Optically driven quantum dots as source of coherent cavity phonons: a proposal for a phonon laser scheme

Near-field-driven thermal phonon lasing

The extreme electromagnetic near-field environment of nanoplasmonic resonators and metamaterials can give rise to unprecedented electromagnetic heating effects, enabling large and manipulable temperature gradients on the order of 101-102 K/nm. In this Letter, by interfacing traditional semiconductor quantum dots with industry-grade plasmonic transducer technology, we demonstrate that the near-field-induced thermal gradient can facilitate the requisite population inversion for coherent phonon amplification and lasing at the nanoscale. Our detailed analysis uncovers both the characteristics and parameter sensitivity of inversion and relaxation oscillations in the system, thereby unveiling hitherto unexplored opportunities for leveraging plasmonic near-field effects in the context of quantum thermodynamics and phononics.

Hard excitation mode of a system with optomechanical instability

Systems with strong photon-phonon interaction and optomechanical instability are perspective for the generation of coherent phonons and photons. We predict the existence of a hard mode of excitation in such systems when a jump-like increase in the photon intensity takes place at the generation threshold. We derive an analytical expression that defines conditions for such an increase. We demonstrate that the hard excitation mode in systems with optomechanical instability arises due to an additional phase condition for the existence of a nonzero solution. We propose to use systems with optomechanical instability operating in the hard excitation mode to create highly sensitive sensors.

Characteristics of the phonon laser in the active levitated optomechanical system

Phonon lasers, coherent oscillations of phonons, have gradually become one of the emerging frontiers in the last decades, and have promising applications in quantum sensing, information processing, and precise measurement. Recently, phonon lasers based on dissipative coupling have been realized in an active levitated optomechanical (LOM) system for the first time. Here, we further investigated the characteristics of the phonon laser in the system above regarding the oscillator amplitude and the phonon laser linewidth. We established both the experimental system and a physical model of the phonon laser. On the basis of simulations and experiments, the influences of pumping power, numerical aperture, the microsphere's diameter and refractive index on the performance of the phonon lasers are sufficiently discussed. Our work is of great significance for the high-quality phonon lasers generated by the appropriate parameters, which is the basis for the in-depth research and practical application.

Subthreshold phonon generation in an optomechanical system with an exceptional point

We consider a phonon laser based on an optomechanical system consisting of two optical modes interacting with each other via a phononic mode. An external wave exciting one of the optical modes plays the role of the pumping. We show that in this system at some amplitude of the external wave an exceptional point exists. When the external wave amplitude is less than one corresponding to the exceptional point, the splitting of the eigenfrequencies takes place. We demonstrate that in this case, the periodic modulation of the external wave amplitude can result in simultaneous generation of photons and phonons even below the threshold of optomechanical instability.

Theory of radial oscillations in metal nanoparticles driven by optically induced electron density gradients

We provide a microscopic approach to describe the onset of radial oscillation of a silver nanoparticle. Using the Heisenberg equation of motion framework, we find that the coupled ultrafast dynamics of coherently excited electron occupation and the coherent phonon amplitude initiate periodic size oscillations of the nanoparticle. Compared to the established interpretation of experiments, our results show a more direct coupling mechanism between the field intensity and coherent phonons. This interaction triggers a size oscillation via an optically induced electron density gradient occurring directly with the optical excitation. This source is more efficient than the incoherent heating process currently discussed in the literature and well-describes the early onset of the oscillations in recent experiments.

Operational regimes of lasers based on gain media with a large Raman scattering cross-section

We report on unusual regimes of operation of a laser with a gain medium with a large Raman scattering cross-section, which is often inherent in new types of gain media such as colloidal and epitaxial quantum dots and perovskite materials. These media are characterized by a strong electron–phonon coupling. Using the Fröhlich Hamiltonian to describe the electron–phonon coupling in such media, we analyze the operation of the system above the lasing threshold. We show that below a critical value of the Fröhlich constant, the laser can only operate in the conventional regime: namely, there are coherent cavity photons but no coherent phonons. Above the critical value, a new pump rate threshold appears. Above this threshold, either joint self-oscillations of coherent phonons in the gain medium and photons in a cavity or a chaotic regime are established. We also find a range of the values of the Fröhlich constant, the pump rate, and the resonator eigenfrequency, in which more than one dynamical regime of the system is stable. In this case the laser dynamics is determined by the initial values of the resonator field, the active medium polarization, the population inversion, and phonon amplitude.

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.

N-Phonon Bundle Emission via the Stokes Process

We demonstrate theoretically the bundle emission of n strongly correlated phonons in an acoustic cavity QED system. The mechanism relies on Stokes resonances that generate super-Rabi oscillations between states with a large difference in their number of excitations, which, combined with dissipation, transfer coherently pure n-phonon states outside of the cavity. This process works with close to perfect purity over a wide range of parameters and is tunable optically with well-resolved operation conditions. This broadens the realm of quantum phononics, with potential applications for on-chip quantum information processing, quantum metrology, and engineering of new types of quantum devices, such as optically heralded n-phonon guns.

Phonon laser in a cavity magnomechanical system

Using phonons to simulate an optical two-level laser action has been the focus of research. We theoretically study phonon laser in a cavity magnomechanical system, which consist of a microwave cavity, a sphere of magnetic material and a uniform external bias magnetic field. This system can realize the phonon-magnon coupling and the cavity photon-magnon coupling via magnetostrictive interaction and magnetic dipole interaction respectively, the magnons are driven directly by a strong microwave field simultaneously. Frist, the intensity of driving magnetic field which can reach the threshold condition of phonon laser is given. Then, we demonstrate that the adjustable external magnetic field can be used as a good control method to the phonon laser. Compared with phonon laser in optomechanical systems, our scheme brings a new degree of freedom of manipulation. Finally, with the experimentally feasible parameters, threshold power in our scheme is close to the case of optomechanical systems. Our study may inspire the field of magnetically controlled phonon lasers.

Enhancement of the Raman Effect by Infrared Pumping

We propose a method for increasing Raman scattering from an ensemble of molecules by up to 4 orders of magnitude. Our method requires an additional coherent source of IR radiation with the half-frequency of the Stokes shift. This radiation excites the molecule electronic subsystem that in turn, via Fröhlich coupling, parametrically excites nuclear oscillations at a resonant frequency. This motion is coherent and leads to a boost of the Raman signal in comparison to the spontaneous signal because its intensity is proportional to the squared number of molecules in the illuminated volume.

Phonon laser in the coupled vector cavity optomechanics

We presented a method to control the intensity of a phonon-laser mode (the vibrational excitations of a mechanical mode) by adjusting the polarization of the pump light based on the experimentally achievable parameters, which provides an additional degree of freedom to control the phonon laser action. Due to orthogonally polarized modes of cavity, the polarization behavior of light field which describes it’s vector nature is introduced to control phonon laser action in our scheme. Compared with the traditional phonon laser scheme, polarization-related phonon laser in the coupled vector cavity optomechanics can be effectively controlled without changing other parameters of the device. This result provides an useful approach for acquiring polarization-related phonon laser by on-chip optical device.

PsiQuaSP-A library for efficient computation of symmetric open quantum systems

In a recent publication we showed that permutation symmetry reduces the numerical complexity of Lindblad quantum master equations for identical multi-level systems from exponential to polynomial scaling. This is important for open system dynamics including realistic system bath interactions and dephasing in, for instance, the Dicke model, multi-Λ system setups etc. Here we present an object-oriented C++ library that allows to setup and solve arbitrary quantum optical Lindblad master equations, especially those that are permutationally symmetric in the multi-level systems. PsiQuaSP (Permutation symmetry for identical Quantum Systems Package) uses the PETSc package for sparse linear algebra methods and differential equations as basis. The aim of PsiQuaSP is to provide flexible, storage efficient and scalable code while being as user friendly as possible. It is easily applied to many quantum optical or quantum information systems with more than one multi-level system. We first review the basics of the permutation symmetry for multi-level systems in quantum master equations. The application of PsiQuaSP to quantum dynamical problems is illustrated with several typical, simple examples of open quantum optical systems.

Horizons of cybernetical physics

The subject and main areas of a new research field-cybernetical physics-are discussed. A brief history of cybernetical physics is outlined. The main areas of activity in cybernetical physics are briefly surveyed, such as control of oscillatory and chaotic behaviour, control of resonance and synchronization, control in thermodynamics, control of distributed systems and networks, quantum control.This article is part of the themed issue 'Horizons of cybernetical physics'.

Deterministic Single-Phonon Source Triggered by a Single Photon

We propose a scheme that enables the deterministic generation of single phonons at gigahertz frequencies triggered by single photons in the near infrared. This process is mediated by a quantum dot embedded on chip in an optomechanical circuit, which allows for the simultaneous control of the relevant photonic and phononic frequencies. We devise new optomechanical circuit elements that constitute the necessary building blocks for the proposed scheme and are readily implementable within the current state-of-the-art of nanofabrication. This will open new avenues for implementing quantum functionalities based on phonons as an on-chip quantum bus.

Gate-controlled electromechanical backaction induced by a quantum dot

Semiconductor-based quantum structures integrated into mechanical resonators have emerged as a unique platform for generating entanglement between macroscopic phononic and mesocopic electronic degrees of freedom. A key challenge to realizing this is the ability to create and control the coupling between two vastly dissimilar systems. Here, such coupling is demonstrated in a hybrid device composed of a gate-defined quantum dot integrated into a piezoelectricity-based mechanical resonator enabling milli-Kelvin phonon states to be detected via charge fluctuations in the quantum dot. Conversely, the single electron transport in the quantum dot can induce a backaction onto the mechanics where appropriate bias of the quantum dot can enable damping and even current-driven amplification of the mechanical motion. Such electron transport induced control of the mechanical resonator dynamics paves the way towards a new class of hybrid semiconductor devices including a current injected phonon laser and an on-demand single phonon emitter.

Ultracompact interference phonon nanocapacitor for storage and lasing of coherent terahertz lattice waves

We introduce a novel ultracompact nanocapacitor of coherent phonons formed by high-finesse interference mirrors based on atomic-scale semiconductor metamaterials. Our molecular dynamics simulations show that the nanocapacitor stores coherent monochromatic terahertz lattice waves, which can be used for phonon lasing-the emission of coherent phonons. Either one- or two-color phonon emission can be realized depending on the geometry of the nanodevice. The two-color regime of the interference phonon nanocapacitor originates from the different incidence-angle dependence of the transmission of longitudinal and transverse phonons at the respective interference antiresonances. Coherent phonon storage can be achieved by an adiabatic cooling the nanocapacitor initially thermalized at room temperature or by the pump-probe optical technique. The linewidth narrowing and the computed relative phonon participation number confirm strong phonon confinement in the ultracompact interference nanocavity by an extremely small amount of resonance defects. The emission of coherent terahertz acoustic beams from the nanocapacitor can be realized by applying a tunable reversible stress, which shifts the frequencies of the interference antiresonances.

Energy transport and coherence properties of acoustic phonons generated by optical excitation of a quantum dot

The energy transport of acoustic phonons generated by the optical excitation of a quantum dot as well as the coherence properties of these phonons are studied theoretically both for the case of a pulsed excitation and for a continuous wave (CW) excitation switched on instantaneously. For a pulsed excitation, depending on pulse area and pulse duration, a finite number of phonon wave packets is emitted, while for the case of a CW excitation a sequence of wave packets with decreasing amplitude is generated after the excitation has been switched on. We show that the energy flow associated with the generated phonons is partly related to coherent phonon oscillations and partly to incoherent phonon emission. The efficiency of the energy transfer to the phonons and the details of the energy flow depend strongly and in a non-monotonic way on the Rabi frequency exhibiting a resonance behavior. However, in the case of CW excitation it turns out that the total energy transferred to the phonons is directly linked in a monotonic way to the Rabi frequency.

Strain-mediated coupling in a quantum dot-mechanical oscillator hybrid system

Recent progress in nanotechnology has allowed the fabrication of new hybrid systems in which a single two-level system is coupled to a mechanical nanoresonator. In such systems the quantum nature of a macroscopic degree of freedom can be revealed and manipulated. This opens up appealing perspectives for quantum information technologies, and for the exploration of the quantum-classical boundary. Here we present the experimental realization of a monolithic solid-state hybrid system governed by material strain: a quantum dot is embedded within a nanowire that features discrete mechanical resonances corresponding to flexural vibration modes. Mechanical vibrations result in a time-varying strain field that modulates the quantum dot transition energy. This approach simultaneously offers a large light-extraction efficiency and a large exciton-phonon coupling strength g0. By means of optical and mechanical spectroscopy, we find that g0/2 π is nearly as large as the mechanical frequency, a criterion that defines the ultrastrong coupling regime.

Single photon delayed feedback: a way to stabilize intrinsic quantum cavity electrodynamics

We propose a scheme to control cavity quantum electrodynamics in the single photon limit by delayed feedback. In our approach a single emitter-cavity system, operating in the weak coupling limit, can be driven into the strong coupling-type regime by an external mirror: The external loop produces Rabi oscillations directly connected to the electron-photon coupling strength. As an expansion of typical cavity quantum electrodynamics, we treat the quantum correlation of external and internal light modes dynamically and demonstrate a possible way to implement a fully quantum mechanical time-delayed feedback. Our theoretical approach proposes a way to experimentally feedback control quantum correlations in the single photon limit.