We experimentally demonstrate PT-symmetric optical lattices with periodical gain and loss profiles in a coherently prepared four-level N-type atomic system. By appropriately tuning the pertinent atomic parameters, the onset of PT-symmetry breaking is observed through measuring an abrupt phase-shift jump between adjacent gain and loss waveguides. The experimental realization of such a readily reconfigurable and effectively controllable PT-symmetric waveguide array structure sets a new stage for further exploiting and better understanding the peculiar physical properties of these non-Hermitian systems in atomic settings.

One of the most fundamental problems in optomechanical cooling is how small the thermal phonon number of a mechanical oscillator can be achieved under the radiation pressure of a proper cavity field. Different from previous theoretical predictions, which were based on an optomechanical system's time-independent steady states, we treat such cooling as a dynamical process of driving the mechanical oscillator from its initial thermal state, due to its thermal equilibrium with the environment, to a stabilized quantum state of higher purity. We find that the stabilized thermal phonon number left in the end actually depends on how fast the cooling process could be. The cooling speed is decided by an effective optomechanical coupling intensity, which constitutes an essential parameter for cooling, in addition to the sideband resolution parameter that has been considered in other theoretical studies. The limiting thermal phonon number that any cooling process cannot surpass exhibits a discontinuous jump across a certain value of the parameter.Preparing the approximate pure quantum states of a sizable mechanical oscillator is a feasible way toward macroscopic quantumness. Practically starting from its thermal equilibrium with the environment, such process is implemented by coupling the oscillator to a cavity field generated by a red-detuned external drive, to reduce the associated thermal phonon number to a low level, similar to cooling the oscillator to a lower temperature. An important feature we will illustrate is that the cooling result depends on how fast the optomechanical system (OMS) evolves to the finally stable quantum state.So far numerous experiments have realized the cooling to a few and even less than one mechanical quanta [1][2][3][4][5][6][7][8][9][10][11][12][13]. Following the earlier study of quantum fluctuations under radiation pressure [14,15], the theoretical description of such optomechanical cooling (see, e.g. [16][17][18][19][20][21][22][23]) was based on a linearization procedure as that described in [24]; that is to decompose the cavity field mode into the sum of the classical mean value α and its quantum fluctuation δâ. The linearized Hamiltonian gives the cooling action as a beamsplitter (BS) type coupling between the mechanical modeb and the fluctuation δâ with their coupling intensity g magnified by α, which was generally treated as a constant of steady-state value. In an actual cooling process, however, the cavity mean field â(t) = α(t) is built up from zero (when the mechanical oscillator is in thermal equilibrium with its environment) and takes time to evolve to stable value. Then the effective coupling strength g|α| used in the previous studies should be more appropriately taken as a variable, since α(t) keeps changing during a cooling process. Due to the impossibility of finding the time-dependent α(t) analytically, it is difficult to study the cooling as a dynamical process if adopting the above-mentioned linearization.In the present work we put forward a quantum dynamical theory for ...

Effective transition between the population-inverted optical eigenmodes of two coupled microcavities carrying mechanical oscillation realizes a phonon analogue of optical two-level laser. By providing an approach that linearizes the dynamical equations of weak nonlinear systems without relying on their steady states, we study such phonon laser action as a realistic dynamical process, which exhibits time-dependent stimulated phonon field amplification especially when one of the cavities is added with optical gain medium. The approach we present explicitly gives the conditions for the optimum phonon lasing, and thermal noise is found to be capable of facilitating the phonon laser action significantly. c J ω − c J

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High-sensitivity sensor of low relative humidity based on overlay on side-polished fibersABSTRACT: In this article, a miniaturized rectangular printed antenna that meets the ultrawideband (UWB) characteristics in terms of bandwidth and reflection coefficient is proposed. The designed antenna with dimension of 25316 mm 2 (L 3 W) was fabricated on FR-4 epoxy dielectric with relative permittivity of 4.4. SMA female connector is used for feeding with characteristic impedance of 50 X. The antenna exhibits good UWB characteristics and has the capability of operating between 3.92 to 11.32 GHz. The proposed antenna has omnidirectional radiation pattern on most of the operating band. The measured reflection coefficient of the proposed antenna is compared with the simulated one; good agreement is observed. Also, radiation pattern and gain of the antenna are presented. All simulations are carried out using the EM commercial simulators, HFSS, and CST.

Since the periodic parity‐time (PT)‐symmetric potential can possess unique properties compared to a single PT cell (with only two coupled components), various schemes have been proposed to realize PT symmetry in optical lattices. Here, a PT‐symmetric optical lattice is experimentally constructed with simultaneous gain and loss in an atomic medium. The gain and loss arrays are created in the four‐level N‐type configurations excited by spatially alternating strong and weak pump fields, respectively, which do not require discrete diffractions and can be realized more easily with more relaxed operating conditions. Also, the gain and loss coefficients can be modified independently. The dynamical behaviors of the system are investigated by measuring the phase difference between two adjacent gain and loss channels. The demonstrated PT‐symmetric lattice with easy accessibility and better tunability shows obvious advantages in exploiting more peculiar properties and great improvements in practical applications of periodic PT‐symmetric potentials.

Breaking Lorentz reciprocity was believed to be a prerequisite for nonreciprocal transmissions of light fields, so the possibility of nonreciprocity by linear optical systems was mostly ignored. We put forward a structure of three mutually coupled microcavities or optical fiber rings to realize optical nonreciprocity. Although its couplings with the fields from two different input ports are constantly equal, such system transmits them nonreciprocally either under the saturation of an optical gain in one of the cavities or with the asymmetric couplings of the circulating fields in different cavities. The structure made up of optical fiber rings can perform nonreciprocal transmissions as a time-independent linear system without breaking Lorentz reciprocity. Optical isolation for inputs simultaneously from two different ports and even approximate optical isolator operations are implementable with the structure.

Coherent optomechanical interaction known as stimulated Brillouin scattering (SBS) can enable ultrahigh resolution signal processing and narrow-linewidth lasers. SBS has recently been studied extensively in integrated waveguides; however, many implementations rely on complicated fabrication schemes. The absence of SBS in standard and mature fabrication platforms prevents its large-scale circuit integration. Notably, SBS in the emerging silicon nitride (Si 3 N 4 ) photonic integration platform is currently out of reach because of the lack of acoustic guidance. Here, we demonstrate advanced control of backward SBS in multilayer Si 3 N 4 waveguides. By optimizing the separation between two Si 3 N 4 layers, we unlock acoustic waveguiding in this platform, potentially leading up to 15× higher Brillouin gain coefficient than previously possible in Si 3 N 4 waveguides. We use the enhanced SBS gain to demonstrate a high-rejection microwave photonic notch filter. This demonstration opens a path to achieving Brillouin-based photonic circuits in a standard, low-loss Si 3 N 4 platform.