SFG – Sum Frequency Generation

Sum Frequency Generation (SFG) is a nonlinear optical process where two input photons with different frequencies (ω₁ and ω₂) combine in a nonlinear crystal to generate a new photon with a frequency equal to their sum (ω₃ = ω₁ + ω₂). Second Harmonic Generation (SHG) is a special case of SFG where both input photons have identical frequencies, resulting in an output photon with exactly twice the frequency (and half the wavelength) of the original beam.

Raicol’s crystal solutions excel in both processes: Our KTP crystals offer SHG coefficients approximately three times higher than KDP, making them ideal for laser frequency doubling applications. For advanced applications, our HGTR KTP crystals enable high-power SHG with average output power densities up to 5 kW/cm² at 532 nm, while our PPKTP crystals provide flexible phase-matching for both SFG and SHG across the 350-4000 nm transparency range. The quasi-phase-matching capability of PPKTP eliminates walk-off effects and offers about three times larger effective nonlinear coefficients compared to bulk KTP.

SFG serves as a powerful tool in advanced spectroscopy and imaging, enabling researchers to probe molecular structures at interfaces and surfaces with exceptional sensitivity and selectivity. This technique is particularly valuable in surface science, biological imaging, chemical analysis, and materials characterization, where it reveals molecular interactions and structural properties that conventional optical methods cannot easily access.

Raicol’s specialized crystals enhance SFG spectroscopy capabilities: Our BBO crystals feature very wide transparency ranges extending into the deep UV (190-1780 nm), making them ideal for broad-spectrum SFG applications. With high damage thresholds (5 GW/cm² at 1064 nm) and excellent optical homogeneity, BBO crystals ensure reliable performance in demanding spectroscopic setups. For mid-infrared applications, our PPLN crystals with their broad transparency range (420-5200 nm) and high nonlinear coefficient (d₃₃ ~27 pm/V) provide exceptional efficiency. Additionally, our Super Polished LBO crystals offer ultra-low surface roughness (≤3Å RMS) and bulk absorption as low as 2 ppm/cm at 1064 nm, minimizing noise and maximizing signal quality in sensitive spectroscopic measurements.

The efficiency and performance of SFG critically depend on selecting the appropriate nonlinear crystal for your specific wavelength range, power requirements, and application needs. Raicol offers a comprehensive portfolio of premium nonlinear crystals optimized for SFG applications:

• Potassium Titanyl Phosphate (KTP) – Our KTP crystals deliver high nonlinear coefficients, excellent thermal stability, and large apertures up to 50×50 mm. With laser damage thresholds of 1 GW/cm² and low absorption (<20 ppm/cm at 1064 nm), they’re ideal for efficient SFG in visible and near-infrared ranges.
• Periodically Poled KTP (PPKTP) – Raicol pioneered PPKTP technology over 20 years ago. Our PPKTP crystals offer flexible quasi-phase matching, enabling SFG, SHG, DFG, and OPO applications from visible to mid-IR without the limitations of critical phase matching. Available in Type-0 or Type-II configurations with custom apertures up to 2×10 mm².
• Beta Barium Borate (BBO) – Featuring very wide transparency (190-1780 nm) and high damage thresholds, our BBO crystals excel in ultraviolet and high-power SFG applications. They support harmonic generation from second through fifth harmonics of Nd:YAG lasers.
• Super Polished LBO – Our proprietary super-polished LBO crystals feature exceptional surface quality (≤3Å RMS roughness) and extremely low absorption, providing superior performance for high-power and precision SFG applications with damage thresholds up to 2500 MW/cm² at 1064 nm.
• PPLN (Periodically Poled Lithium Niobate) – With the highest nonlinear coefficient (d₃₃ ~27 pm/V) and broad transparency (420-5200 nm), our MgO:PPLN crystals are optimal for mid-infrared SFG and frequency conversion in compact, low-power systems.
• ppSLT (Periodically Poled Stoichiometric Lithium Tantalate) – Our advanced ppSLT crystals offer high photorefractive damage resistance and strong nonlinear coefficients (>7.5 pm/V), enabling high-power frequency conversion across UV to mid-IR ranges with exceptional thermal stability.

Achieving maximum SFG efficiency requires precise phase matching to maintain optimal phase relationships between interacting waves throughout the crystal. Raicol provides both critical and quasi-phase matching solutions tailored to your specific requirements:

• Critical Phase Matching (Angle Tuning) – Our bulk crystals like KTP, BBO, and LBO can be precisely oriented to achieve optimal phase matching angles. Raicol’s crystals feature exceptional optical homogeneity and strict angular tolerances (better than 30 arc seconds parallelism for KTP) ensuring consistent phase matching across the entire aperture.
• Quasi-Phase Matching (QPM) – Raicol’s pioneering PPKTP, PPLN, and ppSLT technologies revolutionize phase matching by using periodic domain inversion to compensate for phase mismatch. This approach offers several critical advantages:
  • Eliminates walk-off effects completely
  • Enables phase matching at any wavelength within the crystal’s transparency range
  • Utilizes the largest nonlinear coefficient (d₃₃) of the material
  • Allows Type-0 interactions for optimal beam quality
  • Supports custom poling periods for specific applications
• Advanced QPM Designs – Raicol offers sophisticated poling configurations including:
  • Aperiodic poling (apKTP) for tailored spectral properties and enhanced purity
  • Multi-period structures for simultaneous multiple wavelength conversion
  • Fan-out designs for tunable wavelength conversion
  • 2D poling patterns for complex beam shaping and spatial mode conversion

Our proprietary manufacturing processes ensure poling uniformity and crystal quality that maximize conversion efficiency while minimizing unwanted effects like beam distortion or spectral broadening.

SFG technology enables breakthrough capabilities across diverse industrial and scientific domains, with Raicol’s crystals powering applications worldwide in leading research institutions and commercial systems:

• Biomedical and Life Sciences – Our PPKTP crystals enable advanced nonlinear microscopy techniques for high-resolution biological imaging, allowing researchers to visualize cellular structures and molecular processes with minimal photodamage. The Type-0 and Type-II phase matching options provide flexibility for optimizing signal-to-noise ratios in challenging biological environments.
• Materials Science and Surface Analysis – BBO and LBO crystals from Raicol facilitate precise molecular structure analysis at surfaces and interfaces. The ultra-low absorption and exceptional surface quality of our Super Polished LBO (≤3Å RMS roughness) ensure measurement accuracy in demanding surface science applications.
• Quantum Photonics and Communications – Our quantum-grade PPKTP and apKTP crystals serve as the foundation for photon pair generation, quantum entanglement, and frequency conversion in quantum communication systems. Raicol Quantum division has optimized these crystals for leading quantum research labs globally, supporting applications from quantum computing to satellite-based quantum key distribution.
• Defense and Aerospace Laser Systems – HGTR KTP crystals designed by Raicol enable high-power laser frequency conversion for range finding, LIDAR, and directed energy applications. These crystals withstand power densities up to 5 kW/cm² while maintaining exceptional beam quality and long-term stability in harsh environments.
• Industrial Laser Manufacturing – Our KTP and PPKTP crystals power green laser systems used in precision micromachining, display technology, and materials processing. The non-hygroscopic nature and thermal stability of these crystals ensure reliable performance in industrial environments.
• Spectroscopy and Analytical Instrumentation – PPLN and ppSLT crystals enable mid-infrared frequency generation for chemical analysis, environmental monitoring, and process control applications. The broad transparency ranges and high nonlinear coefficients make possible compact, efficient analytical instruments.
• Telecommunications and Datacom – Raicol’s crystals support wavelength conversion technologies essential for optical networking, enabling efficient frequency translation between communication bands and supporting the infrastructure of modern telecommunications.