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We propose and investigate the quasi-phase matched (QPM) nonlinear optical frequency conversion of optical vortices in periodically poled Lithium Niobate (PPLN). Laguerre-Gaussian (LG) modes are used to represent the orbital angular momentum (OAM) states, characterized with the azimuthal and radial indices. Typical three-wave nonlinear interactions among the involved OAM modes are studied with the help of coupling wave equations. Being different from normal QPM process where the energy and quasi-momentum conservations are satisfied, both of the azimuthal and radial indices of the OAM states keep constant in most of the cases. However, abnormal change of the radial index is observed when there is asynchronous nonlinear conversion in different parts of the beams. The QPM nonlinear evolution of fractional OAM states is also discussed showing some interesting properties. In comparison with the traditional birefringent phase matching (BPM), the QPM technique avoids the undesired walk-off effect to reserve high-quality LG modes. We believe the QPM is an efficient way to convert, amplify and switch OAM states in various optical vortex related applications.

A tunable dual-wavelength filter is experimentally demonstrated in domain-engineered lithium niobate. Application of an electric field on the y-surfaces of the sample results in the optical axes rotating clockwise and anticlockwise, which makes selective polarization rotation. The quasi phase-matching wavelengths could be adjusted through suitable domain design. A unique dual valley spectrum is obtained in a periodically poled lithium niobate structure with a central defect if the sample is placed between two parallel polarizers. The expected bandwidth could be varied from ∼1 nm to ∼40 nm. Moreover, both the spectral response and group delay dispersion could be engineered.

The refractive indices of ordinary and extraordinary waves (no and ne) of congruent LiNbO3 (CLN) in the terahertz (THz) band are measured at different temperatures. Extended Cauchy equations are proposed to describe the temperature- and frequency-dependent refractive indices. With less than 0.5% deviation, both no and ne can be obtained accurately. The values of no and ne are ~7 and ~5.4, respectively, which are much larger than those in the visible band. The extremely high birefringence (~1.6) may lead to some interesting applications of linear, electro-optic, and nonlinear optical effects of CLN. As an example, true zero-order THz waveplates (WPs) could be obtained with considerable low loss based on thin CLN wafers. The working frequency can be tuned by changing the environmental temperature. Moreover, wide bandwidth achromatic THz WPs were also achievable by stacking four CLN wafers together with precise orientation control.

Optical parametric amplification (OPA) of arbitrarily polarized light is proposed in a multi-section periodically poled Lithium Niobate (PPLN). External electric field is applied on selected sections to induce the polarization rotation of involved lights, thus the quasi-phase matched optical parametric processes exhibit polarization insensitivity under suitable voltage. In addition to the amplified signal wave, an idler wave with the same polarization is generated simultaneously. As an example, a ~10 times OPA showing polarization independency is simulated. Applications of this technology are also discussed.

An in-line, all-optical fiber modulator based on a stereo graphene–microfiber structure (GMF) utilizing the lab-on-rod technique was demonstrated in this study. Owing to its unique spring-like geometry, an ultra-long GMF interaction can be achieved, and a modulation depth of ∼7.5 dB (∼2.5 dB) and a modulation efficiency of ∼0.2 dB mW−1 (∼0.07 dB mW−1) were demonstrated for two polarization states. The modulation depth and modulation efficiency are more than one order of magnitude larger than those of other graphene–microfiber hybrid all-optical modulators, although at the cost of a higher insertion loss. By further optimizing the transferring and cleaning process, the upper limit of the modulation depth is mainly determined by the loss from the intrinsic absorption, which depends on the light–graphene interaction. Then, the modulator can quickly switch between the on-state and the off-state with a theoretically maximized modulation depth of tens of decibels. This modulator is compatible with the current fiber-optic communication systems and may be applied in the near future to meet the impending need for ultrafast optical signal processing. A polarization-sensitive all-optical fiber modulator based on a graphene–microfiber structure has been demonstrated by researchers in China. All-optical modulators with high modulation speeds, sufficient modulation depth and wide optical bandwidth are needed to realize ultrafast optical signal processing. Now, Fei Xu and co-workers at Nanjing University have fabricated an all-optical fiber modulator whose modulation depth and efficiency are more than ten times greater than previous all-optical modulators based on graphene and microfibers. These properties originate from its very long interaction length resulting from its spring-like geometry. The researchers consider that even better performance can be achieved by using higher quality graphene and improving the transfer and wrapping of the graphene in the fabrication process. Since the modulator is compatible with existing fiber-optic communication systems, it may find rapid application for ultrafast optical signal processing.

Versatile devices, especially tunable ones, for terahertz imaging, sensing and high-speed communication, are in high demand. Liquid crystal based components are perfect candidates in the optical range; however, they encounter significant challenges in the terahertz band, particularly the lack of highly transparent electrodes and the drawbacks induced by a thick cell. Here, a strategy to overcome all these challenges is proposed: Few-layer porous graphene is employed as an electrode with a transmittance of more than 98%. A subwavelength metal wire grid is utilized as an integrated high-efficiency electrode and polarizer. The homogeneous alignment of a high-birefringence liquid crystal is implemented on both frail electrodes via a non-contact photo-alignment technique. A tunable terahertz waveplate is thus obtained. Its polarization evolution is directly demonstrated. Furthermore, quarter-wave plates that are electrically controllable over the entire testing range are achieved by stacking two cells. The proposed solution may pave a simple and bright road toward the development of various liquid crystal terahertz apparatuses. Light: Science & Applications (2015) 4, e253; doi:10.1038/lsa.2015.26; published online 27 February 2015