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Register a new userQuantum theory of exciton-photon coupling in photonic crystal slabs with embedded quantum wells
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A theoretical description of radiation-matter coupling for semiconductor-based photonic crystal slabs is presented, in which quantum wells are embedded within the waveguide core layer. A full quantum theory is developed, by quantizing both the electromagnetic field with a spatial modulation of the refractive index and the exciton center of mass field in a periodic piecewise constant potential. The second-quantized hamiltonian of the interacting system is diagonalized with a generalized Hopfield method, thus yielding the complex dispersion of mixed exciton-photon modes including losses. The occurrence of both weak and strong coupling regimes is studied, and it is concluded that the new eigenstates of the system are described by quasi-particles called photonic crystal polaritons, which can occur in two situations: (i) below the light line, when a resonance between exciton and non-radiative photon levels occurs (guided polaritons), (ii) above the light line, provided the exciton-photon coupling is larger than the intrinsic radiative damping of the resonant photonic mode (radiative polaritons). For a square lattice of air holes, it is found that the energy minimum of the lower polariton branch can occur around normal incidence. The latter result has potential implications for the realization of polariton parametric interactions in photonic crystal slabs.
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Self-assembled hybrid perovskite quantum wells have attracted attention due to their tunable emission properties, ease of fabrication and device integration. However, the dynamics of excitons in these materials, especially how they couple to phonons remains an open question. Here, we investigate two widely used materials, namely butylammonium lead iodide $(CH_3(CH_2)3NH_3)2PbI_4$ and hexylammonium lead iodide $(CH_3(CH_2)5NH_3)2PbI_4$, both of which exhibit broad photoluminescence tails at room temperature. We performed femtosecond vibrational spectroscopy to obtain a real-time picture of the exciton phonon interaction and directly identified the vibrational modes that couple to excitons. We show that the choice of the organic cation controls which vibrational modes the exciton couples to. In butylammonium lead iodide, excitons dominantly couple to a 100 cm-1 phonon mode, whereas in hexylammonium lead iodide, excitons interact with phonons with frequencies of 88 cm-1 and 137 cm-1. Using the determined optical phonon energies, we analyzed PL broadening mechanisms. At low temperatures (<100 K), the broadening is due to acoustic phonon scattering, whereas at high temperatures, LO phonon-exciton coupling is the dominant mechanism. Our results help explain the broad photoluminescence lineshapes observed in hybrid perovskite quantum wells and provide insights into the mechanism of exciton-phonon coupling in these materials.
Understanding the nature and energy distribution of optical resonances is of central importance in low-dimensional materials$^{1-4}$ and its knowledge is critical for designing efficient optoelectronic devices. Ruddlesden-Popper halide perovskites are 2D solution-processed quantum wells with a general formula A$_2$A$_{n-1}$M$_n$X$_{3n+1}$, where optoelectronic properties can be tuned by varying the perovskite layer thickness (n value), and have recently emerged as efficient semiconductors with technologically relevant stability. However, fundamental questions concerning the nature of optical resonances (excitons or free-carriers) and the exciton reduced mass, and their scaling with quantum well thickness remains unresolved. Here, using optical spectroscopy and 60-Tesla magneto-absorption supported by modelling, we unambiguously demonstrate that the optical resonances arise from tightly bound excitons with unexpectedly high exciton reduced mass (0.20 m0) and binding energies varying from 470 meV to 125 meV with increasing thickness from n=1 to 5. Our work demonstrates the dominant role of Coulomb interactions in 2D solution-processed quantum wells and presents unique opportunities for next-generation optoelectronic and photonic devices.
A theoretical study of photonic bands for one-dimensional (1D) lattices embedded in planar waveguides with strong refractive index contrast is presented. The approach relies on expanding the electromagnetic field on the basis of guided modes of an effective waveguide, and on treating the coupling to radiative modes by perturbation theory. Photonic mode dispersion, gap maps, and intrinsic diffraction losses of quasi-guided modes are calculated for the case of self-standing membranes as well as for Silicon-on-Insulator structures. Photonic band gaps in a waveguide are found to depend strongly on the core thickness and on polarization, so that the gaps for transverse electric and transverse magnetic modes most often do not overlap. Radiative losses of quasi-guided modes above the light line depend in a nontrivial way on structure parameters, mode index and wavevector. The results of this study may be useful for the design of integrated 1D photonic structures with low radiative losses.
Photonic crystals with a finite size can support surface modes when appropriately terminated. We calculate the dispersion curves of surface modes for different terminations using the plane wave expansion method. These non-radiative surface modes can be excited with the help of attenuated total reflection technique. We did experiments and simulations to trace the surface band curve, both in good agreement with the numerical calculations.
We use the third- and fourth-order autocorrelation functions $g^{(3)}(tau_1,tau_2)$ and $g^{(4)}(tau_1,tau_2, tau_3)$ to detect the non-classical character of the light transmitted through a photonic-crystal nanocavity containing a strongly-coupled quantum dot probed with a train of coherent light pulses. We contrast the value of $g^{(3)}(0, 0)$ with the conventionally used $g^{(2)}(0)$ and demonstrate that in addition to being necessary for detecting two-photon states emitted by a low-intensity source, $g^{(3)}$ provides a more clear indication of the non-classical character of a light source. We also present preliminary data that demonstrates bunching in the fourth-order autocorrelation function $g^{(4)}(tau_1,tau_2, tau_3)$ as the first step toward detecting three-photon states.
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