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Laser Physics

Role of tunneling induced coherence in modulation of absorption and dispersion in a quantum dot molecule with symmetrical coupling configuration

Bibhas Kumar Dutta, Pradipta Panchadhyayee, Nityananda Das, P.K. Mahapatra

Published Jul 3, 2018• 0 citations
Publisher

Abstract

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.

DOI
10.1088/1555-6611/aac70f

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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.
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.
Since discrete multilevel transitions of quantum-dot molecules driven by external electromagnetic fields can exhibit quantum coherence effects, such an optical characteristic can be utilized to control propagation of electromagnetic wave through a quantum-dot molecule dielectric film. Since inner-dot tunneling in quantum-dot molecules can be controlled by a gate voltage, destructive quantum coherence among multilevel transitions in quantum-dot molecule would give rise to EIT (electromagnetically induced transparency). In this report, we shall investigate controllable on- and off-resonance tunneling effects of an incident electromagnetic wave through such a quan-tum-dot-molecule dielectric film, of which the optical response is tuned by the switchable gate voltage. We have found from the theoretical mechanism that a high gate voltage can cause the EIT phenomenon of quan-tum-dot-molecule systems, and under the condition of on-resonance light tunneling through the thin film, the probe field will propagation without loss if the probe frequency detuning is zero. By taking advantage of these effects sensitive to the tunable gate voltage, such quantum coherence would be inte-grated in certain photonic structures, and some devices such as photonic switching and transistors can be designed. Transient evolution of optical characteristics in the quantum-dot-molecule dielectric film (once the tunable gate voltage is turned on or off) is also considered in this report.
Exciton-biexciton coherent coupling effects are examined in semiconductor quantum dots. The exciton absorption spectrum is measured with the microphotoluminescence excitation technique in a single InGaAs quantum dot. The spectrum changes from a Lorenztian-type line shape to an unusual dip-shaped line shape with increasing excitation intensities in a higher exciton state where there is a large oscillator strength between the exciton and biexciton states. The intensity dependence of the dip energy width clearly indicates that coherent Rabi oscillation occurs between the exciton and biexciton states. The absorption properties with excitation light of different polarizations show that the dip-shaped spectra only appear when there is a large biexciton state population with linear polarization. A theoretical analysis undertaken with the density matrix method agrees well with experimental results. This agreement reveals that exciton-biexciton coherent interactions lead to unusual absorption spectra and contribute crucially to the optical properties of quantum dots. The exciton-biexciton coherent effects provide a scheme for controlling four distinguishable states, which can be applied to a demonstration of quantum gate operations.
We manipulate the coupling effect between interdot tunneling and the external control field to study the optical diffractive property in a quantum dot molecule (QDM) system. Based on the density matrix approach, an analytical relation is obtained for the optical susceptibility, which shows tunneling-induced transparency (TIT) in the absorption spectra. We propose an innovative optoelectronic device called tunneling-induced grating, which operates based on the diffraction of the forward-transmitted probe laser light. The diffraction integrals for the far-field regime (i.e., Fraunhofer diffraction) are numerically solved, and the effect of interdot tunneling coupling strength on the diffraction patterns is studied. It is shown that the transmission is increased under tunneling-induced quantum coherence. The weak probe light can be diffracted into higher-order directions upon applying a standing-wave control field. The optimal parametric condition for maximum first- and second-order diffractions is obtained. The results find the potential applications in quantum information processing devices, such as all-optical grating and switches at low light levels.
This study investigates the tunable propagation of optical pulses in a three-level system of coupled quantum dots. The system forms an effective closed configuration, supporting strong quantum coherence and controllable light–matter interactions. Using the density matrix formalism, we analyze the quantum dynamical response under the influence of both a weak probe and a strong control field, with inter-dot tunneling acting as a key tunable parameter. Our results reveal that both the tunneling strength (Te) and control field Rabi frequency (Ωc) play crucial roles in shaping the system’s optical properties. We observe the emergence of electromagnetically induced transparency-like windows, the splitting and shifting of dispersion peaks, and the formation of absorption doublets and transparency windows. The real and imaginary parts of the electric permittivity exhibit pronounced sensitivity to these external controls. Furthermore, the group index and group velocity are shown to be highly tunable, enabling transitions between slow-light and fast-light regimes. Detailed analysis of temporal pulse dynamics demonstrates the ability to achieve controllable delay and advancement of optical pulses, depending on the interaction between Te and Ωc. These findings highlight the potential of such quantum dot systems for dynamic photonic control, including applications in optical buffering, reconfigurable delay lines, coherent switching, and quantum information processing. The demonstrated tunability and coherence effects make this platform a promising candidate for next-generation optoelectronic and quantum photonic technologies.
The coherent coupling between three quantum dots via tunneling fields leads to double tunneling-induced transparency and giant Kerr nonlinearity with vanishing absorption. This creates the possibility to achieve self-phase modulation in semiconductor nanostructures via controllable tunneling under the conditions of low light levels. As a result, a considerably high refractive index can be obtained. We suggest an intracavity medium which consists of triple quantum dot molecules to control the Goos–Hänchen (GH) shifts in the reflected light. For a suitable design of a triple quantum dot system, our results show that relatively large positive and negative GH shifts are achievable, without significant distortion, using partially coherent incident light fields.