  Achieving a broadband nonreciprocal device without gain and any external bias
is very challenging and highly desirable for modern photonic technologies and
quantum networks. Here, we theoretically propose a passive and bias-free
all-optical isolator for a femtosecond laser pulse by exploiting a new
mechanism of unidirectional self-induced transparency, obtained with a
nonlinear medium followed by a normal absorbing medium at one side. The
transmission contrast between the forward and backward directions can reach
~14.3 dB for a 2\pi 5 fs laser pulse, implying isolation of a signal with an
ultrabroad bandwidth of 200 THz. The 20 dB bandwidth is about 57 nm, already
comparable with a magneto-optical isolator. This cavity-free optical isolator
may pave the way to integrated nonmagnetic isolation of ultrashort laser
pulses.
