Ultrathin Crystals for Femtosecond Applications

Thin crystals are used in different applications with femtosecond pulses:

  • Harmonic generation (SHG, SFG);
  • Optical parametric generation and amplification (OPG, OPA);
  • Difference frequency generation (DFG);
  • Pulse width measurements by auto and cross correlation;
  • THz frequency generation (in GaSe crystal);
  • Polarization entangled photon pairs generation.

The propagation of a ultrashort optical pulses through the crystal results in a delay of the pulses because of Group Velocities Mismatch (GVM), a duration broadening because of Group Delay Dispersion (GDD) and a frequency chirp. Unfortunately those effects forces to limit nonlinear crystal thickness in frequency generation schemes. For two collinearly propagating pulses with different group velocities their quasistatic interaction length (Lqs) is defined as distance over which they separate by a path equal to the one of the pulses duration (or to the desired pulse duration):

thin

where GVM is the group velocity mismatch and   is the duration of the pulse. GVM calculations are presented for the most popular Type 1 phase matching applications for different crystals in Table 1.Optimal BBO, LBO, KDP and LiIO3 crystal thicknesses which are limited by GVM for Type 1 SHG of 800 nm at different fundamental pulse duration are presented in the Table 2. Also effective coefficients and phase matching angles at room temperature (20°C) are calculated. If longer crystal will be used this will cause second harmonic pulse broadening to the duration longer than fundamental pulse duration (or desired pulse duration). Group delay dispersion (GDD) has an important impact on the propagation of pulses, because a pulse always has certain spectral width, so that dispersion will cause its frequency components to propagate with different velocities. In case of crystals where we have normal dispersion when refractive index decreases with increasing wavelength this leads to a lower group velocity of higher-frequency components, and thus to a positive chirp.The frequency dependence of the group velocity also has an influence on the pulse duration. If the pulse is initially unchirped, dispersion in a crystal will always increase its duration. This is called dispersive pulse broadening. For an originally unchirped Gaussian pulse with the duration  0, the pulse duration is increased according to:

where L - thickness of the crystal in mm; D  - second order group delay dispersion or dispersion parameter. Table 3 gives D parameter for Type 1 phase matching SHG @ 800 nm for 800 nm pulse with „o“ polarization and 400 nm pulse with „e“ polarization in different crystals. 

 

Table 1. Group velocity mismatch between shortest and longest wave pulse for Type 1 phase matching

Crystal SFM
800+266 nm
SFM
800+400 nm
SHG
800 nm
SHG
1030 nm
SHG
1064 nm
DFG 1.26-2.183
→ 3 μm
DFG 1.48-1.74
→ 10 μm
BBO  2074 fs/mm 737 fs/mm 194 fs/mm   94 fs/mm  85 fs/mm   - 
LBO  - 448 fs/mm 123 fs/mm  51 fs/mm  44 fs/mm   - -
KDP  - 370 fs/mm 77 fs/mm 1 fs/mm  -7 fs/mm  -
LiIO3  - - 559 fs/mm  285 fs/mm  262 fs/mm   -
AgGaS2 - - -   - 170 fs/mm -10 fs/mm

Table 2. Quasistatic interaction length for Type 1 SHG of 800 nm
Crystal 200 fs 100 fs 50 fs 20 fs 10 fs Cut angles θ, φ Coefficient deff
BBO 1.0 mm 0.5 mm 0.26 mm 0.1 mm 0.05 mm 29.2°, 90° 2.00 pm/V
LBO 1.6 mm 0.8 mm 0.4 mm 0.16 mm 0.08 mm 90°, 31.7° 0.75 pm/V
KDP 2.6 mm 1.3 mm 0.6 mm 0.26 mm 0.13 mm 44.9°, 45° 0.30 pm/V
LiIO3 0.4 mm 0.18 mm 0.01 mm 0.04 mm 0.018 mm 42.5°, 0° 3.59 pm/V

Table 3. D parameter for Type 1 SHG @ 800 nm orientation crystals for 800 nm (o-pol) and 400 nm (e-pol) pulses
Crystal D (fsec2/mm) at 800 nm D (fsec2/mm) at 400 nm
BBO 75 fsec2/mm 196 fsec2/mm
LBO 47 fsec2/mm 128 fsec2/mm
KDP 27 fsec2/mm 107 fsec2/mm
LilO3 196 fsec2/mm 589 fsec2/mm
We may calculate that spectrum limited initial 30 fsec Gaussian pulse at 400 nm will be broadened to 35 fsec pulse after passing 1 mm thickness BBO crystal.

  BBO, LBO KDP, LiIO3, AgGaS2
Flatness λ/6 @ 633 nm λ/4 @ 633 nm
Parallelism < 10 arcsec < 30 arcsec
Angle tolerance < 15 arcmin < 30 arcmin
Surface quality 10/5 scratch & dig 20/10 scratch & dig

Free Standing Crystals

The crystals of thickness down to 100 µm thickness can be supplied as free standing crystals  not attached to the support. However ring mounts are highly recommended for safe handling of these thin crystals. The tolerance is ±50 µm for crystals of thickness down to 300 µm and ±20 µm for crystals of thickness down to 100 µm.

GaSe crystal is supplied glued in to dia Ø40 mm ring holder only. 

Crystal Minimal aperture Maximal aperture Minimal thickness
BBO 5×5 mm 20×20 mm 0.1 mm
LBO 5×5 mm 30×30 mm 0.1 mm
KDP 4×4 mm 100×100 mm 0.1 mm*
LiIO3 4×4 mm 50×50 mm 0.1 mm*
AgGaS2 5×5 mm 15×15 mm 0.1 mm
GaSe Ø5 mm Ø7 mm 0.01 mm

* The thickness should be about 0.5 mm for max aperture KDP and LiIO3.

 

We provide various AR, BBAR and protective coatings for all free-standing crystals. 

Ring mounts made from anodized aluminium are available for safe and convenient handling of ultrathin crystals. The standard sizes are Ø 1” and 25 or 30 mm. Mount thickness is 10 mm. Mounts of custom shapes and sizes are available also.

 

Optically contacted crystals

 

BBO crystals of thickness less than 100 µm  can be supplied optically contacted on UV Fused Silica substrate sizes 10×10×2 mm or 12×12×2 mm. Other sizes of substrates are also available on request. The tolerance of BBO crystal thickness is +10/-5 µm. 

Crystal Minimal aperture Maximal aperture Minimal thickness
BBO 5×5 mm 10×10 mm 10±5 µm

Coatings are not available for optically contacted crystals.

 

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