Efficient and broadband fourth-harmonic generation in quasi-phase-matched crystals

Area of Science:

• Nonlinear Optics
• Quantum Optics
• Materials Science

Background:

• Quasi-phase-matching (QPM) is crucial for efficient harmonic generation in nonlinear crystals.
• Conventional QPM schemes rely on discrete phase-matching conditions, limiting tunability and robustness.
• Cascaded harmonic generation involves multiple nonlinear processes.

Purpose of the Study:

• To propose and investigate an adiabatic scheme for cascaded fourth-harmonic generation (FHG).
• To achieve efficient and tunable frequency conversion using chirped dual-period QPM crystals.
• To enable robust energy transfer among optical harmonics.

Main Methods:

• Utilizing chirped dual-period quasi-phase-matched (QPM) crystals for fundamental-to-second-harmonic (FF→SH) and second-to-fourth-harmonic (SH→FH) interactions.
• Implementing an adiabatic evolution scheme in the multi-step frequency conversion process.
• Engineering QPM gratings to facilitate continuous adiabatic passage in phase-mismatch space.

Main Results:

• Achieved a maximum FHG efficiency of 94% under realistic conditions.
• Demonstrated a tunable bandwidth of approximately 13 nm.
• Showcased controllable generation of second- and fourth-harmonic signals.
• Observed robust and tunable energy transfer among harmonics.

Conclusions:

• The proposed adiabatic scheme offers a novel and efficient method for cascaded FHG.
• This approach overcomes limitations of conventional QPM by enabling continuous phase-matching.
• The engineered chirped QPM crystals provide tunable and efficient harmonic generation.