Emitter Separation Research Articles

Undamped Rabi oscillations due to polaron-emitter hybrid states in a nonlinear photonic waveguide coupled to emitters

The collective dynamics of two non-interacting two-level emitters, which are coupled to a structured wave guide that supports two-photon bound states, is investigated. Tuning the energy of the two emitters such that they are in resonance with the two-photon bound state energy band, we identify parameter regimes where the system displays fractional populations and essentially undamped Rabi oscillations. The Rabi oscillations, which have no analog in the single-emitter dynamics, are attributed to the existence of a collective polaron-like photonic state that is induced by the emitter-photon coupling. The full dynamics is reproduced by a two-state model, in which the photonic polaron interacts with the state $|e,e,\text{vac} \rangle$ (two emitters in their excited state and empty wave guide) through a Rabi coupling frequency that depends on the emitter separation. Our work demonstrates that emitter-photon coupling can lead to an all-to-all momentum space interaction between two-photon bound states and tunable non-Markovian dynamics, opening up a new direction for emitter arrays coupled to a waveguide.

Two emitters coupled to a bath with Kerr-like nonlinearity: Exponential decay, fractional populations, and Rabi oscillations

We consider two noninteracting two-level emitters that are coupled weakly to a one-dimensional nonlinear waveguide. Due to the Kerr-like nonlinearity, the waveguide considered supports---in addition to the scattering continuum---a two-body bound state. As such, the waveguide models a bath with nontrivial mode structure. Solving the time-dependent Schr\"odinger equation, the radiation dynamics of the two emitters, initially prepared in their excited states, is presented. Changing the emitter frequency such that the two-emitter energy is in resonance with one of the two-body bound states, radiation dynamics ranging from exponential decay to fractional populations to Rabi oscillations is observed. Along with the detuning, the dependence on the separation of the two emitters is investigated. Approximate reduced Hilbert-space formulations, which result in effective emitter separation and momentum dependent interactions, elucidate the underlying physical mechanisms and provide an avenue to showcase the features that would be absent if the one-dimensional waveguide did not contain a nonlinearity. Our theoretical findings apply to a number of experimental platforms and the predictions can be tested with state-of-the-art technology. In addition, the weak-coupling Schr\"odinger equation based results provide critical guidance for the development of master equation approaches.

Breaking the diffraction limit using fluorescence quantum coherence.

The classical optical diffraction limit can be overcome by exploiting the quantum properties of light in several theoretical studies; however, they mostly rely on an entangled light source. Recent experiments have demonstrated that quantum properties are preserved in many fluorophores, which makes it possible to add a new dimension of information for super-resolution fluorescence imaging. Here, we developed a statistical quantum coherence model for fluorescence emitters and proposed a new super-resolution method using fluorescence quantum coherence in fluorescence microscopy. In this study, by exploiting a single-photon avalanche detector (SPAD) array with a time-correlated single-photon-counting technique to perform spatial-temporal photon statistics of fluorescence coherence, the subdiffraction-limited spatial separation of emitters is obtained from the determined coherence. We numerically demonstrate an example of two-photon interference from two common fluorophores using an achievable experimental procedure. Our model provides a bridge between the macroscopic partial coherence theory and the microscopic dephasing and spectral diffusion mechanics of emitters. By fully taking advantage of the spatial-temporal fluctuations of the emitted photons as well as coherence, our quantum-enhanced imaging method has the significant potential to improve the resolution of fluorescence microscopy even when the detected signals are weak.

Evaluation of electron currents from cesium-coated tungsten emitter arrays with inclusion of space charge effects, workfunction changes, and screening

Evaluations of electron current output from tungsten emitter arrays with Cs and CsI coatings are carried out. The approach is based on first-principles calculations of the material physics including evaluation of the internal potentials, electronic wavefunctions, tunneling probabilities, and work function to predict field emission currents. This is coupled to time-dependent kinetic simulations for the assessment of emitter array currents with an inclusion of many-body Coulomb contributions from the electron swarm, geometric field enhancements with shielding based on a line charge model and dynamic screening from the swarm. Our numerical evaluations for arrays with a hexagonal lattice show the expected role of field screening with reductions in emitter separation. For scaling with emitter number, the results indicate nearest neighbor separations of more than 2.5 times the emitter height, in keeping with previous reports.

Many-particle based evaluations for maximum current output from bimodal electron emitter arrays

Evaluations of the current output from emitter arrays have been carried out based on time-dependent kinetic simulations that include many-body Coulombic contributions from the electron swarm, geometric field enhancements with shielding based on a line charge model, and dynamic screening due to the evolution of the swarm in the position and velocity space. Numerical evaluations are applied to different multi-emitter array arrangements having a hexagonal lattice, with a focus on bimodal distributions. Our results show the expected role of field screening with reductions in emitter separation and positional dependence within the array based on connectivity. Different patterns were examined within a hexagonal lattice structure. For a bimodal distribution, output current optimization is shown for alternating arrangements with three or more successive emitters of the same length along primitive axes predicted to have an advantage.

Robust Beamforming-Aided Signal Recovery for MIMO VLC System With Incomplete Channel

Visible light communication (VLC) based on light emitting diodes (LEDs) has attracted much attention because of its high data rate and energy efficiency. However, the shadowing caused by any mobile obstacles could block the light-of-sight (LoS) link which may have a dramatical effect on the indoor VLC channel state information (CSI). Instead of investigating the shadowing channel characteristics, we model the link blockages as the incomplete channel matrix with sparse missing elements and reconstruct the transmitted signal by the l 0 -minimization with additional constraint on shadowing loss. However, the indoor VLC channel is highly correlated especially in cases with small emitter separation. To further improve the accuracy performance, we design the transmit beamforming to reduce the total coherence. Simulation results illustrate the effectiveness of the proposed signal recovery algorithm with incomplete channel and further significant enhancement via the proposed beamforming design.

Waveguide quantum electrodynamics in squeezed vacuum

We study the dynamics of a general multiemitter system coupled to the squeezed vacuum reservoir and derive a master equation for this system based on the Weisskopf-Wigner approximation. In this theory, we include the effect of positions of the squeezing sources which is usually neglected in the previous studies. We apply this theory to a quasi-one-dimensional waveguide case where the squeezing in one dimension is experimentally achievable. We show that while dipole-dipole interaction induced by ordinary vacuum depends on the emitter separation, the two-photon process due to the squeezed vacuum depends on the positions of the emitters with respect to the squeezing sources. The dephasing rate, decay rate, and the resonance fluorescence of the waveguide-QED in the squeezed vacuum are controllable by changing the positions of emitters. Furthermore, we demonstrate that the stationary maximum entangled NOON state for identical emitters can be reached with arbitrary initial state when the center-of-mass position of the emitters satisfies certain conditions.

Measurement of deep-subwavelength emitter separation in a waveguide-QED system.

In the waveguide quantum electrodynamics (QED) system, emitter separation plays an important role for its functionality. Here, we present a method to measure the deep-subwavelength emitter separation in a waveguide-QED system. In this method, we can also determine the number of emitters within one diffraction-limited spot. In addition, we also show that ultrasmall emitter separation change can be detected in this system which may then be used as a waveguide-QED-based sensor to measure tiny local temperature/strain variation.

Dynamical theory of single-photon transport in a one-dimensional waveguide coupled to identical and nonidentical emitters

We develop a general dynamical theory for studying a single photon transport in a one-dimensional (1D) waveguide coupled to multiple emitters which can be either identical or non-identical. In this theory, both the effects of the waveguide and non-waveguide vacuum modes are included. This theory enables us to investigate the propagation of an emitter excitation or an arbitrary single photon pulse along an array of emitters coupled to a 1D waveguide. The dipole-dipole interaction induced by the non-waveguide modes, which is usually neglected in the literatures, can significantly modify the dynamics of the emitter system as well as the characteristics of output field if the emitter separation is much smaller than the resonance wavelength. Non-identical emitters can also strongly couple to each other if their energy difference is smaller than or of the order of the dipole-dipole energy shift. Interestingly, if their energy difference is close but non-zero, a very narrow transparency window around the resonance frequency can appear which does not occur for identical emitters. This phenomenon may find important applications in quantum waveguide devices such as optical switch and ultra narrow single photon frequency comb generator.

Parallel nanomanufacturing via electrohydrodynamic jetting from microfabricated externally-fed emitter arrays

We report the design, fabrication, and characterization of planar arrays of externally-fed silicon electrospinning emitters for high-throughput generation of polymer nanofibers. Arrays with as many as 225 emitters and with emitter density as large as 100 emitters cm−2 were characterized using a solution of dissolved PEO in water and ethanol. Devices with emitter density as high as 25 emitters cm−2 deposit uniform imprints comprising fibers with diameters on the order of a few hundred nanometers. Mass flux rates as high as 417 g hr−1 m−2 were measured, i.e., four times the reported production rate of the leading commercial free-surface electrospinning sources. Throughput increases with increasing array size at constant emitter density, suggesting the design can be scaled up with no loss of productivity. Devices with emitter density equal to 100 emitters cm−2 fail to generate fibers but uniformly generate electrosprayed droplets. For the arrays tested, the largest measured mass flux resulted from arrays with larger emitter separation operating at larger bias voltages, indicating the strong influence of electrical field enhancement on the performance of the devices. Incorporation of a ground electrode surrounding the array tips helps equalize the emitter field enhancement across the array as well as control the spread of the imprints over larger distances.

Reversible dynamics of single quantum emitters near metal-dielectric interfaces

Here we present a systematic study of the dynamics of a single quantum emitter near a flat metal-dielectric interface. We identify the key elements that determine the onset of reversibility in these systems by using a formalism suited for absorbing media and through an exact integration of the dynamics. Moreover, when the quantum emitter separation from the surface is small, we are able to describe the dynamics within a pseudomode description that yields analytical understanding and allows more powerful calculations.

Nanoantenna Arrays for Large-Area Emission Enhancement

The electrodynamic modeling of linear arrays of noble metal nanoantennas reveals that plasmonic Bragg modes allow for long-range and large-area emission enhancement. Significant light extraction is possible at large antenna emitter separation (micrometer range) and unfavorable emitter dipole moment orientation. One order of magnitude average emission enhancement for randomly oriented emitters is possible over areas ≈20 times larger than a single nanoantenna geometrical cross section.

The statistical performance of the MUSIC and the minimum-norm algorithms in resolving plane waves in noise

This paper presents an asymptotic statistical analysis of the null-spectra of two eigen-assisted methods, MUSIC [1] and Minimum-Norm [2], for resolving independent closely spaced plane waves in noise. Particular attention is paid to the average deviation of the null-spectra from zero at the true angles of arrival for the plane waves. These deviations are expressed as functions of signal-to-noise ratios, number of array elements, angular separation of emitters, and the number of snapshots. In the case of MUSIC. an approximate expression is derived for the resolution threshold of two plane waves with equal power in noise. This result is validated by Monte Carlo simulations.