John D. Dow

Bio: John D. Dow is an academic researcher from Princeton University. The author has contributed to research in topics: Extended X-ray absorption fine structure & Exciton. The author has an hindex of 8, co-authored 11 publications receiving 1092 citations.

Toward a Unified Theory of Urbach's Rule and Exponential Absorption Edges

Abstract: Exponential absorption edges $\ensuremath{\alpha}=A{e}^{g(\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{-}\ensuremath{\hbar}{\ensuremath{\omega}}_{0})}$ have been observed in both ionic (Urbach's rule: $g=\frac{\ensuremath{\sigma}}{{k}_{B}{T}^{*}}$ and covalent materials. Arguments are given to show that a unified theory of exponetial absorption edges must (i) rely on electric microfields as the cause, (ii) include exciton effects and the final-state interaction between the electron and the hole, and (iii) ascribe Urbach's rule to the relative, internal motion of the exciton. An approximate calculation has been made in which the nonuniform microfields are replaced by a statistical distribution of uniform microfields; this calculation is a generalization to physically relevant intermediate-strength fields of previous strong- and weak-field theories of Redfield and Dexter. In contrast with the other microfield models, which obtain the exponential spectral shape by averaging over microfield distributions, the present theory obtains a quantitatively exponential edge as an inherent feature. The temperature dependences of the edges in various materials follow qualitatively from the nature of the microfield sources. The specific temperature dependence of Urbach's rule in ionic crystals is obtained from this model, with supplementary arguments to account for nonuniformity of the fields.

Electroabsorption in Semiconductors: The Excitonic Absorption Edge

Abstract: Numerical calculations of the optical-absorption coefficient for direct, excitonic transitions in a uniform applied electric field are presented. The electron-hole scattering is treated within the effective-mass approximation and leads to an absorption coefficient which differs markedly in size and shape from the Franz-Keldysh absorption spectrum. A detailed numerical study of the shape of the absorption-edge spectrum at photon energies somewhat below the zero-field absorption threshold suggests that for small field strengths the dominant asymptotic form of the absorption coefficient is $\mathrm{exp}(\ensuremath{-}\frac{{C}_{0}|E\ensuremath{-}{{E}_{0}}^{\ensuremath{'}}|}{f})$, where $f=\frac{|e|\mathrm{Fa}}{R}$ is the electric field strength in units of exciton Rydbergs per electron-exciton Bohr radius. This result contradicts the existing belief that the electron-hole interaction does not alter the asymptotic form of the Franz-Keldysh shape: $\mathrm{exp}(\ensuremath{-}\frac{{{C}_{0}}^{\ensuremath{'}}{|E\ensuremath{-}{{E}_{0}}^{\ensuremath{'}}|}^{\frac{3}{2}}}{f})$. Physical arguments are presented to show why the exciton effects should be important. A discussion is presented of the interrelationships among the present treatment of electro-absorption and various one-electron, exciton, and many-body formalisms.

Theory of Exponential Absorption Edges in Ionic and Covalent Solids

Abstract: A calculation has been performed which yields a good fit to the exponential absorption edge of Urbach's rule in ionic crystals. The major feature of this calculation is that its underlying physical mechanism offers a unified explanation of exponential absorption edges in all materials: ionic, covalent, and intermediate bonding crystals, as well as amorphous semiconductors.

Optical Response of Semiconductors in Electric Fields: Excitonic Effects

Abstract: The effects of a uniform electric field on the optical response functions of a semiconductor are calculated, by numerically solving the effective-mass equation for a Wannier exciton. The calculations assume an isotropic effective-mass model of direct transitions at a three-dimensional ${M}_{0}$ threshold. The relationship between the real and imaginary parts of the dielectric function are discussed for the purposes of understanding the physics of electroreflection and developing rules of thumb for interpreting electroreflectivity spectra. The theory is compared with the electroreflectivity data at the direct edge of Ge taken by Handler, Jasperson, and Koeppen, and yields very good agreement. The value of the momentum matrix element extracted from the fit of theory to experiment is $0.35\frac{\ensuremath{\hbar}}{{a}_{0}}$, in excellent agreement with experimentally measured values.

Intermediate Coupling Theory: Padé Approximants for Polarons

Abstract: A general method is presented for obtaining an accurate intermediate-coupling theory from weak- and strong-coupling perturbation theory. The method uses two-point Pad\'e approximants to extrapolate (low-order) expansions about the weak- and the strong-coupling limits into the intermediate-coupling regime. The method is used to evaluate the ground-state energy and effective mass of the polaron with gratifying success. In addition, the weak-coupling perturbation theory of the polaron dispersion relation, ground-state energy, and effective mass are extended to fourth, sixth, and fourth order, respectively. A scheme based on two-point Pad\'e approximants is used to obtain an optimal polaron dispersion relation.

Conduction in non-crystalline systems V. Conductivity, optical absorption and photoconductivity in amorphous semiconductors

Abstract: The experimental evidence concerning the density of states in amorphous semiconductors and the ranges of energy in which states are localized is reviewed; this includes d.c. and a.c. conductivity, drift mobility and optical absorption. There is evidence that for some chalcogenide semiconductors the model proposed by Cohen, Fritzsche and Ovshinsky (1969) should be modified by introducing a band of localized states, near the centre of the gap. The values of C, when the d.c. conductivity is expressed as C exp (- E/kT), are considered. The behaviour of the optical absorption coefficient near the absorption edge and its relation to exciton formation are discussed. Finally, an interpretation of some results on photoconductivity is offered.

Electric field dependence of optical absorption near the band gap of quantum-well structures.

Abstract: We report experiments and theory on the effects of electric fields on the optical absorption near the band edge in GaAs/AlGaAs quantum-well structures. We find distinct physical effects for fields parallel and perpendicular to the quantum-well layers. In both cases, we observe large changes in the absorption near the exciton peaks. In the parallel-field case, the excitons broaden with field, disappearing at fields \ensuremath{\sim}${10}^{4}$ V/cm; this behavior is in qualitative agreement with previous theory and in order-of-magnitude agreement with direct theoretical calculations of field ionization rates reported in this paper. This behavior is also qualitatively similar to that seen with three-dimensional semiconductors. For the perpendicular-field case, we see shifts of the exciton peaks to lower energies by up to 2.5 times the zero-field binding energy with the excitons remaining resolved at up to \ensuremath{\sim}${10}^{5}$ V/cm: This behavior is qualitatively different from that of bulk semiconductors and is explained through a mechanism previously briefly described by us [D. A. B. Miller et al., Phys. Rev. Lett. 53, 2173 (1984)] called the quantum-confined Stark effect. In this mechanism the quantum confinement of carriers inhibits the exciton field ionization. To support this mechanism we present detailed calculations of the shift of exciton peaks including (i) exact solutions for single particles in infinite wells, (ii) tunneling resonance calculations for finite wells, and (iii) variational calculations of exciton binding energy in a field. We also calculate the tunneling lifetimes of particles in the wells to check the inhibition of field ionization. The calculations are performed using both the 85:15 split of band-gap discontinuity between conduction and valence bands and the recently proposed 57:43 split. Although the detailed calculations differ in the two cases, the overall shift of the exciton peaks is not very sensitive to split ratio. We find excellent agreement with experiment with no fitted parameters.

States in the gap

Abstract: Experimental evidence about the states in the gap of chalcogenide glasses is discussed. The total concentration of states is estimated from the measurements of the magnetic susceptibility and their density distribution from the optical and photo-emission measurements. Possible models for the interpretation of the experimental facts are considered.

Searching for better plasmonic materials

Abstract: Plasmonics is a research area merging the fields of optics and nanoelectronics by confining light with relatively large free-space wavelength to the nanometer scale - thereby enabling a family of novel devices. Current plasmonic devices at telecommunication and optical frequencies face significant challenges due to losses encountered in the constituent plasmonic materials. These large losses seriously limit the practicality of these metals for many novel applications. This paper provides an overview of alternative plasmonic materials along with motivation for each material choice and important aspects of fabrication. A comparative study of various materials including metals, metal alloys and heavily doped semiconductors is presented. The performance of each material is evaluated based on quality factors defined for each class of plasmonic devices. Most importantly, this paper outlines an approach for realizing optimal plasmonic material properties for specific frequencies and applications, thereby providing a reference for those searching for better plasmonic materials.

Searching for Better Plasmonic Materials

Abstract: Plasmonics is a research area merging the fields of optics and nanoelectronics by confining light with relatively large free-space wavelength to the nanometer scale - thereby enabling a family of novel devices. Current plasmonic devices at telecommunication and optical frequencies face significant challenges due to losses encountered in the constituent plasmonic materials. These large losses seriously limit the practicality of these metals for many novel applications. This paper provides an overview of alternative plasmonic materials along with motivation for each material choice and important aspects of fabrication. A comparative study of various materials including metals, metal alloys and heavily doped semiconductors is presented. The performance of each material is evaluated based on quality factors defined for each class of plasmonic devices. Most importantly, this paper outlines an approach for realizing optimal plasmonic material properties for specific frequencies and applications, thereby providing a reference for those searching for better plasmonic materials.