A microscale photoacoustic sensor uses the detection of ultrasound waves generated by a sample in response to incident light absorption to perform photoacoustic spectroscopy, imaging, and microscopy. The microscale photoacoustic sensor, including components to excite a sample and detect ultrasound waves, may be integrated onto a single chip. The microscale photoacoustic sensor may excite a sample using a metasurface collimator. The metasurface collimator includes an array of diffraction grooves to collimate an excitation beam uniformly out of the plane of the sensor to create a wide and homogeneous beam spot. The microscale photoacoustic sensor may detect ultrasound waves using an optical photoacoustic transducer. The optical photoacoustic transducer includes a resonator on a mechanical membrane to detect ultrasound waves with high sensitivity. The microscale photoacoustic sensor may be used in applications such as deep-tissue neural imaging or microfluidic biological screening.