With the rapid development of portable equipment, the demand for the miniaturization of optical elements is increasing. Metasurfaces are considered as potential planar analogs of conventional devices due to their small size and their extraordinary ability to modulate light. As one of the most important branches of metasurfaces, metalenses attract much attention and are fascinating to develop compact, miniature optical imaging devices. However, the highly chromatic characteristic limits their further applications. Therefore, achromatic metalenses with constant focal lengths over a broad bandwidth are highly desirable. Here, we demonstrate a diffraction-limited achromatic metalens with an octave-wide bandwidth in the infrared. Unlike typical metalenses with periodic unit cells, the proposed metalens comprises well-designed an aperiodic array of elements based on the Pancharatnam-Berry phase. The proposed metalens with a numerical aperture of 0.49 achieves an average focusing efficiency of 37% in a wavelength range from 1.15 to 2.3 μm, which is one octave, and maintains a near-constant focal length of 25 μm. It paves the way for miniaturized and broadband imaging applications.
The formation and manipulation of ultrashort pulses on chip would be of great interest to ultrafast optics and integrated photonics. One of the important issues is dispersion-assisted nonlinear interactions of broadband frequency components. In this paper, we show for the first time that a bilayer waveguide for quasi-TE mode produces a quite flat and saddleshaped dispersion profile. Different from previously reported TE-mode waveguides with flattened dispersion, the proposed waveguide exhibits a greatly simplified structure with no need for a high-aspect-ratio slot and has quite small group delay difference in a wide spectral range with four zero-dispersion wavelengths (ZDWs). For the first time we study supercontinuum generation in hybrid dispersion regime, in which the broadened spectrum covers a bandwidth with all ZDWs. It is found that one can obtain greatly improved spectral flatness in supercontinuum generation, with a power variation as small as 3 dB over a bandwidth of <500 nm. Moreover, the proposed waveguides are particularly suitable for low-distortion pulse propagation over a long distance, which is important for on-chip ultrashort pulse delivery.
Dispersion engineering in integrated waveguides and microresonators has been intensely studied in recent years. The main focus is to achieve desirable adjustment of dispersion value, slope, bandwidth and flatness, which is important for broadband nonlinear applications. Dispersion has been viewed as a control knob to leverage the parameter space provided by high-index-contrast on-chip devices, enabling strong interactions of far apart frequency components over an octavespanning bandwidth. Here, we review recent advances in dispersion engineering in integrated waveguides and microresonators based on various material platforms, with an emphasis on their applications in mid-infrared (IR) photonics.
Photonic integrated circuits suffer from a thermal drift of device performance, which is a key obstacle to the development of commercial optoelectronic products. Temperature-insensitive integrated waveguides and resonators have been demonstrated at a single wavelength, using materials with a negative TOC, which are not suitable for WDM devices and wideband nonlinear devices. Here, we propose two waveguides to realize the generation of broadband athermal features. For one of them, the temperature-insensitivity over a bandwidth of 780 nm (1280 to 2060 nm) with an ultra-small effective-TOC within is ±1×10-6/K. Uniquely, the waveguide has small anomalous dispersion (from 66 to 329 ps/nm/km) over the same band and is suitable for frequency comb generation without being affected by intra-cavity thermal dynamics. We also show another waveguide design with an effective-TOC variation of ±1×10-6/K over a bandwidth of 1060 nm, from 1220 to 2280 nm. The obtained dispersion varies from -232 to -502 ps/nm/km over the same band, which can be used in nonlinear devices.
The high temperature sensitivity of silicon material limits the applications of silicon-based micro-ring resonators in integrated photonics. To realize a low but broadband temperature-dependence-wavelength-shift (TDWS) micro-ring resonator, designing a broadband athermal waveguide becomes a significant task. In this work, we propose a broadband athermal waveguide which shows a low effective thermos-optical coefficient (TOC) of ±1×10-6/K at 1400 nm to 1700 nm. The proposed waveguide shows low-loss performance of 0.01 dB/cm and stable broadband-athermal ability when it’s applied in micro-ring resonators, and the optical loss of micro-ring resonator with a radius of 100 μm using this waveguide is 0.02 dB/cm.
Group-velocity dispersion (GVD) engineering is vital to many nonlinear optical phenomena and has been widely used for nonlinear optics. Aiming at different applications, one need to control and engineer the sign, value, slope of dispersion and the number of zero-dispersion wavelengths (ZDWs). In this work, we demonstrate generation of 5 ZDWs in a new type of bilayer waveguides. Outer layer of this waveguide can be formed by depositing without etching. Material combinations are Ge23Sb7S70 (n≈2.2) and Ge28Sb12Se60 (n≈2.6). In this waveguide, an extremely wideband-low and flat dispersion can be obtained from 2.6 to 15.5 μm (2.6 octaves).
In this paper, we present a stable dark soliton with its spectrum spanning about one octave generated in the silicon nitride microring resonator with normal dispersion. The dark soliton can be generated straightforwardly from noise. Then, we investigate the dependence of bandwidth and power level on pump power, the second-order dispersion, the ratio of coupling and intracavity loss coefficient and free spectral range.
Microresonator-based frequency combs have attracted a great deal of attention in recent years. Traditional generation scheme could be slow due to the operation of tunable lasers and thermal effects. In some spectral ranges, it is also difficult to find a tunable laser with a certain tuning range. In this paper, we propose a fast and simple method for Kerr comb generation without laser detuning and local cooling. In this way, the generation time can be reduced to tens of nanoseconds, three orders of magnitude faster.
Photonic circuits suffer from thermal drift of device performance, which is a key obstacle to the development of commercial optoelectronic products. Temperature-insensitive integrated waveguides and resonators have been demonstrated by using materials with a negative TOC at a single wavelength, which are not suitable for WDM devices and wideband nonlinear devices. Here, we propose a waveguide structure with temperature-insensitivity over a bandwidth of 780 nm (1280 to 2060 nm) with an ultra-small effective TOC within ±1×10-6/K. Uniquely, the waveguide has small anomalous dispersion (from 66 to 329 ps/nm/km) over the same band and is suitable for frequency comb generation without being affected by intra-cavity thermal dynamics.
Frequency comb generation in the mid-infrared (mid-IR)region is attractive recently. Here, we propose the Ge-on-Si microresonator for power-efficient frequency comb generation in the mid-IR. An octave-spanning comb can be obtained with power reduced to 190 mW. The robustness of the frequency comb generation with localized spectral loss is also analyzed. Based on the analysis, we propose a novel architecture of on-chip spectroscopy systems in the mid-IR.
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