  A major limitation of transient optical spectroscopy is that relatively high
laser fluences are required to enable broadband, multichannel detection with
acceptable signal-to-noise levels. Under typical experimental conditions, many
condensed phase and nanoscale materials exhibit fluence-dependent dynamics,
including higher-order effects such as carrier-carrier annihilation. With the
proliferation of commercial laser systems, offering both high repetition rates
and high pulse energies, has come new opportunities for high sensitivity
pump-probe measurements at low pump fluences. However, experimental
considerations needed to fully leverage the statistical advantage of these
laser systems has not been fully described. Here we demonstrate a high
repetition rate, broadband transient spectrometer capable of multichannel
shot-to-shot detection at 90 kHz. Importantly, we find that several high-speed
cameras exhibit a time-domain fixed pattern noise resulting from interleaved
analog-to-digital converters that is particularly detrimental to the
conventional "ON/OFF" modulation scheme used in pump-probe spectroscopy. Using
a modified modulation and data processing scheme, we achieve a noise level of
$10^{-5}$ OD for an integration time of four seconds, an order of magnitude
lower than for commercial 1 kHz transient spectrometers. We leverage the high
sensitivity of this system to measure the differential transmission of
monolayer graphene at low pump fluence. We show that signals on the order of
$10^{-6}$ OD can be measured, enabling a new data acquisition regime for
low-dimensional materials.
