Blue-shifted dispersive waves and broadband UV emission using dual-core SiN waveguides

TL;DR

This work demonstrates that strong coupling in dual-core silicon nitride waveguides can blue-shift high-frequency dispersive waves and enable broadband blue-to-UV emission during supercontinuum generation. By exciting the anti-symmetric supermode, the short-wavelength dispersive wave appears near 540 nm—about 80 nm shorter than in a comparable single-core waveguide—and a blue-UV continuum extending below 350 nm emerges, the latter not observed in single-core devices. Numerical modeling with multimode and single-mode GNLSE frameworks shows good agreement with experiments, attributing the visible DW to AM dynamics and the UV/blue component to both spectral broadening and third-harmonic processes, with THG estimated to be non-phase-matched. The results suggest a general, on-chip strategy for accessing shorter wavelengths in SC spectra, potentially extendable to other materials and cross-sectional designs for compact visible-UV light sources.

Abstract

We show that using strongly coupled dual-core waveguides for supercontinuum generation shifts the wavelength of the high-frequency dispersive waves towards shorter wavelengths, as compared to generation in a single-core waveguide having the same core dimensions. In a demonstration experiment, we launch ultrashort infrared pump pulses at 1-$μ$m wavelength (285-THz frequency) into silicon nitride waveguides, where soliton formation and fission leads to generation of dispersive waves in the visible range. Efficient input coupling and controlled excitation of the fundamental supermodes of the dual-core waveguide is provided with adiabatic tapers and a dual-prong input structure. For the dual-core waveguide, the short-wavelength dispersive wave is located at 540~nm (green, 555~THz), which is blue-shifted by 80~nm (70~THz) compared to that of the single-core waveguide. Simultaneously, the dual-core waveguide generates broadband radiation spanning from the blue into the UV range, reaching to below 350~nm (above 855~THz), with typically a spectral density 25~dB below that of the dispersive wave. The broadband component can be addressed to third harmonic generation and is not observed in single-core supercontinuum generation. Numerical modeling shows good agreement with experimental measurements. The demonstrated dual-core approach and dedicated input coupling appear to hold promise also for other waveguide structures, independent of specific materials or core dimensions, by providing shorter wavelengths than with the respective single-core waveguide.

Figures (10)

• Figure 1: (a) Schematic top view of the dual-core sample. The prongs are equipped with 380 $\mu$m-long inverse tapers providing a MFD of 2 $\mu$m at the facets. The part colored in blue is the strongly coupled dual-core waveguide section with a small gap of 300 nm between the two cores, each 0.8-$\mu$m wide and 0.8-$\mu$m thick. (b) Single-core reference sample having the same core cross section and input/output tapers. (c) Calculated normalized field distributions ($E_x$ component) of the two fundamental supermodes (SM, symmetric, and AM, anti-symmetric) supported by the dual-core section at 1053-nm wavelength. The color code indicates the sign and strength of the electric field normalized to the maximum. The field distribution of the single-core mode TE$_\textrm{00}$ is shown as well. (d) Calculated group velocity dispersion (GVD) profiles. The SM (blue curve) possesses all-normal dispersion, while the TE$_\textrm{00}$ (black dashed curve) and the AM (orange curve) possess anomalous dispersion. With regard to single-core dispersion, the zero-dispersion wavelength (ZDW) of the anti-symmetric supermode is blue-shifted by approximately 50 nm.
• Figure 2: $\Delta\textrm{ZDW}$ (blue curve) between the shortest ZDW of the dual-core waveguide for the AM and that of the corresponding single-core for the TE$_\textrm{00}$ mode, and absolute value of the ZDW (red curve) of the dual-core (AM) as a function of core width. The core height is fixed at 800 nm, and the gap between two cores for the dual-core structure is set to 300 nm. Core widths smaller than 580 nm are not included because the TE$_\textrm{00}$ mode no longer exhibits anomalous dispersion and does no longer have a ZDW.
• Figure 3: Schematic view of the experimental set-up. Pump light is produced by a laser that operates at a center wavelength of 1053 nm with a 3-dB bandwidth of 21 nm, a pulse duration of 100 fs, and a repetition rate of 80 MHz. The laser is set at full power, and the pulse power is varied via the combination of two half-wave plates (HWP) and a polarizing beam splitter (PBS) in between. A beam expander and an aspheric lens are used to couple the laser beam into the input inverse tapers of the waveguides with a coupling loss of about 1 dB. The input aspheric lens is mounted on a piezo-controlled precision translation stage (Thorlabs MAX311D/M interfaced with a Thorlabs BPC203 piezo controller) for reproducible alignment and allows for changing the injection either between one of the two prongs of a dual-core waveguide or the inverse taper of an adjacent single-core waveguide. Lensed fibers (MFD of 2 $\mu$m) are used to collect output light from the waveguides for spectral analysis using an optical spectrum analyzer (OSA) or a spectrometer. When using the spectrometer to record the spectra shorter than 500-nm wavelength, the output of the fiber is sent over a free-space distance of 100 mm to the spectrometer, with a long-pass dichroic mirror (DMLP) placed at a free-space distance of 50 mm behind the fiber output. The purple line represents light guided in free space while the green line represents light guided in fibers.
• Figure 4: Calculated change of mode energy in the SM and the AM versus propagation length for simultaneous excitation with equal pump energies of 375 pJ.
• Figure 5: Comparison of the calculated spectra by MM-GNLSE with solely AM excitation (black curve) and the SMC-GNLSE (red curve) for an input pulse energy of 187.5 pJ. All physical input parameters are the same. DW: dispersive waves.