Here, we study the resonant photonic states of photonic double quantum well (PDQW) heterostructures composed of two different photonic crystals. The heterostructure is denoted as B/A/B/A/B, where photonic crystals A and B act as photonic wells and barriers, respectively. The resulting band structure causes photons to become confined within the wells, where they occupy discrete quantized states. We have obtained an expression for the transmission coefficient of the PDQW heterostructure using the transfer matrix method and have found that resonant states exist within the photonic wells. These resonant states occur in split pairs, due to a coupling between degenerate states shared by each of the photonic wells. It is observed that when the resonance energy lies at a bound photonic state and the two photonic quantum wells are far away from each other, resonant states appear in the transmission spectrum of the PDQW as single peaks. However, when the wells are brought closer together, coupling between bound photonic states causes an energy-splitting effect, and the transmitted states each have two peaks. Essentially, this means that the system can be switched between single and double transparent states. We have also observed that the total number of resonant states can be controlled by varying the width of the photonic wells, and the quality factor of transmitted peaks can be drastically improved by increasing the thickness of the outer photonic barriers. It is anticipated that the resonant states described here can be used to develop new types of photonic-switching devices, optical filters, and other optoelectronic devices.
KeywordsPhotonic crystal heterostructures Resonant tunneling Photonic devices Photonic quantum well Nanophotonics
During the past two decades, photonic crystals [1, 2] have attracted a great deal of attention due to their ability to localize and control the flow of light within their structure. Considerable effort has been placed into finding ways to harness their potential for developing new photonic devices. Toward that end, photonic crystal heterostructures have provided a promising means for turning raw photonic crystals into functional devices [3, 4].
Photonic crystals are materials with a periodically modulating dielectric constant, which may vary in one, two, or three spatial dimensions. Due to this periodic dielectric modulation, a band gap forms in the structure’s photonic dispersion relation. Light possessing a frequency within this gap is prevented from propagating in the photonic crystal. Photonic crystal heterostructures, as their name implies, are formed by joining two or more photonic crystals into a single structure. Combining various photonic crystals with different band gaps into a single heterostructure gives it a more complicated band structure than that of a bulk photonic crystal, which allows for greater customization and control over how light propagates within the overall structure. Recently, photonic crystal heterostructures have been used to develop devices such as high-quality resonant cavities [4–7], low-loss waveguides [8, 9], and high efficiency add-drop filters .
Photonic quantum wells (PQWs) are a class of photonic crystal heterostructures that possess a distinct band structure. A PQW consists of a photonic well imbedded between two photonic barriers. The barriers are photonic crystals with band gaps that may be regarded as potential barriers for photons, whereas the photonic well consists of either a uniform dielectric material or another photonic crystal with a different band gap than that of the barrier. Due to the photonic band mismatch between the well and the barrier, photons become confined within the well and occupy quantized states. This so-called photonic confinement effect has been observed in both theoretical [11–15] and experimental [16, 17] studies and is analogous to the electronic confinement effect that occurs in semiconductor quantum wells.
Furthermore, it has been shown that the phenomenon of resonant photonic tunneling can occur for a PQW with sufficiently thin photonic barriers [11, 12]. Resonant tunneling occurs when a photon with an energy corresponding to a bound state of the PQW tunnels through one of the barriers, where it occupies this bound state within the well for a finite period of time before escaping by tunneling back out; thus the photon is said to have occupied a quasi-bound or resonant state within the PQW . As a consequence of this phenomenon, an incident photon with an energy matching a resonant state of the PQW will undergo perfect transmission through both barriers. In the transmission spectrum of a PQW, resonant states appear as sharp peaks approaching unity [11–18].
Here, we study resonant photonic states in photonic double quantum well (PDQW) heterostructures consisting of two different photonic crystals. A PDQW is formed by simply adding another photonic well and photonic barrier to a single PQW system. Using the transfer matrix method , we have obtained an expression for the transmission coefficient of the PDQW heterostructure. From this, we have performed numerical simulations of the transmission spectra for a number of PDQW heterostructures. In our simulations, we vary the thicknesses of the photonic wells or barriers in order to study their effect on the resonant states of the system. It is anticipated that the resonant states described here will be useful for developing new types of photonic-switching devices, optical filters, and other optoelectronic devices [3, 11–17].
At this point, we denote our PDQW heterostructure as B1/A2/B3/A4/B5 in order to clarify discussions. The width of the photonic wells (photonic crystals A2 and A4) is given as d A , while the width of the outer photonic barriers (crystals B1 and B2) is given by d B . The width of the central photonic barrier (photonic crystal B3, or equivalently the photonic well separation) is denoted as d W . The transmission coefficient of the total PDQW heterostructure is calculated using the transfer matrix method, which matches the electric field and its first derivative at interfaces between adjacent layers of photonic crystals in order to relate the incident and reflected amplitudes in each layer . Using the dispersion relation given in Eq. 1, the transmission coefficient is obtained as a function of incident photon energy and was plotted over the range of energies between the upper band edge of photonic crystal A to that of crystal B in order to determine the resonant states of the system.
It is considered that photonic crystals A and B each consist of a silica background material (such that n A = n B = 1.45) imbedded with air spheres. The radii of the air spheres in each crystal were taken as a A = a B = 125 nm, and the lattice constants for each crystal were taken as L A = 420 nm and L B = 410 nm. The upper photonic band edges of crystals A and B were calculated using Eq. 1as 1.3965 and 1.4382 eV, respectively. We have performed numerical simulations of the transmission coefficient of the PDQW heterostructure considered here. Transmission spectra were produced while varying parameters such as the width of the photonic wells (d A ), the width of the central photonic barrier, i.e. the photonic well separation (d W ), and the width of the outer photonic barriers (d B ).
In this paper, we have studied the resonant photonic tunneling effect in PDQW heterostructures made from two photonic crystals. By performing numerical simulations of the transmission spectra for various PDQW heterostructures, we have demonstrated that resonant photonic tunneling occurs in these structures, whereby photons with energies corresponding to bound states of the system undergo perfect transmission through the entire structure. Our studies have shown that these states occur in split pairs, with energies centered on that of the degenerate states in each individual, isolated photonic well. This spectral splitting is attributed to the coupling of the degenerate states in each photonic well, where the finite photonic barrier separating these wells allows for an overlap of the electromagnetic fields of the degenerate states. Thus, the degree of spectral splitting can be controlled by varying the thickness of the central photonic barrier, and we have shown that the system can, therefore, be switched between one or two transparent states. The energy-splitting phenomenon described here is in qualitative agreement with reported experimental results involving a PDQW heterostructure consisting of 1-D photonic crystals . Our simulations have also shown that the total number of transmitted resonant states can be controlled by modifying the width of the photonic wells in the heterostructure, and the quality factor of transmitted resonant peaks can be greatly improved by increasing the thickness of the outer photonic barriers. It is anticipated that the resonant states described in this study will be useful for developing new types of photonic-switching devices, optical filters, and other optoelectronic devices [3, 11–17].
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