75 research outputs found
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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Magnetic trapping of an atomic 55Mn-52Cr mixture
Atomic manganese 55Mn and chromium 52Cr are simultaneously loaded and confined in a magnetic trap.
Using a cryogenic 3 He buffer gas, 1011 manganese and 1012 chromium atoms are trapped at an initial temperature of 600 mK. The buffer gas is then pumped away, thermally isolating the sample. The Mn-Cr interspecies inelastic rate constant is measured to be GMn,Cr=1.5s±0.2d310−13 cm3 /s.Physic
Molecules Near Absolute Zero and External Field Control of Atomic and Molecular Dynamics
This article reviews the current state of the art in the field of cold and
ultracold molecules and demonstrates that chemical reactions, inelastic
collisions and dissociation of molecules at subKelvin temperatures can be
manipulated with external electric or magnetic fields. The creation of
ultracold molecules may allow for spectroscopy measurements with extremely high
precision and tests of fundamental symmetries of nature, quantum computation
with molecules as qubits, and controlled chemistry. The probability of chemical
reactions and collisional energy transfer can be very large at temperatures
near zero Kelvin. The collision energy of ultracold atoms and molecules is much
smaller than perturbations due to interactions with external electric or
magnetic fields available in the laboratory. External fields may therefore be
used to induce dissociation of weakly bound molecules, stimulate forbidden
electronic transitions, suppress the effect of centrifugal barriers in outgoing
reaction channels or tune Feshbach resonances that enhance chemical reactivity
High order optical sideband generation with Terahertz quantum cascade lasers
Optical sidebands are generated by difference frequency mixing between a resonant bandgap near-infrared beam and a terahertz (THz) wave. This is realized within the cavity of a THz quantum cascade laser using resonantly enhanced non-linearities. Multiple order optical sidebands and conversion efficiencies up to 0.1% are shown
Temperature Dependent Zero-Field Splittings in Graphene
Graphene is a quantum spin Hall insulator with a 45 eV wide non-trivial
topological gap induced by the intrinsic spin-orbit coupling. Even though this
zero-field spin splitting is weak, it makes graphene an attractive candidate
for applications in quantum technologies, given the resulting long spin
relaxation time. On the other side, the staggered sub-lattice potential,
resulting from the coupling of graphene with its boron nitride substrate,
compensates intrinsic spin-orbit coupling and decreases the non-trivial
topological gap, which may lead to the phase transition into trivial band
insulator state. In this work, we present extensive experimental studies of the
zero-field splittings in monolayer and bilayer graphene in a temperature range
2K-12K by means of sub-Terahertz photoconductivity-based electron spin
resonance technique. Surprisingly, we observe a decrease of the spin splittings
with increasing temperature. We discuss the origin of this phenomenon by
considering possible physical mechanisms likely to induce a temperature
dependence of the spin-orbit coupling. These include the difference in the
expansion coefficients between the graphene and the boron nitride substrate or
the metal contacts, the electron-phonon interactions, and the presence of a
magnetic order at low temperature. Our experimental observation expands
knowledge about the non-trivial topological gap in graphene.Comment: Main text with figures (20 pages) and Supplementary Information (14
pages) Accepted in Phys. Rev.
Short pulse generation and dispersion in THz quantum cascade lasers
We demonstrate the generation of short terahertz pulses from spectrally broad metal-metal quantum cascade lasers at 77 K via active mode-locking, and show the limiting role of phase-matching between the terahertz pulse and the microwave modulation. Furthermore a new concept of THz pulse dispersion control is proposed to go beyond the limitation of the current modulation scheme
Terahertz pulse generation from quantum cascade lasers
We demonstrate the generation of 11ps terahertz pulses from metal-metal (MM) quantum cascade lasers (QCLs) at 77K via active mode-locking. Contrary to popular belief that a long gain recovery time is required, we demonstrate that the dominant factor necessary for active pulse generation is in fact the synchronization between the propagating electronic microwave modulation and the generated THz pulses in the QCL. This allows the THz pulse to propagate in phase with the microwave modulation along the gain medium, permitting pulse generation
The Buffer Gas Beam: An Intense, Cold, and Slow Source for Atoms and Molecules
Beams of atoms and molecules are stalwart tools for spectroscopy and studies
of collisional processes. The supersonic expansion technique can create cold
beams of many species of atoms and molecules. However, the resulting beam is
typically moving at a speed of 300-600 m/s in the lab frame, and for a large
class of species has insufficient flux (i.e. brightness) for important
applications. In contrast, buffer gas beams can be a superior method in many
cases, producing cold and relatively slow molecules in the lab frame with high
brightness and great versatility. There are basic differences between
supersonic and buffer gas cooled beams regarding particular technological
advantages and constraints. At present, it is clear that not all of the
possible variations on the buffer gas method have been studied. In this review,
we will present a survey of the current state of the art in buffer gas beams,
and explore some of the possible future directions that these new methods might
take
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