  The higher-multipoles of gravitational wave signals from coalescing compact
binaries play a vital role in the accurate reconstruction of source properties,
bringing about a deeper and nuanced understanding of fundamental physics and
astrophysics. Their effect is most pronounced in systems with asymmetric masses
having an orbital geometry that is not face-on. The detection of
higher-multipoles of GW signals from any single, isolated merger event is
challenging, as there is much less power in comparison to the dominant
quadrupole mode. In this paper, we present a new method for their detection by
combining multiple events observed in interferometric gravitational wave
detectors. Sub-dominant modes present in (the inspiral part of) the signal from
separate events are stacked using time-frequency spectrogram of the data. We
demonstrate that this procedure enhances the signal-to-noise ratio of the
higher-multipole components and thereby leads to increased chances of their
detection. From Monte-Carlo simulations, we estimate that a combination of
$\sim 100$ events observed in two-detector coincidence can lead to the
detection of the higher-multipole components with a $\geq$ 95\% detection
probability. The advanced-LIGO detectors are expected to record these many
binary black hole merger events within a month of operation at design
sensitivity. We also present results from the analysis of data from O1 and O2
science runs containing previously detected events using our new method.
