Cascaded up-conversion of twin-beam OPG in nonlinear photonic crystals

Summary form only given. Purely nonlinear photonic crystals (NPCs) [1], fabricated by 2D periodic poling of ferroelectric materials, offer novel degrees of freedom to engineer the response of single step and cascaded frequency up-converters [2-3]. Recently, we demonstrated a novel configuration for frequency down-conversion in hexagonally poled LiTa03 (hexLT) [4], whereby two lattice resonances are coherently coupled to yield twin-beam optical parametric generation (OPG) with a shared signal or idler, significantly enhancing the OPG spectral and angular responses [5]. In this paper we show how the twin-beam OPG output in turn initiates multiple up-conversion processes (mediated by higher-order RLVs of the lattice) resulting in further enrichment of the spectral and angular range of cascaded up-conversions processes in the visible.Figure 1 illustrates experimental results obtained with a 1 cm-long 4 mm-wide hexLT crystal of period Λ = 22.8μm, pumped at an angle of 0.7° with 806 nm (10-100μJ) 1.7ps-pulses as in Ref. 4. The input beam was focused to 0.1 mm vertical and 0.8 mm horizontal radii for efficient excitation of the 2D lattice. To achieve spectral and angular resolution, the crystal output was collected in the Fourier plane by a fibrecoupled optical spectrum analyser, translated laterally to map the output angles. Figure 1a shows the (visible) crystal output in the far field resulting from up-conversion processes mediated by multiple RLVs, yielding an almost periodic in-plane distribution of red, green and blue lines (sum frequency generation (SFG) of two wavelengths and second harmonics generation (SHG), respectively). In Fig. 1b we provide a spectral and angular map of the measured OPG signals, pump and sum frequency (SF) outputs for this working point. Fig. 1c shows a high-resolution measurement taken under similar conditions of the SF output.Through experiments in hexLT crystals pumped around 800 nm, we investigate the degre- s of freedom of cascaded processes sustained by twin-beam OPG in 2D lattices. The results confirm the flexibility of NPCs also in cascading downand upconversion processes and their appeal for the engineering of novel parametric light sources with enhanced responses in the spectral and spatial domains.