Understanding the nature of ultrasonic propagation, scattering effects and mode conversation for crack diffraction in materials is critical to the effectiveness of ultrasonic characterization and sizing cracks and defects in structures. Thus far, a complete solution for the diffraction of an ultrasonic pulse of arbitrary shape at a crack of arbitrary shape has not yet been achieved. In this work, diffraction of a thermoelastic laser-generated ultrasonic line source at a semi-infinite half-plane is examined using rigorous diffraction theory. Directivity patterns are calculated for a laser line source, for diffraction of a plane wave at a semi-infinite half-plane and for diffraction of an ultrasonic shear wave generated by a laser line. Experimental validation of the theory is performed using laser-generated ultrasonic diffraction of a shear wave at a slot and its subsequent detection with an EMAT receiver sensitive to shear displacements. Results from calculations and experiments show that the directivity of a laser line source is identical to a laser point source in two dimensions if thermal conduction and subsurface optical penetration effects are neglected. Calculations of the directivity pattern for diffraction of an ultrasonic shear wave at a semi-infinite half-plane show that the majority of the diffracted energy propagates in the direction of the incident plane wave with a smaller portion diffracted at all angles. Diffraction of a laser-generated line source at a semi-infinite half-plane shows similar results. Both models indicate that there will not be a sharp shadow boundary for ultrasonic diffraction at a crack. Experiments performed on an aluminum half-cylinder milled with an EDM slot along the radius verify that ultrasonic signals appear in the shadow zone of the crack. Surface scans using the hybrid laser/EMAT system on aluminum plates milled with slots of different sizes show that crack diffraction is a frequency dependent process. Lower frequencies are found to dominate in the shadow zone. These results indicate that crack sizing may be aided by filtering the signals or similar signal processing methods. ©2000 American Institute of Physics.