Ultrafast all-optical switches based on epsilon-near-zero
(ENZ)-enhanced
nonlinear refraction in transparent conducting oxides have achieved
exciting results in realizing large absolute modulations. However,
broad-band, polarization-independent, and wide-angle ultrafast all-optical
switches have been challenging to produce, due to the inherent narrow
band, polarization-dependent, and angle-dependent characteristics
of the ENZ effect. To this end, we propose an ultrafast all-optical
switch based on the enhanced nonlinear absorption of corrugated indium
tin oxide (ITO) thin films. Taking advantage of the perfect absorption
and localized field enhancement of the ENZ and localized surface plasmon
resonance modes, we significantly enhanced the nonlinear absorption
of the corrugated ITO film in the 1450–1650 nm telecom band.
The experimental results show that the nonlinear saturable absorption
coefficient of the corrugated ITO film at 1450 nm was as high as −1.5
× 105 cm GW–1, enabling all-optical
switching to obtain an extinction ratio of 14.32 dB and an ultrafast
switching time of 350 fs at a pump fluence of 18.51 mJ cm–2. Furthermore, the all-optical switch achieved an extinction ratio
of over 15 dB and an insertion loss of approximately 2.6 dB within
the 200 nm absorption band and exhibited polarization-independent
and wide-angle features. The ultrafast temporal response can be attributed
to intraband transient bleaching of the corrugated ITO film. Our findings
demonstrate that corrugated ENZ films can overcome the inherent narrow-band,
polarization-dependent, and angle-dependent problems of natural ENZ
materials without increasing the response time, making them a potential
ENZ ultrafast all-optical switching material platform.
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