arXiv subjects

A. I. Tartakovskii

Publications and source records attributed to A. I. Tartakovskii.

At least 19 recordsLinked to original sources

arXiv2021

Giant effective Zeeman splitting in a monolayer semiconductor realized by spin-selective strong light-matter coupling

Strong coupling between light and the fundamental excitations of a two-dimensional electron gas (2DEG) are of foundational importance both to pure physics and to the understanding and development of future photonic nanotechnologies. Here we study the relationship between spin polarization of a 2DEG in a monolayer semiconductor, MoSe$_2$, and light-matter interactions modified by a zero-dimensional optical microcavity. We find robust spin-susceptibility of the 2DEG to simultaneously enhance and suppress trion-polariton formation in opposite photon helicities. This leads to observation of a giant effective valley Zeeman splitting for trion-polaritons (g-factor >20), exceeding the purely trionic splitting by over five times. Going further, we observe robust effective optical non-linearity arising from the highly non-linear behaviour of the valley-specific strong light-matter coupling regime, and allowing all-optical tuning of the polaritonic Zeeman splitting from 4 to >10 meV. Our experiments lay the groundwork for engineering quantum-Hall-like phases with true unidirectionality in monolayer semiconductors, accompanied by giant effective photonic non-linearities rooted in many-body exciton-electron correlations.

cond-mat.mes-hall↗

arXiv2020

Interplay between spin proximity effect and charge-dependent exciton dynamics in MoSe$_2$ / CrBr$_3$ van der Waals heterostructures

Semiconducting ferromagnet-nonmagnet interfaces in van der Waals heterostructures present a unique opportunity to investigate magnetic proximity interactions dependent upon a multitude of phenomena including valley and layer pseudospins, moiré periodicity, or exceptionally strong Coulomb binding. Here, we report a charge-state dependency of the magnetic proximity effects between MoSe$_2$ and CrBr$_3$ in photoluminescence, whereby the valley polarization of the MoSe$_2$ trion state conforms closely to the local CrBr$_3$ magnetization, while the neutral exciton state remains insensitive to the ferromagnet. We attribute this to spin-dependent interlayer charge transfer occurring on timescales between the exciton and trion radiative lifetimes. Going further, we uncover by both the magneto-optical Kerr effect and photoluminescence a domain-like spatial topography of contrasting valley polarization, which we infer to be labyrinthine or otherwise highly intricate, with features smaller than 400 nm corresponding to our optical resolution. Our findings offer a unique insight into the interplay between short-lived valley excitons and spin-dependent interlayer tunnelling, while also highlighting MoSe$_2$ as a promising candidate to optically interface with exotic spin textures in van der Waals structures.

cond-mat.mes-hall↗

arXiv2020

Spin-valley dynamics in alloy-based transition metal dichalcogenide heterobilayers

Van der Waals heterobilayers based on 2D transition metal dichalcogenides have been recently shown to support robust and long-lived valley polarization for potential valleytronic applications. However, the role of the band structure and alignment of the constituent layers in the underlying dynamics remains largely unexplored. Here we study spin--valley relaxation dynamics in heterobilayers with different band structures engineered via the use of alloyed monolayer semiconductors. Through a combination of time-resolved Kerr rotation spectroscopic measurements and theoretical modelling for Mo$_{1-x}$W$_{x}$Se$_2$/WSe$_2$ samples with different chemical compositions and stacking angles, we uncover the roles of interlayer exciton recombination and charge carrier spin depolarization in the overall valley dynamics. Our results provide insights into the microscopic spin--valley polarization mechanisms in van der Waals heterostructures for the development of future 2D valleytronic devices.

cond-mat.mes-hall↗

arXiv2020

Electrically pumped WSe$_2$-based light-emitting van der Waals heterostructures embedded in monolithic dielectric microcavities

Vertical stacking of atomically thin layered materials opens new possibilities for the fabrication of heterostructures with favorable optoelectronic properties. The combination of graphene, hexagonal boron nitride and semiconducting transition metal dichalcogenides allows fabrication of electroluminescence (EL) devices, compatible with a wide range of substrates. Here, we demonstrate a full integration of an electroluminescent van der Waals heterostructure in a monolithic optical microcavity made of two high reflectivity dielectric distributed Bragg reflectors (DBRs). Owing to the presence of graphene and hexagonal boron nitride protecting the WSe$_2$ during the top mirror deposition, we fully preserve the optoelectronic behaviour of the device. Two bright cavity modes appear in the EL spectrum featuring Q-factors of 250 and 580 respectively: the first is attributed directly to the monolayer area, while the second is ascribed to the portion of emission guided outside the WSe$_2$ island. By embedding the EL device inside the microcavity structure, a significant modification of the directionality of the emitted light is achieved, with the peak intensity increasing by nearly two orders of magnitude at the angle of the maximum emission compared with the same EL device without the top DBR. Furthermore, the coupling of the WSe$_2$ EL to the cavity mode with a dispersion allows a tuning of the peak emission wavelength exceeding 35 nm (80 meV) by varying the angle at which the EL is observed from the microcavity. This work provides a route for the development of compact vertical-cavity surface-emitting devices based on van der Waals heterostructures.

physics.optics↗

arXiv2019

Enhanced light-matter interaction in an atomically thin semiconductor coupled with dielectric nano-antennas

Unique structural and optical properties of atomically thin two-dimensional semiconducting transition metal dichalcogenides enable in principle their efficient coupling to photonic cavities with optical modes volumes close to or below the diffraction limit. Recently, it has become possible to make all-dielectric nano-cavities with reduced mode volumes and negligible non-radiative losses. Here, we realise low-loss high-refractive-index dielectric gallium phosphide (GaP) nano-antennas with small mode volumes coupled to atomic mono- and bilayers of WSe$_2$. We observe a photoluminescence enhancement exceeding 10$^4$ compared with WSe$_2$ placed on planar GaP, and trace its origin to a combination of enhancement of the spontaneous emission rate, favourable modification of the photoluminescence directionality and enhanced optical excitation efficiency. A further effect of the coupling is observed in the photoluminescence polarisation dependence and in the Raman scattering signal enhancement exceeding 10$^3$. Our findings reveal dielectric nano-antennas as a promising platform for engineering light-matter coupling in two-dimensional semiconductors.

physics.app-ph↗

arXiv2019

Large Area Automated Characterisation of Chemical Vapour Deposition Grown Monolayer Transition Metal Dichalcogenides Through Photoluminescence Imaging

CVD growth is capable of producing multiple single crystal islands of atomically thin TMDs over large area substrates, with potential control of their morphology, lateral size, and epitaxial alignment to substrates with hexagonal symmetry. Subsequent merging of epitaxial domains can lead to single-crystal monolayer sheets - a step towards scalable production of high quality TMDs. For CVD growth to be effectively used for such production it is necessary to be able to rapidly assess the quality of material across entire large area substrates. To date characterisation has been limited to sub 0.1 mm2 areas, where the properties measured are not necessarily representative of an entire sample. Here, we apply photoluminescence (PL) imaging and computer vision techniques to create an automated analysis for large area samples of semiconducting TMDs, measuring the properties of island size, density of islands, relative PL intensity and homogeneity, and orientation of triangular domains. The analysis is applied to 20x magnification optical microscopy images that completely map samples of WSe2 on hBN, 5.0 mm x 5.0 mm in size, and MoSe2-WS2 on SiO2/Si, 11.2 mm x 5.8 mm in size. For the latter sample 100,245 objects were identified and their properties measured, with an orientation extracted from 27,779 objects that displayed a triangular morphology. In the substrates studied, two prevailing orientations of epitaxial growth were observed in WSe2 grown on hBN and four predominant orientations were observed in MoSe2, initially grown on c-plane sapphire. The proposed analysis will greatly reduce the time needed to study freshly synthesised material over large area substrates and provide feedback to optimise growth conditions, advancing techniques to produce high quality TMD monolayer sheets for commercial applications.

cond-mat.mtrl-sci↗

arXiv2019

Highly nonlinear trion-polaritons in a monolayer semiconductor

Highly nonlinear optical materials with strong effective photon-photon interactions (Kerr-like nonlinearity) are required in the development of novel quantum sources of light as well as for ultrafast and quantum optical signal processing circuitry. Here we report very large Kerr-like nonlinearities by employing strong optical transitions of charged excitons (trions) observed in semiconducting transition metal dichalcogenides (TMDCs). By hybridising trions in monolayer MoSe$_2$ at low electron densities with a microcavity mode, we realise trion-polaritons exhibiting significant energy shifts at very small photon fluxes due to phase space filling. Most notably, the strong trion-polariton nonlinearity is found to be 10 to 1000 larger than in other polariton systems, including neutral exciton-polaritons in TMDCs. Furthermore it exceeds by factors of $\sim 10^3-10^5$ the magnitude of Kerr nonlinearity in bare TMDCs, graphene and other widely used optical materials (e.g. Si, AlGaAs etc) in weak light-matter coupling regimes. The results are in good agreement with a theory which accounts for the composite nature of excitons and trions and deviation of their statistics from that of ideal bosons and fermions. This work opens a new highly nonlinear system for quantum optics applications enabling in principle scalability and control through nano-engineering of van der Waals heterostructures.

cond-mat.mes-hall↗

arXiv2019

Nonlinear polaritons in monolayer semiconductor coupled to optical bound states in the continuum

Optical bound states in the continuum (BICs) provide a way to engineer very narrow resonances in photonic crystals. The extended interaction time in such systems is particularly promising for enhancement of nonlinear optical processes and development of the next generation of active optical devices. However, the achievable interaction strength is limited by the purely photonic character of optical BICs. Here, we mix optical BIC in a photonic crystal slab with excitons in atomically thin semiconductor MoSe$_2$ to form nonlinear exciton-polaritons with a Rabi splitting of 27~meV, exhibiting large interaction-induced spectral blueshifts. The asymptotic BIC-like suppression of polariton radiation into far-field towards the BIC wavevector, in combination with effective reduction of excitonic disorder through motional narrowing, results in small polariton linewidths below 3~meV. Together with strongly wavevector-dependent Q-factor, this provides for enhancement and control of polariton--polariton interactions and resulting nonlinear optical effects, paving the way towards tunable BIC-based polaritonic devices for sensing, lasing, and nonlinear optics.

cond-mat.mes-hall↗

arXiv2018

The valley Zeeman effect in inter- and intra-valley trions in monolayer WSe$_2$

Monolayer transition metal dichalcogenides (TMDs) hold great promise for future information processing applications utilizing a combination of electron spin and valley pseudospin. This unique spin system has led to observation of the valley Zeeman effect in neutral and charged excitonic resonances under applied magnetic fields. However, reported values of the trion valley Zeeman splitting remain highly inconsistent across studies. Here, we utilize high quality hBN encapsulated monolayer WSe$_2$ to enable simultaneous measurement of both intervalley and intravalley trion photoluminescence. We find the valley Zeeman splitting of each trion state to be describable only by a combination of three distinct g-factors, one arising from the exciton-like valley Zeeman effect, the other two, trion specific, g-factors associated with recoil of the excess electron. This complex picture goes significantly beyond the valley Zeeman effect reported for neutral excitons, and eliminates the ambiguity surrounding the magneto-optical response of trions in tungsten based TMD monolayers.

cond-mat.mes-hall↗

arXiv2018

Valley coherent exciton-polaritons in a monolayer semiconductor

Two-dimensional transition metal dichalcogenide (TMD) semiconductors provide a unique possibility to access the electronic valley degree of freedom using polarized light, opening the way to valley information transfer between distant systems. Excitons with a well-defined valley index (or valley pseudospin) as well as superpositions of the exciton valley states can be created with light having circular and linear polarization, respectively. However, the generated excitons have short lifetimes (ps) and are also subject to the electron-hole exchange interaction leading to fast relaxation of the valley pseudospin and coherence. Here we show that control of these processes can be gained by embedding a monolayer of WSe$_2$ in an optical microcavity, where part-light-part-matter exciton-polaritons are formed in the strong light-matter coupling regime. We demonstrate the optical initialization of the valley coherent polariton populations, exhibiting luminescence with a linear polarization degree up to 3 times higher than that of the excitons. We further control the evolution of the polariton valley coherence using a Faraday magnetic field to rotate the valley pseudospin by an angle defined by the exciton-cavity-mode detuning, which exceeds the rotation angle in the bare exciton. This work provides unique insight into the decoherence mechanisms in TMDs and demonstrates the potential for engineering the valley pseudospin dynamics in monolayer semiconductors embedded in photonic structures.

cond-mat.mes-hall↗

arXiv2017

MOVPE growth, transmission electron microscopy and magneto-optical spectroscopy of individual InAsP/GaInP quantum dots

We report on growth and characterization of individual InAsP/GaInP quantum dots with variable nominal As molar fraction. Magneto-photoluminescence experiments reveal QD emission in a wide range from 1.3 to 1.8 eV confirming incorporation of As into quantum dots. Transmission electron microscopy reveals a core-cap structure of InAsP quantum dots with an InAs-rich core capped by an InP-rich layer. Inside the core, an As molar fraction up to x=0.12 is observed. The heavy hole g-factor is found to be strongly dependent on As molar fraction, while the electron g-factor is close to the InP values. This suggests type-II carrier confinement in the studied InAsP dots with holes (electrons) localized in the core (cap) region. Finally, dynamic nuclear polarization is observed which allows for further insight into structural properties using nuclear magnetic resonance.

cond-mat.mes-hall↗

arXiv2017

Resonantly excited exciton dynamics in two-dimensional MoSe$_2$ monolayers

We report on the exciton and trion density dynamics in a single layer of MoSe$_2$, resonantly excited and probed using three-pulse four-wave mixing (FWM), at temperatures from 300K to 77K . A multi-exponential third-order response function for amplitude and phase of the heterodyne-detected FWM signal including four decay processes is used to model the data. We provide a consistent interpretation within the intrinsic band structure, not requiring the inclusion of extrinsic effects. We find an exciton radiative lifetime in the sub-picosecond range consistent to what has been recently reported. After the dominating radiative decay, the remaining exciton density, which has been scattered from the initially excited bright radiative state into dark states of different nature by exciton-phonon scattering or disorder scattering, shows a slower dynamics, covering 10ps to 10ns timescales. This includes direct bright transitions with larger in-plane momentum, as well as indirect dark transitions to indirect dark states. We find that exciton-exciton annihilation is not relevant in the observed dynamics, in variance from previous finding under non-resonant excitation. The trion density at 77K reveals a decay of the order of 1ps, similar to what is observed for the exciton. After few tens of picoseconds, the trion dynamics resembles the one of the exciton, indicating that trion ionization occurs on this timescale.

cond-mat.mes-hall↗

arXiv2016

Valley addressable exciton-polaritons in atomically thin semiconductors

While conventional semiconductor technology relies on the manipulation of electrical charge for the implementation of computational logic, additional degrees of freedom such as spin and valley offer alternative avenues for the encoding of information. In transition metal dichalcogenide (TMD) monolayers, where spin-valley locking is present, strong retention of valley chirality has been reported for MoS$_2$, WSe$_2$ and WS$_2$ while MoSe$_2$ shows anomalously low valley polarisation retention. In this work, chiral selectivity of MoSe$_2$ cavity polaritons under helical excitation is reported with a polarisation degree that can be controlled by the exciton-cavity detuning. In contrast to the very low circular polarisation degrees seen in MoSe$_2$ exciton and trion resonances, we observe a significant enhancement of up to 7 times when in the polaritonic regime. Here, polaritons introduce a fast decay mechanism which inhibits full valley pseudospin relaxation and thus allows for increased retention of injected polarisation in the emitted light. A dynamical model applicable to cavity-polaritons in any TMD semiconductor, reproduces the detuning dependence through the incorporation of the cavity-modified exciton relaxation, allowing an estimate of the spin relaxation time in MoSe$_2$ which is an order of magnitude faster than those reported in other TMDs. The valley addressable exciton-polaritons reported here offer robust valley polarised states demonstrating the prospect of valleytronic devices based upon TMDs embedded in photonic structures, with significant potential for valley-dependent nonlinear polariton-polariton interactions.

cond-mat.mes-hall↗

arXiv2016

Exciton and trion dynamics in atomically thin MoSe2 and WSe2: effect of localization

We present a detailed investigation of the exciton and trion dynamics in naturally doped MoSe2 and WSe2 single atomic layers as a function of temperature in the range 10-300K under above band-gap laser excitation. By combining time-integrated and time-resolved photoluminescence (PL) spectroscopy we show the importance of exciton and trion localization in both materials at low temperatures. We also reveal the transition to delocalized exciton complexes at higher temperatures where the exciton and trion thermal energy exceeds the typical localization energy. This is accompanied with strong changes in PL including suppression of the trion PL and decrease of the trion PL life-time, as well as significant changes for neutral excitons in the temperature dependence of the PL intensity and appearance of a pronounced slow PL decay component. In MoSe2 and WSe2 studied here, the temperatures where such strong changes occur are observed at around 100 and 200 K, respectively, in agreement with their inhomogeneous PL linewidth of 8 and 20 meV at T~10K. The observed behavior is a result of a complex interplay between influences of the specific energy ordering of bright and dark excitons in MoSe2 and WSe2, sample doping, trion and exciton localization and various temperature-dependent non-radiative processes.

cond-mat.mes-hall↗

arXiv2016

Electrically pumped single-defect light emitters in WSe$_2$

Recent developments in fabrication of van der Waals heterostructures enable new type of devices assembled by stacking atomically thin layers of two-dimensional materials. Using this approach, we fabricate light-emitting devices based on a monolayer WSe$_2$, and also comprising boron nitride tunnelling barriers and graphene electrodes, and observe sharp luminescence spectra from individual defects in WSe$_2$ under both optical and electrical excitation. This paves the way towards the realization of electrically-pumped quantum emitters in atomically thin semiconductors. In addition we demonstrate tuning by more than 1 meV of the emission energy of the defect luminescence by applying a vertical electric field. This provides an estimate of the permanent electric dipole created by the corresponding electron-hole pair. The light-emitting devices investigated in our work can be assembled on a variety of substrates enabling a route to integration of electrically pumped single quantum emitters with existing technologies in nano-photonics and optoelectronics.

physics.optics↗

arXiv2015

WSe2 light-emitting tunneling transistors with enhanced brightness at room temperature

Monolayers of molybdenum and tungsten dichalcogenides are direct bandgap semiconductors, which makes them promising for opto-electronic applications. In particular, van der Waals heterostructures consisting of monolayers of MoS2 sandwiched between atomically thin hexagonal boron nitride (hBN) and graphene electrodes allows one to obtain light emitting quantum wells (LEQWs) with low-temperature external quantum efficiency (EQE) of 1%. However, the EQE of MoS2 and MoSe2-based LEQWs shows behavior common for many other materials: it decreases fast from cryogenic conditions to room temperature, undermining their practical applications. Here we compare MoSe2 and WSe2 LEQWs. We show that the EQE of WSe2 devices grows with temperature, with room temperature EQE reaching 5%, which is 250x more than the previous best performance of MoS2 and MoSe2 quantum wells in ambient conditions. We attribute such a different temperature dependences to the inverted sign of spin-orbit splitting of conduction band states in tungsten and molybdenum dichalcogenides, which makes the lowest-energy exciton in WSe2 dark.

cond-mat.mes-hall↗

arXiv2015

Magneto-phonon resonance in photoluminescence excitation spectra of magneto-excitons in GaAs/AlGaAs Superlattices

Strong increase in the intensity of the peaks of excited magneto-exciton (ME) states in the photoluminescence excitation (PLE) spectra recorded for the ground heavy-hole magneto-excitons (of the 1sHH type) has been found in a GaAs/AlGaAs superlattice in strong magnetic field B applied normal to the sample layers. While varying B the intensities of the PLE peaks have been measured as functions of energy separation $ΔE$ between excited ME peaks and the ground state of the system. The resonance profiles have been found to have maxima at $ΔE_{\rm max}$ close to the energy of the GaAs LO-phonon. However, the value of $ΔE_{\rm max}$ depends on quantum numbers of the excited ME state. The revealed very low quantum efficiency of the investigated sample allows us to ascribe the observed resonance to the enhancement of the non-radiative magneto-exciton relaxation rate arising due to LO-phonon emission. The presented theoretical model, being in a good agreement with experimental observations, provides a method to extract 1sHH magneto-exciton ``in-plane" dispersion from the dependence of $ΔE_{\rm max}$ on the excited ME state quantum numbers.

cond-mat.mes-hall↗

arXiv2015

Photoluminescence and Raman investigation of stability of InSe and GaSe thin films

Layered III-chalcogenide compounds belong to a variety of layered crystals that can be implemented in van der Waals heterostructures. Here we report an optical study of the stability of two of these compounds: indium selenide (InSe) and gallium selenide (GaSe). Micro-photoluminescence (PL) and Raman spectroscopy are used to determine how the properties of thin films of these materials change when they are exposed to air at room temperature. We find that in GaSe films, PL signal decreases on average below 50% over 24 (72) hours of exposure for films with thicknesses 10-25 (48-75) nm. In contrast, weak PL decrease of less than 20% is observed for InSe nm films after exposure of 100 hours. Similar trends are observed in Raman spectroscopy: within a week, the Raman signal decreases by a factor of 10 for a 24 nm thick GaSe, whereas no decrease was found for a 16 nm InSe film. We estimate that when exposed to air, the layers adjacent to the GaSe film surface degrade and become non-luminescent with a rate of 0.14$\pm$0.05 nm/hour. We show that the life-time of the GaSe films can be increased by up to two orders of magnitude (to several months) by encapsulation in dielectric materials such as SiO$_2$ or Si$_x$N$_y$.

cond-mat.mes-hall↗