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The Journal of Physical Chemistry C

Dipole–Dipole Interaction in Two-Photon Spectroscopy of Metallic Nanohybrids

Mahi R. Singh, Patrick D. Persaud

Published Feb 23, 2020• 50 citations
Publisher

Abstract

We have developed a theory for the two-photon fluorescence in nanohybrids made of an ensemble of metallic nanorod shells and an ensemble of quantum emitters. A metallic nanorod shell is made of a metallic rod and dielectric shell. We consider that the quantum emitters are four-level quantum systems. When a probe laser light falls on the metallic nanorod shells, the surface plasmon polariton electric field is produced at the interface between the metallic nanorod and dielectric shell. This electric field, along with the probe field, induces dipoles in the quantum emitters and nanorod shells. These dipoles interact with each other via the dipole–dipole interaction. The two-photon fluorescence has been calculated by using the quantum density matrix method in the presence of the dipole–dipole interaction (coupling). Analytical expressions of the two-photon fluorescence have been derived in the presence of the dipole–dipole interaction. We showed that that the two-photon process is made of two terms. The first term is the two-photon process due to the two probe field photons. On other the hand, the second term is made of one DDI field photon and one probe field photon. It is found that the surface plasmon polariton resonance energy is not resonant with the exciton energy, the two-photon fluorescence spectrum splits from one peak to three peaks. The splitting in the spectrum is due to the presence of the dressed states created in the system due to the strong dipole–dipole interaction. We also compared our theory with the experimental data of the metallic nanohybrid system made of metallic nanorod shells and quantum emitters (T790 molecules) and found a good agreement between the theory and the experiments.

DOI
10.1021/acs.jpcc.9b11614

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We have developed a theory for the Kerr nonlinearity in nanohybrids made of an ensemble of metallic nanoshells and low concentration of quantum emitters. A metallic nanoshell is made of a metallic core sphere and dielectric shell. We consider that quantum emitters are four-level quantum systems. When a probe laser light falls in the metallic nanoshells, the surface plasmon polariton electric field is produced at the interface between the metal sphere and dielectric shell. This electric field along with the probe field induces dipoles in metallic nanoshells. These dipoles interact with each other via the dipole-dipole interaction. The Kerr nonlinearity has been calculated by using the quantum density matrix method in the presence of the dipole-dipole interaction (coupling). We found that in the weak-coupling limit there is an enhancement in the Kerr nonlinearity. On the other hand, in the strong-coupling limit, the peaks in the Kerr coefficient split from two peaks to four peaks when the frequency of the dipole electric field is in the resonance with the exciton frequency. The splitting in the spectrum is due to the presence of the dressed sates created in the system. We showed that heights and locations of peaks are very sensitive to the strength of the dipole-dipole interaction. Physics of the enhancement can be used fabricate Kerr nanosensors. On the other hand, physics of the splitting from two peaks (ON) to four peaks (OFF) can be used to fabricate Kerr nanoswitches.
We have developed a theory for the one-photon and two-photon fluorescence of a nanohybrid composite comprised of an ensemble of quantum dots encased in a matrix of polymer. The quantum dots are comprised of four-level quantum emitters encased within a coating of dielectric material. The fluorescence of the quantum dot system is calculated using the master equation method where dipole–dipole interaction (DDI) coupling is present. The application of a probe field to the nanohybrid composite material in conjunction with the DDI between the quantum dots generates an enhancement of both the single-photon and double-photon fluorescence, and in certain conditions causes spectrum splitting. With a strong DDI field, the two-photon emission spectrum experiences slight quenching and shifts from one peak to two peaks. The dressed states are a result of strong coupling effects between the DDI field and the quantum dots. We generated an analytical expression for the fluorescence spectrum and intensity, and quantitatively compared it to experimental data in which ZnSe quantum dots were embedded within a P3HT polymer film, with our theory strongly correlating with the experiment. We also quantitatively compared our theory with experimental data that demonstrated the single-photon emission spectrum of colloidal quantum dots comprised of a CdZnS/ZnS gradient structure in varying probe field strengths. We found that our theory predicted the spectrum peak splitting that the experiment produced.
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A theory of a switching and sensing mechanisms using the Kerr nonlinearity has been developed for nanohybrids made of an ensemble of three-level quantum emitters and metallic nanoshells. We have calculated the surface plasmon polaritons at interface between the metallic core and dielectric shell of the metallic nanoshell. The Kerr nonlinearity has been calculated by using the quantum density matrix method in the dipole–dipole coupling between quantum emitters. We showed that the switching and sensing mechanisms due to the Kerr nonlinearity are made of four contributions. The first contribution is due to three probe photons and the second one is due to one polariton and two probe photons. On other hand, the third contribution is due to the two polaritons and one probe photon, and the fourth one is due to three polaritons. The enhancement in the Kerr nonlinearity is found, and it is due to second, third, and fourth contributions. This prediction can be used to fabricate all optical Kerr nanosensors. It is also shown that two peaks (ON) in the Kerr coefficient can be switched to four peaks (OFF) due to the strong dipole–dipole coupling. We have also predicted that the Kerr coefficient can be switched from the positive value (ON) to the negative value (OFF) by changing the dipole–dipole coupling. We found that the minimum of the Kerr coefficient changes sign from positive (ON) to negative (OFF) as the dipole–dipole coupling strength increases. The splitting in the Kerr spectrum is due to the presence of the dressed states created in the nanohybrid due the dipole–dipole interaction. These predictions can be used to fabricate all optical Kerr nanoswitches.
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