Intense ultraviolet-visible-infrared full-spectrum laser

Ultraviolet-visible-infrared (UV-Vis-IR) full-spectrum optical spectroscopy is critical for probing microscopic physical, chemical, and biological systems. Conventional approaches rely on multiple discrete coherent sources and mechanical tuning, which is complex. Broad spectrally coherent sources that cover multiple absorption bands are indispensable for resolving multiple dynamic processes across gases, plasmas, liquids, and solids. Despite advances in laser technology and nonlinear optics, directly generating a high-brightness UV-Vis-IR full-spectrum white laser remains challenging. This work aims to realize a high-energy, smooth, multioctave supercontinuum spanning from UV to mid-IR in a single source, addressing spectral gaps and efficiency limitations inherent to prior methods.

Two main approaches have been pursued for supercontinuum white laser generation: (1) Third-order nonlinearity (3rd-NL)-based optical supercontinuum generation (SCG) using self-phase modulation and related effects in microstructured fibers, long-path gas-filled hollow-core fibers, multiple thin silica plates, or bulk media. These face constraints in spectral bandwidth, flatness, and pulse energy due to small mode areas and complex dispersion engineering. (2) Second-order nonlinearity (2nd-NL)-based frequency conversion (SHG, SFG, THG, high harmonic generation) in natural nonlinear crystals or engineered structures such as chirped periodically poled lithium niobate (CPPLN) via phase matching or quasi-phase matching (QPM). These suffer from limited pump bandwidth, constrained QPM working bandwidth, and reduced efficiency for higher-order harmonics, limiting spectral and power scaling.

Concept and conditions: The authors identify two preconditions for gap-free multioctave spectra via high-harmonic generation (HHG): (i) a pump bandwidth satisfying λ2 ≥ 2λ1 (one-octave pump), exemplified by 2500–5000 nm; (ii) a nonlinear crystal providing extremely broad up-conversion bandwidth via multiple QPM reciprocal-lattice vector (RLV) bands. They exploit synergistic 2nd-order (HHG) and 3rd-order (SPM) nonlinearities, where SPM broadens both the pump and the generated harmonics, enhancing bandwidth and spectral smoothness through overlap of broadened harmonic bands.

System architecture: A cascaded HCF–LN–CPPLN module is used. Stage 1: A noble gas-filled hollow-core fiber (HCF) receives an intense 3.9 µm mid-IR femtosecond pump (from an OPCPA, 3.3 mJ) to induce SPM-driven spectral broadening. Stage 2: A bare bulk lithium niobate (LN) plate further broadens the spectrum (SPM) to meet the one-octave requirement. Stage 3: A specially designed chirped periodically poled lithium niobate (CPPLN) crystal enables efficient 2nd–10th HHG via multiple QPM RLV bands, while strong peak powers drive additional SPM in the harmonics, smoothing the final spectrum.

CPPLN design and parameters: The CPPLN has a chirped poling period from 41 µm (input side) to 23.79 µm (output side) with chirp rate Dφ = 5.5 µm⁻². Dimensions: 20 mm × 6 mm × 2 mm. Fabricated via electric poling. The Fourier-transform of the second-order nonlinear susceptibility shows six continuous RLV bands with appreciable effective nonlinear coefficients: B1 [0.1–0.3 µm⁻¹] with χeff ≈ 0.035 d33; B2 [0.3–0.8 µm⁻¹], B3 [0.8–1.06 µm⁻¹], B4 [1.06–1.6 µm⁻¹], B5 [1.6–2.1 µm⁻¹], B6 [2.1–3.14 µm⁻¹] with χeff ≈ 0.003–0.01 d33. The SHG phase mismatch in homogeneous LN is fully covered by B1, ensuring broadband QPM SHG across the pump; the ensemble of bands supports efficient ultrabroadband 2nd–10th HHG via cascaded three-wave mixing.

Experimental setup and conditions: The pump is a home-built OPCPA at 3.9 µm with 3.3 mJ pulse energy. After the HCF (filled with Krypton at 2.2 bar), SPM broadening yields a 1.75 mJ pulse with spectrum from 2.8–4.8 µm. Passing through the bare LN further broadens to cover 2.5–5.0 µm with 1.15 mJ pulse energy, providing the one-octave pump for CPPLN. The CPPLN output exhibits bright white-light emission; a diffraction grating confirms smooth visible components from violet to red. Spectral characterization employs three spectrometers to cover UV–Vis–IR; spectra are normalized for display.

• Achieved a four-octave-spanning supercontinuum from 300–5000 nm at −25 dB relative to peak, with notable spectral flatness, smoothness, and continuity across UV–Vis–IR.
• Output pulse energy after CPPLN: 0.54 mJ per pulse, pumped by a 1.15 mJ octave-spanning mid-IR source (2.5–5.0 µm) produced by HCF + LN SPM broadening.
• HCF stage (Krypton 2.2 bar) broadened the 3.9 µm pump to 2.8–4.8 µm at 1.75 mJ; LN stage extended coverage to the required 2.5–5.0 µm range with improved spectral balance.
• CPPLN with six broad QPM RLV bands enabled efficient simultaneous 2nd–10th HHG; synergistic 2nd-NL and 3rd-NL effects further broadened and smoothed the spectrum.
• Measurement limited to ≤5000 nm by available spectrometers; actual generation likely extends beyond 5000 nm.
• Compared to prior art (fiber SCG with picosecond-level energies or bulk-based narrower bandwidths), this system delivers both high pulse energy and broad, flat multi-octave spectra.

The work fulfills the longstanding goal of a high-brightness, gap-free UV–Vis–IR white laser by meeting two key conditions: an octave-spanning mid-IR pump (2.5–5.0 µm) and a nonlinear medium (CPPLN) with exceptionally broad QPM bandwidth supporting multiple simultaneous harmonic orders. The synergy between second-order HHG and third-order SPM broadening, acting both on the pump and on the generated harmonics, enables spectral overlap and smoothing across harmonic bands. The result is a continuous, flat supercontinuum covering 300–5000 nm at −25 dB with 0.54 mJ pulse energy. This approach overcomes limitations of purely 3rd-NL SCG (energy and dispersion constraints) and purely 2nd-NL QPM schemes (bandwidth and efficiency limits), providing a powerful source for full-spectrum ultrafast spectroscopy with potential impact in physics, chemistry, biology, materials science, industrial processing, and environmental monitoring. The comparison to prior methods underscores the simultaneous achievement of wide bandwidth and high pulse energy.

The authors demonstrate an intense full-spectrum femtosecond laser source spanning 300–5000 nm at −25 dB with 0.54 mJ per pulse by cascading a gas-filled HCF, a bare LN crystal, and a specially designed CPPLN. The system exploits synergistic second- and third-order nonlinearities, with an octave-spanning mid-IR pump and multiple QPM bands enabling efficient 2nd–10th HHG and SPM-induced spectral smoothing. This architecture delivers a high-brightness, flat, multi-octave supercontinuum suitable for broad applications in ultrafast, full-spectrum spectroscopy across many disciplines. The authors note that the generated spectrum likely extends beyond 5000 nm, suggesting further potential for even broader coverage and application reach.

Spectral measurement was limited to wavelengths ≤5000 nm by the available spectrometers, so longer-wavelength components could not be verified despite likely presence. The authors also note that, in general, realizing the required one-octave pump bandwidth and extremely broad QPM bandwidth in practice is technically challenging.