  We demonstrate the ability to guide high-frequency photoacoustic waves
through thick tissue with a water-filled silica-capillary (150 \mu m inner
diameter and 30 mm long). An optical-resolution photoacoustic image of a 30 \mu
m diameter absorbing nylon thread was obtained by guiding the acoustic waves in
the capillary through a 3 cm thick fat layer. The transmission loss through the
capillary was about -20 dB, much lower than the -120 dB acoustic attenuation
through the fat layer. The overwhelming acoustic attenuation of high-frequency
acoustic waves by biological tissue can therefore be avoided by the use of a
small footprint capillary acoustic waveguide for remote detection. We finally
demonstrate that the capillary can be used as a dual optical-in acoustic-out
waveguide, paving the way for the development of minimally invasive
optical-resolution photoacoustic endoscopes free of any acoustic or optical
elements at their imaging tip.
