Mid-infrared-to-ultraviolet supercontinuum generation in low-loss tantalum pentoxide nanophotonic waveguides

Abstract: Optical frequency combs on photonic integrated platforms are revolutionizing precision metrology, bio-imaging, atomic and molecular sensing, and ultrafast photonics, yet most remain confined to the near-infrared. This restriction prevents access to the ultraviolet, visible, and mid-infrared bands critical for a vast array of quantum, atomic, and molecular systems. The fundamental obstacle has been the lack of a nanophotonic waveguide that simultaneously provides an ultra-broad transparency window, engineered dispersion, ultra-low propagation loss, and a strong Kerr nonlinearity, all while suppressing detrimental two-photon absorption at short wavelengths. Here, we overcome this challenge by exploring tantalum pentoxide for ultra-broadband supercontinuum spanning continuously from the ultraviolet to the mid-infrared, leveraging its broad transparency window (300-8000 nm), a high nonlinear refractive index three times larger than that of silicon nitride, and a wide bandgap that suppresses two-photon absorption. Critically, by using a photolithography assisted chemo-mechanical etching process that avoids a lossy SiO2 upper cladding, we achieve dispersion engineered waveguides with record-low propagation losses of 0.066 dB/cm at telecom wavelengths and 0.43 dB/cm at 780 nm, significantly facilitating the supercontinuum spectral extension into the ultraviolet and the mid-infrared. Pumping these anomalous-dispersion waveguides with femtosecond pulses at 1550 nm yields a gap-free, 3.2-octave supercontinuum spanning from 350 to 3200 nm via a soliton-based dynamics at only 54 pJ pulse energy, representing the broadest comb spectrum on this platform. We further demonstrate a relatively flat spectrum with a -30 dB bandwidth of 1182 nm by engineering normal dispersion, validate the comb coherence via heterodyne detection, and achieve soliton-effect pulse self-compression from 126.7 fs to 19.2 fs.

• The paper demonstrates ultra-broadband supercontinuum generation spanning 350–3200nm in Ta₂O₅ waveguides with record-low propagation loss.
• It employs a photolithography-assisted chemo-mechanical etching process to create ultra-smooth, dispersion-engineered waveguides that enable efficient nonlinear dynamics.
• Coherence and soliton-effect pulse compression are validated through heterodyne detection and numerical modeling, advancing chip-scale optical frequency combs.

Ultra-Broadband Supercontinuum Generation in Low-Loss Ta₂O₅ Nanophotonic Waveguides

Overview and Motivation

This paper addresses a fundamental challenge in photonic integrated platforms: the generation of optical frequency combs (OFCs) spanning ultraviolet (UV), visible, near-infrared (NIR), and mid-infrared (mid-IR) bands. Existing solutions are largely confined to the NIR, limiting applicability in quantum information, optical clocks, and molecular spectroscopy where UV and mid-IR access is crucial. The key innovation presented is the use of tantalum pentoxide (Ta₂O₅) waveguides, fabricated using a photolithography-assisted chemo-mechanical etching (PLACE) process, achieving exceptional optical properties combined with record-low propagation loss. This enables supercontinuum generation with unprecedented spectral coverage.

Materials and Fabrication Advances

Ta₂O₅ is demonstrated as an ideal platform, combining broad transparency ($300-8000\,\text{nm}$), high refractive index ($n \approx 2.07$), wide bandgap ($4.3\,\text{eV}$), and substantial third-order nonlinearity ($n_2 = 7.8 \times 10^{-19}\,\text{m}^2/\text{W}$)—three times higher than silicon nitride. The PLACE process achieves sidewall roughness of $0.34\,\text{nm}$, which drastically suppresses scattering loss and facilitates precise dispersion engineering. The rib waveguides exhibit propagation loss of $0.066\,\text{dB/cm}$ at telecom wavelengths, a significant improvement over prior Ta₂O₅ devices, and $0.43\,\text{dB/cm}$ in the visible ($780\,\text{nm}$), doubling the state-of-the-art performance.

Dispersion Engineering and Device Architecture

Dispersion profiles are tailored via geometric control of waveguide parameters, enabling both anomalous and flattened normal dispersion regimes. The fabricated devices utilize adiabatic tapering to optimize fiber-chip coupling while maintaining low overall loss. The ultra-smooth and crack-free Ta₂O₅ films do not require high-temperature annealing or lossy SiO₂ cladding, eliminating OH absorption and allowing extension of the supercontinuum into the mid-IR.

Supercontinuum Generation: Experimental Results

Pump pulses (126.7\,fs duration, centered at $1550\,\text{nm}$, with pulse energies as low as $54\,\text{pJ}$) drive soliton-based nonlinear dynamics in the anomalous-dispersion waveguides. This results in a seamless, gap-free supercontinuum spanning $n \approx 2.07$0 (3.2 octaves), representing the broadest comb spectrum reported for Ta₂O₅ platforms. Efficient spectral broadening at low pulse energies is enabled by minimized propagation loss and high nonlinearity.

Through further dispersion engineering, a flattened supercontinuum spectrum with $n \approx 2.07$1 bandwidth of $n \approx 2.07$2 ($n \approx 2.07$3) is obtained, underlining the agility of spectral shaping afforded by this platform.

Coherence Validation and Pulse Compression

Coherence is validated by heterodyne detection at $n \approx 2.07$4, with beat notes corresponding to the pump laser's repetition rate ($n \approx 2.07$5) and narrow linewidths ($n \approx 2.07$6), confirming inheritance of the comb structure and high coherence of the generated supercontinuum. Soliton-effect pulse compression achieves output pulse durations as short as $n \approx 2.07$7, corresponding to a compression ratio of $n \approx 2.07$8.

Nonlinear Dynamics and Numerical Modeling

Modeling via the nonlinear Schrödinger equation (NLSE), using accurate experimental parameters ($n \approx 2.07$9, dispersion, and pulse energies), reproduces observed spectra and dynamics. The interplay of self-phase modulation (SPM), soliton fission, and dispersive wave generation is elucidated; SPM dominates early spectral broadening, while soliton fission and dispersive waves extend coverage into UV and mid-IR. Third-harmonic generation (THG) is observed via phase-matched interactions between the pump and high-order modes, with distinct peaks near $4.3\,\text{eV}$0 and $4.3\,\text{eV}$1.

Practical and Theoretical Implications

The demonstrated Ta₂O₅ platform establishes a new paradigm for chip-scale OFCs seamlessly bridging UV to mid-IR. Key practical implications include:

• Quantum technologies: Full spectral access enables multiple atomic transitions for state preparation and quantum logic.
• Optical clocks: Wide coverage allows simultaneous referencing and stabilization, with potential for portable, integrated devices.
• Precision spectroscopy: Mid-IR extension facilitates molecule-specific detection and quantification.
• Integrated photonics: Low-loss, high nonlinearity, and efficient fabrication enable scalable and robust devices for on-chip applications.

Theoretically, this work advances understanding of nonlinear dynamics in broadband dispersion-engineered platforms, with implications for soliton control, spectral shaping, and f-3f self-referencing via THG for fully stabilized combs.

Future Directions

The current implementation's mid-IR edge is limited by the silica substrate’s absorption beyond $4.3\,\text{eV}$2, but transfer to sapphire substrates will extend coverage to $4.3\,\text{eV}$3 and beyond, further enhancing utility in molecular spectroscopy. The low-residual-stress, wafer-scale film deposition facilitates monolithic integration and potential 3D stacking. The platform’s robustness and agility in spectral shaping will catalyze the development of multi-octave OFCs for quantum information, metrology, and integrated photonics.

Conclusion

This paper demonstrates ultra-broadband, coherent supercontinuum generation in low-loss, dispersion-engineered Ta₂O₅ nanophotonic waveguides, spanning $4.3\,\text{eV}$4 with high coherence and efficient pulse compression. The synergy of advanced nanofabrication, optimal material properties, and precise dispersion management enables a versatile, scalable platform for chip-scale OFCs, addressing critical needs across quantum, atomic, and molecular photonics. The implications extend to practical and fundamental advances in photonic integration and nonlinear optics.