124 research outputs found
Observation of inter-Landau-level quantum coherence in semiconductor quantum wells
Using three-pulse four-wave-mixing femtosecond spectroscopy, we excite a
non-radiative coherence between the discrete Landau levels of an undoped
quantum well and study its dynamics. We observe quantum beats that reflect the
time evolution of the coherence between the two lowest Landau level
magnetoexcitons. We interpret our observations using a many-body theory and
find that the inter Landau level coherence decays with a new time constant,
substantially longer than the corresponding interband magnetoexciton dephasing
times. Our results indicate a new intraband excitation dynamics that cannot be
described in terms of uncorrelated interband excitations.Comment: 5 pages, 5 figures, to appear in Phys. Rev. B Rapid Communication
Ultrafast dynamics of coherences in the quantum Hall system
Using three-pulse four-wave-mixing optical spectroscopy, we study the
ultrafast dynamics of the quantum Hall system. We observe striking differences
as compared to an undoped system, where the 2D electron gas is absent. In
particular, we observe a large off-resonant signal with strong oscillations.
Using a microscopic theory, we show that these are due to many-particle
coherences created by interactions between photoexcited carriers and collective
excitations of the 2D electron gas. We extract quantitative information about
the dephasing and interference of these coherences.Comment: 4 pages, 4 figures, to be published in Phys. Rev. Let
Engineering Fano resonances in discrete networks
We study transmission properties of discrete networks composed of linear
arrays coupled to systems of N side defects, and demonstrate the basic
principles of the resonant scattering management through engineering Fano
resonances. We find exact solutions for the wave transmission coefficient and
reveal the conditions for the perfect reflections and transmissions due to
either destructive or constructive interferences. We associate these
reflections and transmissions with Fano resonances, and demonstrate how they
can be tuned by introducing nonlinear defects into the network.Comment: 6 pages, 5 figures, accepted for publication in Phys. Rev.
Nonlinear frequency mixing in quantum cascade lasers: Towards broadband wavelength shifting and THz up-conversion
Terahertz (THz) sideband generation on a near-infrared (NIR) carrier has been recently demonstrated using quantum cascade lasers (QCL), with potential applications in wavelength shifting and THz up-conversion. However, the NIR wavelength range and nonlinear efficiency were severely limited by absorption. Here we overcome this drawback through a novel reflection geometry, whilst preserving a large interaction area. As well as insights into the nonlinear mechanism, this allows a much large range of NIR pump energies, relaxing the criteria of using particular excitation wavelengths
Monolithic echo-less photoconductive switches for high-resolution terahertz time-domain spectroscopy
Interdigitated photoconductive (IPC) switches are convenient sources and detectors for terahertz (THz) time domain spectroscopy. However, reflection of the emitted or detected radiation within the device substrate can lead to echoes that inherently limits the spectroscopic resolution achievable. In this work, we design and realize low-temperature-grown-GaAs (LT-GaAs) IPC switches for THz pulse generation and detection that suppresses such unwanted echoes. This is realized through a monolithic geometry of an IPC switch with a metal plane buried at a subwavelength depth below the LT-GaAs surface. Using this device as a detector, and coupling it to an echo-less IPC source, enables echo-free THz-TDS and high-resolution spectroscopy, with a resolution limited only by the temporal length of the measurement governed by the mechanical delay line used
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