NTT-AT to sell samples of highly refractive optical crystal - KT crystal

Background and history

It is well know that high-quality KT crystal is highly transparent, free from optical anisotropy^*1 and highly refractive. However, it has been extremely difficult to grow large, high-quality KT crystals with high reproducibility. For this reason, it has been very difficult to use the material for lenses or optical components on a commercial base. NTT Photonics Laboratories have discovered that the precise control of temperature is extremely important in growing high-quality KT crystal, and have succeeded in growing large, high-quality KT crystals with high reproducibility, making the material applicable to practical lenses.

NTT-AT will start selling samples of KT crystal that has been produced using the technology developed by NTT Photonics Laboratories. It will also accept requests for lenses or other products made of the crystal.

For the time being, samples will be provided to business users who undertake research and development or prototyping.

About KT crystal

KT (KTaO₃) crystal is transparent optical crystal made up of potassium, tantalum and oxygen. The structure of KT crystal is shown in Figure 1. It is a cubic crystal^*2, which has a number of attractive properties, such as absence of optical anisotropy, high refractive index, wide transmission range, and high stability in a hot or damp environment. Figure 2 is a dispersion diagram, showing the refractive index versus wavelength of KT crystal while Figure 3 shows the transmission property. Table 1 shows the refractive index at various wavelengths.

The most important properties of KT crystal are the absence of optical anisotropy and the high refractive index. For example, its refractive index is as high as 2.38374 at a wavelength of 405 nm, the wavelength used by the laser being developed as the optical source for next-generation optical disk systems. Using KT crystal, it is possible to produce a solid immersion lens (SIL)^*3 with a high NA.
The refractive index rises as high as 2.47 at the i-line wavelength (365.015 nm) near the absorption edge. Even at a wavelength of 2,000 nm, the refractive index is still 2 or higher.

The second important property of KT crystal is its wide transmission range. The absorption edge at the shorter wavelength side is about 365nm while that at the longer wavelength side is about 4,000 nm.

The third important property of KT crystal is its stability in the presence of heat or moisture. Since KT crystal remains a cubic crystal irrespective of temperature, its optical isotropy is retained over a wide range of temperature.

Figure 4 shows an example of processing KT crystal into ball lenses. These exhibit no cleavages^*4, which are usually common in crystals. On account of its high workability, KT crystal can be easily processed into lenses.

Fig. 1

KT crystal structure (cubic crystal)

Fig. 2

Dispersion diagram showing refractive index versus wavelength of KT crystal

Fig. 3

In-line transmittance of the crystal(τ₁₀mm) 300~2400nm

Transmission property

Table 1

Refractive index at key wavelengths within the visible region

	Wavelength (nm)	Refractive Index
C	656.273	2.22234
d	587.562	2.24257
e	546.075	2.25945
F	486.133	2.29436
g	435.835	2.34063

Fig. 4

Ball lenses made of KT crystal

Glossary

*1

Optical anisotropy
Property of exhibiting different refractive indexes, depending on the crystal orientation.

*2

Cubic crystal
A cubic crystal has three mutually orthogonal axes. The axis ratio is 1:1:1.

*3

Solid immersion lens (SIL)
A lens shaped in a hemisphere so that the spot is positioned at the end face of the lens (inside the lens) to produce a high NA.
There are proposals to place a solid immersion lens extremely close to the recording medium and use near-field light as a way of increasing the memory density of an optical disk system.

*4

Cleavage
Tendency for a crystal to fracture along a specific orientation.

For inquiries, contact

International Business Promotion Headquarters
NTT Advanced Technology Corporation
TEL: +81 422 36 6715
FAX: +81 422 36 6702