  We employ multiple sinusoid modulated optical tweezers to measure the
frequency dependent rheological parameters of a linear viscoelastic fluid over
five decades of frequency in a single shot, hitherto not achieved using active
microrheology alone. Thus, we spatially modulate a trapped probe particle
embedded in a fluid medium with a combination of a square wave - which is by
definition a superposition of odd sinusoidal harmonics - and a linear
superposition of multiple sinusoids at a wideband frequency range, with
complete control over the amplitude, frequency and relative phase of the
modulating signals. For the latter, we selectively excite the particle by
larger amplitudes at high frequencies where the particle response is low,
thereby enabling wideband active microrheology with large signal-to-noise. This
mitigates the principal issue associated with conventional active microrheology
- which is low bandwidth - and also renders our method better in terms of
signal to noise, and faster compared to passive microrheology. We determine the
complex viscoelastic parameters of the fluid by extracting the phase response
(relative to input excitation) of the probe from the experimentally recorded
time series data of the probe displacement, and employing well-known
theoretical correlations thereafter. We test the efficacy of our method by
studying a linear viscoelastic media (polyacrylamide-water solution) at
different concentrations, and find good agreement of the measured fluid
parameters with known literature values.
