arXiv subjects

Wojciech Pacuski

Publications and source records attributed to Wojciech Pacuski.

At least 37 records · Page 2Linked to original sources

arXiv2023

Magnetic ion relaxation time distribution within a quantum well

Time-resolved optically detected magnetic resonance (ODMR) is a valuable technique to study the local deformation of the crystal lattice around magnetic ion as well as the ion spin relaxation time. Here we utilize selective Mn-doping to additionally enhance the inherent locality of the ODMR technique. We present the time-resolved ODMR studies of single {(Cd,Mg)Te/(Cd,Mn)Te} quantum wells (QWs) with manganese ions located at different positions along the growth axis -- in the center or on the sides of the quantum well. We observe that spin-lattice relaxation of Mn$^{2+}$ significantly depends on the ion-carrier wavefunction overlap at low-magnetic fields. Interestingly, the effect is clearly observed in spite of very low carrier density, which suggests the potential for control of the Mn$^{2+}$ ion relaxation rate by means of the electric field in future experiments.

cond-mat.mes-hall↗

arXiv2023

Molecular Beam Epitaxy growth of MoTe$_2$ on Hexagonal Boron Nitride

Hexagonal boron nitride has already been proven to serve as a decent substrate for high quality epitaxial growth of several 2D materials, such as graphene, MoSe$_{\tiny{\textrm{2}}}$, MoS$_{\tiny{\textrm{2}}}$ or WSe$_{\tiny{\textrm{2}}}$. Here, we present for the first time the molecular beam epitaxy growth of MoTe$_{\tiny{\textrm{2}}}$ on atomically smooth hexagonal boron nitride (hBN) substrate. Occurrence of MoTe$_{\tiny{\textrm{2}}}$ in various crystalline phases such as distorted octahedral 1T' phase with semimetal properties or hexagonal 2H phase with semiconducting properties opens a possibility of realisation of crystal-phase homostructures with tunable properties. Atomic force microscopy studies of MoTe$_{\tiny{\textrm{2}}}$ grown in a single monolayer regime enable us to determine surface morphology as a function of the growth conditions. The diffusion constant of MoTe$_{\tiny{\textrm{2}}}$ grown on hBN can be altered 5 times by annealing after the growth, reaching about 5 $\cdot$ 10$^{-6}$ cm$^{2}$/s. Raman spectroscopy results suggest a coexistence of both 2H and 1T' MoTe$_{\tiny{\textrm{2}}}$ phases in the studied samples.

cond-mat.mtrl-sci↗

arXiv2023

Natural exceptional points in the excitation spectrum of a light-matter system

We observe natural exceptional points in the excitation spectrum of an exciton-polariton system by optically tuning the light-matter interactions. The observed exceptional points do not require any spatial or polarization degrees of freedom and result solely from the transition from weak to strong light-matter coupling. We demonstrate that they do not coincide with the threshold for photon lasing, confirming previous theoretical predictions [Phys. Rev. Lett. 122, 185301 (2019), Optica 7, 1015 (2020) ]. Using a technique where a strong coherent laser pump induces up-converted excitations, we encircle the exceptional point in the parameter space of coupling strength and particle momentum. Our method of local optical control of light-matter coupling paves the way to investigation of fundamental phenomena including dissipative phase transitions and non-Hermitian topological states.

physics.optics↗

arXiv2023

Coherent imaging and dynamics of excitons in MoSe$_2$ monolayers epitaxially grown on hexagonal boron nitride

Using four-wave mixing microscopy, we measure the coherent response and ultrafast dynamics of excitons and trions in MoSe$_2$ monolayers grown by molecular beam epitaxy on thin films of hexagonal boron nitride. We assess inhomogeneous and homogeneous broadenings in the transition spectral lineshape. The impact of phonons on the homogeneous dephasing is inferred via the temperature dependence of the dephasing. Four-wave mixing mapping, combined with the atomic force microscopy, reveals spatial correlations between exciton oscillator strength, inhomogeneous broadening and the sample morphology. The quality of coherent optical response of the epitaxially grown transition metal dichalcogenides becomes now comparable with the samples produced by mechanical exfoliation, enabling coherent nonlinear spectroscopy of innovative materials, like magnetic layers or Janus semiconductors.

cond-mat.mtrl-sci↗

arXiv2022

Structural properties of TaAs Weyl semimetal thin films grown by molecular beam epitaxy on GaAs(001) substrates

Thin crystalline layers of TaAs Weyl semimetal (9 and 18 nm thick) are grown by molecular beam epitaxy on GaAs(001) substrates. The (001) planes of the tetragonal TaAs lattice are parallel to the GaAs(001) substrate, but the corresponding in-plane crystallographic directions of the substrate and the layer are rotated by 45°. In spite of a substantial lattice mismatch (about 19%) between GaAs(001) substrate and TaAs epilayer no misfit dislocations are observed at the GaAs(001)/TaAs(001) interface. Only stacking fault defects in TaAs are detected with transmission electron microscopy. Thorough X-ray diffraction measurements and analysis of the in-situ reflection high energy electron diffraction images indicates that TaAs layers are fully relaxed already at the initial deposition stage. Atomic force microscopy imaging reveals the columnar structure of the layers, with lateral (parallel to the layer surface) columns about 20 nm wide and 200 nm long. Both X-ray diffraction and transmission electron microscopy measurements indicate that the columns share the same orientation and crystalline structure.

cond-mat.mtrl-sci↗

arXiv2022

Coherent dynamics of a single Mn-doped quantum dot revealed by four-wave mixing spectroscopy

For future quantum technologies the combination of a long quantum state lifetime and an efficient interface with external optical excitation are required. In solids, the former is for example achieved by individual spins, while the latter is found in semiconducting artificial atoms combined with modern photonic structures. One possible combination of the two aspects is reached by doping a single quantum dot, providing a strong excitonic dipole, with a magnetic ion, that incorporates a characteristic spin texture. Here, we perform four-wave mixing spectroscopy to study the system's quantum coherence properties. We characterize the optical properties of the undoped CdTe quantum dot and find a strong photon echo formation which demonstrates a significant inhomogeneous spectral broadening. Incorporating the Mn$^{2+}$ ion introduces its spin-5/2 texture to the optical spectra via the exchange interaction, manifesting as six individual spectral lines in the coherent response. The random flips of the Mn-spin result in a special type of spectral wandering between the six transition energies, which is fundamentally different from the quasi-continuous spectral wandering that results in the Gaussian inhomogeneous broadening. Here, the discrete spin-ensemble manifests in additional dephasing and oscillation dynamics.

cond-mat.mes-hall↗

arXiv2022

Neural networks based on ultrafast time-delayed effects in exciton-polaritons

We demonstrate that time-delayed nonlinear effects in exciton-polaritons can be used to construct neural networks where information is coded in optical pulses arriving consecutively on the sample. The highly nonlinear effects are induced by time-dependent interactions with the excitonic reservoir. These nonlinearities allow to create a nonlinear XOR logic gate that can perform operations on the picosecond timescale. An optoelectronic neural network based on the constructed logic gate performs classification of spoken digits with a high accuracy rate.

physics.optics↗

arXiv2021

Heteroepitaxial growth of high optical quality, wafer-scale van der Waals heterostrucutres

Transition metal dichalcogenides (TMDs) are materials that can exhibit intriguing optical properties like a change of the bandgap from indirect to direct when being thinned down to a monolayer. Well-resolved narrow excitonic resonances can be observed for such monolayers, however only for materials of sufficient crystalline quality, so far mostly available in the form of micrometer-sized flakes. A further significant improvement of optical and electrical properties can be achieved by transferring the TMD on hexagonal boron nitride (hBN). To exploit the full potential of TMDs in future applications, epitaxial techniques have to be developed that not only allow to growlarge-scale, high-quality TMD monolayers, but allow to perform the growth directly on large-scale epitaxial hBN. In this work we address this problem and demonstrate that MoSe2 of high optical quality can be directly grown on epitaxial hBN on an entire two-inch wafer. We developed a combined growth theme for which hBN is first synthesized at high temperature by Metal Organic Vapor Phase Epitaxy (MOVPE) and as a second step MoSe2 is deposited on top by Molecular Beam Epitaxy (MBE) at much lower temperatures. We show that this structure exhibits excellent optical properties, manifested by narrow excitonic lines in the photoluminescence spectra. Moreover, the material is homogeneous on the area of the whole two-inch wafer, with only +/-0.14 meV deviation of excitonic energy. Our mixed growth technique may guide the way for future large-scale production of high quality TMD/hBN heterostructures.

cond-mat.mtrl-sci↗

arXiv2021

Molecular Beam Epitaxy of a 2D material nearly lattice matched to a 3D substrate: $NiTe_{2}$ on $GaAs$

The lattice mismatch between interesting 2D materials and commonly available 3D substrates is one of the obstacles in the epitaxial growth of monolithic 2D/3D heterostructures, but a number of 2D materials have not yet been considered for epitaxy. Here we present the first molecular beam epitaxy growth of NiTe$_{2}$ 2D transition metal dichalcogenide. Importantly, the growth is realized on a nearly lattice matched GaAs(111)B substrate. Structural properties of the grown layers are investigated by electron diffraction, X-ray diffraction, and scanning tunnelling microscopy. Surface coverage and atomic scale order is evidenced by images obtained with atomic force, scanning electron, and transmission electron microscopy. Basic transport properties were measured confirming that NiTe$_{2}$ layers are metallic, with the Hall concentration of $10^{20}$cm$^{-3}$ to $10^{23}$cm$^{-3}$, depending on the growth conditions.

cond-mat.mtrl-sci↗

arXiv2021

Polariton lasing and energy-degenerate parametric scattering in non-resonantly driven coupled planar microcavities

Multi-level exciton-polariton systems offer an attractive platform for studies of non-linear optical phenomena. However, studies of such consequential non-linear phenomena as polariton condensation and lasing in planar microcavities have so far been limited to two-level systems, where the condensation takes place in the lowest attainable state. Here, we report non-equilibrium Bose-Einstein condensation of exciton-polaritons and low threshold, dual-wavelength polariton lasing in vertically coupled, double planar microcavities. Moreover, we find that the presence of the non-resonantly driven condensate triggers interbranch exciton-polariton transfer in the form of energy-degenerate parametric scattering. Such an effect has so far been observed only under excitation that is strictly resonant in terms of the energy and incidence angle. We describe theoretically our time-integrated and time-resolved photoluminescence investigations by a set of rate equations involving an open-dissipative Gross-Pitaevskii equation. Our platform's inherent tunability is promising for construction of planar lattices, enabling three-dimensional polariton hopping and realization of photonic devices, such as two-qubit polariton-based logic gates.

cond-mat.mes-hall↗

arXiv2020

Long-Distance Coupling and Energy Transfer between Exciton States in Magnetically Controlled Microcavities

Coupling of quantum emitters in a semiconductor relies, generally, on short-range dipole-dipole or electronic exchange type interactions. Consistently, energy transfer between exciton states, that is, electron-hole pairs bound by Coulomb interaction, is limited to distances of the order of 10~nm. Here, we demonstrate polariton-mediated coupling and energy transfer between excitonic states over a distance exceeding 2~$μ$m. We accomplish this by coupling quantum well-confined excitons through the delocalized mode of two coupled optical microcavities. Use of magnetically doped quantum wells enables us to tune the confined exciton energy by the magnetic field and in this way to control the spatial direction of the transfer. Such controlled, long-distance interaction between coherently coupled quantum emitters opens possibilities of a scalable implementation of quantum networks and quantum simulators based on solid-state, multi-cavity systems.

cond-mat.mes-hall↗

arXiv2019

Readout of a dopant spin in the anisotropic quantum dot with a single magnetic ion

Owing to exchange interaction between the exciton and magnetic ion, a quantum dot embedding a single magnetic ion is a great platform for optical control of individual spin. In particular, a quantum dot provides strong and sharp optical transitions, which give experimental access to spin states of an individual magnetic ion. We show, however, that physics of quantum dot excitons also complicate spin readout and optical spin manipulation in such a system. This is due to electron-hole exchange interaction in anisotropic quantum dots, which affects the polarization of the emission lines. One of the consequences is that the intensity of spectral lines in a single spectrum are not simply proportional to the population of various spin states of magnetic ion. In order to provide a solution of the above problem, we present a method of extracting both the spin polarisation degree of a neutral exciton and magnetic dopant inside a semiconductor quantum dot in an external magnetic field. Our approach is experimentally verified on a system of CdSe/ZnSe quantum dot containing a single Fe$^{2+}$ ion. Both the resonant and non-resonant excitation regimes are explored resulting in a record high optical orientation efficiency of dopant spin in the former case. The proposed solutions can be easily expanded to any other system of quantum dots containing magnetic dopants.

cond-mat.mes-hall↗

arXiv2018

Spin polarized semimagnetic exciton-polariton condensate in magnetic field

Owing to their integer spin, exciton-polaritons in microcavities can be used for observation of non-equilibrium Bose-Einstein condensation in solid state. However, spin-related phenomena of such condensates are difficult to explore due to the relatively small Zeeman effect of standard semiconductor microcavity systems and the strong tendency to sustain an equal population of two spin components, which precludes the observation of condensates with a well defined spin projection along the axis of the system. The enhancement of the Zeeman splitting can be achieved by introducing magnetic ions to the quantum wells, and consequently forming semimagnetic polaritons. In this system, increasing magnetic field can induce polariton condensation at constant excitation power. Here we evidence the spin polarization of a semimagnetic polaritons condensate exhibiting a circularly polarized emission over 95% even in a moderate magnetic field of about 3 T. Furthermore, we show that unlike nonmagnetic polaritons, an increase on excitation power results in an increase of the semimagnetic polaritons condensate spin polarization. These properties open new possibilities for testing theoretically predicted phenomena of spin polarized condensate.

cond-mat.mes-hall↗

arXiv2017

Origin of luminescence quenching in structures containing CdSe/ZnSe quantum dots with a few Mn$^{2+}$ ions

We present a detailed spectroscopic study of the photoluminescence quenching in an epitaxial structures containing CdSe/ZnSe quantum dots doped with low concentration of Mn$^{2+}$ ions. Our time-resolved and time-integrated experiments reveal the origin of the quenching observed in macro-photoluminescence studies of ensembles of such dots. We show that incorporation of even a few ions to an individual dot does not quench its luminescence effectively, although some fingerprints of expected spin-dependent quenching are visible. At the same time, the presence of Mn$^{2+}$ ions in the sample significantly affects the luminescence intensity of the wetting layer, resulting in a quenching of the global luminescence from studied structure. On the other hand, the luminescence decay dynamics is found to be independent of the presence of Mn$^{2+}$ ions, which suggests that the observed quenching occurs for the excited excitonic states.

cond-mat.mes-hall↗

arXiv2016

Effect of electron-hole separation on optical properties of individual Cd(Se,Te) Quantum Dots

Cd(Se,Te) Quantum Dots (QD) in ZnSe barrier typically exhibit a very high spectral density, which precludes investigation of single dot photoluminescence. We design, grow and study individual Cd(Se,Te)/ZnSe QDs of low spectral density of emission lines achieved by implementation of a Mn-assisted epitaxial growth. We find an unusually large variation of exciton-biexciton energy difference (3 meV $\leq$ $Δ\mathrm{E_{X-XX}}$ $\leq$ 26 meV) and of exciton radiative recombination rate in the statistics of QDs. We observe a strong correlation between the exciton-biexciton energy difference, exciton recombination rate, splitting between dark and bright exciton, and additionally the exciton fine structure splitting $δ_1$ and Landé factor. Above results indicate that values of the $δ_1$ and of the Landé factor in the studied QDs are dictated primarily by the electron and hole respective spatial shift and wavefunctions overlap, which vary from dot to dot due to a different degree of localization of electrons and holes in, respectively, CdSe and CdTe rich QD regions.

cond-mat.mes-hall↗

arXiv2015

Strong coupling and polariton lasing in Te based microcavities embedding (Cd,Zn)Te quantum wells

We report on properties of an optical microcavity based on (Cd,Zn,Mg)Te layers and embedding (Cd,Zn)Te quantum wells. The key point of the structure design is the lattice matching of the whole structure to MgTe, which eliminates the internal strain and allows one to embed an arbitrary number of unstrained quantum wells in the microcavity. We evidence the strong light-matter coupling regime already for the structure containing a single quantum well. Embedding four unstrained quantum wells results in further enhancement of the exciton-photon coupling and the polariton lasing in the strong coupling regime.

cond-mat.mes-hall↗

arXiv2015

Single-color, in situ photolithography marking of individual CdTe/ZnTe Quantum Dots containing a single Mn2+ ion

A simple, single-color method for permanent marking of the position of individual self-assembled semiconductor Quantum Dots (QDs) at cryogenic temperatures is reported. The method combines in situ photolithography with standard micro-photoluminescence spectroscopy. Its utility is proven by a systematic magnetooptical study of a single CdTe/ZnTe QD containing a Mn2+ ion, where a magnetic field of up to 10 T in two orthogonal, Faraday and Voigt, configurations is applied to the same QD. The presented approach can be applied to a wide range of solid state nanoemitters.

cond-mat.mes-hall↗

arXiv2014

Influence of ZnTe based distributed Bragg reflectors on the yellow range luminescence of self assembled CdTe QDs

The influence of a distributed Bragg reflector composed of ZnTe, MgTe, and MgSe superlattices on photoluminescence of self assembled CdTe quantum dots (QD) emitting in the yellow spectral range is investigated. In the case of QDs grown on a distributed Bragg reflector the photoluminescence intensity is enhanced by more than one order of magnitude, whereas the single QD lines are broadened as compared to the case of QDs grown on a ZnTe buffer. Structural and chemical analysis reveal an unintentional formation of a thin ZnSe layer induced by the growth interruption needed for the deposition of the QDs sheet. Sharp emission lines from individual quantum dots are recovered in the case of a thicker ZnTe layer grown prior to the QDs. This indicates that growth interruptions might be responsible for the QD emission line broadening.

cond-mat.mes-hall↗