Thorlabs' PPKTP crystals for Spontaneous Parametric Down Conversion (SPDC) are designed for type-0 or type-II SPDC, a nonlinear process where a single high-energy pump photon is down converted into a signal and idler pair of lower-energy photons, conserving both energy and momentum. SPDC is commonly used as a source for polarization-entangled photon pairs or as a heralded single-photon source. Type-0 SPDC outputs signal and idler photons with the same polarization as the pump polarization; where the polarizations for the pump, signal, and idler photons are all oriented in the vertical direction. Type-II SPDC outputs signal and idler photons with polarizations perpendicular to each other. Crystals are available with lengths from 5 mm to 30 mm for type-0 or type-II SPDC with pump wavelengths of either 405 nm or 775 nm.
Features:
- Periodically Poled Potassium Titanyl Phosphate (PPKTP) Nonlinear Crystals
- Optimized for Type-0 or Type-II Spontaneous Parametric Down Conversion
- Drop-In Compatibility with Nonlinear Crystal Ovens
- Source for Polarization Entangled Photons or Heralded Single Photons from 405 nm or 775 nm Pump Lasers
In the SPDC process, a small fraction of pump photons are converted by the crystal into a pair of signal and idler photons. To distinguish between pump and signal/idler photons (especially for collinear SPDC), a high optical density long-pass filter can be used to suppress the pump. Thorlabs recommends our high extinction filters, which have an OD of 10 or greater at the pump wavelengths. Once the pump is filtered out, a highly sensitive detector is required to resolve the single-photon output, like our SPDMA single photon detection module. For a pre-configured SPDC solution, these crystals are integrated into our turnkey correlated photon-pair sources.
Image above: An NLCK16 SPDC crystal is shown mounted in an NLCH1 nonlinear crystal oven for temperature control.
SPDC and Crystal Length Dependence
SPDC is the reverse process of sum frequency generation (SFG) and, in the degenerate case, second harmonic generation (SHG). The periodically poled design of these crystals results in greater photon generation efficiency (more signal/idler photon counts) for longer crystal lengths. Shorter crystals exhibit broader bandwidth and require a tighter focus for optimal efficiency. The properties of the pump laser also play an important role in the efficiency of the SPDC process. For more information on SPDC dependence on the pump laser, periodic poling, and SPDC crystal design, please see the SPDC Tutorial tab. For more details on experimental conditions and considerations to optimize SPDC efficiency and heralding ratio, see Bennink 20101
Mounting and Temperature Control
The crystals are each mounted in housings with a clear aperture of >80% of the 1.0 mm x 2.0 mm crystal face area. We also offer empty versions of these housings for use with other crystals. Both the mounted crystals and the empty housings may be secured to a kinematic mount (sold below) for angle of incidence adjustment.
Each PPKTP crystal is optimized for degenerate SPDC at around 60 °C with the design pump wavelength and therefore generally requires an oven for use. We offer the NLCH1(/M) and NLCH2(/M) nonlinear crystal ovens that are designed for simple drop-in compatibility with these mounted PPKTP crystals (see image above) in order to hold the temperature of the crystals constant for repeatable SPDC photon generation.
References
1. Ryan S. Bennink, "Optimal collinear Gaussian beams for spontaneous parametric down-conversion". Physical Review A 81, 053805 (2010)
Typical Applications:
- Entangled Photon Pair Generation
- Heralded Single-Photon Sources
- Quantum Key Distribution (QKD) and Quantum Communications
- Squeezed-Light Generation and Quantum Metrology
Key Common Specifications
- Material PPKTP (Periodically Poled Potassium Titanyl Phosphate)
- Crystal Face Dimensions 1.0 mm x 2.0 mm
- Clear Aperture >80% of Crystal Face Area
- Surface Flatness λ/6 @ 633 nm
- Surface Quality 10-5 Scratch-Dig
- Phase Matching Temperature for Degeneracy1 60 ± 10 °C
- This is the theoretical temperature where the signal and idler photons will have the same wavelength when pumped at the design wavelength. Actual degeneracy temperature will vary.
For further information, please contact