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Register a new userUltrathin broadband reflective optical limiter
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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).
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Analog computing has emerged as a promising candidate for real-time and parallel continuous data processing. This paper presents a reciprocal way for realizing asymmetric optical transfer functions (OTFs) in the reflection side of the on-axis processing channels. It is rigorously demonstrated that the presence of Cross-polarization Exciting Normal Polarizabilities (CPENP) of a reciprocal metasurface circumvents the famous challenge of Greens function approach in implementation of on-axis reflective optical signal processing while providing dual computing channels under orthogonal polarizations. Following a comprehensive theoretical discussion and as a proof of concept, an all-dielectric optical metasurface is elaborately designed to exhibit the desired surface polarizabilities, thereby reflecting the first derivative and extracting the edges of images impinging from normal direction. The proposed study offers a flexible design method for on-axis metasurface-based optical signal processing and also, dramatically facilitates the experimental setup required for ultrafast analog computation and image processing.
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.
The relationships between material constructions and reflective spectrum patterns are important properties of photonic crystals. One particular interesting reflectance profile is a high-intensity and uniform three-peak pattern with peak positions right located at the red, green, and blue (RGB, three original colors) region. For ease of construction, a seek for using one-dimensional photonic crystals to achieve RGB triple reflective bands is a meaningful endeavor. Only very limited previous studies exist, all relying on traditional periodic photonic crystals (PPCs) and of large thickness. The underlying physical principles remain elusive, leaving the question of thickness limit to achieve RGB bands unaddressed. Here, we present the first detailed work to explore the thickness limit issue based on both theoretical and experimental investigation. A set of heuristically derived design principles are used to uncover that the break of translational symmetry, thus introducing heterostructure photonic crystals (HPCs), is essential to reduce the total optical path difference (OPD) to ~ 3200nm (the theoretical limit) while still exhibiting high-quality RGB bands. A systematic experiment based on a 12-layer heterostructure construction was performed and well confirmed the theoretical predictions. The associated three-peak properties are successfully used to realize quantum dot fluorescent enhancement phenomena. Furthermore, the HPC exhibits unusually stability against solvent stimulus, in strong contrast to typical behaviors reported in PPCs. Our work for the first time proposes and verifies important rational rules for designing ultrathin HPCs toward RGB reflective bands, and provides insights for a wider range of explorations of light manipulation in photonic crystals.
Advances in integrated photonics open exciting opportunities for batch-fabricated optical sensors using high quality factor nanophotonic cavities to achieve ultra-high sensitivities and bandwidths. The sensitivity improves with higher optical power, however, localized absorption and heating within a micrometer-scale mode volume prominently distorts the cavity resonances and strongly couples the sensor response to thermal dynamics, limiting the sensitivity and hindering the measurement of broadband time-dependent signals. Here, we derive a frequency-dependent photonic sensor transfer function that accounts for thermo-optical dynamics and quantitatively describes the measured broadband optomechanical signal from an integrated photonic atomic-force-microscopy nanomechanical probe. Using this transfer function, the probe can be operated in the high optical power, strongly thermo-optically nonlinear regime, reaching a sensitivity of $approx$ 0.4 fm/Hz$^{1/2}$, an improvement of $approx 10times$ relative to the best performance in the linear regime. Counterintuitively, we discover that higher transduction gain and sensitivity are obtained with lower quality factor optical modes for low signal frequencies. Not limited to optomechanical transducers, the derived transfer function is generally valid for describing small-signal dynamic response of a broad range of technologically important photonic sensors subject to the thermo-optical effect.
We demonstrate serrodyne frequency shifting of light from 200 MHz to 1.2 GHz with an efficiency of better than 60 percent. The frequency shift is imparted by an electro-optic phase modulator driven by a high-frequency, high-fidelity sawtooth waveform that is passively generated by a commercially available Non-Linear Transmission Line (NLTL). We also implement a push-pull configuration using two serrodyne-driven phase modulators allowing for continuous tuning between -1.6 GHz and +1.6 GHz. Compared to competing technologies, this technique is simple and robust, and offers the largest available tuning range in this frequency band.
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