We demonstrate a PT -symmetry-breaking chaos in optomechanical system (OMS), which features an ultralow driving threshold. In principle, this chaos will emerge once a driving laser is applied to the cavity mode and lasts for a period of time. The driving strength is inversely proportional to the starting time of chaos. This originally comes from the dynamical enhancement of nonlinearity by field localization in PT -symmetry-breaking phase (PT BP). Moreover, this chaos is switchable by tuning the system parameters so that a PT -symmetry phase transition occurs. This work may fundamentally broaden the regimes of cavity optomechanics and nonlinear optics. It offers the prospect of exploring ultralow-power-laser triggered chaos and its potential applications in secret communication.PACS numbers: 07.10.Cm Cavity optomechanics is a rapidly developing research field exploring the radiation-pressure interaction between the electromagnetic and mechanical systems [1]. Thanks to this nonlinear interaction, the optomechanical system (OMS) provides an alternative platform for implementing many interesting quantum [2][3][4][5][6][7][8] and classical [9-17] nonlinearity phenomena. In particular, one can strong driving the cavity so that the OMS enters into a regime of self-induced oscillations [9][10][11][12], where the backaction-induced mechanical gain overcomes mechanical loss. Further increasing the strength of the driving laser, chaotic motion emerges both in the optical and mechanical modes requiring no external feedback or modulation [13][14][15][16]. It is useful for generating random numbers [18] and implementing secret information processing [19][20][21]. However, to apply chaos into the secret communication scheme requiring low-power optical interconnects, the chaos threshold should be reduced dramatically [22,23].The notion of parity-time (PT ) symmetry, initially proposed in quantum mechanics [24], attracts recently enormous attention in the field of optics [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39]. It is known [24] that PT symmetric Hamiltonians can exhibit a real eigenvalue spectrum in spite of the fact that they can be non-Hermitian. These systems have the interesting property to undergo an abrupt phase-transition where the system loses its PT-symmetry. At the exceptional point (EP) pairs of eigenvalues collide and become complex. Typically, the transition between PT symmetric phase (real spectrum) to spontaneously broken PT symmetry (complex spectrum) occurs as a parameter in the Hamiltonian (which somehow controls the degree of non-Hermiticity) is varied [25][26][27]. This phase transition has been demonstrated in synthetic waveguides and microcavities, and can induce unique optical phenomena including loss-induced transparency [28], power oscillations violating left-right symmetry [29], low-power optical diodes [35] and single-mode laser [36,37]. A natural question is whether it could influence the chaos dynamics (especially the chaos threshold) significantly. Moreover, the crossover b...

Descriptor Number: 6.01 Word Count: 3,295 At a Glance Commentary:Scientific Knowledge on the Subject: Short-term exposure to air pollution has been associated with increased risk of mortality from a variety of causes including respiratory diseases. As a common chronic respiratory disease, asthma affects 339 million people worldwide. Substantial evidence suggests that short-term exposure to air pollution causes asthma symptoms and leads to exacerbation and increased health-care use of asthma, but its effect on asthma mortality remains largely unknown. What This Study Adds to the Field:In this large case-crossover study, we found that short-term exposures to PM 2.5 , NO 2 and O 3 were positively associated with asthma mortality. For each IQR increase of exposures to PM 2.5 , NO 2 and O 3 , the odds of asthma mortality significantly increased by 7%, 11% and 9%, respectively. Our findings suggest that air pollution increase the risk of asthma mortality and highlight the needs to take effect measures to protect asthmatic individuals by reducing exposure to air pollution.This article has an online data supplement, which is accessible from this issue's table of content online at www.atsjournals.org. AbstractRationale: Short-term exposure to air pollution has been associated with asthma exacerbation and increased health-care use due to asthma, but its effect on asthma mortality remains largely unknown. Objectives:To quantitatively assess the association between short-term exposure to air pollution and asthma mortality. Methods:We investigated 4,454 individuals who lived in Hubei province, China and died from asthma between 2013 and 2018. A case-crossover design and conditional logistic regression models were applied for data analyses. Exposures to particulate matter ≤ 2.5 µm in aerodynamic diameter (PM 2.5 ), PM 10 , sulfur dioxide (SO 2 ), nitrogen dioxide (NO 2 ), carbon monoxide (CO) and ozone (O 3 ) were estimated by inverse distance weighted averages of all monitoring stations within 50 km from each case's home address. Measurements and Main Results:Each interquartile range (IQR) increase of PM 2.5 (lag 3; IQR: 47.1 µg/m 3 ), NO 2 (lag 03; IQR: 26.3 µg/m 3 ) and O 3 (lag 3; IQR: 52.9 µg/m 3 ) were positively associated with asthma mortality, with odds ratios (ORs) of 1.07 (95% confidence interval [CI]: 1.01-1.12), 1.11 (95% CI: 1.01-1.22) and 1.09 (95% CI: 1.01-1.18), respectively. There was no evidence of departure from linearity for these associations. Further adjustment for other pollutants did not change the associations materially. We did not observe significant associations between PM 10 , SO 2 , CO exposures and asthma mortality. Overall, the estimates remained consistent in various sensitivity analyses. Conclusions:Our results provide new evidence that short-term exposures to PM 2.5 , NO 2 and O 3 may increase asthma mortality risk. Further studies are needed to confirm our findings in other populations. Page 4 of 47 2 Word Count: 249

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STUDY QUESTION What is the relationship between abnormal BMI and semen quality? SUMMARY ANSWER Underweight was significantly associated with lower sperm concentration, total sperm number and total motile sperm count, while overweight was significantly associated with lower semen volume, total sperm number and total motile sperm count. WHAT IS KNOWN ALREADY Abnormal BMI has been associated with lower semen quality, but the results remain somewhat controversial. In addition, most previous studies have focused on the influence of obesity or overweight on semen quality, and evidence on the association between underweight and semen quality is rare. STUDY DESIGN, SIZE, DURATION This research was an observational study investigating 3966 sperm donors from a large sperm bank in Wuhan city, China. These donors passed the screening for sperm donation and underwent 29 949 semen examinations between 1 January 2013 and 9 April 2018. PARTICIPANTS/MATERIALS, SETTING, METHODS BMI was categorized into four groups: underweight (<18.5 kg/m2), normal weight (18.5–24.9 kg/m2), overweight (25–29.9 kg/m2) and obese (≥30 kg/m2). Semen volume, sperm concentration, total sperm number, total motility, progressive motility and total motile sperm count were determined by trained clinical technicians. Linear mixed models were used to conduct dose–response analyses between BMI and semen quality parameters. MAIN RESULTS AND THE ROLE OF CHANCE Underweight was significantly associated with a 3.0% (95% CI: 0.1%, 5.8%), 6.7% (1.9%, 11.3%) and 7.4% (2.2%, 12.4%) reduction in sperm concentration, total sperm number and total motile sperm count, respectively. Overweight was significantly associated with a 4.2% (1.6%, 6.8%), 3.9% (0.9%, 6.9%) and 3.6% (0.2%, 6.9%) reduction in semen volume, total sperm number and total motile sperm count, respectively. Non-linear models including continuous BMI as a natural cubic spline function yielded similar results. LIMITATIONS, REASONS FOR CAUTION Our study subjects were sperm donors who are typically young and healthy, and therefore not representative of the general male population. Caution should be paid in generalizing our results to other populations. Furthermore, we did not measure the donors’ weight repeatedly along with each semen donation; instead, we only measured it once during the screening, which may cause bias due to the variations of weight across time. WIDER IMPLICATIONS OF THE FINDINGS Our study provides evidence that underweight and overweight are associated with lower semen quality, and highlights the importance of maintaining a normal weight for men. STUDY FUNDING/COMPETING INTEREST(S) This work was supported by the Health and Family Planning Commission of Hubei Province (Grant number WJ2015MA027), the Hubei Provincial Committee of the Communist Youth League of China, and Center for Global and Regional Environmental Research at the University of Iowa. The authors declare that there are no conflicts of interest. TRIAL REGISTRATION NUMBER N/A.

Induced transparency is a common but remarkable effect in optics. It occurs when a strong driving field is used to render an otherwise opaque material transparent. The effect is known as electromagnetically induced transparency in atomic media and optomechanically induced transparency in systems that consist of coupled optical and mechanical resonators. In this work, we introduce the concept of photothermally induced transparency (PTIT). It happens when an optical resonator exhibits non-linear behavior due to optical heating of the resonator or its mirrors. Similar to the established mechanisms for induced transparency, PTIT can suppress the coupling between an optical resonator and a traveling optical field. We further show that the dispersion of the resonator can be modified to exhibit slow or fast light. Because of the relatively slow thermal response, we observe the bandwidth of the PTIT to be 2π × 15.9 Hz which theoretically suggests a group velocity of as low as 5 m/s.

Abstract. We investigate the order of the r-th, 1 ≤ r < +∞, central moment of the length of the longest common subsequence of two independent random words of size n whose letters are identically distributed and independently drawn from a finite alphabet. When all but one of the letters are drawn with small probabilities, which depend on the size of the alphabet, a lower bound is shown to be of order n r/2 . This result complements a generic upper bound also of order n r/2 .

We propose a potentially valuable scheme to measure the properties of an external time-harmonic-driving force with frequency ω via investigating its interaction with the combination of a pump field and a probe field in a generic optomechanical system. We show that the spectra of both the cavity field and output field in the configuration of optomechanically induced transparency are greatly modified by such an external force, leading to many interesting linear and non-linear effects, such as the asymmetric structure of absorption in the frequency domain and the antisymmetry breaking of dispersion near ω = ωm. Furthermore, we find that our scheme can be used to measure the initial phase of the external force. More importantly, this setup may eliminate the negative impact of thermal noise on the measurement of the weak external force in virtue of the process of interference between the probe field and the external force. Finally, we show that our configuration can be employed to improve the measurement resolution of the radiation force produced by a weak ultrasonic wave.

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