DKDP Crystal: "Brilliant and Enduring"​

DKDP Crystal: "Brilliant and Enduring"​​

Development History​

​

DKDP is a high-performance crystal possessing both electro-optic and nonlinear characteristics. As a typical synthetic crystal, the growth and application of DKDP have a considerable history, dating back to its predecessor, KDP (Potassium Dihydrogen Phosphate) crystal. KDP was one of the first functional crystals to gain significant attention, with its application history traceable to the 1950s, primarily for manufacturing sonar and civilian piezoelectric transducers.

With the advent of laser technology in the 1960s, KDP crystal, known for its large nonlinear optical coefficient, high laser-induced damage threshold, broad transmission range, and high electro-optic coefficient, became commonly used as a frequency doubling material and electro-optic material for Nd:YAG lasers. However, as laser power increased, severe stimulated Raman scattering damage and high infrared absorption in KDP crystals were identified, significantly limiting the advancement of laser technology.

Subsequent extensive research revealed a strong correlation between stimulated Raman scattering and hydrogen ions. This led to attempts to grow crystals in heavy water solutions by replacing the hydrogen in the KDP crystal with deuterium. The resulting new crystal was named DKDP. As anticipated, testing showed that DKDP crystals exhibited substantially reduced stimulated Raman scattering damage. Furthermore, due to the isotope effect, the infrared absorption coefficient of DKDP crystals was reduced by an order of magnitude compared to KDP crystals.

Additionally, DKDP crystals offer a wider laser transmission band, superior electro-optic coefficients, and a lower half-wave voltage (see Table 1) compared to KDP crystals. Consequently, DKDP crystals are more suitable than KDP crystals for fabricating various electro-optic devices, holding significant experimental value and scientific importance for laser applications [1].

Basic Properties of DKDP Crystals​​

Potassium Dideuterium Phosphate (abbreviated as DKDP or KD*P) exhibits two crystalline phases. The commonly grown and utilized form is the tetragonal phase, which belongs to the same symmetry class as KDP crystals. The ideal crystal morphology of DKDP consists of a combination of a tetragonal prism and a tetragonal bipyramid. DKDP crystals demonstrate excellent nonlinear and electro-optic properties, including low half-wave voltage, high linear electro-optic coefficients, a broad transmission bandwidth, and the capability to grow large-sized crystals with high optical quality [2].

Table 1. Basic Properties of DKDP and KDP

Applications of DKDP ​​

DKDP crystals are primarily applied in two areas:

As ​​electro-optic crystals​​ for manufacturing electro-optic modulators.

As ​​nonlinear crystals​​ for producing large-aperture, high-energy laser frequency doublers.

1. Excellent Electro-Optic Material with Transmission from UV to NIR

When utilized as an electro-optic crystal, DKDP operates based on the primary electro-optic effect. Applying an electric field in a specific direction alters the refractive index ellipsoid of the crystal. Due to this change in refractive index distribution, the ordinary (o) and extraordinary (e) rays travel at different velocities within the crystal, resulting in a phase difference at the same position. This phase difference depends on the applied electric field strength, allowing DKDP to function as an optical waveplate with tunable phase retardation, thereby modulating polarized light. Figure 1 shows the working schematic of an electro-optic Q-switching setup with DKDP crystal in the laser cavity.

Figure 1. The working schematic of an electro-optic Q-switching setup with DKDP crystal

Owing to its exceptional performance, DKDP crystals are widely used in intracavity Q-switching, high-speed optical shutters, pulse picking, and regenerative amplification.

CASTECH can design and manufacture DKDP electro-optic modulators with custom clear apertures, lengths, and operational wavelengths (covering 343–1600 nm) to meet specific client requirements.

Figure 2. DKDP Electro-optic Modulators Fabricated by CASTECH

2. Application in High-Power Laser Frequency Doublers

Over the past half-century, advancements in synthetic crystals have led to the emergence of various new materials, posing significant challenges to DKDP crystals. Crystals such as LBO and BBO, with their larger nonlinear coefficients and higher laser-induced damage thresholds, have gradually replaced DKDP in nonlinear applications.

However, with the development of Inertial Confinement Fusion (ICF) technology, DKDP has regained prominence due to its unique capability to grow large-aperture crystals (exceeding 400 mm). Considering various performance metrics and growth conditions, high-quality large-sized DKDP crystals have become the preferred nonlinear optical material for ICF facilities. Currently, the United States, Russia, and China are actively researching the growth technology of high-quality large-sized DKDP crystals to meet ICF engineering demands. Thus, the growth of large-aperture DKDP crystals has become a cutting-edge research focus.

As early as the 1960s, researchers at the Fujian Institute of Research on the Structure of Matter(FIRSM) began studying the growth of KDP/DKDP-type crystals, achieving remarkable results. Since its establishment, CASTECH has commercialized DKDP crystals. While ensuring stable batch production, CASTECH remains committed to technological innovation. In 2002, the company successfully grew a DKDP crystal with an aperture of 150 mm × 150 mm, meeting all technical targets and passing the acceptance organized by the Fujian Provincial Science and Technology Commission for the project "Development of High-Quality, Large-Aperture DKDP Crystals." CASTECH's R&D team continues to strive for advancements, addressing the gap in large-aperture DKDP crystal growth technology and dedicating efforts to developing even larger aperture DKDP crystals.

Figure 3 DKDP crystal Grown by CASTECH

Overcoming Processing Challenges​​

​​

In practical laser applications, whether used as electro-optic or nonlinear crystals, DKDP crystals require not only large size and high internal quality but also precise deuterium content, processing metrics, and coating quality to ensure excellent optical performance and long lifespan.

• High Deuterium Content

Deuterium substitution for hydrogen effectively reduces stimulated Raman scattering damage in DKDP crystals. CASTECH-grown DKDP crystals can achieve deuterium content exceeding 98%, tailored to client requirements.

• High Demands on Technique and Skill

DKDP is highly soluble in water and relatively soft and brittle, with a Mohs hardness of approximately 2.5. These properties make cutting, rough grinding, and polishing processes extremely challenging, requiring advanced techniques and skilled personnel. For rod-shaped products commonly used in electro-optic applications, improper force control and monitoring during the rounding process can lead to significant deviations between the axis and the normal to the light-passing surface, affecting optical path alignment. Through years of technical accumulation, CASTECH has mastered the batch processing technology for DKDP crystals, meeting all required specifications. The directional accuracy of rounded products is controlled within ±0.5°, and for special requirements, it can be maintained within ±0.15°.

Table 2. Specifications of finished DKDP

• High-Quality Coating Technology

DKDP crystals are highly hygroscopic and prone to phase transition above 100°C, leading to performance degradation. Conventional crystal coating processes typically involve temperatures above 200°C, making it essential to develop alternative methods. The challenge lies in preparing optical films on DKDP crystals that offer excellent spectral characteristics (e.g., high transmittance), mechanical properties (e.g., adhesion, film hardness), and high laser-induced damage thresholds, all while preventing deliquescence and phase transition. CASTECH's engineering team has developed specialized high-transmittance coatings for DKDP crystals through years of dedicated research, making CASTECH one of the few optoelectronic enterprises globally to master high-quality DKDP coating technology.

Figure 5. DKDP Crystal Coating Specifications: AR-1064/(1040-1150)(630-660) nm

Over the past half-century, numerous optical crystals have emerged, yet the "veteran" DKDP crystal remains "Brilliant and Enduring". This enduring popularity stems from DKDP's excellent optical properties and continuous advancements in processing technologies. CASTECH is proud to have played a significant role in the development of DKDP technology.

References​​

[1] ZHANG Ke Cong, ZHANG Le Hui. Crystal Growth Science and Technology[M]. Science Press, 1997.

[2] ZHANG Ke Cong. Fundamentals of Modern Crystallography[M]. Science Press, 1987.