Phonon Emission Research Articles

Fine Structure of the Photoluminescence Spectrum of Diamond under the Multiple Emission of an Optical Phonon during the Autolocalization of Photoexcited Electrons

The autolocalization of electrons photoexcited by laser pulses with a wavelength of 525 nm in the visible range and with a duration of 150 fs in an electron-hole plasma (density ∼1021 cm−3) in natural diamond at room temperature occurs unsteadily through step-by-step multiple (up to nine acts) emission of an optical phonon at subpicosecond times. This process is manifested for the probe photoluminescence signal in its fine periodic structure in the band gap beginning from the interband absorption edge. The intensity of multiphonon photoluminescence peaks reaches a maximum after four to six emission acts, where the energy of an electron-softened optical phonon is recovered and the broadening of peaks increases monotonically. Attributing the broadening of peaks to the kinetics of the emission of a successive phonon, one can compare their widths to the time dynamic scale of the electron-hole plasma and multiphonon emission during autolocalization. Then, the initial increase in the intensity of the peaks demonstrates the emission of phonons during autolocalization, whereas saturation and subsequent decrease are due to the subpicosecond Auger recombination of the plasma, which significantly suppresses photoluminescence because of the sharp decrease in the hole density (to 1019 cm−3), but not the process of autolocalization itself.

Magneto-optical transport properties of parabolic potential GaAs/Ga1−Al As wells under the influence of Al concentration and hydrostatic pressure

Abstract Magneto-optical transport properties of GaAs/Ga1−κAlκAs parabolic potential quantum-wells (PQWs) under the effects of Al-contents, hydrostatic pressure, and temperature is examined by studying their effects on the absorption power (AP) for magnetophonon resonance (MPR) and on the full width at half maximum (FWHM) of the optically-detected-MPR (ODMPR) peak due to modes confinement represented by slab-mode model, Huang-Zhu (HZ) model, guided-mode model. The numerical results indicate the Al-contents, pressure, as well as temperature which have great effect on the AP and FWHM in PQWs. Moreover, the results are also compared to bulk phonon modes for both two processes of phonons (emission and absorption). The results denote that phonon absorption process is always much more dominant compared to phonon emission process for four types of above phonons. In particular, the influences of pressure, Al-concentration in Ga1−κAlκAs barriers on the magneto-phonon resonance AP and FWHM in PQWs are more pronounced as confined modes are taken into account.

Berry-phase-like effect of thermo-phonon transport in optomechanics

We investigate thermo-phonon transport and its nontrivial Berry-phase-like effect in an optomechanical system with a squeezed vacuum injection. By taking the cumulant generation function approach the exact expressions of the thermo-phonon flux and optomechanical Berry phase are derived analytically. Further, the quantum master equation approach is invoked to verify the analytical results of the transport properties. It is shown that the steady-state thermo-phonon flux can be modulated by varying optically the nonequilibrium characteristics of the system via the squeezed vacuum. In particular, an adiabatic modulation of squeezing parameters induces an optomechanical Berry-phase-like effect and as a result provides an additional geometric phonon response across the macroscopic mechanical motion near the quantum regime, which can also be seen as a consequence of the asymmetric jumping probability between transition associated with phonon absorption and emission in a thermal bath. The present method and results are general and can be straightforwardly extended to any multimode oscillator systems and therefore pave the way to the thermal noise energy harvesting and rectification in coupled oscillator systems with inertial terms.

Photon-avalanche-like upconversion in NdAl3(BO3)4 nanoparticles excited at 1064 nm

We report efficient nonresonant ground-state excitation at 1064 nm of trivalent neodymium (Nd3+) ions in stoichiometric neodymium aluminum borate NdAl3(BO3)4 nanoparticles, which are crystalline and, besides the large content of Nd3+ ions, present excellent photoluminescence properties. Upconversion (UC) emission was observed, and the energy pathways were identified starting from multi-phonon assisted ground-state absorption (4I9/2 → 4F3/2) and excited-state absorption (4I11/2 → 4F3/2) with the population of the 4I11/2 level by thermal coupling with the ground state. The excited state 4I11/2 is also populated by relaxations of the Nd3+ ions at upper-lying levels. Cross-relaxation between two Nd3+ ions (4F3/2, 4I9/2) → (4I15/2, 4I15/2) with subsequent phonon emission leads to two ions at the 4I11/2 level, with every iteration triggering a photon-avalanche-like emission, which greatly enhances the efficiency of the UC emissions. Ladder thermal excitation 4F3/2 → (4F5/2, 2H9/2) → (4F7/2, 4S3/2) → 4F9/2 was achieved, and the ground-state relaxation from these levels provided emission at 880 nm, 810 nm, 750 nm, and 690 nm, respectively. Energy transfer UC (Auger) between Nd3+ ions at the 4F3/2 level allowed the population of (2G3/2, 4G7/2) from which relaxations to the 4I9/2, 4I11/2, and 4I13/2 states provided emissions around 536 nm, 600 nm, and 660 nm, respectively. Associated with the nonradiative relaxations, we observed the heating of the nanoparticles (22 °C–240 °C) with subsequent thermal enhancement of the frequency UC emissions due to population redistribution between the coupled energy levels of the Nd3+ ions. The present results have potential applications in super-resolution imaging and nanothermometry.

Numerical Study of Thermal Dissipation Processes in Silicon

Abstract Heat dissipation in nanoelectronics has become a major bottleneck to further scaling in next-generation integrated circuits. In order to address this problem and develop more energy-efficient nanoelectronic transistor, sensor, and storage devices, we must understand thermal processes at the atomic scale, which requires numerical simulation of the interaction between electrons and heat, carried by quantized lattice vibrations called phonons. Here we examine in detail the phonon emission and absorption spectra in silicon at several elevated values for the electron temperature. The effect of electric field on the electron distribution and equivalent electron temperature is obtained from full-band Monte Carlo simulation for bulk silicon. The electron distributions are used to numerically compute the phonon emission and absorption spectra and discover trends in their behavior at high electron temperatures. The concept of electron temperature is used to understand the relationship between field and heat emission, and it is found that longitudinal acoustic (LA) phonon emission increases at high electron temperatures. It is also found that emission of slower zone-edge phonons increases for all phonon branches at high electron temperatures. These conclusions at high electric fields can be used to enable heat-conscious design of future silicon devices.

Mitigating decoherence in hot electron interferometry

Due to their high energy, hot electrons in quantum Hall edge (QHE) states can be considered as single particles that have the potential to be used for quantum optics-like experiments. Unlike photons, however, electrons typically undergo scattering processes in transport, which results in a loss of coherence and limits their ability to show quantum-coherent behaviour. Here we study theoretically the decoherence mechanisms of hot electrons in a Mach–Zehnder interferometer (MZI), and highlight the role played by both acoustic and optical phonon emission. We discuss optimal choices of experimental parameters and show that high visibilities of ≳ 85% are achievable in hot-electron devices over relatively long distances of 10 μm. We also discuss energy filtration techniques to remove decoherent electrons and show that this can increase visibilities to over 95%. This represents an improvement over Fermi-level electron quantum optics, and suggests hot-electron charge pumps as a platform for realising quantum-coherent nanoelectronic devices.

Thermodynamic analysis of GaInN-based light-emitting diodes operated by quasi-resonant optical excitation

In this study, we aim to understand the thermodynamics inside the junctions of GaInN-based LEDs through optical operation of the device (the wavelength of the pumping laser is 405 nm for quasi-resonant optical excitation). First, to achieve this goal, the short-circuit current vs the open-circuit voltage curve and photoluminescence spectrum are carefully analyzed and compared with the current–voltage curve and electroluminescence spectrum. By this comparative study, we experimentally demonstrate the electrical–optical energy conversion efficiency (ECE) > 100% under optical excitation. Second, the intra-band phonon absorption and emission inside and/or outside the junction (PPeltier and PHC) are investigated through the power-loss analysis. In so doing, we ascertain that the carriers in the multiple quantum wells are energized by phonons, which introduces the ECE > 100%. Specifically, after the energization, the Boltzmann carrier distribution is no longer controlled by the ambient lattice temperature but by higher temperature, namely, that of the carriers themselves. Based on the analysis, we propose a thermally enhanced effective temperature for the Boltzmann distribution. Finally, we compare the junction efficiency by photoluminescence (PL), composed of the internal quantum efficiency by PL and the ECE by PL, of the sample at various temperatures to investigate the effect of PHC on the overall PL efficiency.

Polariton-driven phonon laser

Efficient generation of phonons is an important ingredient for a prospective electrically-driven phonon laser. Hybrid quantum systems combining cavity quantum electrodynamics and optomechanics constitute a novel platform with potential for operation at the extremely high frequency range (30–300 GHz). We report on laser-like phonon emission in a hybrid system that optomechanically couples polariton Bose-Einstein condensates (BECs) with phonons in a semiconductor microcavity. The studied system comprises GaAs/AlAs quantum wells coupled to cavity-confined optical and vibrational modes. The non-resonant continuous wave laser excitation of a polariton BEC in an individual trap of a trap array, induces coherent mechanical self-oscillation, leading to the formation of spectral sidebands displaced by harmonics of the fundamental 20 GHz mode vibration frequency. This phonon “lasing” enhances the phonon occupation five orders of magnitude above the thermal value when tunable neighbor traps are red-shifted with respect to the pumped trap BEC emission at even harmonics of the vibration mode. These experiments, supported by a theoretical model, constitute the first demonstration of coherent cavity optomechanical phenomena with exciton polaritons, paving the way for new hybrid designs for quantum technologies, phonon lasers, and phonon-photon bidirectional translators.

Influences of dispersion, dielectric constant and polarization on the double-parameter asymmetric Gaussian potential quantum dot qubit

Selecting the double-parameter asymmetric Gaussian (AG) potential to describe the confinement effects of electrons in quantum dot (QD), the effect of dielectric constant, dispersion and polarization on the properties of the QD qubit is studied by LLP–Pekar transformation variation method. By discussing the oscillation period of the qubit with the AG potential, the mechanism of influence of the material parameters such as the dielectric constant ratio and the electron–phonon coupling (EPC) constant on qubit survival time and information storage is clarified. Based on the Fermi gold rule and the even-order approximation, the qubit decoherence with AG potential is studied. It is revealed that increasing the dielectric constant ratio, the dispersion or the EPC constant of materials is all ways to solve the qubit decoherence due to spontaneous emission of phonons. The theoretical basis for phase rotation control of qubit spheres is given by improving dispersion coefficient or dielectric constant ratio of materials.

Phonon spectral density in a GaAs/AlGaAs double quantum dot

We study phonon emission in a GaAs/AlGaAs double quantum dot by monitoring the tunneling of a single electron between the two dots. We prepare the system such that a known amount of energy is emitted in the transition process. The energy is converted into lattice vibrations and the resulting tunneling rate depends strongly on the phonon scattering and its effective phonon spectral density. We are able to fit the measured transition rates and see imprints of interference of phonons with themselves causing oscillations in the transition rates.

Simultaneous improvement in emission intensity and spectral purity of red photon upconversion in Gd2O3:Yb3+/Er3+ phosphors under 1550 nm excitation

Upconversion luminescent nano-materials with both excitation and emission peaks in the “optical window” is of significant importance for application in deeper bio-imaging, due to the weak effects of light absorption, light scattering, and auto-fluorescence. This paper reports an Er3+ self-sensitized red up-converting nanophosphors with improved emission intensity and spectral purity. The influence of Yb3+ concentration on upconversion luminescence properties is investigated comprehensively. Compared with the sample without Yb3+ doping, the luminescence intensity in the sample with 10% Yb3+ is increased by 12.9 times. Moreover, the dominant wavelength at 1550 nm excitation is about 60 nm larger than that excited at 980 nm. It has been demonstrated that Yb3+ co-doping can not only improve the photon absorption at 1550 nm and the probability of red emission, but also suppress other light emissions and phonon emission through the selective population of red-emitting level.

PT-symmetric phonon laser under gain saturation effect

As an analog of optical laser, phonon laser has potential applications in various areas. We study a type of phonon laser implemented by two coupled micro-cavities, one of which carries optical gain medium. The phonon laser operation is under a blue detuned external drive leading to dynamical instability. The saturation of the optical gain is considered, and its induced nonlinearity results in more complicated behaviors in stimulated phonon emission. To deal with such complex dynamics, we apply a composite numerical approach, in addition to a previously used dynamical approach, to the time evolution of the system. The workable phonon laser operation is found to be achievable by choosing the proper system parameters. Moreover, low threshold for the phonon laser operation is possible with the suitable coupling between the cavities and an optimum damping rate in one cavity.

Superfluid Flow of Polaron Polaritons above Landau's Critical Velocity.

We develop a theory for the interaction of light with superfluid optical media, describing the motion of quantum impurities that are created and dragged through the liquid by propagating photons. It is well known that a mobile impurity suffers dissipation due to phonon emission as soon as it moves faster than the speed of sound in the superfluid-Landau's critical velocity. Surprisingly we find that in the present hybrid light-matter setting, polaritonic impurities can be protected against environmental decoherence and be allowed to propagate well above the Landau velocity without jeopardizing the superfluid response of the medium.

Investigation on the origin of friction and superlubricity

Friction consumes about a third of the world’s primary energy, wear causes about 80% of machine parts to fail, and the economic losses caused by friction and wear account for about 2%–7% of its gross domestic product (GDP) for different countries every year. For large manufacturing countries such as China, the loss caused by friction and wear is relatively high. If we only calculate by 5%, the loss caused by friction and wear in China in 2019 may reach 4.95 trillion RMB. Therefore, how to reduce friction and wear is a widely concerned problem, and it is also one of the key factors that affect the human society to go green, environmental protection, and effective use of resources and energy. In present paper, advancements in the research of the origin of friction and in superlubricity have been reviewed, in which most is from my group. In the part of the origin of friction, the energy dissipation mechanism of micro-friction has been introduced firstly. The works on such area by molecular dynamics simulation showed that the friction generation process will be accompanied by the energy dissipation of phonons, electrons and acoustic-electric coupling modes. The dissipative form of sliding kinetic energy is mainly that the sliding process produces lattice vibration, which makes the energy irreversibly dissipated in the form of phonons. However, the phonon dissipation measurements of the tribo-process have not been reported at present and all these results need to be proved. Our group developed a friction energy dissipation measurement system, which can realize the measurement of various emission of friction process, phonon and electron dissipation process, molecular orientation change, micro friction force and ultra-low friction coefficient. By such system, it is found that the defects will trap neutral excitons and cause the excitons to dissipate energy faster, and also can reduce the exciton diffusion length significantly. Superlubricity is a new technique that reduces the friction coefficient by several orders of magnitude, accompanied by orders of magnitude decrease in wear rate. It is divided into three sides, one is solid superlubricity, and another is liquid superlubricity, and then is the combine superlubricity of liquids and solids. In the part of solid superlubricity, the introduction of the concept of superlubricity and the latest research progress on the superlubricity properties of 2D materials, such as molybdenum disulfide, boron nitride, graphite, and graphene, are introduced. The lowest friction coefficient for 2D materials about 0.00002 has been gotten in my group. In the amorphous carbon system we achieve superlubricity state with friction coefficient about 0.0001 under low temperature (−110°C) and high pressure (>2 GPa). In the field of liquid superlubricity, it has developed very rapidly in the past 20 years. We conclude three mechanisms for liquid superlubricity: Fluid hydrodynamic effects, double layer force and hydration force. Sometimes one mechanism can lead to superlubricity, and sometimes it requires the joint action of two or three mechanisms. Supported by these three mechanisms, the liquid superlubricity bearing capacity has been improved from about 10 MPa 20 years ago to more than 300 MPa; the system composed of water agent has been extended to solutions (acid, alkali, salt), alcohol-based liquids, oil-based system etc.; and the materials of friction pair are extended from mica, ceramics to sapphire, metal, DLC (diamond-like carbon) film, etc. In addition, recently 2D materials as additives have been added to liquid to form a liquid-solid combined superlubricity state in my group and the bearing pressure has greatly increased to the GPa level, which is a big step forward for the application of liquid superlubricity. Therefore, the time has come for the engineering application of superlubricity, and the concept of “superlubricitive engineering” has been proposed.

Combined effects of temperature, hydrostatic pressure, and aluminum concentration on the magneto-optical properties of GaAs/Al Ga1−μAs quantum wells when phonons are confined

Abstract The combined influence of temperature, hydrostatic pressure, and applied aluminum concentration on the magneto-optical properties of GaAs/AlμGa1−μAs quantum wells (QWs) is presented by considering the combined influences of them on the magnetophonon resonance absorption power (AP) and the full width at half maximum (FWHM) of the optically detected magneto-phonon resonance (ODMPR) peaks for both phonon absorption and emission processes when phonons are confined to describe by Huang and Zhu. The numerical results denote that the AP and the FWHM have great influence (strong dependence) on the applied aluminum concentration, the temperature, and hydrostatic pressure. Moreover, the results also indicate the value and variation of the FWHM with applied aluminum concentration, hydrostatic pressure, as well as temperature of the ODMPR peak for phonon absorption processes is always larger and faster in comparison with that phonon emission process for both the confined and bulk phonons. Especially, when the phonons are confined, the more pronounced these properties are.

Role of lattice trapping for sliding friction

Using molecular dynamics we study the dependence of the friction force on the sliding speed when an elastic slab (block) is sliding on a rigid substrate with a surface height profile. The friction force is nearly velocity independent due to phonon emission at the closing and opening crack tips, where rapid atomic snap-in and -out events occur during sliding. The rapid events result from lattice trapping and are closely related to the velocity gap and hysteresis effects observed in model studies of crack propagation in solids. This indicates that the friction force is dominated by processes occurring at the edges of the contact area, which is confirmed by calculations showing that the friction force is independent of the normal force. The friction force increases drastically when the sliding velocity approaches the solid transverse sound velocity, as expected from the theory of cracks.

Parity-to-charge conversion for readout of topological Majorana qubits

We theoretically study a scheme to distinguish the two ground states of a one-dimensional topological superconductor, which could serve as a basis for the readout of Majorana qubits. The scheme is based on parity-to-charge conversion, i.e., the ground-state parity of the superconductor is converted to the charge occupation on a tunnel-coupled auxiliary quantum dot. We describe how certain error mechanisms degrade the quality of the parity-to-charge conversion process. We consider (i) leakage due to a strong readout tunnel pulse, (ii) incomplete charge Rabi oscillations due to slow charge noise, and (iii) charge relaxation due to phonon emission and absorption. To describe these effects, we use simple model Hamiltonians based on the ideal Kitaev chain, and draw conclusions to generic one-dimensional topological superconductors wherever possible. In general, the effects of the error mechanisms can be minimized by choosing a smooth shape and an optimal strength for the readout tunnel pulse. In a case study based on InAs heterostructure device parameters, we estimate that the parity-to-charge conversion error is mainly due to slow charge noise for weak tunnel pulses and leakage for strong tunnel pulses.

Transiently changing shape of the photon number distribution in a quantum-dot–cavity system driven by chirped laser pulses

We have simulated the time evolution of the photon number distribution in a semiconductor quantum dot-microcavity system driven by chirped laser pulses and compare with unchirped results. When phonon interactions with the dot are disregarded - thus corresponding to the limit of atomic cavity systems - chirped pulses generate photon number distributions that change their shape drastically in the course of time. Phonons have a strong and qualitative impact on the photon statistics. The asymmetry between phonon absorption and emission destroys the symmetry of the photon distributions obtained for positive and negative chirps. While for negative chirps transient distributions resembling thermal ones are observed, for positive chirps the photon number distribution still resembles its phonon-free counterpart but with overall smoother shapes. In sharp contrast, using unchirped pulses of the same pulse area and duration wave-packets are found that move up and down the Jaynes-Cummings ladder with a bell-shape that changes little in time. For shorter pulses and lower driving strength Rabi-like oscillations occur between low photon number states. For all considered excitation conditions transitions between sub- and super-Poissonian statistics are found at certain times. For resonant driving with low intensity the Mandel parameter oscillates and is mostly negative, which indicates a non-classical state in the cavity field. Finally, we show that it is possible that the Mandel parameter dynamically approaches zero and still the photon distribution exhibits two maxima and thus is far from being a Poissonian.

Pressure-induced first order phase transition in bulk GeSe

The phase transition mechanism in germanium selenide (GeSe) from the rhombohedral to face-centered cubic (FCC) phase is extensively analysed using density functional theory (DFT). The energy profile of GeSe reveals that the rhombohedral structure lies at the minimum energy, and the enthalpy differences predict its transition to the FCC phase at 7.3 GPa. Furthermore, we confirm the phase transition by calculating the phonon dispersion curves at various pressures. The rhombohedral phase of GeSe is dynamically stable at 0 GPa, whereas the FCC phase shows stability at 7.3 GPa. The rhombohedral to FCC phase transition is scrutinized by the degeneracy and splitting of the phonon modes along with the components of Born effective charge and dielectric constant. Investigation of the lattice transport properties of GeSe in both phases shows that FCC possesses high thermal conductivity (8.06 W/m K) as compared to the rhombohedral phase (4.88 W/m K) due to the three phonon emission process for scattering channels. The electronic band structure of the FCC GeSe under pressure shows its topological nature, and similar behavior was observed in the rhombohedral phase at 7.3 GPa. The rhombohedral phase of GeSe manifests better thermoelectric performance at 0 GPa with the thermoelectric figure of merit of 0.82 at 300 k much higher than its FCC phase at 7.3 GPa, suggesting its application in thermoelectric energy conversion.

Long Carrier Diffusion Length and Slow Hot Carrier Cooling in Thin Film Mixed Halide Perovskite

The long diffusion length is the key to achieve remarkable power conversion efficiency and delayed hot carriers (HCs) relaxation helps to overcome the theoretical Shockley–Queisser limit in perovskite photovoltaic. Here, we investigate the incident photon density dependent bandedge bleaching and HCs dynamics in thin film methyl-ammonium lead iodide-chloride (mixed halide) perovskite using transient absorption spectroscopy. The bandedge bleaching dynamics shows the radiative recombination rate constant of the order of ∼10−11 cm−3 s−1 results in the maximum carrier diffusion length of the order of ∼10 μm. The HCs dynamics indicates cooling occurs by longitudinal-optical (LO) phonon emission in early ∼800 fs via Frohlich electron–phonon interaction at low-carrier concentration (∼1.7 × 1018 cm−3) and an efficient phonon bottleneck due to the delayed LO-phonon decay by two order of magnitude about ∼11 ± 1.3 ps at high carrier density (∼1019 cm−3). Our finding shows the potential of thin film mixed halide perovskite in HCs photovoltaic and light emission application.