Editing Microscopy (section)

==Photoacoustic microscopy==
{{main|Photoacoustic microscopy}}
[[File:HumanRBCsPAM.png|thumb|Photoacoustic micrograph of human red blood cells.]]

A microscopy technique relying on the [[photoacoustic effect]],<ref>{{Cite journal| doi = 10.2475/ajs.s3-20.118.305| title = On the production and reproduction of sound by light| journal = American Journal of Science| issue = 118| pages = 305–324| year = 1880| last1 = Bell| first1 = A. G.| volume = s3-20| url = https://zenodo.org/record/1450056| bibcode = 1880AmJS...20..305B| s2cid = 130048089| access-date = 2019-06-28| archive-date = 2022-11-27| archive-url = https://web.archive.org/web/20221127193008/https://zenodo.org/record/1450056| url-status = live}}</ref> i.e. the generation of (ultra)sound caused by light absorption.
A focused and intensity modulated laser beam is raster scanned over a sample. The generated (ultra)sound is detected via an ultrasound transducer. Commonly piezoelectric ultrasound transducers are employed.<ref>{{Cite journal | doi = 10.1002/lpor.201200060| title = Photoacoustic Microscopy| journal = Laser Photonics Rev.| volume = 7| issue = 5| pages = 1–36| year = 2013| last1 = Yao | first1 = J.| last2 = Wang | first2 = L. V.| pmc = 3887369 | pmid=24416085| bibcode = 2013LPRv....7..758Y}}</ref>

The image contrast is related to the sample's absorption coefficient <math>\alpha</math>. This is in contrast to bright or dark field microscopy, where the image contrast is due to transmittance or scattering. In principle, the contrast of fluorescence microscopy is proportional to the sample's absorption too. However, in fluorescence microscopy the fluorescence quantum yield <math>\eta_{fl}</math> needs to be unequal to zero in order that a signal can be detected. In photoacoustic microscopy, however, every absorbing substance gives a photoacoustic signal <math>PA</math> which is proportional to

<math> PA \propto \alpha * \Gamma *( (1-\eta_{fl})*E_g + (E_{laser} - E_g) )</math>

Here <math>\Gamma</math>  is the Grüneisen coefficient, <math>E_{laser}</math> is the laser's photon energy and <math>E_g</math> is the sample's band gap energy. Therefore, photoacoustic microscopy seems well suited as a complementary technique to fluorescence microscopy, as a high fluorescence quantum yield leads to high fluorescence signals and a low fluorescence quantum yield leads to high photoacoustic signals.

Neglecting non-linear effects, the lateral resolution {{math|<var>dx</var>}} is limited by the [[Abbe diffraction limit]]:

<math> dx =\lambda / (2*NA) </math>

where <math>\lambda</math> is the wavelength of the excitation laser and {{math|<var>NA</var>}} is the numerical aperture of the objective lens. The Abbe diffraction limit holds if the incoming wave front is parallel. In reality, however, the laser beam profile is Gaussian. Therefore, in order to the calculate the achievable resolution, formulas for truncated Gaussian beams have to be used.<ref>{{Cite journal | doi = 10.1364/BOE.7.002692| pmid = 27446698 | pmc = 4948622 | title = Frequency domain photoacoustic and fluorescence microscopy | journal = Biomedical Optics Express| volume = 7 | issue = 7| pages = 2692–702| year = 2016| last1 = Langer | first1 = G.| last2 = Buchegger | first2 = B.| last3 = Jacak | first3 = J. | last4 = Klar | first4 = T. A. | last5 = Berer | first5 = T.}}</ref>