A light emitter is formed from nanoparticles including a compound semiconductor containing an Ag component, In component, and Se component. The peak wavelength of the emission intensity falls within the range of 700 to 1400 nm, and the half-value width ΔH for the peak wavelength is 100 nm or less. The light emitted is configured to emit strong light in the near-infrared region, and which is capable of detecting biological information, and is preferred for bioimaging.