  Given the long subclinical stage of Alzheimer's disease (AD), the study of
biomarkers is relevant both for early diagnosis and the fundamental
understanding of the pathophysiology of AD. Biomarkers provided by
amyloid-${\unicode[Times]{x3B2}}$ (A${\unicode[Times]{x3B2}}$) plaques have led
to an increasing interest in characterizing this hallmark of AD due to its
promising potential. In this work, we characterize A${\unicode[Times]{x3B2}}$
plaques by label-free multimodal imaging: we combine two-photon excitation
autofluorescence (TPEA), second harmonic generation (SHG), spontaneous Raman
scattering (SpRS), coherent anti-Stokes Raman scattering (CARS), and stimulated
Raman scattering (SRS) to describe and compare high-resolution images of
A${\unicode[Times]{x3B2}}$ plaques in brain tissues of an AD mouse model.
Comparing single-laser techniques images, we discuss the origin of the SHG,
which can be used to locate the plaque core reliably. We study both the core
and the halo with vibrational microscopy and compare SpRS and SRS microscopies
for different frequencies. We also combine SpRS spectroscopy with SRS
microscopy and present two core biomarkers unexplored with SRS microscopy:
phenylalanine and amide B. We provide high-resolution SRS images with the
spatial distribution of these biomarkers in the plaque and compared them with
images of the amide I distribution. The obtained spatial correlation
corroborates the feasibility of these biomarkers in the study of
A${\unicode[Times]{x3B2}}$ plaques. Furthermore, since amide B enables rapid
imaging, we discuss its potential as a novel fingerprint for diagnostic
applications.
