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Nano Letters

Plasmon Squeezing in Single-Molecule Junctions

Lei‐Lei Nian, Tao Wang, Jing‐Tao Lü

Published Nov 30, 2022• 4 citations
Publisher

Abstract

Scanning tunneling microscope (STM)-induced luminescence provides an ideal platform for electrical generation and the atomic-scale manipulation of nonclassical states of light. However, despite its extreme importance in quantum technologies, squeezed light emission with reduced quantum fluctuations has hitherto not been demonstrated in such a platform. Here, we theoretically predict that the emitted light from the plasmon mode can be squeezed in an STM single molecular junction subject to an external laser drive. Going beyond the traditional paradigm that generates squeezing with the quadratic interaction of photons, our prediction explores the molecular coherence involved in an anharmonic energy spectrum of a coupled plasmon-molecule-exciton system. Furthermore, we show that, by selectively exciting the energy ladder, the squeezed plasmon can show either sub- or super-Poissonian statistical properties. We also demonstrate that, following the same principle, the molecular excitonic mode can be squeezed simultaneously.

DOI
10.1021/acs.nanolett.2c03371

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Scanning tunneling microscope (STM)-induced luminescence provides an ideal platform for electrical generation and the atomic-scale manipulation of nonclassical states of light. However, despite its extreme importance in quantum technologies, squeezed light emission with reduced quantum fluctuations has hitherto not been demonstrated in such a platform. Here, we theoretically predict that the emitted light from the plasmon mode can be squeezed in an STM single molecular junction subject to an external laser drive. Going beyond the traditional paradigm that generates squeezing with the quadratic interaction of photons, our prediction explores the molecular coherence involved in an anharmonic energy spectrum of a coupled plasmon-molecule-exciton system. Furthermore, we show that, by selectively exciting the energy ladder, the squeezed plasmon can show either sub- or super-Poissonian statistical properties. We also demonstrate that, following the same principle, the molecular excitonic mode can be squeezed simultaneously.
Recent scanning tunneling microscopy (STM) experiments reported single-molecule fluorescence induced by tunneling currents in the nanoplasmonic cavity formed by the STM tip and the substrate. The electric field of the cavity mode couples with the current-induced charge fluctuations of the molecule, allowing the excitation of photons. We investigate theoretically this system for the experimentally relevant limit of large damping rate κ for the cavity mode and arbitrary coupling strength to a single-electronic level. We find that for bias voltages close to the first inelastic threshold of photon emission, the emitted light displays antibunching behavior with vanishing second-order photon correlation function. At the same time, the current and the intensity of emitted light display Franck-Condon steps at multiples of the cavity frequency ω_{c} with a width controlled by κ rather than the temperature T. For large bias voltages, we predict strong photon bunching of the order of κ/Γ where Γ is the electronic tunneling rate. Our theory thus predicts that strong coupling to a single level allows current-driven nonclassical light emission.
Strong light-matter coupling offers a powerful framework for modifying molecular structure, dynamics, and optical response, yet achieving this regime at the single-molecule level remains a central challenge. Scanning Tunneling Microscope Break Junctions (STM-BJ) provide a promising route by combining single-molecule addressability with extreme nanoscale field confinement. However, the metallic environment that enables such confinement also introduces substantial plasmonic and excitonic losses, raising the question of how strong coupling can be realized in a metal-molecule-metal junction. Here, we identify the voltage as the key control parameter. We show that the applied bias not only drives the formation of an interfacial exciton through resonant charge transport but also suppresses its coupling to metallic loss channels. Simultaneously, the plasmonic cavity retains extreme mode confinement while the availability of modes is moderately tuned by voltage. Together, these results establish voltage-driven STM-BJs as electrically tunable single-molecule plasmonic cavities and open new routes for using the powerful STM-BJ toolbox, including quantum transport, tracking chemical dynamics, optoelectronics, and molecular-scale quantum control under strong light-matter coupling.
We demonstrate single-photon superradiance from artificially constructed nonbonded zinc-phthalocyanine molecular chains of up to 12 molecules. We excite the system via electron tunneling in a plasmonic nanocavity and quantitatively investigate the interaction of the localized plasmon with single-exciton superradiant states resulting from dipole-dipole coupling. Dumbbell-like patterns obtained by subnanometer resolved spectroscopic imaging disclose the coherent nature of the coupling associated with superradiant states while second-order photon correlation measurements demonstrate single-photon emission. The combination of spatially resolved spectral measurements with theoretical considerations reveals that nanocavity plasmons dramatically modify the linewidth and intensity of emission from the molecular chains, but they do not dictate the intrinsic coherence of the superradiant states. Our studies shed light on the optical properties of molecular collective states and their interaction with nanoscopically localized plasmons.
We theoretically investigate the photon statistics in a cavity quantum electrodynamics system of a single quantum dot (QD) coupled to photonic molecule. Our previous work [Wen Zhang et.al, Phys. Rev. A 89, 043832 (2014)] has shown that QD-bimodal cavity system can generate ultrastrongly sub-Poissonian light by regulating the ratio between driving strengths of two cavity modes. Here we study two coupled single-mode nanocavities with a QD coupling to one of them as a photonic molecule system. Statistical character of photon emission is presented by evaluating the zero-delay secondorder correlation function g2(0). When both cavities with/without QD are driven, the sub-Poissonian character can be optimized by regulating the ratio between driving strengths of each cavities. We also present the dependences of other system parameters on the photon statistics. The physical mechanism of both effects is to optimize the combination of super-Poissonian and coherent light which results in sub-Poissonian light generation. As a result g2(0) can be reduced up to several orders of magnitude compared with the QD coupled one-mode-cavity system.
We calculate the quantum state of the plasmon field excited by an ensemble of molecular emitters, which are driven by exchange of electrons with metallic nanoparticle electrodes. Assuming identical emitters that are coupled collectively to the plasmon mode but are otherwise subject to independent relaxation channels, we show that symmetry constraints on the total system density matrix imply a drastic reduction in the numerical complexity. For Nm{N}_{\text{m}}Nm​ three-level molecules, we may thus represent the density matrix by a number of terms scaling as (Nm+8)!/(8!Nm!)({N}_{\mathrm{m}}+8)!/(8!{N}_{\mathrm{m}}!)(Nm​+8)!/(8!Nm​!) instead of 9Nm{9}^{{N}_{\text{m}}}9Nm​, and this allows exact simulations of up to Nm=10{N}_{\text{m}}=10Nm​=10 molecules. Our simulations demonstrate that many emitters compensate strong plasmon damping and lead to the population of high plasmon number states as well as a narrowed emission, which indicates a transition from spontaneous emission to amplified spontaneous emission. For large Nm{N}_{\text{m}}Nm​, our exact results are reproduced by an approximate approach based on the plasmon reduced density matrix. With this approach, we have extended the simulations to more than 100 molecules and shown that the plasmon number state population follows a Poisson-like distribution, which indicates the formation of a plasmon coherent state. This clearly indicates a transition from amplified spontaneous emission to lasing. An alternative approach based on nonlinear rate equations for the molecular state populations and the mean plasmon number also reproduce the main lasing characteristics of the system.
We propose a molecular optomechanical platform to generate robust entanglement among bosonic modes-photons, phonons, and plasmons-under ambient conditions. The system integrates a high-Q whispering-gallery-mode (WGM) optical resonator with a plasmonic nanocavity formed by a metallic nanoparticle and a single molecule. This hybrid architecture offers two critical advantages over stand-alone plasmonic systems: (i) efficient redirection of Stokes photons from the lossy plasmonic mode into the long-lived WGM resonator and (ii) suppression of molecular absorption and sustaining vibrational ground states via plasmon-WGM interactions. These features enable entanglement to redistribute from the fragile plasmon-phonon bipartition to a robust photon-phonon bipartition in the blue-detuned regime, yielding robust stationary entanglement resilient to environmental noise. Remarkably, the achieved entanglement surpasses the theoretical bound for conventional two-mode squeezing in certain parameter regimes. Our scheme establishes a universal approach to safeguard entanglement in open quantum systems and opens avenues for noise-resilient quantum information technologies.