Phonon Emission Research Articles

Intrapulse Correlated Dynamics of Self-Phase Modulation and Spontaneous Raman Scattering in Synthetic Diamond Excited and Probed by Positively Chirped Ultrashort Laser Pulses

In synthetic diamond plates, the intrapulse-correlated dynamics of self-phase modulation and spontaneous nonresonant Raman scattering by center-zone optical phonons were for the first time directly investigated for tightly focused (focusing numerical aperture NA = 0.25) positively chirped visible-range high-intensity laser pulses with variable durations (0.3–9.5 ps) and energies transmitted through the sample. The observed self-phase modulation broadening and modulation of the transmitted light and Stokes Raman spectra for the (sub)picosecond pulse durations indicate the considerable Raman–Kerr contribution to the nonlinear polarization. The latter appears through plasma emission of the optical phonons, which emerges on the (sub)picosecond timescale and dominates at ≈1 ps. Later, this phonon contribution is eventually suppressed in the material due to picosecond-scale electron-lattice thermalization and the related thermally enhanced symmetrical decay of optical phonons into lower-frequency acoustic ones.

Influence of the internal degrees of freedom of coronene molecules on the nonlinear dynamics of a columnar chain.

The nonlinear dynamics of a one-dimensional molecular crystal in the form of a chain of planar coronene molecules is analyzed. Using molecular dynamics, it is shown that a chain of coronene molecules supports acoustic solitons, rotobreathers, and discrete breathers. An increase in the size of planar molecules in a chain leads to an increase in the number of internal degrees of freedom. This results in an increase in the rate of emission of phonons from spatially localized nonlinear excitations and a decrease in their lifetime. Presented results contribute to the understanding of the effect of the rotational and internal vibrational modes of molecules on the nonlinear dynamics of molecular crystals.

Detection and Modeling of Hole Capture by Single Point Defects under Variable Electric Fields

Understanding carrier trapping in solids has proven key to semiconductor technologies, but observations thus far have relied on ensembles of point defects, where the impact of neighboring traps or carrier screening is often important. Here, we investigate the capture of photogenerated holes by an individual negatively charged nitrogen-vacancy (NV) center in diamond at room temperature. Using an externally gated potential to minimize space-charge effects, we find the capture probability under electric fields of variable sign and amplitude shows an asymmetric-bell-shaped response with maximum at zero voltage. To interpret these observations, we run semiclassical Monte Carlo simulations modeling carrier trapping through a cascade process of phonon emission and obtain electric-field-dependent capture probabilities in good agreement with experiment. Because the mechanisms at play are insensitive to the characteristics of the trap, we anticipate the capture cross sections we observe─largely exceeding those derived from ensemble measurements─may also be present in materials platforms other than diamond.

Generation and manipulation of phonon lasering in a two-drive cavity magnomechanical system

A simple and feasible scheme for the generation and manipulation of phonon lasering is proposed and investigated based on a generic three-mode cavity magnomechanical system, in which a magnon mode couples simultaneously with a microwave cavity mode and a phonon mode. In sharp contrast to all previous phonon lasering schemes with only a single drive, the input pump field for the system in the proposed scheme is split into two microwave driving fields to drive the microwave cavity mode and the magnon mode, respectively. The impact of changing relative phase and relative amplitude ratio of the two microwave drives on mechanical gain, stimulated emitted phonon number, threshold power, and phonon emission line shape are theoretically and numerically investigated. The results indicate that the phonon laser action can be effectively controlled simply by adjusting the relative phase and relative amplitude ratio, so additional and tunable degrees of freedom are introduced to control the phonon laser. Considering the experimental feasibility of the generic cavity magnomechanical system and the two-drive approach, the present scheme provides a potentially practical route for the development of tunable phonon lasering devices with low-threshold, high-gain, and narrow-linewidth properties based on the platform of cavity magnomechanics.

Absorption and emission of longitudinal optical phonons by confined electrons in Pöschl–Teller quantum wells under the effects of the aluminum concentration, temperature, and hydrostatic pressure

In this study, we investigated the simultaneous effects of the alloy concentration, electron temperature, and hydrostatic pressure on the magneto-optical transport properties (MOTPs) of GaAs/AlηGa1−ηAs Pöschl–Teller quantum wells (QW) by using the operator projection method and profile technique. The optical absorption power and full width at half maximum were calculated and compared for the optically detected magneto-phonon resonance peaks with phonon absorption and emission. In addition, the main results were compared with those for the rectangular potential QW. The results showed that the alloy concentration, electron temperature, and hydrostatic pressure greatly influenced the MOTPs of the GaAs/AlηGa1−ηAs Pöschl–Teller QW in both cases with the absorption and emission of phonons. In addition, these effects were greater when the degree of asymmetry was stronger for the Pöschl-Teller QW. Our results also demonstrated that the combined effects of the alloy concentration, electron temperature, and hydrostatic pressure on the MOTPs of the GaAs/AlηGa1−ηAs Pöschl-Teller QW were always greater in the phonon absorption case than the phonon emission case. Furthermore, the combined effects of these three parameters on the MOTPs of the Pöschl-Teller potential QW were greater than those on the rectangular potential QW in both the phonon emission and absorption processes.

Low-energy solar neutrino detection utilizing advanced germanium detectors

We explore the possibility to use advanced germanium (Ge) detectors as a low-energy solar neutrino observatory by means of neutrino-nucleus elastic scattering. A Ge detector utilizing internal charge amplification for the charge carriers created by the ionization of impurities is a novel technology with experimental sensitivity for detecting low-energy solar neutrinos. Ge internal charge amplification (GeICA) detectors will amplify the charge carriers induced by neutrino interacting with Ge atoms through the emission of phonons. It is those phonons that will create charge carriers through the ionization of impurities to achieve an extremely low energy threshold of ∼0.01 eV. We demonstrate the phonon absorption, excitation, and ionization probability of impurities in a Ge detector with impurity levels of 3 × 1010 cm−3, 9 × 1010 cm−3, and 2 × 1011 cm−3. We present the sensitivity of such a Ge experiment for detecting solar neutrinos in the low-energy region. We show that, if GeICA technology becomes available, then a new opportunity arises to observe pp and 7Be solar neutrinos. Such a novel detector with only 1 kg of high-purity Ge will give ∼10 events per year for pp neutrinos and ∼5 events per year for 7Be neutrinos with a detection energy threshold of 0.01 eV.

Three-Dimensional Mechanistic Modeling of Time-Dependent Dielectric Breakdown in Polycrystalline Thin Films

Time-dependent dielectric breakdown (TDDB) is a crucial issue for the dielectric reliability. In this work, we present a full three-dimensional mechanistic model for calculation of the TDDB process in polycrystalline thin films. The model is based on the multiphonon trap-assisted tunneling theory and takes into account the intrinsic three-dimensional discreteness of traps at the dielectric grain boundaries. The leakage current density is calculated by solving coupled three-dimensional master equation and Poisson equation. The net phonon emission associated with each charge trapping and release event is treated as a local point heat source, which then enters the Fourier heat equation for three-dimensional temperature distribution calculation. The generated trap is determined by local temperature and electric field, which is subsequently included in the next round of calculation of electric and thermal properties. A positive feedback loop gradually leads to an increase of trap density, temperature, and leakage current density, and finally the dielectric breakdown. Our model can, to a good approximation, reproduce the experimental leakage current density-voltage characteristics and the Weibull distribution of time to breakdown at different dielectric thicknesses, stress voltages, and environmental temperatures. We find that in realistic devices, the three-dimensional trap-to-trap transport of electrons contributes a non-negligible part to the leakage current when the dielectric approaches breakdown. Our approach of three-dimensional mechanistic simulation is computationally efficient such that evolution of ${10}^{3}$ traps during the TDDB process can be easily performed on a standard desktop computer.

Optical switching a photon-avalanche-like mechanism in NdAl3(BO3)4 particles excited at 1064 nm by an auxiliary beam at 808 nm.

In recent years, an unconventional excitation of trivalent neodymium ions (N d 3+) at 1064nm, not resonant with ground-state transitions, has been investigated with the unprecedented demonstration of a photon-avalanche-like (PA-like) mechanism, in which the temperature increase plays a fundamental role. As a proof-of-concept, N d A l 3(B O 3)4 particles were used. A consequence of the PA-like mechanism is the absorption enhancement of excitation photons providing light emission at a broad range covering the visible and near-infrared spectra. In the first study, the temperature increase was due to intrinsic nonradiative relaxations from the N d 3+ and the PA-like mechanism ensued at a given excitation power threshold (P t h ). Subsequently, an external heating source was used to trigger the PA-like mechanism while keeping the excitation power below P t h at room temperature. Here, we demonstrate the switching on of the PA-like mechanism by an auxiliary beam at 808nm, which is in resonance with the N d 3+ ground-state transition 4 I 9/2→{4 F 5/2,2 H 9/2}. It comprises the first, to the best of our knowledge, demonstration of an optical switched PA, and the underlying physical mechanism is the additional heating of the particles due to the phonon emissions from the N d 3+ relaxation pathways when exciting at 808nm. The present results have potential applications in controlled heating and remote temperature sensing.

Influence of magnetic field on the dielectric characteristics in manganese sulfide substituted with thulium

The properties of a solid solution of thulium-substituted manganese sulfide prepared by melting the initial pure samples of manganese sulfide and thulium sulfide are studied. X-ray diffraction analysis was carried out. The face-centered cubic lattice is determined, and the decrease in the intensity of the peaks is found. Dielectric permittivity was measured in the frequency range 102106 Hz and temperatures 300-500 K in magnetic fields up to 12 kOe in TmxMn1xS solid solutions (0 x 0,15). From the frequency dependence of the permittivity, the relaxation time and the relaxation mechanism of the dipole moments are found. The leakage current is excluded, and the contribution of migration polarization due to charges in the region with chemical phase separation is estimated. Die real and imaginary components of the permittivity of TmxMn1xS samples are described in the Debye model. The maximum of the imaginary component of the permittivity shifts towards higher frequencies upon heating, and the relaxation time is described by the Arrhenius function. Dielectric losses are caused by electron-phonon interaction. The frequency of the crossover from Debye relaxation to relaxation conduction associated with the absorption and emission of phonons is found. Die crossover frequency increases as the sample is heated according to a power law. In a magnetic field, the permittivity decreases and the largest change of 2.5 % is achieved at T = 450 K. At other temperatures, the magnetocapacitance does not exceed 0.5 %. The shift of the maximum of the imaginary component of the permittivity to low frequencies in a magnetic field leads to a change in the sign of dielectric losses from positive to negative. A qualitative difference in () is established in the vicinity of the concentration of thulium ion flow through the FCC lattice, where the permittivity is not described in the Debye model and there is no magnetocapacitance. The mechanism for reducing dielectric losses in a magnetic field is determined.

Bidirectional phonon emission in two-dimensional heterostructures triggered by ultrafast charge transfer.

Photoinduced charge transfer in van der Waals heterostructures occurs on the 100 fs timescale despite weak interlayer coupling and momentum mismatch. However, little is understood about the microscopic mechanism behind this ultrafast process and the role of the lattice in mediating it. Here, using femtosecond electron diffraction, we directly visualize lattice dynamics in photoexcited heterostructures of WSe2/WS2 monolayers. Following the selective excitation of WSe2, we measure the concurrent heating of both WSe2 and WS2 on a picosecond timescale-an observation that is not explained by phonon transport across the interface. Using first-principles calculations, we identify a fast channel involving an electronic state hybridized across the heterostructure, enabling phonon-assisted interlayer transfer of photoexcited electrons. Phonons are emitted in both layers on the femtosecond timescale via this channel, consistent with the simultaneous lattice heating observed experimentally. Taken together, our work indicates strong electron-phonon coupling via layer-hybridized electronic states-a novel route to control energy transport across atomic junctions.

Non-Radiative Transitions of Holes on Mercury Vacancies in Narrow-Gap HgCdTe

Mercury vacancies, acting as double acceptors, are the dominant point defects in ternary HgCdTe alloys. Though HgCdTe is one of the leading materials in infrared optoelectronics, the energy spectra of the vacancies are still a matter of some debate. This work investigated the rates at which holes are captured to a singly ionized mercury vacancy via acoustic phonon emission in narrow-gap Hg1−xCdxTe with technologically relevant x~0.22. Combined with the calculated rates of intracenter transitions, the data allow one to predict the most pronounced optical transitions in the emission spectrum of a double-charged acceptor. The results are sustained by the photoluminescence spectroscopy in the terahertz domain, allowing one to identify the emission band that is related to neutral vacancies.

Unveiling a Counterintuitive Intermode Interplay in a Prototype Plasmonic Nanosystem.

We demonstrate a counterintuitive intermode interplay in the plasmonic system of gold nanorods, i.e., energy transfer (EnT) from the lower-energy longitudinal (L) mode to the higher-energy transverse (T) mode. The opening of this EnT(L→T) channel is enabled by an energy upconversion process with the L mode, in which the solvent environment plays a critical role. Switching from a strong thermal-conductivity solvent (i.e., water) to a much weaker one (i.e., chloroform) brings on prolongation of plasmonic hot-electron lifetime and enhancement of phonon emission, thereby increasing the probability of L-mode energy upconversion assisted by self-absorption of phonon emission. The pertinent justification and further manipulation of EnT(L→T) are provided by control experiments mainly from ultrafast spectroscopy. Besides, a subtle intermode dynamic screening effect in this unary plasmonic system is also addressed. This work refreshes our knowledge about the elusive intermode interplay in plasmonic systems and offers implementable strategies to harness hot electrons toward plasmon-mediated applications.

Local bistability under microwave heating for spatially mapping disordered superconductors

We theoretically study a strongly disordered superconducting layer heated by near-field microwave radiation from a nanometric metallic tip. The microwaves heat up the quasiparticles, which cool by phonon emission and conduction away from the heated area. Due to a bistability with two stable states of the electron temperature under the tip, the heating can be tuned to induce a submicrometer-sized normal region bounded by a sharp domain wall between high- and low-temperature states. We propose this as a local probe to access different physics from existing methods, for example, to map out inhomogeneous superfluid flow in the layer. The bistability-induced domain wall can significantly improve its spatial resolution.

Numerical simulation of homojunction p-i-n In0.4Ga0.6N solar cell with different absorber layer configurations

The effects of thickness, carrier densities, and absorber layer (p-In0.4Ga0.6N) configurations were evaluated. Numerical studies were performed to optimize the homojunction p-i-n In0.4Ga0.6N solar cell using SCAPS-1D. With the optimized thicknesses and carrier densities, the p-i-n In0.4Ga0.6N solar cell shows a maximum conversion efficiency of 18.74 %. By changing the fixed indium composition of the absorber layer into step- and linear-graded configurations, the conversion efficiency, open-circuit voltage, and short-circuit current density of the solar cell were further improved. The results showed that the solar cells with step- and linear-graded absorbers attained maximum conversion efficiency of 19.77 % and 19.84 %, respectively. These results implied that the absorber with the graded compositional design helps to reduce the barrier height of electron-hole transport for band-to-band and inter-band absorption. This eventually reduces the energy loss caused by phonon emission and heat thermalization.

Valley Relaxation of the Moiré Excitons in a WSe2/MoSe2 Heterobilayer.

The moiré superlattice consisting of lattice- or angular-mismatched van der Waals heterostructures drastically changes the physical properties of constituent atomically thin materials by confinement of the exciton by the moiré potential, which is promising for next-generation quantum optics. The moiré superlattice also affects the valley degrees of freedom of the monolayer transition-metal dichalcogenides (TMDs) and the valley-dependent optical selection rule, which results in the characteristic circular polarized light emission of the moiré exciton. However, the valley relaxation process of excitons in the moiré superlattice remains to be understood. Here, we studied valley relaxation of moiré excitons in a twisted WSe2/MoSe2 heterobilayer by circularly polarized photoluminescence and photoluminescence excitation (PLE) spectroscopy. The experimentally observed circularly polarized emission strongly depends on the excitation power density, which contrasts with the case of two-dimensional monolayer TMDs. The excitation power-dependent circularly polarized emission suggests the characteristic valley relaxation of the moiré exciton with a small density of states in zero-dimensional systems. In addition, the resonant PLE measurement reveals the intravalley relaxation process from the triplet to singlet state of the moiré exciton via Γ5 phonon emission. Our findings clarified the valley relaxation of the moiré excitons, which would lead to the application of the circularly polarized quantum light emitter in twisted semiconducting heterobilayers.

Ultrafast electronic heat dissipation through surface-to-bulk Coulomb coupling in quantum materials

The timescale of electronic cooling is an important parameter controlling the performance of devices based on quantum materials for optoelectronic, thermoelectric and thermal management applications. In most conventional materials, cooling proceeds via the emission of phonons, a relatively slow process that can bottleneck the carrier relaxation dynamics, thus degrading the device performance. Here we present the theory of near-field radiative heat transfer, that occurs when a two-dimensional electron system is coupled via the non-retarded Coulomb interaction to a three-dimensional bulk that can behave as a very efficient electronic heat sink. We apply our theory to study the cooling dynamics of surface states of three dimensional topological insulators, and of graphene in proximity to small-gap bulk materials. The ``Coulomb cooling'' we introduce is alternative to the conventional phonon-mediated cooling, can be very efficient and dominate the cooling dynamics under certain circumstances. We show that this cooling mechanism can lead to a sub-picosecond time scale, significantly faster than the cooling dynamics normally observed in Dirac materials.

Unidirectional Coherent Phonon Emission in an Optomechanic Nanobeam Containing Coupled Cavities

Nonreciprocal phonon emission is predicted theoretically from the coherent excitation of two coupled optomechanical cavities arranged along a phoxonic crystal nanobeam. The latter consists of a periodic array of holes and stubs and exhibits simultaneous photonic and phononic bandgaps. It is shown that nonreciprocal phonon emission arises from a combined effect of the spatial symmetry of the cavities and their underlying coupled phononic modes and the temporal phase shift between the excitation sources. This demonstration paves the way for the development of advanced integrated phonon networks and circuits, in which mechanical waves connect different elements in phononic and optomechanical structures.

Vortex shedding frequency of a moving obstacle in a Bose–Einstein condensate

We experimentally investigate the periodic vortex shedding dynamics in a highly oblate Bose–Einstein condensate using a moving penetrable Gaussian obstacle. The shedding frequency f v is measured as a function of the obstacle velocity v and characterized by a linear relationship of f v = a(v − v c) with v c being the critical velocity. The proportionality constant a is linearly decreased with a decrease in the obstacle strength, whereas v c approaches the speed of sound. When the obstacle size increases, both a and v c are decreased. We discuss a possible association of a with the Strouhal number in the context of universal shedding dynamics of a superfluid. The critical vortex shedding is further investigated for an oscillating obstacle and found to be consistent with the measured f v. When the obstacle’s maximum velocity exceeds v c but its oscillation amplitude is not large enough to create a vortex dipole, we observe that vortices are generated in the low-density boundary region of the trapped condensate, which is attributed to the phonon emission from the oscillating obstacle.

Impact of compositional disorder on electron migration in lutetium–yttrium oxyorthosilicate scintillator

A general description of the dynamics of nonequilibrium carriers in multicomponent activated scintillation materials with a compositional disorder of the crystalline matrix is developed and applied for studying the excitation transfer and timing properties of lutetium–yttrium oxyorthosilicate (LYSO). The energy structure, the density of states, and the effective potential of LSO and YSO crystals have been calculated by using the Quantum Espresso package. An analytical form of the potential fluctuations due to compositional disorder is suggested in the pseudopotential approximation. The spatial distribution of lutetium and yttrium cations in the LYSO crystal has been simulated by the Monte Carlo method using the thermodynamic approach for three qualitatively different cases of cation distribution: uniform, heterogeneous neighboring, and clustered. The impact of the compositional disorder on electron migration is found to be qualitatively different in four typical regions of electron energy. The density of localized states in LYSO calculated using the coherent potential approximation (CPA) and the quasiclassical approach is comparable to the density of secondary carriers expected in an ionization track and might have significant influence on the migration of thermalized carriers. The transport mean free path of nonlocalized electrons limited by elastic scattering on pseudopotential fluctuations is shown to be substantially longer than that due to longitudinal optical phonon emission in the low-energy region (calculated using CPA) and the high-energy region (calculated using the Born approximation). The scattering on pseudopotential fluctuations is important for intermediate-energy electrons due to a substantial influence of the core potential fluctuations on high-energy branches.

Nitrogen vacancy luminescence and their phonon sidebands characteristics in nitrogen-doped diamond

In this work, the nitrogen vacancy (NV) luminescence properties and their phonon sidebands in both high and low nitrogen diamonds after electron irradiation and subsequent annealing were thoroughly investigated. From the extracted results, it was demonstrated that the phonon sidebands were caused by the phonon emission behavior of the electrons located in the NV center during the induced transition process from the laser excitation. On top of that, the phonon energies of the NV0 (neutral) and NV— (negatively charged) centers were estimated at about 45 meV and 63 meV, respectively. As the measurement temperature increased, the excited electrons were more likely to return to the ground state by employing non-radiative transitions without generating phonon sidebands. As a result, the synchronous quenching of the NV luminescence and their phonon sidebands effect takes place. The complex lattice environment of the NV centers in the high nitrogen diamond leads also to a smaller thermal quenching activation energy, while the phonon process at a relatively high measurement temperature results in the manifestation of more obvious phonon sidebands of the NV centers.