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Register a new userA Low Power Threshold, Ultrathin Optical Limiter Based on a Nonlinear Zone Plate
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Ultrathin optical limiters are needed to protect light sensitive components in miniaturized optical systems. However, it has proven challenging to achieve a sufficiently low optical limiting threshold. In this work, we theoretically show that an ultrathin optical limiter with low threshold intensity can be realized using a nonlinear zone plate. The zone plate is embedded with nonlinear saturable absorbing materials that allow the device to focus low intensity light, while high intensity light is transmitted as a plane wave without a focal spot. Based on this proposed mechanism, we use the finite-difference time-domain method to computationally design a zone plate embedded with InAs quantum dots as the saturable absorbing material. The device has a thickness of just 0.5 $mu m$ and exhibits good optical limiting behavior with a threshold intensity as low as 0.45 kW/$cm^2$, which is several orders of magnitude lower than current ultrathin flat-optics-based optical limiters. This design can be optimized for different operating wavelengths and threshold intensities by using different saturable absorbing materials. Additionally, the diameter and focal length of the nonlinear zone plate can be easily adjusted to fit different systems and applications. Due to its flexible design, low power threshold, and ultrathin thickness, this optical limiting concept may be promising for application in miniaturized optical systems.
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Optical limiters are nonlinear devices that feature decreasing transmittance with increasing incident optical intensity, and thus can protect sensitive components from high-intensity illumination. The ideal optical limiter reflects rather than absorbs light in its active (limiting) state, minimizing risk of damage to the limiter itself. Previous efforts to realize reflective limiters were based on embedding nonlinear layers into relatively thick multilayer photonic structures, resulting in substantial fabrication complexity, reduced speed and, in some instances, limited working bandwidth. We overcome these tradeoffs by using the insulator-to-metal transition in vanadium dioxide (VO2) to achieve intensity-dependent modulation of resonant transmission through aperture antennas. Due to the dramatic change of optical properties across the insulator-to-metal transition, low-quality-factor resonators were sufficient to achieve high on-off ratios in device transmittance. As a result, our ultra-thin reflective limiter (thickness ~1/100 of the free-space wavelength) is broadband in terms of operating wavelength (> 2 um at 10 um) and angle of incidence (up to ~50$deg$ away from the normal).
Second-order nonlinear optical processes are used to convert light from one wavelength to another and to generate quantum entanglement. Creating chip-scale devices to more efficiently realize and control these interactions greatly increases the reach of photonics. Optical crystals and guided wave devices made from lithium niobate and potassium titanyl phosphate are typically used to realize second-order processes but face significant drawbacks in scalability, power, and tailorability when compared to emerging integrated photonic systems. Silicon or silicon nitride integrated photonic circuits enhance and control the third-order optical nonlinearity by confining light in dispersion-engineered waveguides and resonators. An analogous platform for second-order nonlinear optics remains an outstanding challenge in photonics. It would enable stronger interactions at lower power and reduce the number of competing nonlinear processes that emerge. Here we demonstrate efficient frequency doubling and parametric oscillation in a thin-film lithium niobate photonic circuit. Our device combines recent progress on periodically poled thin-film lithium niobate waveguidesand low-loss microresonators. Here we realize efficient >10% second-harmonic generation and parametric oscillation with microwatts of optical power using a periodically-poled thin-film lithium niobate microresonator. The operating regimes of this system are controlled using the relative detuning of the intracavity resonances. During nondegenerate oscillation, the emission wavelength is tuned over terahertz by varying the pump frequency by 100s of megahertz. We observe highly-enhanced effective third-order nonlinearities caused by cascaded second-order processes resulting in parametric oscillation. These resonant second-order nonlinear circuits will form a crucial part of the emerging nonlinear and quantum photonics platforms.
Strict requirements were imposed on the sizes of testing sample in the previously suggested scheme of hard X-ray Fourier-transform holography based on a two-block Fresnel zone plate interferometer with common optical axis. The failure of these requirements leads to appearance of noise in the reconstructed image. In this work, the mechanism of noise formation, as well as possibility of its suppression are considered.
Despite fundamentally challenging in integrated (nano)photonics, achieving chip-based light nonreciprocity becomes increasingly urgent in signal processing and optical communications. Because of material incompatibilities in conventional approaches based on Faraday effects, alternative solutions have resorted to nonlinear processes to obtain one-way transmission. However, revealed dynamic reciprocity in a recent theoretical analysis has pinned down the functionalities of these nonlinear isolators. To overcome this dynamic reciprocity, we here report the first demonstration of a nonlinear optical isolator on a silicon chip enforced by phase-matched parametric amplification. Using a high-Q microtoroid resonator, we realize highly nonreciprocal transport at the 1,550 nm wavelength when waves are simultaneously launched in both forward and backward directions. Our design, compatible with current CMOS technique, yields convincing isolation performance with sufficiently low insertion loss for a wide range of input power levels. Moreover, our work evidences the possibility of designing chip-based real nonlinear isolators for information processing and laser protection.
The topological lasers, which are immune to imperfections and disorders, have been recently demonstrated based on many kinds of robust edge states, being mostly at microscale. The realization of 2D on-chip topological nanolasers, having the small footprint, low threshold and high energy efficiency, is still to be explored. Here, we report on the first experimental demonstration of the topological nanolaser with high performance in 2D photonic crystal slab. Based on the generalized 2D Su-Schrieffer-Heeger model, a topological nanocavity is formed with the help of the Wannier-type 0D corner state. Laser behaviors with low threshold about 1 $mu W$ and high spontaneous emission coupling factor of 0.25 are observed with quantum dots as the active material. Such performance is much better than that of topological edge lasers and comparable to conventional photonic crystal nanolasers. Our experimental demonstration of the low-threshold topological nanolaser will be of great significance to the development of topological nanophotonic circuitry for manipulation of photons in classical and quantum regimes.
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