The optical properties of wurtzite-structured InN grown on sapphire substrates by molecular beam epitaxy have been studied using optical absorption, photoluminescence (PL), and photo-modulated reflectance (PR) techniques. These techniques show an energy gap for InN between 0.7 and 0.8 eV, much lower than the commonly accepted value of 1.9 eV. The PL peak energy is sensitive to the free electron concentration and exhibits weak hydrostatic pressure dependence and a small blueshift with increasing temperature. In contrast, early studies of InN thin films interpreted the optical absorption as consistent with a fundamental energy gap of about 2 eV. However, recent studies show that InN has a much smaller band gap, with PL peaks around 1 eV. Theoretical calculations also predict a smaller band gap for InN, with some predicting a gap as low as 1.39 eV. The optical absorption and PL measurements are consistent with an intrinsic fundamental band gap of InN of about 0.8 eV. The band gap exhibits unusual temperature and pressure dependencies. InN films were grown on sapphire with an AlN buffer layer by molecular beam epitaxy. The samples were characterized by optical absorption, PL, and PR spectroscopy. The optical absorption curve shows an onset at ~0.78 eV, with a high absorption coefficient. The PL peak energy decreases with increasing temperature, and the PL spectra show a small blueshift. The PL peak energy is also sensitive to the free electron concentration. The PL peak energy is found to be around 0.8 eV, consistent with the fundamental band gap of InN. The PL peak energy is also found to be sensitive to the excitation power. The PL peak energy does not shift with excitation power, indicating that the PL originates from fundamental interband transitions. The hydrostatic pressure dependence of the PL peak energy is small, indicating a low pressure coefficient. The temperature dependence of the band gap is weak, indicating a small temperature coefficient. The band gap of InN is significantly lower than previously reported values. The PL peak energy is found to be around 0.8 eV, consistent with the fundamental band gap of InN. The PL peak energy is also found to be sensitive to the free electron concentration. The PL peak energy is found to be around 0.8 eV, consistent with the fundamental band gap of InN. The PL peak energy is also found to be sensitive to the excitation power. The PL peak energy does not shift with excitation power, indicating that the PL originates from fundamental interband transitions. The hydrostatic pressure dependence of the PL peak energy is small, indicating a low pressure coefficient. The temperature dependence of the band gap is weak, indicating a small temperature coefficient. The band gap of InN is significantly lower than previously reported values.The optical properties of wurtzite-structured InN grown on sapphire substrates by molecular beam epitaxy have been studied using optical absorption, photoluminescence (PL), and photo-modulated reflectance (PR) techniques. These techniques show an energy gap for InN between 0.7 and 0.8 eV, much lower than the commonly accepted value of 1.9 eV. The PL peak energy is sensitive to the free electron concentration and exhibits weak hydrostatic pressure dependence and a small blueshift with increasing temperature. In contrast, early studies of InN thin films interpreted the optical absorption as consistent with a fundamental energy gap of about 2 eV. However, recent studies show that InN has a much smaller band gap, with PL peaks around 1 eV. Theoretical calculations also predict a smaller band gap for InN, with some predicting a gap as low as 1.39 eV. The optical absorption and PL measurements are consistent with an intrinsic fundamental band gap of InN of about 0.8 eV. The band gap exhibits unusual temperature and pressure dependencies. InN films were grown on sapphire with an AlN buffer layer by molecular beam epitaxy. The samples were characterized by optical absorption, PL, and PR spectroscopy. The optical absorption curve shows an onset at ~0.78 eV, with a high absorption coefficient. The PL peak energy decreases with increasing temperature, and the PL spectra show a small blueshift. The PL peak energy is also sensitive to the free electron concentration. The PL peak energy is found to be around 0.8 eV, consistent with the fundamental band gap of InN. The PL peak energy is also found to be sensitive to the excitation power. The PL peak energy does not shift with excitation power, indicating that the PL originates from fundamental interband transitions. The hydrostatic pressure dependence of the PL peak energy is small, indicating a low pressure coefficient. The temperature dependence of the band gap is weak, indicating a small temperature coefficient. The band gap of InN is significantly lower than previously reported values. The PL peak energy is found to be around 0.8 eV, consistent with the fundamental band gap of InN. The PL peak energy is also found to be sensitive to the free electron concentration. The PL peak energy is found to be around 0.8 eV, consistent with the fundamental band gap of InN. The PL peak energy is also found to be sensitive to the excitation power. The PL peak energy does not shift with excitation power, indicating that the PL originates from fundamental interband transitions. The hydrostatic pressure dependence of the PL peak energy is small, indicating a low pressure coefficient. The temperature dependence of the band gap is weak, indicating a small temperature coefficient. The band gap of InN is significantly lower than previously reported values.