Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Crystal Symmetry and Polarization of High-order Harmonics in ZnO   We carried out a joint theoretical and experimental study of the polarization
of high-order harmonics generated from ZnO by intense infrared laser pulses.
Experimentally we found that the dependence of parallel and perpendicular
polarizations on the crystal orientation for all odd harmonics are nearly
identical, but they are quite different from even harmonics which also show
little order dependence. A one-dimensional two-band model, combined with a
linear coupled excitation model, is shown to be able to explain the observed
polarization behavior, including low-order harmonics. We further note that the
same odd/even order contrast have been reported in a number of other crystals,
despite that the harmonics were perceived to be generated via entirely
different mechanisms. We demonstrated that this universality is governed by
crystal symmetry, not by specific mechanisms. Thus, polarization measurements
of harmonics offers a powerful pure optical method for determining the crystal
axes as well as monitoring their ultrafast changes when crystals are undergoing
deformation. In addition, the ellipticity of harmonic has been studied. It
shows that ellipticity of high-order harmonics from solids can be tuned
precisely by changing the bond structure of the sample.

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The categories of this paper are: econ
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The categories of this paper are: plasm-ph
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The categories of this paper are: nucl-ex
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The categories of this paper are: mtrl-th
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The categories of this paper are: astro-ph
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The categories of this paper are: adap-org
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The categories of this paper are: chao-dyn
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The categories of this paper are: bayes-an
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The categories of this paper are: q-fin
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The categories of this paper are: solv-int
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The categories of this paper are: math-ph
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The categories of this paper are: q-alg
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The categories of this paper are: q-bio
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The categories of this paper are: quant-ph
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The categories of this paper are: funct-an
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The categories of this paper are: ao-sci