Abstract: A device for photoexcitation of a photo-responsive material, such as a photochromic material, includes a pen-type housing having a UV lamp at one end and a visible light lamp at the other end. The device includes a microcontroller, a power supply, and an input device such as button, microphone, temperature sensor, or accelerometer. The beam width of the lamps can be adjusted by a lens adjacent to the respective lamps. The device is designed to photoexcite the photo-responsive material into various states depending on the type and wavelength of light emitted from the device. The device enables writing and erasing on a substrate, such as human skin, that contains or is embedded with the photo-responsive material, by exposing the material to light emitted from the device.