  Non-perturbative and phase-sensitive light-matter interactions have led to
the generation of attosecond pulses of light and the control electrical
currents on the same timescale. Traditionally, probing these effects via high
harmonic generation has involved complicated lasers and apparatuses to generate
the few-cycle and high peak power pulses needed to obtain and measure spectra
that are sensitive to the phase of the light wave. Instead, we show that
nonlinear effects dependent on the carrier-envelope phase can be accessed in
solid state crystals with simple low-energy frequency combs that we combine
with high-sensitivity demodulation techniques to measure harmonic spectral
modulations. Central to this advance is the use of a scalable 100 MHz
Erbium-fiber frequency comb at 1550 nm to produce 10 nJ, 20 fs pulses which are
focused to the TW/cm2 level. In a single pass through a 500 {\mu}m ZnO crystal
this yields harmonic spectra as short as 200 nm. With this system, we introduce
a technique of carrier-envelope amplitude modulation spectroscopy (CAMS) and
use it to characterize the phase-sensitive modulation of the ultraviolet
harmonics with 85 dB signal-to-noise ratio. We further verify the
non-perturbative nature of the harmonic generation through polarization gating
of the driving pulse to increase the effects of the carrier-envelope phase. Our
work demonstrates robust and ultra-sensitive methods for generating and
characterizing harmonic generation at 100 MHz rates that should provide
advantages in the study of attosecond nonlinear processes in solid state
systems. Additionally, as a simple and low-noise frequency comb, this broadband
source will be useful for precision dual-comb spectroscopy of a range of
physical systems across the ultraviolet and visible spectral regions (200 - 650
nm).
