Generation of 280 THz-spanning near-ultraviolet light in lithium niobate-on-insulator waveguides with sub-100 pJ pulses

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, dispersion-engineered sub-micron silicon nitride (SiN) waveguides operating in the visible (VIS), short-wave infrared (SWIR) and mid-infrared (MIR) regimes^4–6 as well as in silicon (Si) waveguides in the SWIR and MIR regimes.^7,8

While SiN and Si can generate octave-spanning SCG, the resulting spectra are typically centered around the pump wavelength. 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 near-ultraviolet (NUV) range in waveguides of aluminum nitride either by pumping at 1560 nm⁹ or at 780 nm¹⁰ 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^12–14 or quantum memories.^15,16

Another non-centrosymmetric material is lithium niobate, which possesses a non-linearity of |d₃₃| ∼ 33 pm/V¹⁷ and exhibits a transparency window in the range of ∼330 nm–4500 nm.^18–20 Bulk optical lithium niobate waveguides are mostly fabricated by titanium in-diffusion^21–23 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.^28–30 However, these systems have a low index contrast that diminishes the optical confinement and use pulse energies larger than 1 nJ.

For smaller pulse energies, stronger optical confinement is required. The lithium niobate-on-insulator (LNOI) platform typically consists of a <600 nm lithium niobate thin-film bonded onto an oxide layer that results in a refractive index contrast of Δn ∼ 0.7. LNOI combines the non-linearities of lithium niobate with high modal confinement in sub-micron waveguides,^31–33 which enables dispersion engineering.^34–36 Octave-spanning SCG has already been shown on the LNOI platform,^37–39 but so far, broadening the optical spectrum in the NUV region has not been the focus.

Here, we optimize the group velocity dispersion (GVD) in the NIR range for an LNOI waveguide with a cross section of 700 × 400 nm². After dispersion optimization, we pump a 14 mm-long waveguide at 950 nm with 220 fs pulses and a low pulse energy of 67 pJ in the waveguide (peak power of 266 W). We report a broadening of the NIR pump signal of Δλ_NIR = 518 nm (Δω_NIR = 179 THz), and by second-harmonic generation, this NIR SCG signal results in cascaded SCG with a NUV bandwidth of Δλ_NUV = 173 nm (Δω_NUV = 280 THz). In addition, 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.

Figures 1(a) and 1(b) show an x-cut LNOI rib waveguide with a top width of 700 nm, ridge-height of 250 nm, and remaining thin-film thickness of 150 nm. The waveguide is fabricated using standard e-beam lithography and optimized ICP/RIE dry-etching. Subsequent wet-etching in a buffered oxide etch (BOE) and potassium hydroxide (KOH) removes the remaining mask and dry-etching residuals. Figure 1(c) shows the input side of a set of 14 mm-long waveguides.

Figure 1(d) depicts the experimental setup, which consists of a femtosecond (fs) pulsed oscillator with an 80 MHz pulse-train at 950 nm and a pulse width of 220 fs together with a power control unit consisting of a polarization beam splitter plus a half-wave plate. We use a half-wave plate to control the polarization of the fs pulses, which are then focused onto the LNOI sample by a 50× focusing objective with 11.2 dB/facet coupling losses. After collection of the optical signal with a lensed fiber (Oz Optics TPMJ-3A-1550-8), the optical signal is sent to an optical spectrum analyzer (OSA, Advantest Q8381A) with a detection range from 350 nm to 1750 nm and the same resolution of 5 nm over the full wavelength span. One measurement covers the spectral broadening in the NIR and the cascaded spectral broadening in the NUV, close to the bandgap of lithium niobate.^18–20 The 20× imaging objective is used for sample positioning and alignment in combination with a VIS lens and camera. Figure 1(e) shows the chip mounted on a vacuum chuck within the experimental setup, with blue and white light resulting from the SCG process.

In waveguides with high modal confinement, the material dispersion ∝dn/dλ can be compensated with the waveguide dispersion ∝dn_neff/dλ that arises from engineering the waveguide cross section. Here, n (n_eff) is the (effective) refractive index of the guiding medium, and λ is the wavelength of the light. Using the waveguide dispersion, one can achieve a net anomalous dispersion in the system. In photonic integrated circuits, this method was originally implemented in Si and SiN waveguides^40,41 and is also applicable for the LNOI platform.^34–36 

The waveguide dispersion is more commonly given in frequency space as the group velocity dispersion (GVD),

with β = k₀ · n_eff and ω_P being the pump frequency, related by $ωP=2πcλP$ to the pump wavelength λ_P. As shown in Fig. 2(a), we use finite element method (FEM) simulations to calculate and maximize the anomalous dispersion range (GVD < 0) of the fundamental mode at 950 nm in a 700 × 400 nm² trapezoidal waveguide with sidewall angles of 60° and a remaining thin-film thickness of 150 nm.

Figure 2(a) shows the calculated anomalous and normal GVD of the TM₀₀ and TE₀₀ modes. Here, we select the TM₀₀ optical mode with two zero-crossings at 730 nm and 1040 nm that ensure the presence of dispersive waves at both ends of the spectrum, provided the pump wavelength is in the anomalous dispersion regime. The goal is to have a broad spectrum in the NIR that in turn generates a broad signal in the NUV through cascaded second-harmonic generation, which we refer to as cascaded SCG.

To select the broadest dispersion regime, we now maximize the bandwidth of the dispersion operator,^42,43

where $βn=∂nβ∂ωn$⁠, with respect to the pump frequency ω_P. The zero-crossings of $D^$ allow us to visualize the dispersive waves and therefore the NIR broadening of the SCG. Figure 2(b) shows the calculated dispersion operator $D^$ of the TM₀₀ mode for different pump wavelengths. We select the pump wavelength of 950 nm since it maximizes the spectral separation of the dispersive waves, which are located at 760 nm and 1100 nm. To support the dispersive wave calculations, we use the split-step Fourier method to simulate the generalized nonlinear Schrödinger equation.^5,42,43Figure 2(c) shows the computed output optical spectra depending on the input peak power, where the dispersive waves appear at a peak power of ∼200 W at wavelengths of 790 nm and 1120 nm, a deviation of 3.9% and 1.8% from the results shown in Fig. 2(b).

Figure 3 shows the generation of the SCG pumped at λ_P = 950 nm and the cascaded SCG at λ_P/2 = 475 nm for pulse energies ranging from 13 pJ to 67 pJ in the waveguide, which corresponds to peak powers inside the waveguide from 53 W to 266 W. The broadening of the NIR pump increases for increasing pulse energy largely due to self-phase modulation until the appearance of dispersive waves with a pulse energy of 53 pJ and a 30 dB-NIR bandwidth of Δλ_NIR = 278 nm (Δω_NIR = 94 THz), while for a pulse energy of 67 pJ, we measure a 30 dB-NIR bandwidth of Δλ_NIR = 518 nm (Δω_NIR = 179 THz). We observe the dispersive waves at 725 nm and 1175 nm, which deviate by 4.6% and 6.4% from the results shown in Fig. 2(b), respectively. We attribute this difference to the trenching of the LNOI waveguides and the ±20 nm thickness variation of the thin-film over the full chip. The trenches can be observed in Fig. 1(b) between the waveguide sidewall and the remaining thin-film. Trenching is a by-product of the dry-etching that we employ to fabricate the LNOI waveguides. The dry-etching causes redeposition, which leads to the appearance of trenches. The redeposition rate differs from the edge of the sample to the center of it, which translates into inhomogeneous trenches. The inhomogeneous trenching slightly modifies the cross section, which in turn affects the dispersion operator and the spectral location of the dispersive waves.

The onset of the self-phase modulation at 27 pJ for both pump and SHG signals shows the cascaded nature of the process, and the broadening of the SHG signal is consistent with the SCG broadening in the NIR range. For 67 pJ, the NUV signal extends to the short-wavelength side down to 352 nm, which is close to the bandgap of lithium niobate, and shows a 30 dB-NUV bandwidth of Δλ_NIR = 173 nm (Δω_NIR = 280 THz), which is more than two times broader than previous results.¹⁰ The dispersive waves limit the maximal spectral bandwidth; therefore, pulse energies higher than 53 pJ do not broaden the pump spectrum any further. The two kinks at wavelengths 580 nm and 1040 nm (clearly visible in the 27 pJ measurement of Fig. 3) are artifacts of the OSA.

The detected average power is ∼3 μW (−25 dBm) around the NIR pump wavelength of 950 nm. For NUV signals, we detect power levels of ∼10 nW (−50 dBm) at a wavelength of 475 nm and 3.2 nW (−55 dBm) at 360 nm. The drop in detected powers between NIR and NUV ranges is ∼25 dB, which suggests a more efficient broadband generation than power ratios of ∼50 dB in LNOI waveguides that are pumped further in the SWIR range (∼1500 nm).³⁸ In terms of narrowband SHG conversion efficiency, before self-phase modulation occurs, we estimate a conversion efficiency of only ∼1% W⁻¹ cm⁻², which is lower than the conversion efficiency of 4600% W⁻¹ cm⁻² in periodically poled LNOI waveguides.⁴⁴ We expected this result since we engineered the dispersion to broaden the SCG pump spectrum, not to enhance the SHG modal phase matching of particular modes. Further work could optimize SHG phase matching by employing chirp-tapered waveguides,¹⁰ periodic modulations on the waveguide width,⁴⁵ or periodically poled lithium niobate thin-films.⁴⁶ 

We report an optical damage threshold to the LNOI waveguide input of ∼8 kW peak power in the coupling process after >10 min of exposure, which agrees with previous results suggesting a >7 kW damage threshold in LNOI waveguides.³⁸ However, an exhaustive study on this topic as performed for bulk lithium niobate^47–49 is still required. In addition, undoped LNOI waveguides exhibit a photorefractive effect in a fast response of ∼21 ms in the NIR range, which is more than three orders of magnitude faster than that in bulk lithium niobate devices.⁵⁰ Doping the lithium niobate thin-film with, for instance, magnesium would diminish the optical damage threshold in the VIS and NIR range, but the same might not apply to the NUV range.^51,52

All the optical spectra shown in Fig. 3 have a discontinuity between 550 nm and 580 nm. To overlap the NIR SCG with its second-harmonic signal in the NUV range, the dispersion operator should become broader than that shown in Fig. 2(b), which can be achieved by increasing the index contrast between the waveguide and its surroundings. For that purpose, the LNOI waveguides can undergo a partial³⁷ or full³⁹ etching of the silicon dioxide buffer layer underneath the LNOI waveguide.

To better understand the behavior of NUV broadening, we conduct a wavelength sweep of the NIR pump from 940 nm to 960 nm. As shown in Fig. 2(b), the pump wavelength modifies the bandwidth of the dispersion operator, and hence, the SCG and its cascaded signal are also modified. Figure 4(a) shows the generated NUV supercontinuum spectra at different pump wavelengths for the same device as presented before. The corresponding 30 dB-bandwidths are highlighted, demonstrating the dependence on the pump wavelength, shown as expected in Fig. 2(b).

For a waveguide with a slightly larger cross section (800 nm top width and 250 nm ridge height), the dispersion operator is red-shifted with respect to the one shown in Fig. 2(b). Then, the optimal pump wavelength is also red-shifted, which Fig. 4(b) shows to be 990 nm (303 THz), with a broadening of Δλ_NUV = 181 nm (Δω_NUV = 235 THz) in the NUV regime. By selecting the waveguide cross section and pump wavelength, the broadening of the second-harmonic signal can be tailored to specific needs in the NUV or more toward the VIS range.

In a non-centrosymmetric LNOI waveguide, we demonstrated broad NUV generation through direct second-harmonic generation from SCG in the NIR range. The NUV signal has a 30 dB-bandwidth of 280 THz (173 nm) when pumped at 950 nm with a pulse energy of 67 pJ inside the waveguide. The results present sub-100 pJ, nonlinear systems as broadband NUV sources for NUV fluorescence and comb applications. However, since the generated NUV light is close to the bandgap of lithium niobate, doped lithium niobate thin-films (i.e., with magnesium) should be considered for mitigating the photorefractive effect in the VIS–NIR ranges even at pulse energies lower than 67 pJ.

M.R.E. and F.K. equally contributed to this work.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

We acknowledge support for nanofabrication from the Scientific Center of Optical and Electron Microscopy (ScopeM) and from the cleanroom facilities BRNC and FIRST of ETH Zurich. This work was supported by the Swiss National Science Foundation, Grant No. 179099, and by the European Union’s Horizon 2020 Research and Innovation Program from the European Research Council, under Grant Agreement No. 714837 (Chi2-nano-oxides).