Phonon Dephasing Research Articles

Slow Dephasing of Coherent Optical Phonons in Two-Dimensional Lead Organic Chalcogenides.

Hybrid organic-inorganic semiconductors with strong electron-phonon interactions provide a programmable platform for developing a variety of electronic, optoelectronic, and quantum materials by controlling these interactions. However, in current hybrid semiconductors such as halide perovskites, anharmonic vibrations with rapid dephasing hinder the ability to coherently manipulate phonons. Here, we report the observation of long-lived coherent phonons in lead organic chalcogenides (LOCs), a new family of hybrid two-dimensional semiconductors. These materials feature harmonic phonon dynamics despite distorted lattices, combining long phonon dephasing times with tunable semiconducting properties. A dephasing time -up to 75 ps at 10 K, with up to ∼500 cycles of phonon oscillation between scattering events, was observed, corresponding to a dimensionless harmonicity parameter that is more than an order of magnitude larger than that of halide perovskites. The phonon dephasing time is significantly influenced by anharmonicity and centrosymmetry, both of which can be tuned through the design of the organic ligands enabled by the direct bonding between the organic and inorganic motifs. This research opens new opportunities for the manipulation of electronic properties with coherent phonons in hybrid semiconductors.

Elucidating phonon dephasing mechanisms in layered perovskites with coherent Raman spectroscopies.

Organic-inorganic hybrid perovskite quantum wells exhibit electronic structures with properties intermediate between those of inorganic semiconductors and molecular crystals. In these systems, periodic layers of organic spacer molecules occupy the interstitial spaces between perovskite sheets, thereby confining electronic excitations to two dimensions. Here, we investigate spectroscopic line broadening mechanisms for phonons coupled to excitons in lead-iodide layered perovskites with phenyl ethyl ammonium (PEA) and azobenzene ethyl ammonium (AzoEA) spacer cations. Using a modified Elliot line shape analysis for the absorbance and photoluminescence spectra, polaron binding energies of 11.2 and 17.5meV are calculated for (PEA)2PbI4 and (AzoEA)2PbI4, respectively. To determine whether the polaron stabilization processes influence the dephasing mechanisms of coupled phonons, five-pulse coherent Raman spectroscopies are applied to the two systems under electronically resonant conditions. The prominence of inhomogeneous line broadening mechanisms detected in (AzoEA)2PbI4 suggests that thermal fluctuations involving the deformable organic phase broaden the distributions of phonon frequencies within the quantum wells. In addition, our data indicate that polaron stabilization primarily involves photoinduced reorganization of the organic phases for both systems, whereas the impulsively excited phonons represent less than 10% of the total polaron binding energy. The signal generation mechanisms associated with our fifth-order coherent Raman experiments are explored with a perturbative model in which cumulant expansions are used to account for time-coincident vibrational dephasing and polaron stabilization processes.

Engineering the Impact of Phonon Dephasing on the Coherence of a WSe_{2} Single-Photon Source via Cavity Quantum Electrodynamics.

Emitter dephasing is one of the key issues in the performance of solid-state single-photon sources. Among the various sources of dephasing, acoustic phonons play a central role in adding decoherence to the single-photon emission. Here, we demonstrate that it is possible to tune and engineer the coherence of photons emitted from a single WSe_{2} monolayer quantum dot via selectively coupling it to a spectral cavity resonance. We utilize an open cavity to demonstrate spectral enhancement, leveling, and suppression of the highly asymmetric phonon sideband, finding excellent agreement with a microscopic description of the exciton-phonon dephasing in a truly two-dimensional system. Moreover, the impact of cavity tuning on the dephasing is directly assessed via optical interferometry, which points out the capability to utilize light-matter coupling to steer and design dephasing and coherence of quantum emitters in atomically thin crystals.

Coherent phonon signal amplification technique based on a numerical differentiation method

Coherent optical phonon (COP) and coherent acoustic phonon (CAP) play vital roles in probing ultrafast dynamics across various materials and systems. Their applications range from metamaterial diagnosis to manipulating ferroelectric polarization and self-trapping of carriers in perovskites. However, the inherently small signals associated with COP and CAP pose challenges in extracting valuable information. In this paper, we address this challenge by presenting a numerical differential method for signal amplification and deduction of phonon dephasing time. In addition, application of this method has been demonstrated in a superlattice system, establishing the validity and effectiveness of this technique.

Approaching transform-limited photons from nanowire quantum dots using excitation above the band gap

We demonstrate that, even when employing above band gap excitation, photons emitted from semiconductor quantum dots can have linewidths that approach their transform-limited values. This is accomplished by using quantum dots embedded within bottom-up photonic nanowires, an approach which mitigates several potential mechanisms that can result in linewidth broadening: (i) Only a single quantum dot is present in each device, (ii) dot nucleation proceeds without the formation of a wetting layer, and (iii) the sidewalls of the photonic nanowire are comprised not of etched facets, but of epitaxially grown crystal planes. Using these structures we achieve linewidths of $2\ifmmode\times\else\texttimes\fi{}$ the transform limit for above band gap excitation. We also demonstrate a highly nonlinear dependence of the linewidth on both excitation power and temperature which can be described by an independent boson model that considers both deformation and piezoelectric exciton-phonon coupling. We find that for sufficiently low excitation powers and temperatures, the observed excess broadening is not dominated by phonon dephasing, a surprising result considering the high phonon occupation that occurs with above band gap excitation.

Two-fluid dynamics in driven YBa2Cu3O6.48

Coherent optical excitation of certain phonon modes in YBa$_2$Cu$_3$O$_{6+x}$ has been shown to induce superconducting-like interlayer coherence at temperatures higher than $T_c$. Recent work has associated these phenomena to a parametric excitation and amplification of Josephson plasma polaritons, which are overdamped above $T_c$ but are made coherent by the phonon drive. However, the dissipative response of uncondensed quasiparticles, which do not couple in the same way to the phonon drive, has not been addressed. Here, we investigate both the enhancement of the superfluid density, $\omega\sigma_2(\omega)$, and the dissipative response of quasiparticles, $\sigma_1(\omega)$, by systematically tuning the duration and energy of the mid-infrared pulse while keeping the peak field fixed. We find that the photo-induced superfluid density saturates to the zero-temperature equilibrium value for pulses made longer than the phonon dephasing time, whilst the dissipative component continues to grow with increasing pulse duration. We show that superfluid and dissipation remain uncoupled as long as the drive is on, and identify an optimal regime of pump pulse durations for which the superconducting response is maximum and dissipation is minimized.

Theory on Electron–Phonon Spin Dephasing in GaAs Multi‐Electron Double Quantum Dots

AbstractRecent studies reveal that a double‐quantum‐dot system hosting more than two electrons may be superior in certain aspects as compared to the traditional case in which only two electrons are confined (a singlet–triplet qubit). The electron–phonon dephasing occurring in a GaAs multi‐electron double‐quantum‐dot system is studied, in a biased case in which the singlet state is hybridized, as well as in an unbiased case in which the hybridization is absent. It is found that while the electron–phonon dephasing rate increases with the number of electrons confined in the unbiased case, this does not hold in the biased case. A merit figure is defined as a ratio between the exchange energy and the dephasing rate, and have shown that in experimentally relevant range of the exchange energy, the merit figure actually increases with the number of electrons in the biased case. The results show that the multi‐electron quantum‐dot system has another advantage in mitigating the effect of electron–phonon dephasing, which is previously under‐appreciated in the literature.

Resonance-enhanced excitation and relaxation dynamics of coherent phonons in Fe1.14Te.

Lattice dynamics plays a significant role in manipulating the unique physical properties of materials. In this work, femtosecond transient optical spectroscopy is used to investigate the generation mechanism and relaxation dynamics of coherent phonons in Fe1.14Te-a parent compound of chalcogenide superconductors. The reflectivity time series consist of the exponential decay component due to hot carriers and damped oscillations caused by the A1g phonon vibration. The vibrational frequency and dephasing time of the A1g phonons are obtained as a function of temperature. With increasing temperature, the phonon frequency decreases and can be well described with the anharmonicity model. Dephasing time is independent of temperature, indicating that the phonon dephasing is dominated by phonon-defect scattering. The impulsive stimulated Raman scattering mechanism is responsible for the coherent phonon generation. Owing to the resonance Raman effect, the maximum photosusceptibility of the A1g phonons occurs at 1.590 eV, corresponding to an electronic transition in Fe1.14Te.

Quantum Control of the Tin-Vacancy Spin Qubit in Diamond

Group-IV color centers in diamond are a promising light-matter interface for quantum networking devices. The negatively charged tin-vacancy center (SnV) is particularly interesting, as its large spin-orbit coupling offers strong protection against phonon dephasing and robust cyclicity of its optical transitions towards spin-photon entanglement schemes. Here, we demonstrate multi-axis coherent control of the SnV spin qubit via an all-optical stimulated Raman drive between the ground and excited states. We use coherent population trapping and optically driven electronic spin resonance to confirm coherent access to the qubit at 1.7 K, and obtain spin Rabi oscillations at a rate of $\Omega/2\pi$=3.6(1) MHz. All-optical Ramsey interferometry reveals a spin dephasing time of $T_2^*$=1.3(3)$\mu$s and two-pulse dynamical decoupling already extends the spin coherence time to $T_2$=0.33(14) ms. Combined with transform-limited photons and integration into photonic nanostructures, our results make the SnV a competitive spin-photon building block for quantum networks.

Phonon dephasing and spectral diffusion of quantum emitters in hexagonal boron nitride

Quantum emitters in hexagonal boron nitride (hBN) are emerging as bright and robust sources of single photons for applications in quantum optics. In this work we present detailed studies on the limiting factors to achieve Fourier transform limited spectral lines. Specifically, we study phonon dephasing and spectral diffusion of quantum emitters in hBN via resonant excitation spectroscopy at cryogenic temperatures. We show that the linewidths of hBN quantum emitters are phonon broadened, even at 5 K, with typical values of the order of ∼ 1 G H z . While spectral diffusion dominates at increasing pump powers, it can be minimized by working well below saturation excitation power. Our results are important for future utilization of quantum emitters in hBN for quantum interference experiments.

Elastic phonon dephasing effect on spin transport in 2D hexagonal lattice topological insulator

2D hexagonal lattice Topological Insulator (TI) shows topologically protected spin polarized edge transport while the bulk of the material remains insulating. The suppression of backscattering of electrons makes the edge transport of electrons robust against any perturbation. Elastic phonon dephasing represents a generalized scattering and dephasing mechanism. In this work, we have illustrated the effect of elastic phonon dephasing on the transport of electron in 2D hexagonal lattice TI using Non Equilibrium Green's Function (NEGF) formalism. The transmission was found to be unity (1) and the phase of the electron remained coherent even after strong phonon dephasing. The results of this paper indicate the suitability of TI in future dissipationless coherent nanoelectronic devices. • TIs suppress backscattering of electrons and shows robust transport against any perturbation. • Elastic phonon dephasing represents a generalized scattering and dephasing mechanism. • The Effect of elastic phonon dephasing on the transport of electron in 2D hexagonal lattice TI was found. • Non Equilibrium Green’s Function (NEGF) formalism has been used. • The transmission was found to be unity (1) and the phase of the electron remained coherent.

Longitudinal-optical phonon absorption and dephasing in three-level terahertz quantum cascade structures with different injector anticrossings

The effects of longitudinal-optical phonon scattering and dephasing in quantum cascade structures are studied. Three-level longitudinal-optical phonon depopulated terahertz structures are investigated using a density matrix Monte Carlo method. Ideally, these structures do not contain energy states above the upper lasing state, which in principle, can reduce parasitic leakage. The light output and current density as a function of lattice temperature are calculated and shown to be consistent with experiment for a recently reported structure that is confirmed to be a good approximation to a three-level structure. The pure dephasing time is self-consistently found to be relatively constant over the temperature range, which differs from the previous analysis of other structures. At higher lattice temperatures, particularly at elevated temperatures beyond the lasing point, the reduction in current density is rather due largely to rapid longitudinal-optical phonon absorption, which reduces the lifetime of the ground state. It is shown that the operating temperature limit may further be improved by adequately increasing the injector anticrossing.

The effect of ion irradiation on dephasing of coherent optical phonons in GaP

The dephasing of coherent longitudinal optical (LO) phonons in ion-irradiated GaP has been investigated with a femtosecond pump-probe technique based on electro-optic sampling. The dephasing time of the coherent LO phonon is found to be dramatically prolonged by the introduction of a small amount of defects by means of Ga-ion irradiation. The maximum dephasing time observed at room temperature is 9.1 ps at a Ga+ ion dose of 1013/cm2, which is significantly longer than the value of 8.3 ps for GaP before ion irradiation. The longer dephasing time is explained in terms of the suppression of electron-LO-phonon scattering by the presence of defect-induced deep levels.

On the Origin of Frictional Energy Dissipation

The origin of the friction between sliding bodies establishes an outstanding scientific problem. In this article, we demonstrate that the energy loss in each microscopic slip event between the bodies readily follows from the dephasing of phonons that are generated in the slip process. The dephasing mechanism directly links the typical timescales of the lattice vibrations with those of the experienced energy ‘dissipation’ and manifests itself as if the slip-induced motion were close to critically damped.Graphical abstract

Control of the Dephasing of the Coherent Phonons Excited by Femtosecond Laser Pulses in Opaque Crystals

The dephasing of coherent phonons in semimetals and semiconductors is studied using nondegenerate excitation and probing by femtosecond laser pulses. The dependences of the lifetimes of phonons of various symmetries on the exciting-pulse duration, the crystal temperature, and the coherent amplitude, which are determined by coherent control and the exciting-pulse duration, are investigated. These dependences demonstrate that the dephasing of coherent phonons in a linear mode is controlled by the state of environment (reservoir), which is taken to be the set of lattice and electronic states not involved in coherent motion.

Carrier-Induced Coherent Phonon Softening in Bismuth

With performing time-resolved pump-probe experiments with ultrashort pulses from a mode-locked Ti:sapphire laser, we report on the carrier-induced softening and dephasing of coherent phonon oscillations in the bismuth crystal. As the lattice displacement becomes severe at high pump power, a frequency shift can occur due to the lattice anharmonicity term. We clarified from the measurements done by varying the pulse duration that the phonon softening at high pump intensity in bismuth originates from the electron-interaction of photoexcited carriers not from the phonon anharmonicity. We also find that the coherent phonon dephasing is accelerated at high carrier densities.

A Self Energy Model of Dephasing in Molecular Junctions

Quantum decoherence plays an important role in the charge transport characteristics of molecular wires at room temperature. In this paper we propose a generalization of an electron–phonon dephasing model to non orthogonal LCAO basis. We implemented the model in combination with a density functional-based tight binding (DFTB) theory framework and utilized it to model charge transport characteristics of an anthraquinone (AQ) based molecular wire. We demonstrate a modulation of Quantum Interference (QI) effects compatible with experiments and confirm the robustness of QI signatures with respect to dephasing. An analysis of the spatial localization of the dephasing process reveals that both the QI and the dephasing process are localized in the AQ region, hence justifying the general robustness of the transmission temperature dependence in different AQ-based systems.

Dephasing in gapless carbon nanotubes and nanostrips and the suppression of interference in a quantum interferometer based on them

The phonon dephasing of an electron in a finite carbon nanotube and a nanostrip with a gapless spectrum has been studied. It has been shown that its main mechanism is the interaction with soft phonons whose frequencies correspond to the phonon time of transmission through the nanotube. The derived formulas have been used to analyze the suppression of interference in an interferometer consisting of two nanotubes or nanostrips connecting two-dimensional electron seas. Dephasing because of zero-point and thermal vibrations has been considered.

Coherent phonon dynamics in micro- and nanocrystalline diamond

We report on the time-resolved coherent anti-Stokes Raman spectroscopy of phonon dephasing in micro- and nanocrystalline diamond films. The dephasing times T(2) were found to be dependent on the morphology of diamond films (average size of crystals and content of nondiamond carbon phase) and changed from 0.7 to 1.72 ps. The dephasing times were found to be temperature independent in the range 10-295 K. In addition to diamond Raman active phonon mode at 1332 cm(-1), we investigated also the dynamics of a broad Raman peak at 1530 cm(-1) which is present in samples with higher content of nondiamond sp(2) hybridized carbon phase. This peak was found to be homogenously broadened with very fast dephasing (T(2)~50 fs).

Coherent transverse-optical phonon generation induced by lattice defects in nitrogen-ion-implanted GaAs

We describe our observation of coherent phonon oscillations of X-point zone-boundary transverse-optical (TO) mode, TO(X), in nitrogen-ion-implanted GaAs that has been annealed at high temperatures. With the TO(X) mode being forbidden from the Raman selection rule in pure zinc-blende GaAs, the lattice defects have provided additional momentum for phonon generation. Annealing-induced structural modifications were demonstrated through X-ray diffraction, transmission electron microscopy, and Raman scattering measurements. The polarization dependence of the TO(X) mode was compared with that of the longitudinal optical mode, and the temperature dependence of the TO(X) phonon dephasing was also investigated.