Photoacoustic signal detection via atomic force microscopy cantilevers

Abstract

In this thesis, we first present a custom-designed photoacoustic microscopy system (PAM) which includes two separate excitation lasers. One of these tools is a novel fiber-based laser system developed particularly for multiwavelength optical resolution photoacoustic microscopy. The fiber laser has adjustable properties including pulse duration (5-10 ns), pulse energy (up to 10 µJ) and repetition frequency (up to 1 MHz) independently from each other. The whole laser system covers a broad spectral region from 450 to 1100 nm by its supercontinuum output and also can emit wavelengths of 532, 355, and 266 nm from its harmonic generation unit. The other laser source in the PAM system is a commercial tunable optical parametric oscillator (OPO) laser. The lateral resolution of the PAM system is approximately 2 µm. We present various biological (horse hair, human red blood cells, Skmel-28 cancer cells) and non-biological (resolution test target, India inks, cadmium telluride quantum dots) photoacoustic images obtained with our PAM system. In addition, we present a custom-developed atomic force microscopy (AFM) head to detect both acoustic and photoacoustic sig nals using microcantilevers as the pressure sensor. Here, as a preliminary study for the AFM-head system, we analyzed microcantilevers to investigate their responses to pulsed-laser-induced photoacoustic waves. The results indicate that microcantilevers can be used as photoacoustic detection tools by presenting steady-state oscillations when the repetition frequency of the excitation laser matches to the resonant frequency of the cantilever. Finally, for the proof-of-concept studies, we utilized the AFM-head system to detect acoustic waves at various frequencies arising from a personal headset, and also photoacoustic waves from a black ink phantom illuminated by the OPO laser. The AFM-head system is compact, portable, and very sensitive to external forces.