I was using a type 2 BBO crystal that converts 800 to 400nm. But I don't see any SHG forming. Please give me some suggestions as what should I do. My laser is pulsed 780 nm with repetition rate of 1MHZ and power 7 mW.
Your power is very low, therefore you need to focus the radiation, e.g. with 10 cm focal length lens. With type 2 crystal, you need to put 45 degree to the polarization of 780 nm beam and try to find the phase matching direction.
Thank you for your help Sir. Gagik G. Gurzadyan . I have used a 10 cm lens to focus the beam onto the BBO and also a filter to cut out the fundamental beam. I have the laser beam S polarized and by changing the polarization to 45 degrees you mean change the polarization of the laser beam right?
thanks again!
SHG is based on the phase matching effect. In the case phase velocities of laser wave and second harmonic wave have to be strongly the same. This condition can be reached in birefringant crystal like BBO, but just for definite angle around of optical axis for definite laser wavelength. Practically this direction is normal to input planes of nonlinear crystal. So You have to know for what laser wavelength Your BBO crystal has been manufactured. After that do increase the incident angle of Your laser beam on BBO plane relative to normal (zero) value.
The efficiency of The second harmonics depend on :-
the input power, cross section area of incident beam, phase matching condition for any wavelength for any types nonlinear crystals .
Regards
The main issue as mentioned before is the phase matching angle. You need to know how is your crystal cut, usually the crystals are cut to have phase-matching at a given wavelength at normal incidence. If the wavelength you use differs from this, you need to find the phase-matching angle, and yes, the right polarization as well. For such short pulses, and focusing conditions you should see something
Thank you Raul Rangel-Rojo . my crystal is type 2 BBO for 800 to 400nm. and i have also done the right polarization . So should i change the phase matching angle since the beam is 780nm?
Yes, phase matching conditions are definitely different for 800 and 780 nm. Scan the incidence angle with a lightly focused beam (f= 20 cm or longer) , with care. Fo a 2-3 mm thick crystal, the angular tolerance is only a couple of degrees
Thank yousRaul Rangel-Rojo for your help.
Thank you everyone who commented. I appreciate all your help. Thanks again!
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