分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： The cavity electromechanical devices with radiation-pressure-interaction induced Kerr-like nonlinearity are promising candidates to generate microwave frequency combs. We construct a silicon-nitride-membrane-based superconducting cavity electromechanical device and study two mechanical modes mediated synergistic frequency combs. Around the threshold of intracavity field instability, we firstly show independent frequency combs with tooth spacing equalling to each mechanical mode frequency. At the overlap boundaries of these two individual mechanical mode mediated instability thresholds, we observe hybridization of frequency combs based on the cavity field mediated indirect coupling between these two mechanical modes. The spectrum lines turn to be unequally spaced but can be recognized into combinations of the coexisting frequency combs. Beyond the boundary, the comb reverts to the single-mode case, and which mechanical mode frequency will the tooth spacing depend on the mode competition. Our work demonstrates mechanical mode competition enabled switchability of frequency comb tooth spacing and can be extended to other devices with multiple nonlinearities.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： Nonreciprocal elements, such as isolators and circulators, play an important role in classical and quantum information processing. Recently, strong nonreciprocal effects have been experimentally demonstrated in cavity optomechanical systems. In these approaches, the bandwidth of the nonreciprocal photon transmission is limited by the mechanical resonator linewidth, which is arguably much smaller than the linewidths of the cavity modes in most electromechanical or optomechanical devices. In this work, we demonstrate broadband nonreciprocal photon transmission in the \emph{reversed-dissipation} regime, where the mechanical mode with a large decay rate can be adiabatically eliminated while mediating anti-$\mathcal{PT}$-symmetric dissipative coupling with two kinds of phase factors. Adjusting the relative phases allows the observation of \emph{periodic} Riemann-sheet structures with distributed exceptional points (Eps). At the Eps, destructive quantum interference breaks both the $\mathcal{T}$- and $\mathcal{P}$-inversion symmetry, resulting in unidirectional and chiral photon transmissions. In the reversed-dissipation regime, the nonreciprocal bandwidth is no longer limited by the mechanical mode linewidth but is improved to the linewidth of the cavity resonance. Furthermore, we find that the direction of the unidirectional and chiral energy transfer could be reversed by changing the \emph{parity} of the Eps. Extending non-Hermitian couplings to a three-cavity model, the broken anti-$\mathcal{PT}$-symmetry allows us to observe high-order Eps, at which a parity-dependent chiral circulator is demonstrated. The driving-phase controlled periodical Riemann sheets allow observation of the parity-dependent unidirectional and chiral energy transfer and thus provide a useful cell for building up nonreciprocal array and realizing topological, e.g., isolators, circulators, or amplifiers.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： Singularities that symbolize abrupt changes and exhibit extraordinary behavior are of broad interest. We experimentally study optomechanically induced singularities in a compound system consisting of a three-dimensional aluminum superconducting cavity and a metalized high-coherence silicon nitride membrane resonator. Mechanically-induced coherent perfect absorption and anti-lasing occur simultaneously under a critical optomechanical coupling strength. Meanwhile, the phase around the cavity resonance undergoes an abrupt $\pi$-phase transition, which further flips the phase slope in the frequency dependence. The observed infinite-discontinuity in the phase slope defines a singularity, at which the group velocity is dramatically changed. Around the singularity, an abrupt transition from an infinite group advance to delay is demonstrated by measuring a Gaussian-shaped waveform propagating. Our experiment may broaden the scope of realizing extremely long group delays by taking advantage of singularities.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Qichun Liu
Jie Lin
Xiaofeng Wang
Shenghong Gu
Jianrong Shi
Liyun Zhang
Gaobo Xi
Jun Mo
Yongzhi Cai
Liyang Chen
Zhihao Chen
Fangzhou Guo
Xiaojun Jiang
Gaici Li
Wenxiong Li
Han Lin
Weili Lin
Jialian Liu
Cheng Miao
Xiaoran Ma
摘要： Tsinghua University-Ma Huateng Telescopes for Survey (TMTS) aims to detect fast-evolving transients in the Universe, which has led to the discovery of thousands of short-period variables and eclipsing binaries since 2020. In this paper, we present the observed properties of 125 flare stars identified by the TMTS within the first two years, with an attempt to constrain their eruption physics. As expected, most of these flares were recorded in late-type red stars with $G_{\rm BP}-G_{\rm RP}$ > 2.0 mag, however, the flares associated with bluer stars tend to be on average more energetic and have broader profiles. The peak flux (F_peak) of the flare is found to depend strongly on the equivalent duration (ED) of the energy release, i.e., $F_{{\rm peak}} \propto {\rm ED}^{0.72\pm0.04}$, which is consistent with results derived from the Kepler and Evryscope samples. This relation is likely related to the magnetic loop emission, while -- for the more popular non-thermal electron heating model -- a specific time evolution may be required to generate this relation. We notice that flares produced by hotter stars have a flatter $F_{{\rm peak}} \propto {\rm ED}$ relation compared to that from cooler stars. This is related to the statistical discrepancy in light-curve shape of flare events with different colors. In spectra from LAMOST, we find that flare stars have apparently stronger H alpha emission than inactive stars, especially at the low temperature end, suggesting that chromospheric activity plays an important role in producing flares. On the other hand, the subclass having frequent flares are found to show H alpha emission of similar strength in their spectra to that recorded with only a single flare but similar effective temperature, implying that the chromospheric activity may not be the only trigger for eruptions.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Qichun Liu
Jie Lin
Xiaofeng Wang
Shenghong Gu
Jianrong Shi
Liyun Zhang
Gaobo Xi
Jun Mo
Yongzhi Cai
Liyang Chen
Zhihao Chen
Fangzhou Guo
Xiaojun Jiang
Gaici Li
Wenxiong Li
Han Lin
Weili Lin
Jialian Liu
Cheng Miao
Xiaoran Ma
摘要： Tsinghua University-Ma Huateng Telescopes for Survey (TMTS) aims to detect fast-evolving transients in the Universe, which has led to the discovery of thousands of short-period variables and eclipsing binaries since 2020. In this paper, we present the observed properties of 125 flare stars identified by the TMTS within the first two years, with an attempt to constrain their eruption physics. As expected, most of these flares were recorded in late-type red stars with $G_{\rm BP}-G_{\rm RP}$ > 2.0 mag, however, the flares associated with bluer stars tend to be on average more energetic and have broader profiles. The peak flux (F_peak) of the flare is found to depend strongly on the equivalent duration (ED) of the energy release, i.e., $F_{{\rm peak}} \propto {\rm ED}^{0.72\pm0.04}$, which is consistent with results derived from the Kepler and Evryscope samples. This relation is likely related to the magnetic loop emission, while -- for the more popular non-thermal electron heating model -- a specific time evolution may be required to generate this relation. We notice that flares produced by hotter stars have a flatter $F_{{\rm peak}} \propto {\rm ED}$ relation compared to that from cooler stars. This is related to the statistical discrepancy in light-curve shape of flare events with different colors. In spectra from LAMOST, we find that flare stars have apparently stronger H alpha emission than inactive stars, especially at the low temperature end, suggesting that chromospheric activity plays an important role in producing flares. On the other hand, the subclass having frequent flares are found to show H alpha emission of similar strength in their spectra to that recorded with only a single flare but similar effective temperature, implying that the chromospheric activity may not be the only trigger for eruptions.
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