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Results in Optics

The effect of geometrical shape on the nonlinear optical absorption spectrum of double, triple, and quadruple quantum dot molecules

Nader Daneshfar, P. Chavoshani, G. Heydarpour

Published Mar 1, 2026• 0 citations
Publisher

Abstract

In the present study, the effect of the geometrical shape of quantum dots on the nonlinear absorption coefficient of semiconductor quantum dot molecules (QDMs) is theoretically investigated. The nonlinear optical response of QDMs is analyzed using the density matrix formalism, because this theoretical framework provides a rigorous and unified method to describe the interaction between electromagnetic fields and confined carriers, enabling the derivation of both linear and higher-order nonlinear susceptibilities. Accordingly, the nonlinear optical absorption of double quantum dot molecules, triple quantum dot molecules, and quadruple quantum dot molecules coupled through tunneling is studied and compared. We show that the optical spectral response of QDMs is significantly affected by their shape and the strength of tunneling between coupled quantum dots of a QDM. The results indicate that the shape of quantum dots can significantly affect nonlinear optical absorption. In addition, the tunneling intensity between the closely quantum dots strongly affects the nonlinear absorption, and the width and number of transparency windows.

DOI
10.1016/j.rio.2026.101035

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In the present study, the effect of the geometrical shape of quantum dots on the nonlinear absorption coefficient of semiconductor quantum dot molecules (QDMs) is theoretically investigated. The nonlinear optical response of QDMs is analyzed using the density matrix formalism, because this theoretical framework provides a rigorous and unified method to describe the interaction between electromagnetic fields and confined carriers, enabling the derivation of both linear and higher-order nonlinear susceptibilities. Accordingly, the nonlinear optical absorption of double quantum dot molecules, triple quantum dot molecules, and quadruple quantum dot molecules coupled through tunneling is studied and compared. We show that the optical spectral response of QDMs is significantly affected by their shape and the strength of tunneling between coupled quantum dots of a QDM. The results indicate that the shape of quantum dots can significantly affect nonlinear optical absorption. In addition, the tunneling intensity between the closely quantum dots strongly affects the nonlinear absorption, and the width and number of transparency windows.
In the present work, the linear and nonlinear optical properties of quantum dot molecules composed of two quantum dots as well as three quantum dots molecules, respectively known as double quantum dot (DQD) molecule and triple quantum dot (TQD) molecules are studied using the probability amplitude method, and the obtained results are discussed and compared. By solving the state equations of these systems in the interaction picture, under the rotating wave approximation and the dipole approximation, the first and third-order susceptibilities for DQD and TQD molecules are calculated. A phenomenon called tunneling-induced transparency (TIT) which is similar to electromagnetically induced transparency (EIT) is studied in these types of quantum dot molecules. The effect of key system parameters such as tunneling couplings between quantum dots and the tunneling intensity on the linear and nonlinear optical response of multiple quantum dot molecules DQD and TQD is investigated. It was shown that the position and the width of the TIT window created in these quantum dot molecules can be tuned by changing the physical parameters.
A scheme for giant enhancement of the Kerr nonlinearity in linear triple quantum dot molecules is proposed. In such a system, the tunneling-induced transparency window obtained in double quantum dot molecules splits into two windows, due to the coupling with the third quantum dot. And most important, the Kerr nonlinearity can be enhanced by several orders of magnitude, compared with that generated in double quantum dot molecules. With proper detuning of the tunneling, giant Kerr nonlinearity accompanied by vanishing absorption can be realized, which opens the possibility to enhance self-phase modulation in tunneling controllable semiconductor nanostructures under conditions of low light levels. Quantitative analysis shows that the giant Kerr nonlinearity is attributed to the interacting double dark resonances induced by the tunneling between the triple quantum dots, therefore no extra laser fields are required.
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A scheme for giant Kerr nonlinearity via tunneling in triangular triple quantum dot molecules is proposed. In such a system, the linear absorption and the Kerr nonlinearity depend critically on the energy splitting of the excited states and the tunneling intensity. With proper parameters, giant Kerr nonlinearity accompanied by vanishing absorption can be realized. The enhancement of Kerr nonlinearity is attributed to the interacting double dark resonances induced by the tunneling between the quantum dots, requiring no extra coupling laser fields.
Abstract Coherent control of optical effects are studied in a quantum dot molecule comprising triple quantum dots with the consideration of symmetric tunneling induced coupling. We have analyzed the origin of such coherence effects in terms of perturbative influence of the array of tunneling induced coherence terms associated with the semiclassical density matrix element relating to optically induced exciton generation. It has been shown that the coherence terms can significantly modify the absorptive and dispersive line shapes due to the change of condition of weak tunneling to moderate tunneling limited by the coherence dephasing effect. We have illustrated the evolution of Fano-like resonance controlling the absorptive line shapes under the conditions mentioned above. We have shown how, depending upon the variation of gate-biasing voltage, dispersion like features evolve in the absorptive case, while absorption-peak like features originate in case of dispersion. This is due to the appearance of non-linear Fano resonance inherent to this model. Our results highlight the appearance of a negative refractive index like pattern in the dispersive response of the line shape under such conditions.