94 research outputs found
Ultrafast photocurrents in MoSe probed by terahertz spectroscopy
We use the terahertz (THz) emission spectroscopy to study femtosecond
photocurrent dynamics in the prototypical 2D semiconductor, transition metal
dichalcogenide MoSe. We identify several distinct mechanisms producing THz
radiation in response to an ultrashort (fs) optical excitation in a
bilayer (BL) and a multilayer (ML) sample. In the ML, the THz radiation is
generated at a picosecond timescale by out-of-plane currents due to the drift
of photoexcited charge carriers in the surface electric field. The BL emission
is generated by an in-plane shift current. Finally, we observe oscillations at
about THz in the emission from the BL sample. We attribute the
oscillations to quantum beats between two excitonic states with energetic
separation of meV.Comment: This is the Accepted Manuscript version of an article accepted for
publication in 2D Materials. IOP Publishing Ltd is not responsible for any
errors or omissions in this version of the manuscript or any version derived
from it. The Version of Record is available online at
https://doi.org/10.1088/2053-1583/abd527 7 pages, 9 figure
Ultrafast photocurrents in MoSe2 probed by terahertz spectroscopy
We use the terahertz (THz) emission spectroscopy to study femtosecond photocurrent dynamics in the prototypical 2D semiconductor, transition metal dichalcogenide MoSe2. We identify several distinct mechanisms producing THz radiation in response to an ultrashort (30 fs) optical excitation in a bilayer (BL) and a multilayer (ML) sample. In the ML, the THz radiation is generated at a picosecond timescale by out-of-plane currents due to the drift of photoexcited charge carriers in the surface electric field. The BL emission is generated by an in-plane shift current. Finally, we observe oscillations at about 23 THz in the emission from the BL sample. We attribute the oscillations to quantum beats between two excitonic states with energetic separation of similar to 100 meV
Probing the Formation of Dark Interlayer Excitons via Ultrafast Photocurrent
Optically dark excitons determine a wide range of properties of photoexcited semiconductors yet are hard to access via conventional time-resolved spectroscopies. Here, we develop a time-resolved ultrafast photocurrent technique (trPC) to probe the formation dynamics of optically dark excitons. The nonlinear nature of the trPC makes it particularly sensitive to the formation of excitons occurring at the femtosecond time scale after the excitation. As a proof of principle, we extract the interlayer exciton formation time of 0.4 ps at 160 μJ/cm2 fluence in a MoS2/MoSe2 heterostructure and show that this time decreases with fluence. In addition, our approach provides access to the dynamics of carriers and their interlayer transport. Overall, our work establishes trPC as a technique to study dark excitons in various systems that are hard to probe by other approaches
THEORETICAL STUDIES FOR THE USE OF "BIOPLATEAU" FOR DEFFERIZATION OF WATER TREATMENT
The aim of the research is to determine the possibility of using bioplateau with higher aquatic vegetation in order to reduce the concentrations of iron in mine waters before releasing to the surface water body.Methods. The use of the results of theoretical and empirical studies determined during the analysis of literature data made it possible to evaluate the feasibility of using higher aquatic vegetation to accumulate iron ions depending on the water temperature, as well as the ability to form iron compounds in the aqueous system as well as their further precipitation.Results. The materials obtained during the research made it possible to determine the size and design of bioplate in order to purify the water from iron ions taking into account the use of higher aquatic vegetation and minimizing the area of land resources used for the device.Conclusions. A technological scheme for cleaning mine water from iron has been developed, including filters loaded with crushed stone for immobilization of iron bacteria and bioplateau. The bottom of the bioplateau is made of waterproof clay. At the bottom, hydrophytes are planted in the ground. According to the foregoing, it is advisable to plant narrowleaved cattail or reed. The facilities, on the basis of phytotechnology, work as a self-regulating system
Nanomechanical spectroscopy of 2D materials
We introduce a nanomechanical platform for fast and sensitive measurements of the spectrally resolved optical dielectric function of 2D materials. At the heart of our approach is a suspended 2D material integrated into a high Q silicon nitride nanomechanical resonator illuminated by a wavelength-tunable laser source. From the heating-related frequency shift of the resonator as well as its optical reflection measured as a function of photon energy, we obtain the real and imaginary parts of the dielectric function. Our measurements are unaffected by substrate-related screening and do not require any assumptions on the underling optical constants. This fast (τrise ∼ 135 ns), sensitive (noise-equivalent power = 90pW√Hz), and broadband (1.2–3.1 eV, extendable to UV–THz) method provides an attractive alternative to spectroscopic or ellipsometric characterization techniques
Strain control of hybridization between dark and localized excitons in a 2D semiconductor
Mechanical strain is a powerful tuning knob for excitons, Coulomb-bound
electron-hole complexes dominating optical properties of two-dimensional
semiconductors. While the strain response of bright free excitons is broadly
understood, the behavior of dark free excitons (long-lived excitations that
generally do not couple to light due to spin and momentum conservation) or
localized excitons related to defects remains mostly unexplored. Here, we
develop a technique capable of straining pristine suspended WSe2 kept at
cryogenic temperatures up to 3\% to study the strain behavior of these fragile
many-body states. We find that under the application of strain, dark and
localized excitons in monolayer WSe2 - a prototypical 2D semiconductor - are
brought into energetic resonance, forming a new hybrid state that inherits the
properties of the constituent species. The characteristics of the hybridized
state, including an order-of-magnitude enhanced light/matter coupling,
avoided-crossing energy shifts, and strain tunability of many-body
interactions, are all supported by first-principles calculations. The
hybridized exciton reported here may play a critical role in the operation of
single quantum emitters based on WSe2. Furthermore, the techniques we developed
may be used to fingerprint unidentified excitonic statesComment: 15 pages, 5 figure
Strain control of hybridization between dark and localized excitons in a 2D semiconductor
The interface between a ferro- or ferrimagnetic insulator and a normal metal can support spin currents polarized collinear with and perpendicular to the magnetization direction. The flow of angular momentum perpendicular to the magnetization direction (“transverse” spin current) takes place via spin torque and spin pumping. The flow of angular momentum collinear with the magnetization (“longitudinal” spin current) requires the excitation of magnons. In this article we extend the existing theory of longitudinal spin transport [Bender and Tserkovnyak, Phys. Rev. B 91, 140402(R) (2015)] in the zero-frequency weak-coupling limit in two directions: We calculate the longitudinal spin conductance nonperturbatively (but in the low-frequency limit) and at finite frequency (but in the limit of low interface transparency). For the paradigmatic spintronic material system YIG|Pt, we find that nonperturbative effects lead to a longitudinal spin conductance that is ca. 40% smaller than the perturbative limit, whereas finite-frequency corrections are relevant at low temperatures ≲100K only, when only few magnon modes are thermally occupied
Nanomechanical absorption spectroscopy of 2D materials with femtowatt sensitivity
Nanomechanical spectroscopy (NMS) is a recently developed approach to determine optical absorption spectra of nanoscale materials via mechanical measurements. It is based on measuring changes in the resonance frequency of a membrane resonator vs. the photon energy of incoming light. This method is a direct measurement of absorption, which has practical advantages compared to common optical spectroscopy approaches. In the case of two-dimensional (2D) materials, NMS overcomes limitations inherent to conventional optical methods, such as the complications associated with measurements at high magnetic fields and low temperatures. In this work, we develop a protocol for NMS of 2D materials that yields two orders of magnitude improved sensitivity compared to previous approaches, while being simpler to use. To this end, we use mechanical sample actuation, which simplifies the experiment and provides a reliable calibration for greater accuracy. Additionally, the use of low-stress silicon nitride membranes as our substrate reduces the noise-equivalent power to fW , comparable to commercial semiconductor photodetectors. We use our approach to spectroscopically characterize a 2D transition metal dichalcogenide (WS2), a layered magnetic semiconductor (CrPS4), and a plasmonic super-crystal consisting of gold nanoparticles
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