The dataset contains emission spectra from optically pumped photonic-crystal nanobeam lasers incorporating nanoscale buried heterostructures (BHs). The devices have been designed with BH sizes ranging from 155 x 155 nm2 to 250 x 250 nm2. The actual BH dimensions are estimated from measurements of the HSQ mask used to define the buried heterostructures during fabrication. The resulting size estimates are provided in the accompanying Excel file.The lasers were characterised under continuous-wave optical pumping at 980 nm. Emission spectra were recorded as the pump power was varied, enabling analysis of lasing thresholds and spectral evolution in nanoscale semiconductor lasers.Measurements were performed in two complementary experimental setups. In the first setup, the emission was recorded with a grating spectrometer. This setup has a high signal-to-noise ratio enabling reliable detection at low pump powers. In the second setup, the emission from the smallest buried heterostructure laser was measured using an optical spectrum analyser (OSA), which provides higher spectral resolution and enables more accurate determination of the lasing wavelength and linewidth.The dataset contains raw spectral measurements, extracted spectral parameters, and estimates of the actual buried heterostructure sizes used in the analysis reported in the associated publication.