Abstract
Integrated optics has weak ultraviolet and near-ultraviolet (NUV) light conversion due to its strong material dispersion and large propagation losses. To reach this spectral range, we use non-centrosymmetric waveguides that convert near-infrared (NIR) supercontinuum light into broadband NUV light. We measure a 280 THz span that reaches the upper frequency of 851 THz (352 nm) in a 14-mm long rib waveguide of lithium niobate-on-insulator, with an engineered dispersion for supercontinuum generation in the NIR range. The results on broadband NUV signals promote integrated optics for spectroscopy and fluorescence applications such as atomic clocks and chemical sensors.
Highlights
The field of supercontinuum generation (SCG) evolved from first observations in bulk silica1 to applications in standard silica and photonic fibers.2,3 In later years, integrated photonics has demonstrated octave-spanning SCG in CMOS-compatible, dispersionengineered sub-micron silicon nitride (SiN) waveguides operating in the visible (VIS), short-wave infrared (SWIR) and mid-infrared (MIR) regimes4–6 as well as in silicon (Si) waveguides in the SWIR and MIR regimes.7,8While SiN and Si can generate octave-spanning SCG, the resulting spectra are typically centered around the pump wavelength
We report a broadening of the NIR pump signal of ΔλNIR = 518 nm (ΔωNIR = 179 THz), and by second-harmonic generation, this NIR SCG signal results in cascaded SCG with a NUV bandwidth of ΔλNUV = 173 nm (ΔωNUV = 280 THz)
The waveguide dispersion is more commonly given in frequency space as the group velocity dispersion (GVD), FIG. 1. [(a) and (b)] Scanning electron microscopy (SEM) image of the device showing the 700 nm × 400 nm trapezoidal waveguide with a sidewall angle of 60○ and remaining thin-film thickness of 150 nm, which results in a ridge-height of 250 nm, (c) the optical microscope image of the lithium niobate-on-insulator (LNOI) chip, (d) the experimental setup, where a femtosecond-laser and free-space objective are used for light coupling and a lensed fiber and an optical spectrum analyzer are used for broadband detection (350 nm–1700 nm), and (e) the picture showing a single LNOI waveguide pumped at 950 nm
Summary
The field of supercontinuum generation (SCG) evolved from first observations in bulk silica to applications in standard silica and photonic fibers. In later years, integrated photonics has demonstrated octave-spanning SCG in CMOS-compatible, dispersionengineered sub-micron silicon nitride (SiN) waveguides operating in the visible (VIS), short-wave infrared (SWIR) and mid-infrared (MIR) regimes as well as in silicon (Si) waveguides in the SWIR and MIR regimes.. Using non-centrosymmetric materials with a non-vanishing χ(2)tensor allows extending the continuum into the blue by use of cascaded second-harmonic generation while pumping at near-infrared (NIR) or SWIR wavelengths Recent results expand this to the nearultraviolet (NUV) range in waveguides of aluminum nitride either by pumping at 1560 nm or at 780 nm with a non-linearity of |d33| ∼ 8.4 pm/V,11 showing a frequency comb spanning of more than 100 THz. Recent results expand this to the nearultraviolet (NUV) range in waveguides of aluminum nitride either by pumping at 1560 nm or at 780 nm with a non-linearity of |d33| ∼ 8.4 pm/V,11 showing a frequency comb spanning of more than 100 THz Such devices are interesting for a range of frequency comb applications such as atomic clocks or quantum memories.. Bulk optical lithium niobate waveguides are mostly fabricated by titanium in-diffusion or proton exchange methods.24–27 In such waveguide systems, octave-spanning SCG has been shown in periodically poled devices operating in the SWIR and MIR regimes.. By selecting the pump wavelength within the range of 940 nm–1000 nm, we tailor the cascaded SCG from the NUV toward the VIS range
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