$\DeclareMathOperator\PSL{PSL}$ Let $ \mathbb{Z} $ be the ring of integers and $ \mathbb{R} $ the field of real numbers. Let $ \Sigma\_g $ be a surface of genus $ g \geq 2 $. Let $ \pi\_1(\Sigma\_g) $ be the fundamental group of the surface. There are many way to embed $ \pi\_1(\Sigma\_g) $ into $\PSL\_2(\mathbb{R}) $. There are, however, no ways to embed $ \pi\_1(\Sigma\_g) $ into $ \PSL\_2(\mathbb{Z}) $. Given some $ g \geq 2 $, is there a good way (an algorithm) to find a real algebraic integer $ \alpha $ such that $ \pi\_1(\Sigma\_g) $ embeds in $ \PSL\_2(R) $? Here $ R $ is the ring
$$
R:=\mathbb{Z}[\alpha]
$$
(Preferably $ \alpha $ is just a (real) quadratic extension. That is, $ \alpha $ is the root of some polynomial $ x^2+bx+c $ where $ b^2-4c \geq 0 $ and $ b,c \in \mathbb{Z} $.)


History of the question: The original question claimed that surface groups do not embed in $ \PSL\_2(\mathbb{Q}) $ and asked for embeddings into $ \PSL\_2(\mathbb{F}) $ where $ \mathbb{F} $ is a finite degree field extension of $ \mathbb{Q} $. The claim that surface groups do not embed in $ \PSL\_2(\mathbb{Q}) $ is false. In fact there are many such embeddings.
The first edit of the question fixed this and asked instead for an embedding into $ \PSL\_2(R) $ for $ R $ a finite rank extension of $ \mathbb{Z} $ by algebraic integers. The current version of the question is the second edit.



Do surface groups embed into PSL_2 over a real quadratic integer ring?
For the new question an answer is given by arithmetic Fuchsian groups. For example it is well-known that the reflection group associated with the regular right-angled pentagon in $\mathbb H^2$ contains every surface group as it contains the fundamental group of the non-orientable surface of Euler characteristic -1 as an index-4 subgroup. On the other hand it is an index-10 subgroup in the (2,4,5)-triangle group $\Delta$ (by dividing the pentagon into triangles from the center with vertices on the pentagon's and on the middles of its edges). The latter is known to be arithmetic by a result of Takeuchi (see for example item 6 in the table in section 13.3 of Maclachlan--Reid <https://zbmath.org/?q=an%3A1025.57001>). Its trace field is $\mathbb Q(\sqrt 5)$, which means that $\Delta$ will be realisable as a subgroup of $\mathrm{PSL}\_2$ over a quadratic extension $F$ of $\mathbb Q(\sqrt 5)$, and contained in the ring of integers $\mathbb Z\_F$. There are many choices for such an extension, for example one can take $F = \mathbb Q(\sqrt 2, \sqrt 5)$ since the quaternion algebra ramifies only at primes dividing 2. So in principle you get an embedding of $\Delta$, and hence of any surface group, into $\mathrm{PSL}\_2(\mathbb Z\_F)$. You can ask Sage to compute an integral basis for the latter to get a complete answer to your question.


Another possibility to avoid the arithmetic machinery would be to use hyperbolic geometry to compute generators for the triangle group directly and see where they lie, this has probably been done but I don't know a reference (and I'm not sure how to choose the walls as to get something which would lie where it should, i.e. $\mathbb Z\_F$).


This may be not optimal, maybe all surface groups embed into $\mathrm{PSL}\_2$ over a real quadratic field. For example the (2, 4, 6)-triangle group is arithmetic with trace field $\mathbb Q$, it embeds into $\mathrm{PSL}\_2(\mathbb Z[\sqrt 2])$ if I'm not mistaken (see the table in Maclachlan--Reid) so its surface subgroups do as well. However I don't know which genera will be realised this way.




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**EDIT** For the new question a positive general answer is given by computations of John Voight which he recorded in <https://arxiv.org/pdf/0802.0911.pdf>. The paper gives a complete list of all "Shimura curves" (a particular family of hyperbolic 2-orbifolds) whose underlying surface has genus at most 2. In particular his table 4.1 indicates that the curve associated to the full unit group of the unique maximal order in the $\mathbb Q$-quaternion algebra of discriminant 26 has genus 2 and no singularities, so the image of this group in $\mathrm{PSL}\_2(\mathbb R)$ is isomorphic to the surface group of genus 2 and it is contained in $\mathrm{PSL}\_2(\mathbb Z[\sqrt 26])$.


This representation should be computable explicitely using software developed by Voight and others, that i'm not too familiar with (i don't think it's available in Sage currently).


