Strong Mode Coupling Research Articles

Unconventional plastic optical fiber design for very short multimode fiber link.

We introduce a graded-index plastic optical fiber (GI POF) design for very short-distance household applications, in which the transmission quality is predominantly determined by system noise rather than the loss and bandwidth. The developed GI POF has strong mode coupling with low accompanying scattering loss, which is closely related to the specific microscopic heterogeneities in the core material. Such characteristic mode coupling significantly decreases reflection noise, improving the transmission quality compared with silica GI multimode fiber (MMF) for lengths below 30 m. Moreover, in the GI POF link, the transmission quality tends to improve with increasing fiber length, despite the increased loss and decreased bandwidth. This feature suggests that the system noise can be controlled by the microscopic heterogeneous properties of the GI POF for a very short MMF link, where the fiber loss and bandwidth are sufficiently low and high, respectively. This unconventional concept for optical-fiber design can advance fiber-optic communication in emerging applications in households located near optical network terminals.

Strong vibrational coupling in room temperature plasmonic resonators

Strong vibrational coupling has been realized in a variety of mechanical systems. However, there have been no experimental observations of strong coupling of the acoustic modes of plasmonic nanostructures, due to rapid energy dissipation in these systems. Here we realized strong vibrational coupling in ultra-high frequency plasmonic nanoresonators by increasing the vibrational quality factors by an order of magnitude. We achieved the highest frequency quality factor products of f × Q = 1.0 × 1013 Hz for the fundamental mechanical modes, which exceeds the value of 0.6 × 1013 Hz required for ground state cooling. Avoided crossing was observed between vibrational modes of two plasmonic nanoresonators with a coupling rate of g = 7.5 ± 1.2 GHz, an order of magnitude larger than the dissipation rates. The intermodal strong coupling was consistent with theoretical calculations using a coupled oscillator model. Our results enabled a platform for future observation and control of the quantum behavior of phonon modes in metallic nanoparticles.

Extremely high dispersions in heterogeneously coupled waveguides.

We present a heterogeneously coupled Si/SiO2/SiN waveguide structure that can achieve extremely high dispersions (> | ± 107| ps · nm-1km-1). A strong mode coupling between the Si and SiN waveguides introduces a normal dispersion to symmetric mode and an anomalous dispersion to anti-symmetric mode, and the large group velocity difference between the two waveguides results in such high dispersions. Geometric parameters of the structure control the peak dispersions and the central wavelength of the mode coupling, and these engineering capabilities are studied numerically. Analytical representations on the heterogeneously coupled waveguides are also introduced and these equations explain the effects of geometric parameters. This extremely dispersive waveguide scheme can be constructed with other material combinations as well and should be of interest in ultrafast signal processing and spectroscopic applications.

Luminescence mechanism and energy transfer in cesium metavanadate CsVO3

Photoluminescence properties and relaxation dynamics of electronic excitations in cesium vanadate CsVO3 have been studied upon pulse laser excitation in the wide temperature range of 6.5–300 K. A vibronic structure observed in low-temperature PL spectra is considered and interpreted. Peculiarities of luminescence relaxation dynamics and emission spectra of CsVO3 along with the appearance of the vibronic structure are explained in terms of strong electron-phonon coupling of excited electronic states and centrosymmetric vibrational modes in [VO4]3- center. A new approach in an interpretation of luminescence processes in vanadates is demonstrated.

Tunable Valley Polarized Plasmon-Exciton Polaritons in Two-Dimensional Semiconductors.

Monolayers of transition-metal dicalcogenides have emerged as two-dimensional semiconductors with direct bandgaps at degenerate but inequivalent electronic "valleys", supporting distinct excitons that can be selectively excited by polarized light. These valley-addressable excitons, when strongly coupled with optical resonances, lead to the formation of half-light half-matter quasiparticles, known as polaritons. Here we report self-assembled plasmonic crystals that support tungsten disulfide monolayers, in which the strong coupling of semiconductor excitons and plasmon lattice modes results in a Rabi splitting of ∼160 meV in transmission spectra as well as valley-polarized photoluminescence at room temperature. More importantly we find that one can flexibly tune the degree of valley polarization by changing either the emission angle or the excitation angle of the pump beam. Our results provide a platform that allows the detection, control, and processing of optical spin and valley information at the nanoscale under ambient conditions.

Tilting a ground-state reactivity landscape by vibrational strong coupling.

Many chemical methods have been developed to favor a particular product in transformations of compounds that have two or more reactive sites. We explored a different approach to site selectivity using vibrational strong coupling (VSC) between a reactant and the vacuum field of a microfluidic optical cavity. Specifically, we studied the reactivity of a compound bearing two possible silyl bond cleavage sites-Si-C and Si-O, respectively-as a function of VSC of three distinct vibrational modes in the dark. The results show that VSC can indeed tilt the reactivity landscape to favor one product over the other. Thermodynamic parameters reveal the presence of a large activation barrier and substantial changes to the activation entropy, confirming the modified chemical landscape under strong coupling.

Intrinsically Stabilized Plastic Optical Fiber Link Subject to External Optical Feedback

We develop a graded-index plastic optical fiber (GI POF) that can significantly stabilize a multimode fiber (MMF) link with a vertical-cavity surface-emitting laser (VCSEL) subject to external optical feedback. It is demonstrated that the dominant mechanism for this stabilization effect is the strong mode coupling of the GI POF, which is closely related to the polymer-specific microscopic heterogeneities in the GI POF core material. Such mode coupling decreases correlation between the problematic reflected light field and VCSEL cavity field, increasing tolerance of the MMF link for external optical feedback. Our developed GI POF with specific microscopic heterogeneities prevents the external-optical-feedback-induced critical destabilization observed for silica GI MMFs. These results suggest that the stabilization effect can be controlled by microscopic properties regardless of the fiber attenuations and core refractive indices.

Bound states in the continuum and Fano resonances in the strong mode coupling regime

The study of resonant dielectric nanostructures with high refractive index is a new research direction in nanoscale optics and metamaterial-inspired nanophotonics. Because of the unique optically-induced electric and magnetic Mie resonances, high-index nanoscale structures are expected to complement or even replace different plasmonic components in a range of potential applications. Here we study strong coupling between modes of a single subwavelength high-index dielectric resonator and analyse the mode transformation and Fano resonances when resonator's aspect ratio varies. We demonstrate that strong mode coupling results in resonances with high quality factors, which are related to the physics of bound states in the continuum when the radiative losses are almost suppressed due to the Friedrich-Wintgen scenario of destructive interference. We explain the physics of these states in terms of multipole decomposition and show that their appearance is accompanied by drastic change of the far-field radiation pattern. We reveal a fundamental link between the formation of the high-quality resonances and peculiarities of the Fano parameter in the scattering cross-section spectra. Our theoretical findings are confirmed by microwave experiments for the scattering of a high-index cylindrical resonators with a tunable aspect ratio. The proposed mechanism of the strong mode coupling in single subwavelength high-index resonators accompanied by resonances with high quality factor helps to extend substantially functionalities of all-dielectric nanophotonics that opens new horizons for active and passive nanoscale metadevices.

Saturable mode conversion of parametric amplification in a multimode optical fiber

The intimate interaction between phase-matched parametric amplification and strong nonlinear mode coupling in a multimode optical fiber results in a saturable mode conversion in the anti-Stokes component of the phase-matched four-wave mixing process. This novel saturable mode conversion can potentially be used in a variety of applications including the generation of exotic quantum states.

High-Q states and Strong mode coupling in high-index dielectric resonators.

We study strong coupling between eigenmodes of a single subwavelength high-index dielectric resonator and analyse the mode transformation and Fano resonances by varying resonator’s aspect ratio. We demonstrate that the strong mode coupling is associated with the physics of bound states in the continuum when the radiative losses are almost suppressed due to the Friedrich-Wintgen scenario of destructive interference. We confirm our theoretical findings with microwave experiments by using a high-index cylindrical resonator with tunable aspect ratio.

A ghost imaging modality in a random waveguide

We study the imaging of a penetrable scatterer, aka target, in a waveguide with randomly perturbed boundary. The target is located between a partially coherent source which transmits the wave, and a detector which measures the spatially integrated energy flux of the wave. The imaging is impeded by random boundary scattering effects that accumulate as the wave propagates. We consider a very large distance (range) between the target and the detector, where that cumulative scattering is so strong that it distributes the energy evenly among the components (modes) of the wave. Conventional imaging is impossible in this equipartition regime. Nevertheless, we show that the target can be located with a ghost imaging modality. This forms an image using the cross-correlation of the measured energy flux, integrated over the aperture of the detector, with the time and space resolved energy flux in a reference waveguide, at the search range. We consider two reference waveguides: the waveguide with unperturbed boundary, in which we can calculate the energy flux, and the actual random waveguide, before the presence of the target, in which the energy flux should be measured. We analyze the ghost imaging modality from first principles and show that it can be efficient in a random waveguide geometry in which there is both strong modal dispersion and mode coupling induced by scattering, provided that the standard ghost imaging function is modified and integrated over a suitable time offset window in order to compensate for dispersion and diffusion. The analysis quantifies the resolution of the image in terms of the source and detector aperture, the range offset between the source and the target, and the duration of the measurements.

Low-Noise Graded-Index Plastic Optical Fiber for Significantly Stable and Robust Data Transmission

Internet traffic continues to grow with the increase of network-connected smartphones, tablets, televisions, and monitoring devices in the Internet-of-Things (IoT) era. This trend will be significantly accelerated by the introduction of ultrahigh-definition (UHD) imaging technologies in various applications, and existing optical networks including datacenter networks are under urgent development to accommodate this traffic. However, optical fibers have not been introduced into households located in optical network terminal areas, even though UHD device connections require decompressed high-volume data transmission at more than 100 Gb/s, where a multilevel modulation scheme is vital. In UHD applications, very short optical cables (typically less than several meters) are connected and disconnected by consumers in a manner similar to metal interface cables. Under such conditions, a data transmission quality is predominantly limited by noise and instability rather than bandwidth and loss. Here, we propose low-noise graded-index plastic optical fibers (GI POFs) that enable significantly stable and robust data transmission through strong mode coupling, whose mechanism is fundamentally different from that of silica optical fibers. The low-noise GI POF link eliminates the need for the precise fiber alignment, angled fiber facets, and optical isolators typically used in conventional links. Our proposed GI POF material paves the way for quick optical fiber connections for multilevel UHD video transmission, becoming the first “capillaries of light” from optical network terminals in the IoT era.

Mode coupling and competition in square-racetrack hybrid-cavity semiconductor lasers

We propose and demonstrate square-racetrack hybrid-cavity semiconductor lasers to achieve lasing mode control. Strong mode coupling between the modes of the square and racetrack cavities is predicted by finite element method numerical simulation and confirmed by experimental lasing spectra. Mode coupling leads to anti-crossing of the mode wavelengths, crossing of the mode quality factors, and hybrid modes with the field distributing in both square and racetrack cavities, and will affect the competition of lasing modes together with gain distribution induced by separately injected current of the two cavities. Single-mode lasing with a side-mode suppression ratio up to 38 dB and dual-mode lasing with an intensity ratio down to 1 dB are realized by tuning the injection current. Furthermore, increasing racetrack cavity size affects the mode competition process and results in racetrack-mode-dominated lasing for square-racetrack hybrid-cavity semiconductor lasers.

Enhanced water splitting under modal strong coupling conditions.

Strong coupling between plasmons and optical modes, such as waveguide or resonator modes, gives rise to a splitting in the plasmon absorption band. As a result, two new hybrid modes are formed that exhibit near-field enhancement effects. These hybrid modes have been exploited to improve light absorption in a number of systems. Here we show that this modal strong coupling between a Fabry-Pérot nanocavity mode and a localized surface plasmon resonance (LSPR) facilitates water splitting reactions. We use a gold nanoparticle (Au-NP)/TiO2/Au-film structure as a photoanode. This structure exhibits modal strong coupling between the Fabry-Pérot nanocavity modes of the TiO2 thin film/Au film and LSPR of the Au NPs. Electronic excitation of the Au NPs is promoted by the optical hybrid modes across a broad range of wavelengths, followed by a hot electron transfer to TiO2. A key feature of our structure is that the Au NPs are partially inlaid in the TiO2 layer, which results in an enhancement of the coupling strength and water-oxidation efficiency. We observe an 11-fold increase in the incident photon-to-current conversion efficiency with respect to a photoanode structure with no Au film. Also, the internal quantum efficiency is enhanced 1.5 times under a strong coupling over that under uncoupled conditions.

Fully degeneracy-lifted bow-tie elliptical ring-core multi-mode fiber.

We present a bow-tie elliptical ring-core multi-mode fiber (BT-ERC-MMF) that features an elliptical ring-core structure and two symmetrical bow-tie stress-applying parts (SAPs). This special fiber design breaking geometry and stress symmetry fully separates the two- or four-fold degenerate modes in traditional circular symmetry fiber with strong mode coupling during mode-division multiplexing (MDM) transmission. The designed fiber is able to support 53 fully degeneracy-lifted eigenmodes with minimum effective index difference between adjacent modes larger than 1.59 × 10-4 at 1550 nm, facilitating potential fiber eigenmode multiplexing transmission without using multiple-input multiple-output digital signal processing (MIMO-DSP) technique. The effect of bending on the designed fiber is investigated based on conformal mapping. Broadband performance including effective modal index (neff), effective index difference (Δneff), effective mode area (Aeff), nonlinearity and chromatic dispersion (D) is also comprehensively studied over the whole C band ranging from 1530 to 1565 nm. The designed fiber targets emerging applications in low-crosstalk direct fiber eigenmode-division multiplexing combined with the mature wavelength-division multiplexing (WDM) technique to increase transmission capacity and spectral efficiency.

Cavity-mediated interactions between the guided mode resonance and surface phonon polaritons in a hybrid photonic–phononic system

A strong mode coupling between the guided mode resonance (GMR) and surface phonon polaritons (SPhPs) is numerically demonstrated. The photonic–phononic mode coupling can be mediated by a homogeneous spacer which works as a Fabry–Pérot (FP) cavity, and its strength, manifested by mode splitting and bending of dispersion curves, can be tuned flexibly by adjusting the thickness of the FP cavity. This mode-coupling effect can be used to control the near-field localization sites in this hybrid structure, which provide useful implications for manipulating infrared electromagnetic near fields in nanophotonics devices.

CO2 laser-written long-period fiber grating with a high diffractive order cladding mode near the turning point.

We demonstrate the fabrication of a long-period fiber grating (LPFG) in a boron-doped single-mode fiber with a high-diffractive-order cladding mode (HDCM) near the turning point (TP). The simulations show that an LPFG with less than 0.2 duty cycles can couple light to the HDCM. An LPFG with a period of more than 400μm can achieve strong mode coupling between the fundamental mode and the HDCM near the TP. The effect of the external refractive index on the transmission spectrum of a LPFG with different grating periods is investigated by simulations and experiments. With an increase in grating period, the spectral dip corresponding to the HDCM travels faster than the conventional dip, and overlapped dips appear in the transmission spectrum. High sensitivities of up to 13,497.7 nm/RIU and 0.77 nm/°C of, respectively, RI and temperature sensing can be achieved. Such LPFGs could be potentially used as optical filters and high-sensitivity sensors.

Orientational Ordering, Locking-in, and Distortion of CH4 Molecules in Methane Hydrate III under High Pressure

We investigate the effects of high pressure on the reorientational and vibrational dynamics of methane molecules embedded in methane hydrate III—the stable form of methane for pressures above 2 GPa at room temperature—by combining high-pressure Raman spectroscopy with ab initio simulations including nuclear quantum effects. We observe a clear evolution of the system from a gas-filled ice structure, where methane molecules occupy the channels of the ice skeleton and rotate almost freely, to a CH4:D2O compound where methane rotations are hindered, and methane and water dynamics are tightly coupled. The gradual orientational ordering of the guest molecules results in a complete locking-in at approximately 20 GPa. This happens along with a progressive distortion of the guest molecules. Finally, as pressure increases beyond 20 GPa, the system enters a strong mode coupling regime where methane guests and water hosts dynamics are intimately paired.

Ultra-Broadband Directional Scattering by Colloidally Lithographed High-Index Mie Resonant Oligomers and Their Energy-Harvesting Applications.

Emerging high-index all-dielectric nanostructures, capable of manipulating light on the subwavelength scale, empower designing and implementing novel antireflection and light-trapping layers in many photonic and optoelectronic devices. However, their performance and practicality are compromised by relatively narrow bandwidths and highly sophisticated fabrications. In this paper, we demonstrate an ultra-broadband (300-1200 nm) directional light scattering strategy using high-index surface silicon oligomer resonators fabricated by a facile, scalable, and low-cost colloidal lithography technique. The exceptional broadband forward scattering stems from a combined effect of strongly intercoupled Mie resonances within the oligomers composed of randomly positioned nanodisks in the visible region and a strong electric mode coupling between the oligomers and the high-index substrate in the red-to-near-infrared region. By implementing this efficient approach in silicon solar cells, the integrated optical reflection loss across the wavelength range 300-1200 nm can be as low as 7%. Consequently, the short-circuit current density determined from the external quantum efficiency of solar cells can be increased to 35.1 from 25.1 mA/cm2, representing an enhancement of 40%, with a demonstrated energy conversion efficiency exceeding 15.0%. The insights in this paper hold great potentials for new classes of light management and steering photonic devices with drastically improved practicality.

High-dimensional fitting of sparse datasets of CCSD(T) electronic energies and MP2 dipole moments, illustrated for the formic acid dimer and its complex IR spectrum.

We present high-level, coupled-mode calculations of the infrared spectrum of the cyclic formic acid dimer. The calculations make use of full-dimensional, ab initio potential energy and dipole moment surfaces. The potential is a linear least-squares fit to 13 475 CCSD(T)-F12a/haTZ (haTZ means aug-cc-pVTZ basis set for O and C, and cc-pVTZ for H) energies, and the dipole moment surface is a fit to the dipole components, calculated at the MP2/haTZ level of theory. The variables of both fits are all (45) internuclear distances (actually Morse variables). The potential, which is fully permutationally invariant, is the one published recently and the dipole moment surface is newly reported here. Details of the fits, especially the dipole moment, and the database of configurations are given. The infrared spectrum of the dimer is calculated by solving the nuclear Schrödinger equation using a vibrational self-consistent field and virtual-state configuration interaction method, with subsets of the 24 normal modes, up to 15 modes. The calculations indicate strong mode-coupling in the C-H and O-H stretching region of the spectrum. Comparisons are made with experiments and the complexity of the experimental spectrum in the C-H and O-H stretching region is successfully reproduced.