19 research outputs found
Spin-Photon Entanglement and Quantum Optics with Single Quantum Dots.
InAs quantum dots (QDs) can be used as optically coupled quantum storage devices for quantum information applications. The QD can be charged with a single electron, where the spin state (up or down) provides a long lived quantum bit (qubit). The QD's optically excited states are used to initialize, manipulate, and read out the electron spin state with laser pulses. However, most practical quantum information applications require many interacting qubits, forming a quantum network. Since QDs are based on semiconductor technology, and are compatible with standard nano-fabrication processing, there is promise that they can provide a solid state platform where a scalable quantum information architecture is realizable. We focus on scaling the QD system to multiple qubits using intermediate spin-photon entangled states.
In this work, experimental and theoretical techniques are developed to study the QD-light matter interaction at the single photon level. Resonance fluorescence from a single QD is experimentally realized, and the single photon nature of the scattered radiation is verified through intensity correlation experiments. Transient fluorescence measurements on resonantly excited QDs are performed using time correlated single photon counting techniques to measure the excited state lifetime. High speed electro-optic modulators are used to time gate narrow bandwidth lasers, so that a QD can be driven under step-wise excitation, allowing for the direct observation of time-dependent optical Rabi oscillations. From these measurements, we are able to extract a decoherence rate which is consistent with the lifetime limit, indicating that pure dephasing is negligible in this system.
These techniques are applied to the QD spin system to demonstrate a spin-photon entangled state, by performing correlation measurements on the spin and photon state in two bases. A lower bound on the entanglement fidelity of 0.59(4) is achieved, which exceeds the classical limit of 0.5 by more than two standard deviations. The entanglement fidelity is limited primarily by the finite timing resolution of available single photon detectors. Taking this into account, we achieve 84% of the apparatus limited fidelity. These spin-entangled photons can be used to mediate entanglement between distant QD spins, providing the basis of an optically coupled QD spin network.PhDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/99785/1/jschaibl_1.pd
Population pulsation resonances of excitons in monolayer MoSe2 with sub 1 {\mu}eV linewidth
Monolayer transition metal dichalcogenides, a new class of atomically thin
semiconductors, possess optically coupled 2D valley excitons. The nature of
exciton relaxation in these systems is currently poorly understood. Here, we
investigate exciton relaxation in monolayer MoSe2 using polarization-resolved
coherent nonlinear optical spectroscopy with high spectral resolution. We
report strikingly narrow population pulsation resonances with two different
characteristic linewidths of 1 {\mu}eV and <0.2 {\mu}eV at low-temperature.
These linewidths are more than three orders of magnitude narrower than the
photoluminescence and absorption linewidth, and indicate that a component of
the exciton relaxation dynamics occurs on timescales longer than 1 ns. The
ultra-narrow resonance (<0.2 {\mu}eV) emerges with increasing excitation
intensity, and implies the existence of a long-lived state whose lifetime
exceeds 6 ns.Comment: (PRL, in press
Single exciton trapping in an electrostatically defined 2D semiconductor quantum dot
Interlayer excitons (IXs) in 2D semiconductors have long lifetimes and
spin-valley coupled physics, with a long-standing goal of single exciton
trapping for valleytronic applications. In this work, we use a nano-patterned
graphene gate to create an electrostatic IX trap. We measure a unique
power-dependent blue-shift of IX energy, where narrow linewidth emission
exhibits discrete energy jumps. We attribute these jumps to quantized increases
of the number occupancy of IXs within the trap and compare to a theoretical
model to assign the lowest energy emission line to single IX recombination
Phonon-assisted oscillatory exciton dynamics in monolayer MoSe2
In monolayer semiconductor transition metal dichalcogenides, the
exciton-phonon interaction is expected to strongly affect the photocarrier
dynamics. Here, we report on an unusual oscillatory enhancement of the neutral
exciton photoluminescence with the excitation laser frequency in monolayer
MoSe2. The frequency of oscillation matches that of the M-point longitudinal
acoustic phonon, LA(M). Oscillatory behavior is also observed in the
steady-state emission linewidth and in timeresolved photoluminescence
excitation data, which reveals variation with excitation energy in the exciton
lifetime. These results clearly expose the key role played by phonons in the
exciton formation and relaxation dynamics of two-dimensional van der Waals
semiconductors.Comment: Published in npj 2D Materials and Applications.
https://www.nature.com/articles/s41699-017-0035-
Impact of Boron doping to the tunneling magnetoresistance of Heusler alloy Co2FeAl
Heusler alloys based magnetic tunnel junctions can potentially provide high
magnetoresistance, small damping and fast switching. Here junctions with
Co2FeAl as a ferromagnetic electrode are fabricated by room temperature
sputtering on Si/SiO2 substrates. The doping of Boron in Co2FeAl is found to
have a large positive impact on the structural, magnetic and transport
properties of the junctions, with a reduced interfacial roughness and
substantial improved tunneling magnetoresistance. A two-level magnetoresistance
is also observed in samples annealed at low temperature, which is believed to
be related to the memristive effect of the tunnel barrier with impurities.Comment: 9 pages, 4 figure
Directional Interlayer Spin-Valley Transfer in Two-Dimensional Heterostructures
Van der Waals heterostructures formed by two different monolayer
semiconductors have emerged as a promising platform for new optoelectronic and
spin/valleytronic applications. In addition to its atomically thin nature, a
two-dimensional semiconductor heterostructure is distinct from its
three-dimensional counterparts due to the unique coupled spin-valley physics of
its constituent monolayers. Here, we report the direct observation that an
optically generated spin-valley polarization in one monolayer can be
transferred between layers of a two-dimensional MoSe2-WSe2 heterostructure.
Using nondegenerate optical circular dichroism spectroscopy, we show that
charge transfer between two monolayers conserves spin-valley polarization and
is only weakly dependent on the twist angle between layers. Our work points to
a new spin-valley pumping scheme in nanoscale devices, provides a fundamental
understanding of spin-valley transfer across the two-dimensional interface, and
shows the potential use of two-dimensional semiconductors as a spin-valley
generator in 2D spin/valleytronic devices for storing and processing
information
2D Semiconductor Nonlinear Plasmonic Modulators
A plasmonic modulator is a device that controls the amplitude or phase of
propagating plasmons. In a pure plasmonic modulator, the presence or absence of
a pump plasmonic wave controls the amplitude of a probe plasmonic wave through
a channel. This control has to be mediated by an interaction between disparate
plasmonic waves, typically requiring the integration of a nonlinear material.
In this work, we demonstrate the first 2D semiconductor nonlinear plasmonic
modulator based on a WSe2 monolayer integrated on top of a lithographically
defined metallic waveguide. We utilize the strong coupling between the surface
plasmon polaritons, SPPs, and excitons in the WSe2 to give a 73 percent change
in transmission through the device. We demonstrate control of the propagating
SPPs using both optical and SPP pumps, realizing the first demonstration of a
2D semiconductor nonlinear plasmonic modulator, with a modulation depth of 4.1
percent, and an ultralow switching energy estimated to be 40 aJ
Electrical Control of Second-Harmonic Generation in a WSe2 Monolayer Transistor
Nonlinear optical frequency conversion, in which optical fields interact with
a nonlinear medium to produce new field frequencies, is ubiquitous in modern
photonic systems. However, the nonlinear electric susceptibilities that give
rise to such phenomena are often challenging to tune in a given material, and
so far, dynamical control of optical nonlinearities remains confined to
research labs as a spectroscopic tool. Here, we report a mechanism to
electrically control second-order optical nonlinearities in monolayer WSe2, an
atomically thin semiconductor. We show that the intensity of second-harmonic
generation at the A-exciton resonance is tunable by over an order of magnitude
at low temperature and nearly a factor of 4 at room temperature through
electrostatic doping in a field-effect transistor. Such tunability arises from
the strong exciton charging effects in monolayer semiconductors, which allow
for exceptional control over the oscillator strengths at the exciton and trion
resonances. The exciton-enhanced second-harmonic generation is
counter-circularly polarized to the excitation laser, arising from the
combination of the two-photon and one-photon valley selection rules that have
opposite helicity in the monolayer. Our study paves the way towards a new
platform for chip-scale, electrically tunable nonlinear optical devices based
on two-dimensional semiconductors.Comment: Published in Nature Nanotechnolog