Spontaneous emission

About: Spontaneous emission is a research topic. Over the lifetime, 12855 publications have been published within this topic receiving 323684 citations.

Observation of two-photon emission from semiconductors

Abstract: It is possible that when an electron relaxes from an excited state, it generates not one but two photons. Such two–photon emission has been seen in atomic systems, but never in semiconductors, until now. The experimental observation could have intriguing implications for quantum optics.

Quasi-Fermi level splitting and sub-bandgap absorptivity from semiconductor photoluminescence

Abstract: A unified model for the direct gap absorption coefficient (band-edge and sub-bandgap) is developed that encompasses the functional forms of the Urbach, Thomas-Fermi, screened Thomas-Fermi, and Franz-Keldysh models of sub-bandgap absorption as specific cases. We combine this model of absorption with an occupation-corrected non-equilibrium Planck law for the spontaneous emission of photons to yield a model of photoluminescence (PL) with broad applicability to band-band photoluminescence from intrinsic, heavily doped, and strongly compensated semiconductors. The utility of the model is that it is amenable to full-spectrum fitting of absolute intensity PL data and yields: (1) the quasi-Fermi level splitting, (2) the local lattice temperature, (3) the direct bandgap, (4) the functional form of the sub-bandgap absorption, and (5) the energy broadening parameter (Urbach energy, magnitude of potential fluctuations, etc.). The accuracy of the model is demonstrated by fitting the room temperature PL spectrum of GaAs. It is then applied to Cu(In,Ga)(S,Se)2 (CIGSSe) and Cu2ZnSn(S,Se)4 (CZTSSe) to reveal the nature of their tail states. For GaAs, the model fit is excellent, and fitted parameters match literature values for the bandgap (1.42 eV), functional form of the sub-bandgap states (purely Urbach in nature), and energy broadening parameter (Urbach energy of 9.4 meV). For CIGSSe and CZTSSe, the model fits yield quasi-Fermi leveling splittings that match well with the open circuit voltages measured on devices made from the same materials and bandgaps that match well with those extracted from EQE measurements on the devices. The power of the exponential decay of the absorption coefficient into the bandgap is found to be in the range of 1.2 to 1.6, suggesting that tunneling in the presence of local electrostatic potential fluctuations is a dominant factor contributing to the sub-bandgap absorption by either purely electrostatic (screened Thomas-Fermi) or a photon-assisted tunneling mechanism (Franz-Keldysh). A Gaussian distribution of bandgaps (local Eg fluctuation) is found to be inconsistent with the data. The sub-bandgap absorption of the CZTSSe absorber is found to be larger than that for CIGSSe for materials that yield roughly equivalent photovoltaic devices (8% efficient). Further, it is shown that fitting only portions of the PL spectrum (e.g., low energy for energy broadening parameter and high energy for quasi-Fermi level splitting) may lead to significant errors for materials with substantial sub-bandgap absorption and emission.

A new double‐heterostructure optoelectronic switching device using molecular‐beam epitaxy

Abstract: Two‐terminal switching action is observed in a new optoelectronic device structure. The device has a high‐impedance state without light emission and a low‐impedance state characterized by strong spontaneous emission. The transition from either state to the other may be induced by the appropriate optical or electrical input. It is clear that with the appropriate optical cavity construction the switching device will operate as a laser in the on state rather than in the spontaneous mode reported here. In principle, the device offers large digital optical gain determined by its optical sensitivity and its maximum output power.

Decay of excited atoms in absorbing dielectrics

Abstract: We present calculations of the rates of decay of an excited atom embedded in an absorbing dielectric. Decay can occur by spontaneous emission into transverse radiative modes of the electromagnetic field and by Joule heating via longitudinal coupling of the atom to the dielectric. The spontaneous emission (transverse) decay rate is modified in a dielectric, being the free-space rate multiplied by the real part of the refractive index at the transition frequency of the atom. There is a further modification due to the difference between the macroscopic dielectric field and the local field at the position of the atom. In addition there is a longitudinal decay rate which is proportional to the imaginary part of the dielectric constant and therefore vanishes in non-absorbing media. We derive expressions for each of these rates of decay and discuss the physical mechanisms leading to them.

Evanescent light-wave atom mirrors, resonators, waveguides, and traps

Abstract: Publisher Summary This chapter presents an overview of atom mirrors, resonators, waveguides, and traps that operate for the most part on the evanescent light-wave mechanism for atom manipulation. For many years, it has been known that light can be used to trap and manipulate small dielectric particles and atoms. In particular, the intense coherent light of lasers has been used to cool neutral atoms down to the micro-Kelvin and now even the nano-Kelvin regimes. The chapter discusses several convex, evanescent light-wave traps or guides in which at least one field is red-detuned, and hence attractive but a centrifugal force or a blue-detuned field provides a repulsive counterforce to allow the atoms to remain confined in stable orbits around the convex, dielectric, and optical resonator. The chapter focuses on the use of the evanescent field for making atom mirrors, resonators, waveguides, and traps. One of the principal experimental drawbacks of the evanescent light-wave mirror is that it requires quite high laser power to produce a sufficiently large potential barrier to reflect atoms with any realistic component of velocity normal to the surface, while not introducing an unacceptable degree of spontaneous emission probability.