Author: Dr. Noa Voloch Bloch and Ori Levin
Raicol, a renowned leader in PPKTP manufacturing, is committed to propelling the quantum industry forward through state-of-the-art solutions. Over the past few years, our relentless pursuit of excellence has led to significant enhancements in our manufacturing capabilities. Notably, we have dedicated substantial efforts to refining our proficiency in producing shorter poling periods.
While achieving short poling periods for individual elements with low yields is indeed feasible, the real challenge lies in scaling this capability for mass production and making it accessible to our valued customers at a competitive price point. Our engineer, Mr. Michael Schindler, has achieved a breakthrough by developing a process that enables extremely short poling periods (a full cycle of approximately 1 µm). As a result, we are proud to offer a standard poling period of less than 3 µm.
At Raicol, we remain committed to pushing the boundaries of what’s possible in the quantum industry, and our dedication to innovation continues to drive our success.
Narrow bandwidth SPDC
Raicol recently experienced a significant increase in demand for PPKTP crystals with extremely short poling periods. This demand was primarily driven by applications in narrow-band “Counter Propagation SPDC,” which can achieve bandwidths as low as 0.06 nm [1,2]. The importance of these narrow bandwidths lies in their potential for integrating SPDC sources with quantum memories, where matching the bandwidths of atomic transitions and exciting radiation is crucial. The relationship between crystal length and SPDC bandwidth is inverse, with longer crystals producing narrower bandwidths. While Raicol continues to improve its capabilities in this area, the company currently offers PPKTP crystals up to 30 mm in length. This development demonstrates Raicol’s ongoing commitment to advancing quantum technologies and meeting the evolving needs of the photonics industry. As the field of quantum optics continues to progress, Raicol remains dedicated to exploring innovative solutions and pushing the boundaries of PPKTP crystal manufacturing. The company’s efforts in producing crystals with extremely short poling periods and longer lengths are contributing to the advancement of quantum memory integration and other cutting-edge applications in quantum technologies.
Production validation to short poling
As a leader in nonlinear crystal manufacturing we maintain rigorous quality control procedures, especially when developing new crystals or cutting-edge technologies. When trying to implement short poling we faced challenge in characterizing and validating short-polled KTP crystals. While Raicol’s measuring testing systems are typically designed for forward-propagating Second Harmonic Generation (SHG) and Spontaneous Parametric Down-Conversion (SPDC) across various wavelengths. However, the short-period polled KTP crystals presented a distinct obstacle. The absence of forward SHG/SPDC processes corresponding to the short period, due to their occurrence near the edge of the KTP transmission window, made conventional collinear second harmonic processes unsuitable for evaluation of 1.2 µm polled crystals. This situation underscores the complexity of validating advanced crystal technologies and highlights Raicol’s commitment to ensuring product performance through innovative testing methodologies.
A. Backward second harmonic generation
There are two distinct types of collinear second harmonic generation SHG processes [3]: forward and backward propagation:
Forward Second Harmonic Generation: In this process, the beam at the doubled frequency propagates in the same direction as the beam at the fundamental frequency. In the case of PPKTP the shortest poling period enabling efficient frequency doubling from 800 nm pump to 400 nm is approximately 3 µm.
Backward Second Harmonic Generation: In this process, the generated wave propagates in the opposite direction of the pump.
However, the periods necessary for efficient backward second harmonic generation for visible wavelengths are extremely short and pose significant manufacturing challenges. For example, generating a second harmonic of 829 nm light in the first-order QPM requires a period of 0.109 µm.
B. Backward propagation test bench
In order to characterize our 1.2 short poling crystals we measure SH backward propagation, we evaluated the ΔK that matches the phase matching values of 1.2 µm grating periods. For the ΔK values shown in the graph there is no forward process. But for the 829 nm converted to 414.4 nm at 33 deg PPKTP
The ΔK is 5.754⋅10^7 [1/m] specifically suites phase matching of the 11-th order shown in the graph. Backward second harmonic generation of 829 nm converted to 414.5nm [3].
To overcome this challenge, we implement a backward propagation test system, as outlined below: The 829 nm pump beam enters the crystal and the SH which propagates backward is deflected by a dichroic mirror to the detector.
[1] Liu, YC., Guo, DJ., Ren, KQ. et al. Observation of frequency-uncorrelated photon pairs generated by counter-propagating spontaneous parametric down-conversion. Sci Rep 11, 12628 (2021).
[2] Zhang, H., Jin, XM., Yang, J. et al. Preparation and storage of frequency-uncorrelated entangled photons from cavity-enhanced spontaneous parametric downconversion. Nature Photon 5, 628–632 (2011).
[3] S. Moscovich, A. Arie, R. Urenski, A. Agronin, G. Rosenman and T. Rosenwaks, “Noncollinear second harmonic generation in sub-micrometer poled RbTiOPO4”, Optics Express, 12, 2236-2242 (2004).
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