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Optics Letters

Efficient two-photon excitation by photonic dimers

Yao Zhou, Zihao Chen, Lihong V. Wang, Jung‐Tsung Shen

Published Jan 15, 2019• 19 citations
Publisher

Abstract

In this Letter, we show that photonic dimers, the quantum mechanical bound states of two photons, enable efficient nonlinear two-photon excitation, primarily due to the Lorentzian energy anti-correlation and to the temporal proximity between the constituent photons. We analytically and numerically demonstrate the order-of-magnitude improvement of the excitation efficiency per photon by the photonic dimers over the ultrashort pulses from a laser. We further show that, owing to the high excitation efficiency, the two-photon transition rate by photonic dimers deviates from the well-known linear intensity dependence at low intensities. Possible approaches for generating the photonic dimers in semiconductor platforms are also investigated.

DOI
10.1364/ol.44.000475

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In this Letter, we show that photonic dimers, the quantum mechanical bound states of two photons, enable efficient nonlinear two-photon excitation, primarily due to the Lorentzian energy anti-correlation and to the temporal proximity between the constituent photons. We analytically and numerically demonstrate the order-of-magnitude improvement of the excitation efficiency per photon by the photonic dimers over the ultrashort pulses from a laser. We further show that, owing to the high excitation efficiency, the two-photon transition rate by photonic dimers deviates from the well-known linear intensity dependence at low intensities. Possible approaches for generating the photonic dimers in semiconductor platforms are also investigated.
Photonic Molecule, also called Photonic Dimer, is a quantum bound state of two photons. A photonic bound state has a specific entanglement of a Lorentzian anti-correlation in frequency space and two photons are in proximity due to its binding nature.Such signatures of a photonic molecule illuminates a potential tool that increases the two-photon microscopy efficiency to orders of magnitude higher. Here we numerically and analytically demonstrate the two-photon excitation efficiency between photonic molecules, long uncorrelated light pulses and ultrashort light pulses. The high excitation efficiency of a photonic molecule enables a saturation of fluorophores, such that the linear dependence of two-photon excitation crosssection does not necessarily hold. Also, we exhibits two possible methods to obtain the photonic molecules, as a fundamental possibility for a continuous photonic molecule source.
In recent years, the study of quantum electrodynamics (QED) in light-matter interactions has discovered various interesting phenomenons that orient many applications. However, due to the ambient entanglement among photons and atoms, few-particle dynamics remains challenging to analyze precisely and limits the progress in several fields. In few-particle systems, different number of atoms interacting with the light field generates drastically different results, even when there is only a single photon involved in the system. The interference between individual atom’s spontaneous emission wavefunctions can cooperatively alter the effective atom-light coupling strength. Depending on the spatial distance between individual of atoms and the involved cavities in the interaction progresses, the collective spontaneous emission rate can alter drastically from superradiance, where the emission rate is enhanced to a high value proportional to the atom number, to subradiance, where the excited atom ensemble stays in a dark state with the light trapped forever. Superradiance provides a promising mechanism to create an ultra-strong light-matter coupling environment, which is an important ingredient for the design of ultra-fast optical devices, frequency comb generation and quantum mirrors. The dark state of subradiance, on the other hand, is essential for the quantum memory implementation. Also, nontrivial phenomenons also occur when multiple photons interacting with a single atom simultaneously. When a finite number of photons incidents on an atom, their scattering through an atom is highly nonlinear, and one special eigenstate component of such nonlinear process, namely photonic bound states, exhibits many interesting properties in nonlinear quantum optical systems. The photons in such state are strongly bunched with each other, as if a virtual attractive force is in effect. Also, the frequency of photons are anti-correlated so that their total frequency remains a constant value, no matter how arbitrary the frequency of any individual is. Thesefeatures benefit certain nonlinear quantum processes for quantum computing and quantum imaging. Specially for two-photon excitation imaging, the same-parity two-level fluorophore molecules suffers from extremely low excitation efficiencies due to the far off-resonant intermediate states. However, by replacing the current commonly used ultrashort laser pulse illuminations with two photon bound state photon pairs, namely photonic dimers, the efficiency can be enhanced to orders of magnitude higher. Such exceptionally high excitation efficiency can potentially bring the current in vivo deep brain imaging depth from millimeter level to several centimeters. To obtain a stable source of such photonic dimers, it is found that a laser structure with gain medium of two-photon emission materials is capable of generating the coherent state of photonic dimer and can serve as a promising light source for nonlinear quantum optical devices.
We theoretically investigate the two-photon excitation of a molecular vibronic state by correlated photons with energy anticorrelation. A Morse oscillator having three sets of vibronic states is used, as an example, to evaluate the selectivity and efficiency of two-photon excitation. We show that a vibrational mode can be selectively excited with high efficiency by the correlated photons, without phase manipulation or pulse-shaping techniques. This can be achieved by controlling the quantum correlation so that the photon pair concurrently has two pulse widths, namely, a temporally narrow width and a spectrally narrow width. Though this concurrence is seemingly contradictory, we can create such a photon pair by tailoring the quantum correlation between two photons.
We theoretically investigate the dynamics of two-photon excitation by correlated photons with energy anticorrelation in terms of how the excitation efficiency depends on incident pulse width. A three-level atomic system having an intermediate state is used to evaluate the efficiency of two-photon excitation. It is shown that for shorter pulses closer to a monocycle pulse the excitation efficiency by correlated photons is enhanced to become 100100100 times as large as that by uncorrelated photons.
We study the efficiency of different two-photon states of light to induce the simultaneous excitation of two atoms of different kinds when the sum of the energies of the two photons matches the sum of the energies of the two atomic transitions, while no photons are resonant with each individual transition. We find that entangled two-photon states produced by an atomic cascade are indeed capable of enhancing by a large factor the simultaneous excitation probability as compared to uncorrelated photons, as predicted some years ago by Muthukrishnan et al., but that several unentangled, separable, correlated states, produced either by an atomic cascade or parametric down-conversion, or even appropriate combinations of coherent states, have comparable efficiencies. We show that the key ingredient for the increase of simultaneous excitation probability is the presence of strong frequency anticorrelation and neither time correlation nor time-frequency entanglement.