Transition-Metal-Dichalcogenide Tunable Quantum Relay Device

Transition-Metal-Dichalcogenide Tunable Quantum Relay Device Anshika Upadhyay M.S. Electrical and Computer Engineering (Optics and Photonics) University of Michigan, Ann Arbor, USA Abstract: One of the biggest challenges in implementation of Quantum circuits or Photonic Integrated Circuits in general is the inability to create efficient relay devices due to small decoherence time, high delays and poor interconnections that worsen the decoherence and delay problems. A feasible relay device design is proposed that can work in conjunction with various sources, be highly tunable over a reasonably large bandwidth and operate with negligible switching delays. This is achieved with the help of the Transition Metal Dichalcogenide (TMDC) MoSe2/WSe2 heterojunction and a modulating chirped strain/acoustic wave is applied to control the operating frequency and other operational characteristics of the device. The “chirping” of strain wave is meant to control the exciton transport across the device. The switching delay is proposed to reduce by exploiting the exciton dynamics so that the carrier non-equilibrium is never disturbed and therefore no delay occurs while switching on or off. MoSe2/WSe2 heterojunction is chosen because of their binding energy nearly two orders greater than GaAs quantum wells because of which they demonstrate high exciton response which is electrically tunable even at room temperature. Also, it is possible to grow TMDC films on Si based substrates using particular techniques so that their properties are intact. Such a device can find applications in wide range of components in photonic/quantum optoelectronic integrated circuits such as switches, logic gates, sensors and buffers. Introduction The binding energy of an exciton (which is dependent on the spatial size of the exciton, and is negative) is what makes it different from the electrons and holes in a medium. The different exciton spatial distances (and hence, binding energy) lead to interesting physics and many exciting applications. One example is the use of excitons in photovoltaics where absorption of appropriate amount of energy leads to creation of exciton pair and conversely, excitons may annihilate under stimulating conditions and emit a photon. Exciton physics is being exploited in variety of semiconductor applications for various applications like design of photonic memory [1], laser with ultralow threshold [2], etc. Monolayer heterostructures are considered ideal exciton media as they have long-lived excitons compared to their single-layered counterparts [3]. These layers stay together by weak Vander Waals forces (and not grown on top of another). This kind of structure does away with the problems associated with lattice mismatch and allows one to choose any two materials for bandgap engineering. The combination of MoSe2 – WSe2 is taken here for the application. MoSe2 – WSe2 heterojunction forms a Staggered (type II) band alignment (figure 1) [4].

Figure 1. Band alignment of MoSe2 – WSe2 heterojunction Because TMDCs have a high Coulomb binding energy of several hundred meV (much greater than typical semiconductors and nearly two orders greater than GaAs quantum wells), these semiconductors demonstrate high exciton response that is electrically tunable even at room temperature. Also, it is possible to grow TMDC films on Si based substrates using particular techniques so that their properties are intact [5]. The excitons are spatially indirect as the electron will be confined in the conduction band of MoSe2 while the hole will be confined in the valence band of WSe2. This is because the conduction band maximum for electron in MoSe2 and valence band minimum for hole is going to occur at different spatial frequencies. This spatial indirectness is also directly measured in [3]. Such spatially indirect excitons live longer than the direct excitons and thus, such structures are very useful to exploit the exciton dynamics. The ultimate goal of this paper is to engineer the strain signal to cause particular kind of exciton dynamics, exploiting the beneficial properties of TMDC heterojunction, to be able to use it in desired applications.

Figure 2. Proposed design of the device Theory, Results and Discussion Binding energy, 2 2 . 2 bohr B E r      where rbohr is the bohr radius and µ is the reduced mass. Hamiltonian of a simple unperturbed exciton is given by Ho = He + Hh + He. Hamiltonian of electron, He = 2 2 * 2 ( ) 2 CB e V z m z      , Hamiltonian of hole, Hh = 2 2 * 2 ( ) 2 VB h V z m z      . Therefore, Hamiltonian of the electron-hole interaction, He-h = 2 2 2 4 e h P e r         where re-h is the electron-hole separation and is the in-plane reduced mass of the electron- hole pair. Pis the momentum of electron-hole pair. P = 2 2 2 2 2 ( ) x y       [6] Adding spin-orbital coupling in

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