Integrated microwave photonic notch filter using a heterogeneously integrated Brillouin and active-silicon photonic circuit.

Microwave photonics (MWP) has unlocked a new paradigm for Radio Frequency (RF) signal processing by harnessing the inherent broadband and tunable nature of photonic components. Despite numerous efforts made to implement integrated MWP filters, a key RF processing functionality, it remains a long-standing challenge to achieve a fully integrated photonic circuit that can merge the megahertz-level spectral resolution required for RF applications with key electro-optic components. Here, we overcome this challenge by introducing a compact 5 mm × 5 mm chip-scale MWP filter with active E-O components, demonstrating 37 MHz spectral resolution. We achieved this device by heterogeneously integrating chalcogenide waveguides, which provide Brillouin gain, in a complementary metal-oxide-semiconductor (CMOS) foundry-manufactured silicon photonic chip containing integrated modulators and photodetectors. This work paves the way towards a new generation of compact, high-resolution RF photonic filters with wideband frequency tunability demanded by future applications, such as air and spaceborne RF communication payloads.

Diameter-dependent thermal transport in individual ZnO nanowires and its correlation with surface coating and defects.

A systematic study of the thermal transport properties of individual single-crystal zinc oxide (ZnO) nanowires (NWs) with diameters in the range of ∼50-210 nm is presented. The thermal conductivity of the NWs is found to be dramatically reduced by at least an order of magnitude compared to bulk values, due to enhanced phonon-boundary scattering with a reduction in sample size. While the conventional phonon transport model can qualitatively explain the temperature dependence, it fails to account for the diameter dependence. An empirical relationship for assessing diameter-dependent thermal properties is observed, which shows an approximately linear dependence of the thermal conductivity on the cross-sectional area of the NWs in the measured diameter range. Furthermore, it is found that an amorphous-carbon layer coating on the NWs does not perturb the thermal properties of the NW cores, whereas 30 keV Ga(+) ion irradiation at low dose (∼4 × 10(14) cm(-2)) leads to a remarkable reduction of the thermal conductivity of the ZnO NWs.

Thermal transport in suspended and supported few-layer graphene.

We report thermal conductivity (κ) measurements from 77 to 350 K on both suspended and supported few-layer graphene using a thermal-bridge configuration. The room temperature value of κ is comparable to that of bulk graphite for the largest flake, but reduces significantly for smaller flakes. The presence of a substrate lowers the value of κ, but the effect diminishes for the thermal transport in the top layers away from the substrate. For the suspended sample, the temperature dependence of κ follows a power law with an exponent of 1.4 ± 0.1, suggesting that the flexural phonon modes contribute significantly to the thermal transport of the suspended graphene. The measured values of κ are generally lower than those from theoretical studies. We attribute this deviation to the phonon-boundary scattering at the graphene-contact interfaces, which is shown to significantly reduce the apparent measured thermal conductance of graphene.