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Superlattices: problems and new opportunities, nanosolids.

Tsu R - Nanoscale Res Lett (2011)

Bottom Line: Superlattice is simply a way of forming a uniform continuum for whatever purpose at hand.However, new opportunities in component-based nanostructures may lead the field of endeavor to new heights.The all important translational symmetry of solids is relaxed and local symmetry is needed in nanosolids.

View Article: PubMed Central - HTML - PubMed

Affiliation: University of North Carolina at Charlotte, Charlotte, NC 28223 USA. Tsu@uncc.edu.

ABSTRACT
Superlattices were introduced 40 years ago as man-made solids to enrich the class of materials for electronic and optoelectronic applications. The field metamorphosed to quantum wells and quantum dots, with ever decreasing dimensions dictated by the technological advancements in nanometer regime. In recent years, the field has gone beyond semiconductors to metals and organic solids. Superlattice is simply a way of forming a uniform continuum for whatever purpose at hand. There are problems with doping, defect-induced random switching, and I/O involving quantum dots. However, new opportunities in component-based nanostructures may lead the field of endeavor to new heights. The all important translational symmetry of solids is relaxed and local symmetry is needed in nanosolids.

No MeSH data available.


The in-phase, Re〈v〉1 and out-of-phase Im〈v〉1 components of the linear response function for a superlattice with an applied electric field of F = F0 + 2F1cosωt, ωB = eF0 d/ħ, and ωB1 = eF1 d/ħ.
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Figure 1: The in-phase, Re〈v〉1 and out-of-phase Im〈v〉1 components of the linear response function for a superlattice with an applied electric field of F = F0 + 2F1cosωt, ωB = eF0 d/ħ, and ωB1 = eF1 d/ħ.

Mentions: In Figure 1, for ωBτ = 1, Re〈v〉 is always positive indicating the lack of gain or self-oscillation. The Im〈v〉 has a maximum at ω = ωB. For ωBτ = 2, Re〈v〉 has a minimum at ω = ωB/2 and is negative, but Im〈v〉 has a peak at ω = ωB. With a further increase to ωBτ = 3, Re〈v〉 has a maximum negative value at ω = 2ωB/3 and the Im〈v〉 has a peak at ω = ωB. Thus the peak in Im〈v〉 always appears at ω = ωB, substantiating the intuitive understanding that the system is oscillating at the Bloch frequency. The question of gain or loss is another matter as we need to focus on Re〈v〉. Note that Re〈v〉 always has a maximum negative value below ωB, indicating that self-oscillation that occurs at the maximum gain is never at the Bloch frequency. Only as ωBτ → ∞ does the maximum gain coincide with the Bloch frequency. For both ωBτ ≫1 and ωτ ≫ 1, it is seen that Re〈v〉3 can have a substantial region that is negative, indicating that in the region of nonlinear optics, an intense optical field is needed for gain. What is happening is that higher energy photons cause transitions between mini-bands, providing additional nonlinear response. This is because k is conserved to within multiples of the reciprocal lattice vector, as in umklapprozesse. In the usual solids, optical nonlinearity arises from small non-parabolicity of the E-k relation as treated by Jha and Bloembergen [10]. However, in man-made suprelattices, non-parabolicity is huge, leading to substantial 2nd and 3rd harmonics [11].


Superlattices: problems and new opportunities, nanosolids.

Tsu R - Nanoscale Res Lett (2011)

The in-phase, Re〈v〉1 and out-of-phase Im〈v〉1 components of the linear response function for a superlattice with an applied electric field of F = F0 + 2F1cosωt, ωB = eF0 d/ħ, and ωB1 = eF1 d/ħ.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211173&req=5

Figure 1: The in-phase, Re〈v〉1 and out-of-phase Im〈v〉1 components of the linear response function for a superlattice with an applied electric field of F = F0 + 2F1cosωt, ωB = eF0 d/ħ, and ωB1 = eF1 d/ħ.
Mentions: In Figure 1, for ωBτ = 1, Re〈v〉 is always positive indicating the lack of gain or self-oscillation. The Im〈v〉 has a maximum at ω = ωB. For ωBτ = 2, Re〈v〉 has a minimum at ω = ωB/2 and is negative, but Im〈v〉 has a peak at ω = ωB. With a further increase to ωBτ = 3, Re〈v〉 has a maximum negative value at ω = 2ωB/3 and the Im〈v〉 has a peak at ω = ωB. Thus the peak in Im〈v〉 always appears at ω = ωB, substantiating the intuitive understanding that the system is oscillating at the Bloch frequency. The question of gain or loss is another matter as we need to focus on Re〈v〉. Note that Re〈v〉 always has a maximum negative value below ωB, indicating that self-oscillation that occurs at the maximum gain is never at the Bloch frequency. Only as ωBτ → ∞ does the maximum gain coincide with the Bloch frequency. For both ωBτ ≫1 and ωτ ≫ 1, it is seen that Re〈v〉3 can have a substantial region that is negative, indicating that in the region of nonlinear optics, an intense optical field is needed for gain. What is happening is that higher energy photons cause transitions between mini-bands, providing additional nonlinear response. This is because k is conserved to within multiples of the reciprocal lattice vector, as in umklapprozesse. In the usual solids, optical nonlinearity arises from small non-parabolicity of the E-k relation as treated by Jha and Bloembergen [10]. However, in man-made suprelattices, non-parabolicity is huge, leading to substantial 2nd and 3rd harmonics [11].

Bottom Line: Superlattice is simply a way of forming a uniform continuum for whatever purpose at hand.However, new opportunities in component-based nanostructures may lead the field of endeavor to new heights.The all important translational symmetry of solids is relaxed and local symmetry is needed in nanosolids.

View Article: PubMed Central - HTML - PubMed

Affiliation: University of North Carolina at Charlotte, Charlotte, NC 28223 USA. Tsu@uncc.edu.

ABSTRACT
Superlattices were introduced 40 years ago as man-made solids to enrich the class of materials for electronic and optoelectronic applications. The field metamorphosed to quantum wells and quantum dots, with ever decreasing dimensions dictated by the technological advancements in nanometer regime. In recent years, the field has gone beyond semiconductors to metals and organic solids. Superlattice is simply a way of forming a uniform continuum for whatever purpose at hand. There are problems with doping, defect-induced random switching, and I/O involving quantum dots. However, new opportunities in component-based nanostructures may lead the field of endeavor to new heights. The all important translational symmetry of solids is relaxed and local symmetry is needed in nanosolids.

No MeSH data available.