Second-harmonic conical refraction:observation of free and forced harmonic waves

Observation of second-harmonic conical refraction in KTiOPO4 crystal is reported. Two second-harmonic conical patterns, namely forced and free harmonic waves, are identified and registered with exceptional clarity and level of details. The forced second-harmonic wave evolves into the characteristic ring intensity distribution while the free-harmonic wave gives a single ray direction, resulting in a spot.