Transient grating spectroscopy (TGS) based on diffraction gratings is a powerful optical method for studying the transport of energy carriers such as phonons and electrons. The diffraction grating in a TGS system is a key component to form a large-area interference pattern, i.e., transient grating, and to study the mean free path distribution of energy carriers. In this work, a design method for polarization-insensitive diffraction gratings with periods in the range 2-50 µm for TGS by a combination of rigorous coupled wave analysis and genetic algorithm was discussed. The method was tested for pump/probe wavelength of 515/532 or 1030/808 nm. Each ±1st diffraction order carries 35%-40% of the incident energy and the diffraction efficiencies of the other orders are lower than 10%. The optimized diffraction gratings were fabricated by a combination of photolithography and inductively coupled plasma etching, with the processing parameters introduced in detail, and their optical characteristics were evaluated. Finally, as a demonstration, the diffraction gratings for 1030/808 nm were applied to TGS to study the thermal transport properties of Ge. This work provides a useful guide for future applications and the development of TGS.