Abstract
We report nonlinear Cherenkov radiations (NCRs) in a Ti in-diffused LiNbO3 planar waveguide. The radiations were modulated exploiting different polarizations and orders of the guided modes, the fundamental wavelengths and the working temperatures. Some characteristics related to NCRs, such as radiation angles and relative intensities were investigated in detail. The experimental results matched well with theoretical calculations.
Highlights
When the velocity of a charged particle exceeds that of light in medium, coherent light would be emitted at a specific angle, which is called Cherenkov radiation in conventional particle physics [1]
Similar effect was applied to nonlinear optics as the nonlinear Cherenkov radiation (NCR), which requires that the phase velocity of the nonlinear polarization wave (NPW) exceeds that of the harmonic wave
Four groups of NCRs should be observed in total, which corresponded to the four processes utilizing different nonlinear coefficients described in Eq (1)
Summary
When the velocity of a charged particle exceeds that of light in medium, coherent light would be emitted at a specific angle, which is called Cherenkov radiation in conventional particle physics [1]. NCR can be observed in both bulk ferroelectric crystals where the radiation is enhanced by the domain walls, and in nonlinear waveguides where the value of the momentum uncertainty in the thin film is more than the transverse momentum mismatch due to uncertainty principle [2, 3] In both material platforms, several approaches can be exploited to modulate the behavior of NCRs. Forward and backward reciprocal vectors parallel to the direction of the NPW can deaccelerate or accelerate the phase velocity of the NPWs in periodically-poled nonlinear waveguides [4] or two-dimensional nonlinear photonic crystals [5, 6], changing of the radiation angles as well as the nonlinear conversion efficiencies. When the interacting waves are set to be at different polarization states, i.e., using the birefringence of ferroelectric crystals, the phase velocity of the NPW can be modulated as well By this means, Cherenkov type second harmonic generation (SHG), was experimentally demonstrated in an anomalous dispersive ultra-thin MgO doped LiNbO3 without phase velocity threshold [7]. We will report our results on NCR modulation in a Ti in-diffused LiNbO3 planar waveguide by mode dispersion
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