Crystal KBBF is capable of producing DUV and even vacuum-UV coherent light by direct second harmonic generation, but a major problem is its plate-like growth nature that makes it very difficult to grow samples thicker than4 mmalong the c-axis, and KBBF crystal is cleaves easily, so it is impossible to cut the crystal along the phase-matching angle.

With a directly incident pump, the external incident angle will increase to 90° for a second harmonic wavelength of 235 nm. To solve the problem of producing coherent laser light below 235 nm, a special prism-coupling technique ( PCT ) was developed to avoid cutting the crystals along the phase-matching angle.

A KBBF crystal is sandwiched between two prisms made of UV-fused silica, which has almost the same refractive indices as KBBF in the visible to UV spectral region.

Using PCT devices, the maximum aperture is about 6×6 mm, the thickness of KBBF is about1.5mm.

**4****th**** harmonic generation of Nd:YAG laser at 266nm**

(1) Advantages of KBBF over BBO and CLBO

●Much higher damage threshold

●Much smaller two-photon absorption

●No photorefractive effect

●No moisture

(2)Disadvantages of KBBF over BBO and CLBO

● Deff (KBBF) = 1.53(KDP)

● Deff (BBO) = 2.72(KDP)

● Deff (CLBO) = 1.97KDP) at 266nm

**Targets the KBBF can realize:**

● Watt-level 193nm output power with 4th HG of Ti:sapphire Laser

● 100-500mw QCW 177.3nm output power with 6th HG of Nd:YAG Laser

● Wide tunable 2.0mw coherent light output from 220nm-175.0nm with 4th HG of tunable Ti:sapphire Laser

● average 7.8 watt-level 266nm output power

● stable mw-level 165-167nm output power

**Optical Properties**

**Deff :**

KBBF crystal belongs to the space group R32, so from the point group symmetry of D3 and the IEEE/ANSI standard definition of the dij coefficient, it has only two nonzero dij coefficients, namely, d11 and d14. The matrix form of the coefficients can be written as follows:

where d14 is very small according to both theoretical calculation and experiments. On the other hand, the effective deff coefficients of KBBF are:

deff=d11cosqcos3f (type I)

deff=d11cos2qsin3f (type = 2 \* ROMAN II)

By comparing the fringe envelope of the d11 coefficient with the d36 coefficient in KDP, the d11 of KBBF is deduced to be (0.47+0.01) pm/V (d36(KDP)= 0.39 pm/V).

**Transparency
range ****：**

The cutoff wavelength on the UV side is located at 150 nm, and on the IR side around3.5mm. There is virtually no absorption in this region.

**Sellmeier
coefficients ****：**

Sellmeier equations as follows: (l is in units of mm)

no2=2.1738+0.00764λ2-0.01641-0.01174λ2

ne2=1.9319+0.00623λ2-0.00400-0.00356λ2

**Damage threshold ****：**