Electric field dependence of optical absorption near the band gap of quantum-well structures.
TL;DR: Detailed calculations of the shift of exciton peaks are presented including (i) exact solutions for single particles in infinite wells, (ii) tunneling resonance calculations for finite wells, and (iii) variational calculations ofexciton binding energy in a field.
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.
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10 Jun 2009
TL;DR: The current performance and future demands of interconnects to and on silicon chips are examined and the requirements for optoelectronic and optical devices are project if optics is to solve the major problems of interConnects for future high-performance silicon chips.
Abstract: We examine the current performance and future demands of interconnects to and on silicon chips. We compare electrical and optical interconnects and project the requirements for optoelectronic and optical devices if optics is to solve the major problems of interconnects for future high-performance silicon chips. Optics has potential benefits in interconnect density, energy, and timing. The necessity of low interconnect energy imposes low limits especially on the energy of the optical output devices, with a ~ 10 fJ/bit device energy target emerging. Some optical modulators and radical laser approaches may meet this requirement. Low (e.g., a few femtofarads or less) photodetector capacitance is important. Very compact wavelength splitters are essential for connecting the information to fibers. Dense waveguides are necessary on-chip or on boards for guided wave optical approaches, especially if very high clock rates or dense wavelength-division multiplexing (WDM) is to be avoided. Free-space optics potentially can handle the necessary bandwidths even without fast clocks or WDM. With such technology, however, optics may enable the continued scaling of interconnect capacity required by future chips.
1,959 citations
TL;DR: In this paper, the design requirements of SESAM's for stable pulse generation in both the mode-locked and Q-switched regime were reviewed, and the combination of device structure and material parameters provided sufficient design freedom to choose key parameters such as recovery time, saturation intensity, and saturation fluence.
Abstract: Intracavity semiconductor saturable absorber mirrors (SESAM's) offer unique and exciting possibilities for passively pulsed solid-state laser systems, extending from Q-switched pulses in the nanosecond and picosecond regime to mode-locked pulses from 10's of picoseconds to sub-10 fs. This paper reviews the design requirements of SESAM's for stable pulse generation in both the mode-locked and Q-switched regime. The combination of device structure and material parameters for SESAM's provide sufficient design freedom to choose key parameters such as recovery time, saturation intensity, and saturation fluence, in a compact structure with low insertion loss. We have been able to demonstrate, for example, passive modelocking (with no Q-switching) using an intracavity saturable absorber in solid-state lasers with long upper state lifetimes (e.g., 1-/spl mu/m neodymium transitions), Kerr lens modelocking assisted with pulsewidths as short as 6.5 fs from a Ti:sapphire laser-the shortest pulses ever produced directly out of a laser without any external pulse compression, and passive Q-switching with pulses as short as 56 ps-the shortest pulses ever produced directly from a Q-switched solid-state laser. Diode-pumping of such lasers is leading to practical, real-world ultrafast sources, and we will review results on diode-pumped Cr:LiSAF, Nd:glass, Yb:YAG, Nd:YAG, Nd:YLF, Nd:LSB, and Nd:YVO/sub 4/.
1,866 citations
TL;DR: This paper is a review of recent progress made in organic thin films grown in ultrahigh vacuum or using other vapor-phase deposition methods and describes the most important work which has been published in this field since the emergence of OMBD in the mid-1980s.
Abstract: During the past decade, enormous progress has been made in growing ultrathin organic films and multilayer structures with a wide range of exciting optoelectronic properties. This progress has been made possible by several important advances in our understanding of organic films and their modes of growth. Perhaps the single most important advance has been the use of ultrahigh vacuum (UHV) as a means to achieve, for the first time, monolayer control over the growth of organic thin films with extremely high chemical purity and structural precision.1-3 Such monolayer control has been possible for many years using well-known techniques such as Langmuir-Blodgett film deposition,4 and more recently, self-assembled monolayers from solution have also been achieved.5 However, ultrahighvacuum growth, sometimes referred to as organic molecular beam deposition (OMBD) or organic molecular beam epitaxy (OMBE), has the advantage of providing both layer thickness control and an atomically clean environment and substrate. When combined with the ability to perform in situ highresolution structural diagnostics of the films as they are being deposited, techniques such as OMBD have provided an entirely new prospect for understanding many of the fundamental structural and optoelectronic properties of ultrathin organic film systems. Since such systems are both of intrinsic as well as practical interest, substantial effort worldwide has been invested in attempting to grow and investigate the properties of such thin-film systems. This paper is a review of recent progress made in organic thin films grown in ultrahigh vacuum or using other vapor-phase deposition methods. We will describe the most important work which has been published in this field since the emergence of OMBD in the mid-1980s. Both the nature of thin-film growth and structural ordering will be discussed, as well as some of the more interesting consequences to the physical properties of such organic thin-film systems will be considered both from a theoretical as well as an experimental viewpoint. Indeed, it will 1793 Chem. Rev. 1997, 97, 1793−1896
1,809 citations
TL;DR: In this article, the latest advances in valley-tronics have largely been enabled by the isolation of 2D materials (such as graphene and semiconducting transition metal dichalcogenides) that host an easily accessible electronic valley degree of freedom, allowing for dynamic control.
Abstract: Semiconductor technology is currently based on the manipulation of electronic charge; however, electrons have additional degrees of freedom, such as spin and valley, that can be used to encode and process information. Over the past several decades, there has been significant progress in manipulating electron spin for semiconductor spintronic devices, motivated by potential spin-based information processing and storage applications. However, experimental progress towards manipulating the valley degree of freedom for potential valleytronic devices has been limited until very recently. We review the latest advances in valleytronics, which have largely been enabled by the isolation of 2D materials (such as graphene and semiconducting transition metal dichalcogenides) that host an easily accessible electronic valley degree of freedom, allowing for dynamic control. The energy extrema of an electronic band are referred to as valleys. In 2D materials, two distinguishable valleys can be used to encode information and explore other valleytronic applications.
1,799 citations
01 Jun 2000
TL;DR: Optical interconnects to silicon CMOS chips are discussed in this paper, where various arguments for introducing optical interconnections to silicon chips are summarized, and the challenges for optical, optoelectronic, and integration technologies are discussed.
Abstract: The various arguments for introducing optical interconnections to silicon CMOS chips are summarized, and the challenges for optical, optoelectronic, and integration technologies are discussed. Optics could solve many physical problems of interconnects, including precise clock distribution, system synchronization (allowing larger synchronous zones, both on-chip and between chips), bandwidth and density of long interconnections, and reduction of power dissipation. Optics may relieve a broad range of design problems, such as crosstalk, voltage isolation, wave reflection, impedence matching, and pin inductance. It may allow continued scaling of existing architectures and enable novel highly interconnected or high-bandwidth architectures. No physical breakthrough is required to implement dense optical interconnects to silicon chips, though substantial technological work remains. Cost is a significant barrier to practical introduction, though revolutionary approaches exist that might achieve economies of scale. An Appendix analyzes scaling of on-chop global electrical interconnects, including line inductance and the skin effect, both of which impose significant additional constraints on future interconnects.
1,233 citations
References
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TL;DR: In this article, the authors present theory and extended experimental results for the large shift in optical absorption in GaAs-AlGaAs quantum well structures with electric field perpendicular to the layers.
Abstract: We present theory and extended experimental results for the large shift in optical absorption in GaAs-AlGaAs quantum well structures with electric field perpendicular to the layers. In contrast to the Stark effect on atoms or on excitons in bulk semiconductors, the exciton resonances remain resolved even for shifts much larger than the zero-field binding energy and fields g 50 times the classical ionization field. The model explains these results as a consequence of the quantum confinement of carriers.
1,604 citations
TL;DR: In this article, a simple rectangular potential well with a depth of ≈088ΔE g, where ΔE g is the difference in the semiconductor energy gaps is defined.
Abstract: Quantum levels associated with the confinement of carriers in very thin, molecular-beam—grown AlxGa1−xAs−GaAs-Alx Ga1−xAs heterostructures result in pronounced structure in the GaAs optical absorption spectrum Up to eight resolved exciton transitions, associated with different bound-electron and bound-hole states, have been observed The heterostructure behaves as a simple rectangular potential well with a depth of ≈088ΔE g , for confining electrons and ≈012ΔE g for confining holes, where ΔE g is the difference in the semiconductor energy gaps
976 citations
TL;DR: The exponentiell abfallende Flanke der Absorptionskante erleidet eine Verschiebung nach kleineren Frequenzen, welche proportional zum Quadrat der Feldstärke and zu der gemittelten reziproken effektiven Masse der Elektronen and der Löcher ist as discussed by the authors.
Abstract: In einem elektrischen Feld der Größenordnung 105 bis 106 V/cm hat man eine Veränderung der Absorptionskanten zu erwarten, welche in einer Verbreiterung des Kantenfußes auf Kosten höherer Frequenzen besteht. Man kann den Effekt verstehen entweder aus dem Eindringen von Elektronen in das verbotene Band (Vorstufe der inneren Feldemission) oder aus der Frequenzmodulation, welche der optische Übergang infolge der Beschleunigung der Elektronenzustände erfährt. Die exponentiell abfallende Flanke der Absorptionskante erleidet eine Verschiebung nach kleineren Frequenzen, welche proportional zum Quadrat der Feldstärke und zu der gemittelten reziproken effektiven Masse der Elektronen und der Löcher ist. Die Beobachtung der Flankenverschiebung liefert daher eine Bestimmung dieser effektiven Masse.
723 citations
TL;DR: In this article, the authors present detailed experimental studies and modeling of the nonlinear absorption and refraction of GaAs/AlGaAs multiple quantum well structures (MQWS) in the small signal regime.
Abstract: We present detailed experimental studies and modeling of the nonlinear absorption and refraction of GaAs/AlGaAs multiple quantum well structures (MQWS) in the small signal regime. Nonlinear absorption and degenerate four-wave mixing in the vicinity of the room temperature exciton resonances are observed and analyzed. Spectra of the real and imaginary parts of the nonlinear cross section as a function of wavelength are obtained, and these are in excellent agreement with experimental data. A simple model for excitonic absorption saturation is proposed; it accounts qualitatively for the very low saturation intensities of room temperature excitons in MQWS.
645 citations
IBM1
TL;DR: In this paper, the eigenstate of an isolated quantum well subject to an external electric field was analyzed and a quadratic Stark shift was found whose magnitude depended strongly on the finite well depth.
Abstract: We present variational calculations of the eigenstates in an isolated-quantum-well structure subjected to an external electric field. At weak fields a quadratic Stark shift is found whose magnitude depends strongly on the finite well depth. In addition, the electric field induces a spatial shift of the particle wave function along or opposite to the field direction, depending on the sign of the particle mass. This field-induced spatial separation of conduction and valence electrons in GaAs quantum wells decreases the overlap between their associated wave functions, leading to a reduction of interband recombination.
644 citations