Photonic analogs of the moiré superlattices mediated by interlayer electromagnetic coupling are expected to give rise to rich phenomena, such as nontrivial flatband topology. Here, we propose and demonstrate a scheme to tune the flatbands in a bilayer moiré superlattice by employing a band offset. The band offset is changed by fixing the bands of one slab while shifting those of the other slab, which is accomplished by modifying the thickness of the latter slab. Our results show that the band-offset tuning not only makes some flatbands emerge and disappear but also leads to two sets of flatbands that are robustly formed even with the change of band offset over a broad range. These robust flatbands form either at the AA-stack site or at the AB-stack site, and as a result, a single-cell superlattice can support a pair of high-quality localized modes with tunable frequencies. Moreover, we develop a diagrammatic model to provide an intuitive insight into the formation of the robust flatbands. Our work demonstrates a simple yet efficient way to design and control complex moiré flatbands, providing new opportunities to utilize photonic moiré superlattices for advanced light–matter interaction, including lasing and nonlinear harmonic generation.
Lithium niobate (LN) thin film has received much attention as an integrated photonic platform, due to its rich and great photoelectric characteristics, based on which various functional photonic devices, such as electro-optic modulators and nonlinear wavelength converters, have been demonstrated with impressive performance. As an important part of the integrated photonic system, the long-awaited laser and amplifier on the LN thin-film platform have made a series of breakthroughs and important progress recently. In this review paper, the research progress of lasers and amplifiers realized on lithium niobate thin film platforms is reviewed comprehensively. Specifically, the research progress on optically pumped lasers and amplifiers based on rare-earth ions doping of LN thin films is introduced. Some important parameters and existing limitations of the current development are discussed. In addition, the implementation scheme and research progress of electrically pumped lasers and amplifiers on LN thin-film platforms are summarized. The advantages and disadvantages of optically and electrically pumped LN thin film light sources are analyzed. Finally, the applications of LN thin film lasers and amplifiers and other on-chip functional devices are envisaged.
We propose an ultra-thin silicon metasurface supporting a high-quality leaky mode which is formed by partially breaking a bound-state-in-the-continuum (BIC) generated by the collective magnetic dipole (MD) resonance excited in the subdiffractive periodic systems. Such a quasi-BIC MD state leads to a robust near-field enhancement and a significant boost of the nonlinear process, resulting in measured 500-fold enhancement of third-harmonic emission in comparison to the conventional silicon disk metasurface. We further experimentally demonstrate the highly-efficient nonlinear image tuning via polarisation and wavelength control, opening the way for various applications in high-performance nonlinear metadevices including tunable laser, tunable displays, nonlinear imaging.
Microresonators with high quality factors have recently attracted much attention due to their ability to dramatically enhance light intensity by confining light within a small mode volume for a long period of time. They provide a versatile platform for researching on fundamental physics and practical applications ranging from nonlinear and quantum optics to ultrasensitive sensing. Lithium niobate (LN) is a artificial crystalline material with large electro-optical coefficients and high second-order nonlinearity, therefore, it is a good candidate for active photonic devices. Here, we report on our recent progresses on the mass fabrication of monocrystalline LN microdisk resonators with Q factors higher than 1e6 and LN-silica hybrid microdisk resonators with Q factors of the order of 1e5. The active tunable characteristics of the resonance wavelengths of the fabricated LN microdisk resonators and its based transmission modulations were demonstrated based on the electro-optic and thermo-optic effects of LN crystal.
In this paper, we reviewed the theoretical and experimental studies on the manipulation of the group delay of
light based on the transverse phase modulation effect induced by a Gaussian beam. We introduced the basic
theory of slow and fast lights in a thin nonlinear material based on the transverse phase modulation effect.
We introduced a simple but effective technique to actively and chromatically control the group velocity of light
at arbitrary wavelength, therefore, eliminating the requirements on the optical nonlinearity and the photonic
resonance at the signal wavelength. Furthermore, a technique to improve the transverse-modulation-induced
relative delay of light in nonlinear media through the combination of an optical nonlinearity and a resonant
Fabry-Perot cavity was introduced and theoretically demonstrated in ruby as an example. The introduction of
a resonant Fabry-Perot cavity can improve the relative delay by orders of magnitude. The techniques of active
chromatic manipulation and resonant improvement of the group delay of light may have potential applications
in optical information processing and optical communication network.
Transverse localization of light in one-dimensional waveguide arrays with width disorder has been studied in
both linear and nonlinear regimes. Defect mode is generated in the bandgap of the disordered waveguide array
when introducing refractive index modulation into a single waveguide, and its localization strength depends on
the width disorder level of the waveguide array. The evolution of the nonlinear disordered modes with either the
self-focusing or the self-defocusing optical nonlinearities has been studied. The results show that the nonlinear
disordered modes may be delocalized significantly due to the resonant interaction with the nearby eigen modes
in the width-disordered waveguide array.
We have studied theoretically the steady-state nonvolatile two-step, two-color holographic recording performance for both the congruent and the near-stoichiometric LiNbO3:Fe based on the two-center model (the deep-trap and the shallow-trap centers are Fe2+/Fe3+ and NbLi4+/NbLi5+, respectively). We compare the amplitudes of the total space-charge fields and the recording sensitivities between the congruent and the near-stoichiometric LiNbO3:Fe. The results show that the amplitudes of the total space-charge fields in near-stoichiometric LiNbO3:Fe are larger than that of congruent ones by at least one order of magnitude, while the recording sensitivities of the near-stoichiometric LiNbO3:Fe are confirmed theoretically to be around two-order in magnitude higher than that of the congruent ones with the same experimental conditions at low intensity region.
We propose and demonstrate a new method to fabricate volume phase mask in LiNbO3:Fe crystal by photorefractive effect, for the first time to our knowledge. First, we image an amplitude mask with corresponding intensity distribution at the incident face of a photorefractive LiNbO3:Fe crystal. After an exposure of proper time, due to photorefractive effect, the distribution of refractive index in the crystal is changed. Therefore we get the volume phase mask we want. The influence of the anisotropy of photorefractive nonlinearity on volume phase mask can be overcome by controlling experimental conditions. We can see an intensity-modulation image in the image plane of the volume phase mask. The intensity distribution of the image of the volume phase mask is inverse from the one of input amplitude mask. It can be explained by waveguide theory. Volume phase mask can be used to get modulated amplitude pattern by modulating the phase of incident light. It has the advantage of low energy loss over amplitude mask and real-time. Our method is very simple. It can be used to fabricate many kinds of complex phase masks and phase components used in optical information processing and Integrated Optics.
Based on the Li set vacancy model, we analyze the incorporation process of Mg ion in LiNbO3:Mg crystals with increasing MgO doping concentration. The obtained results are in good agreement with the experimental data reported in the literature. The congruent melting compositions are predicted according to our suggested incorporation model when the (Li2O-MgO-Nb2O5) system is considered as a ternary one.A procedure is suggested to find out the congruent melting compositions of LiNbO3:Mg crystals.
To the best of our knowledge, we have demonstrated for the first time that ring waveguides can be written and stored in photovoltaic and self-defocusing photorefractive medium LiNbO3:Fe crystals. The interactions between the ring solitons, such as the fusion and the birth of ring solitons, have been experimentally observed.
In this paper, we analyze the characters of all damage- resistant dopants' valences and electronic shell configurations found up to now in LiNbO3 crystals. We show that all damage-resistant dopants have only one valence state. We also show that these damage-resistant ions' electronic shell configurations are filled fully, as are the cases of the inert elements. From this point of view, we conclude that the anti-site NbLi5+ should be involved in the charge transport process, while the Li vacancy VLi is not involved in the charge transport process. It also can give us some insights about how to select a damage-resistant dopant for LiNbO3 crystal.
In this paper we demonstrated the advantage of the fanning- noise-free double doped photorefractive LiNbO3 crystals for the 3D storage. The fanning noise can be completely suppressed. The SNR and the line resolution of the holographic reconstructed patterns in these kinds of crystal are very high. It is simple and convenient to practice using double doped LiNbO3 crystal to suppress the fanning noise.
In this paper, we introduce a new technology for picking-up images and one-way aberration- free image communication through a phase-disturbing medium using photorefractive four- wave mixing and theoretically analyze this photorefractive process using the photorefractive four-wave coupling equations. We also experimentally realize this operation in the LiNbO3:Fe crystal.
In this paper, we report a new effect -- the threshold effect of incident light intensity for the photorefractive light-induced scattering in LiNbO3:Fe, Mg crystals, which could be used as a simple, effective technique to suppress the photorefractively light-induced scattering and is useful for us to get the noise-free photorefractive devices.
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