Gallium nitride (GaN) as a wide bandgap material is widely used in solid-state lighting. Thanks to its high nonlinearity and high refractive index contrast, GaN-on-insulator (GaNOI) is also a promising platform for nonlinear optical applications. Despite its intriguing optical proprieties, nonlinear applications of GaN are rarely studied owing to the relatively high optical loss of GaN waveguides (typically ≈2 dB cm −1 ). In this paper, GaNOI microresonators with intrinsic quality factor over 2.5 million are reported, corresponding to an optical loss of 0.17 dB cm −1 . Parametric oscillation threshold power as low as 6.2 mW is demonstrated, and the experimentally extracted nonlinear index of GaN at telecom wavelengths is estimated to be n 2 = 1.4 × 10 −18 m 2 W −1 , which is several times larger than that of commonly used platform such as Si 3 N 4 , LiNbO 3 , and AlN. Single soliton generation in GaN is implemented by an auxiliary laser pumping scheme, so as to mitigate the high thermorefractive effect in GaN. The large intrinsic nonlinear refractive index, together with its broadband transparency window and high refractive index contrast, make GaNOI a promising platform for chip-scale nonlinear applications.
The biological actions of artemisinin (ART), an antimalarial drug derived from Artemisia annua, remain poorly understood and controversial. Besides potent antimalarial activity, some of artemisinin derivatives (together with artemisinin, hereafter referred to as ARTs), in particular dihydroartemisinin (DHA), are also associated with anticancer and other antiparasitic activities. In this study, we used baker’s yeast Saccharomyces cerevisiae as cellular and genetic model to investigate the molecular and cellular properties of ARTs. Two clearly separable pathways exist. While all ARTs exhibit potent anti-mitochondrial actions as shown before, DHA exerts an additional strong heme-dependent, likely mitochondria-independent inhibitory action. More importantly, heme antagonizes the mitochondria-dependent cellcidal action. Indeed, when heme synthesis was inhibited, the mitochondria-dependent cellcidal action of ARTs could be dramatically strengthened, and significant yeast growth inhibition at as low as 100 nM ART, an increase of about 25 folds in sensitivity, was observed. We conclude that ARTs are endowed with two major and distinct types of properties: a potent and specific mitochondria-dependent reaction and a more general and less specific heme-mediated reaction. The competitive nature of these two actions could be explained by their shared source of the consumable ARTs, so that inhibition of the heme-mediated degradation pathway would enable more ARTs to be available for the mitochondrial action. These properties of ARTs can be used to interpret the divergent antimalarial and anticancer actions of ARTs.
Abstract:Avalanche photodiode (APD) has been intensively investigated as a promising candidate to replace the bulky and fragile photomultiplier tubes (PMT) for weak light detection. However, intrinsic limits in semiconductors make the former still inferior to the latter on device performance up to now. In conventional APDs, a large portion of carrier energy drawn from the electric field is thermalized, and the multiplication efficiencies of electron and hole are low and close. In order to achieve high gain, the device has to work under breakdown bias, wherein carrier multiplication proceeds bi-directionally to form a positive feedback multiplication circle. In this case, APDs should work under Geiger mode as a compromise between sustainable detection and high gain. On the other hand, PMT can achieve stable high gain under constant bias (linear mode). Here, we demonstrate an APD works like a PMT, which means it can work under constant bias and holds high gain without breakdown simultaneously. The device is based on a GaN/AlN periodically-stacked-structure (PSS). For the PSS holds the intrinsic features that there are deep Γ valleys and larger band offset in conduction band, electron encountered much less scatterings during transport in PSS APD. Electron holds much higher efficiency than hole to draw energy from the electric field, and avalanche happens uni-directionally with high efficiency. Extremely high ionization coefficient (3.96×10 5 /cm) of electron and large ionization coefficient ratio (over 100) between electron and hole is calculated in the PSS APD by Monte-Carlo simulations and a recorded high gain (10 4 ) tested under constant bias without breakdown is obtained in a prototype device, wherein the stable gain can be determined by the periodicity of the GaN/AlN PSS and no quenching circuits are needed for sustainable detection. This work not only brings a new light into avalanche multiplication mechanism, but also paves a technological path to realize highly sensitive APD working under constant bias like PMT.Keywords: Avalanche photodiode; GaN; Ionization; Periodically stacked structure; Transport Introduction:
Semiconductor devices capable of generating a vortex beam with a specific orbital angular momentum (OAM) order are highly attractive for applications ranging from nanoparticle manipulation, imaging and microscopy to fiber and quantum communications. In this work, an electrically pumped integrated OAM emitter operating at telecom wavelengths is fabricated by monolithically integrating an optical vortex emitter with a distributed feedback laser on the same InGaAsP/InP epitaxial wafer. A single-step dry-etching process is adopted to complete the OAM emitter, equipped with specially designed top gratings. The vortex beam emitted by the integrated device is captured and its OAM mode purity characterized. The integrated OAM emitter eliminates the external laser required by silicon- or silicon-on-insulator-based OAM emitters, thus demonstrating great potential for applications in communication systems and the quantum domain.
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