‐based miniaturized photonic circuits, asymmetric Mach–Zehnder interferometers (AMZIs) are fabricated with TC nanofibers by micromanipulation. The AMZIs with a footprint of ≈20 μm × 20 μm exhibit a visibility of nearly unity and function as channel drop filters with the considerably high extinction ratio of up to ≈15 dB. Such high‐performance and ultracompact channel drop filters operating in the visible wavelength region have rarely been developed with other waveguide technologies. The coherent properties of the EPs in the nanofibers are investigated using time‐resolved experiments. The coherent properties provide useful information for designing EP‐based photonic circuits and for understanding EP dynamics in a nanofiber.

Exciton mediated self-organization in glass driven by ultrashort light pulses

We propose an exciton-polariton-mediated self-organization effect in transparent SiO2 glass under intense femtosecond light irradiation. Interference and dipole-dipole interaction of polaritons causes formation of gratings of dielectric polarization. Due to an ultrafast exciton self-localization into a quasicrystal structure, the polariton gratings remain frozen in glass and a permanent three-dimensional image of exciton-polariton gas is created. We show that coherent effects in propagation of exciton-polaritons can serve as a tool for nanostructuring and fabrication of 5-dimensional optical memories in glass, opening new horizons for polaritronics.