We report our recent work on the development of a highly sensitive gas detection technique-photothermal interferometry spectroscopy with hollow-core optical fibers. The basic principle of operation, generation and detection of dynamic photothermal phase modulation, and method to improve the response time of the hollow-core fiber sensors are described. The technique has ultra-high sensitivity and dynamic range, and the measurement is not affected by reflection/scattering and other non-absorbing losses. Sensors based on such a technique could be made compact in size with remote detection, multiplexing and networking capability, which would enable a range of high performance applications in environmental, medical and safety monitoring.