Fabrication limits of waveguides in nonlinear crystals and their impact on quantum optics applications

Abstract

Waveguides in nonlinear materials are a key component for photon pair sources and offer promising solutions to interface quantum memories through frequency conversion. To bring these technologies closer to every-day life, it is still necessary to guarantee a reliable and efficient fabrication of these devices. Therefore, a thorough understanding of the technological limitations of nonlinear waveguiding devices is paramount. In this paper, we study the link between fabrication errors of waveguides in nonlinear crystals and the final performance of such devices. In particular, we first derive a mathematical expression to qualitatively assess the technological limitations of any nonlinear waveguide. We apply this tool to study the impact of fabrication imperfections on the phasematching properties of different quantum processes realized in titanium-diffused lithium niobate waveguides. Finally, we analyse the effect of waveguide imperfections on quantum state generation and manipulation for few selected cases. Studying the impact of fabrication errors on the waveguide widths, we find that the presence of correlated noise plays a major role in the degradation of the phasematching and we suggest different possible strategies to reduce the impact of fabrication imperfections.

Characterisation of fabrication inhomogeneities in Ti:LiNbO3 waveguides

“…variation along the sample could be used to directly reconstruct the measured, highly distorted phase matching profile of the full-length sample. These results confirm the theoretical model presented in [16,18], thus verifying the validity of the conclusions presented therein. The measured inhomogeneities in these titanium-indiffused LN waveguides report the current state of the art and provide crucial information that can be used to optimise the design of new samples.…”

“…Δβ(z, λ)≈Δβ 0 (λ)+δβ(z). In this case, the output intensity spectrum of the SH process is given by [10,14,16]…”

“…and thus depends on the specific δβ(z) profile. However, if the waveguide is sufficiently short, the impact of the variations δβ(z) are not strong enough to appreciably distort the phase matching spectrum, as shown in [16].…”

“…They are still expected, however, to affect positioning of the centre of the spectrum. Previous work in this waveguide system has shown that we expect to see nearly ideal phase matching spectra for waveguides shorter than 1cm in length [16]. Therefore, it should be possible to retrieve the variation of δβ(z) along the sample by dicing a long sample down to ∼1 cm long pieces and monitoring the shift in the position (in wavelength) of the phase matching spectrum.…”

“…While waveguiding systems offer many advantages, these systems also generally exhibit higher losses and are much more sensitive to device imperfections, when compared to their bulk counterparts. It is known that small device imperfections can dramatically reduce device performance [9][10][11][12][13][14][15][16]. For this reason, it is critical to assess the limits of the current fabrication technology to identify the classes of nonlinear processes physically achievable and to devise strategies to overcome such limitations.…”