Watt-level second harmonic generation in periodically poled thin-film lithium tantalate

Abstract

Second-harmonic generation (SHG) is a fundamental tool in modern laser technology, enabling coherent frequency conversion to remote optical bands, serving as the basis for self-referenced femtosecond lasers and quadrature-squeezed light sources. State-of-the-art SHG relies on bulk crystals and ridge waveguides, although continuous-wave (CW) SH efficiency in bulk crystals is limited by short interaction lengths and large mode areas. Ridge waveguides offer better performance with lower pump power requirements, yet must span several centimeters to deliver high output power, complicating fabrication and narrowing the bandwidth. Recently, SHG in periodically poled thin-film lithium niobate integrated photonic circuits has attracted significant interest, offering orders-of-magnitude improvement in SHG under CW pumping due to the stronger optical mode confinement. However, lithium niobate has a low optical damage threshold, even in MgO-doped substrates, which limits SH power output to well below the watt level. Here, we overcome this challenge and demonstrate 7 mm-long periodically poled thin-film lithium tantalate (PPLT) waveguides that achieve high SH output in the CW regime, with generated power exceeding 1 W and off-chip output above 0.5 W at 775 nm under 4.5 W pump power. PPLT offers a higher optical damage threshold than PPLN and supports watt-level operation. By optimizing electrode geometry and poling conditions, we obtain reproducible poling despite lithium tantalate's coercive field being nearly four times higher than that of MgO-doped lithium niobate. Although its effective nonlinearity is more than five times lower, we achieve watt-level CW output with a short waveguide, demonstrating the potential of PPLT circuits for high-power applications in integrated lasers, quantum photonics, AMO physics, optical clocks, and frequency metrology.

Figures (4)

• Figure 1: Second harmonic generation in periodically poled thin-film lithium tantalate waveguides.(a) Numerical simulations of the effective index of the first six modes in the LiTaO$_3$ waveguide reported in this work. The data for the fundamental TE mode is highlighted. Colors indicate the polarization ratio of each mode. (b, c) Mode field profiles for pump and SH waves, respectively. (d, e) Dependence of the poling period on the waveguide width (the slab thickness is fixed) and on the slab thickness (the waveguide width is fixed), respectively; the waveguide height is fixed in both cases. (f) A focus-stacked image of a PPLT chip emitting red light produced by the SHG process in a thin-film PPLT waveguide. A PPLT chip, designed for a pump wavelength of 1307nm, is presented in this image, as emission at 775nm is already cut by filters preinstalled in modern cameras and cannot be captured. (g, h) Performance comparison of a ridge PPLN waveguide (based on the data from Ref. kashiwazaki_high-gain_2021) and the thin-film PPLT waveguide from this work, respectively.
• Figure 2: Periodic poling of thin-film LiTaO$_3$.(a) Type 0 SHG process in crystals or waveguides made of non-centrosymmetric materials with different phase-matching conditions. Inset on the left -- energy conservation schematic in the SHG process. Insets on the right -- momentum conservation conditions for different types of phase-matching. $\Lambda$ is a single QPM period. (b) Comparison of typical coercive field magnitudes reported in literature for bulk stoichiometric (S), congruent (C), and MgO-doped congruent (xMgC) compounds of LiNbO$_3$ (LN) and LiTaO$_3$ (LT) at room temperature kim_coercive_2002ishizuki_periodical_2003ishizuki_mg-doped_2008. (c) A schematic of the setup for periodic poling. AFG: arbitrary function generator. HVA: high-voltage amplifier. EC: electric circuit. OSC: oscilloscope. (d) A typical signal recorded on the oscilloscope during poling, indicating a sequence of applied poling pulses and the collected current produced by domain inversion and the strain circuit capacitance. (e) A focus-stacked image of a set of test poling electrodes, and two electric probe needles brought into a contact with the electrode pads. (f) Four main steps of the waveguide fabrication process, and the material stack of the sample at each step. First, the electrodes are patterned and entire chip is covered with photoresist; in the next step, the pads for probes are opened and poling is performed. Finally, the waveguides are fabricated using dry etching and, optionally, cladded with silicon dioxide. (g) Two-photon microscope images showing a few domain-inverted structures obtained, among numerous others, during poling optimization which includes the optimization of both the electrode shape and the high-voltage poling signal. First images show chaotic inversion, and the rightmost image demonstrates one of the best poling results that can be routinely achieved after our poling technique has been optimized. (h) An optical microscope image of comb-shaped electrodes for periodic poling. A testing 1mm-long electrode is shown; 7mm-long electrodes were used for the actual waveguides -- no dependence on the electrode length is observed for periodic poling with the optimized conditions. The edges of the photoresist layer are visible along the boundaries of the pads where the photoresist is removed. (i) A false-colored SEM image of a part of the same electrode shown in (h).
• Figure 3: Inspection of thin-film PPLT waveguides and SHG efficiency measurements.(a) A two-photon microscope image of inverted domains before the waveguide fabrication. (b) Same as (a), but after the waveguide fabrication was complete. (c) A false-colored SEM image of the waveguide cross-section. (d) A false-colored SEM image of the waveguide sidewalls, demonstrating the absence of etching-induced corrugation or roughness. (e) An SEM image of a waveguide that allows fast high-resolution observation of periodic domain inversion. (f) Same as (e), but with a different sample, a different angle, and different magnification. (g) Experimental setup used to measure the SHG efficiency. DVOA: digital variable optical attenuator. PC: polarization controller. COL: collimator. PM: powermeter. (h) SH power measured and calibrated after varying the pump power. Efficiency is extracted from the slope of the red solid curve.
• Figure 4: Generation and measurement of watt-level CW SH emission.(a) Extended experimental setup for high-power CW optical pumping. EDFA: erbium-doped fiber amplifier. TBPF: tunable bandpass filter. VOA: variable optical attenuator. PC: polarization controller. COL: collimator. DM: dichroic mirror. PM: powermeter. (b) Direct off-chip power measurements of SH and residual pump. (c) Fully-calibrated on-chip power measurements of SH and residual pump, and results of numerical modeling (solid lines). (d) SH spectra measured in different waveguides with varied poling periods. (e) Numerical simulations of potential SHG performance with improved efficiency in waveguides of different lengths under varying pump powers. The red star indicates the results in this work.