Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ultrafast two-dimensional field spectroscopy of terahertz intersubband
  saturable absorbers   Intersubband (ISB) transitions in semiconductor multi-quantum well (MQW)
structures are promising candidates for the development of saturable absorbers
at terahertz (THz) frequencies. Here, we exploit amplitude and phase-resolved
two-dimensional (2D) THz spectroscopy on the sub-cycle time scale to observe
directly the saturation dynamics and coherent control of ISB transitions in a
metal-insulator MQW structure. Clear signatures of incoherent pump-probe and
coherent four-wave mixing signals are recorded as a function of the peak
electric field of the single-cycle THz pulses. All nonlinear signals reach a
pronounced maximum for a THz electric field amplitude of 11 kV/cm and decrease
for higher fields. We demonstrate that this behavior is a fingerprint of
THz-driven carrier-wave Rabi flopping. A numerical solution of the
Maxwell-Bloch equations reproduces our experimental findings quantitatively and
traces the trajectory of the Bloch vector. This microscopic model allows us to
design tailored MQW structures with optimized dynamical properties for
saturable absorbers that could be used in future compact semiconductor-based
single-cycle THz sources.

5
The categories of this paper are: gr-qc
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The categories of this paper are: astro-ph
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The categories of this paper are: chao-dyn
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The categories of this paper are: alg-geom
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The categories of this paper are: math-ph
29
The categories of this paper are: funct-an
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The categories of this paper are: nucl-th
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The categories of this paper are: hep-th
32
The categories of this paper are: atom-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: econ
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The categories of this paper are: dg-ga
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The categories of this paper are: eess