  The quest for efficient and versatile microwave and optical isolators has
recently spawned several novel space-time-modulated isolator structures.
However, such space-time isolators suffer from a large profile and complex
architecture caused by progressive nonreciprocal space-time coupling
properties. To overcome these limitations, we propose a nonmagnetic
phase-engineered temporal-loop-based isolator featuring large isolation levels,
weak undesired time harmonics, and a low profile. The proposed isolator is
composed of two temporal loops that provide desired constructive and
destructive interferences of different time harmonics. Furthermore, these two
loops are designed in a way to assure that the circulation and reflection of
different time harmonics strengthen a low insertion loss unidirectional signal
transmission. An experimental demonstration of the proposed time-modulated
isolator at microwave frequencies is provided, featuring strong unidirectional
wave transmission through the isolator with more than 27 dB contrast between
the forward and backward waves across a fractional bandwidth of 14.3%. The
proposed isolator outperforms the nonlinear-based and transistor-based
isolators by featuring a highly linear response with OP1dB of higher than 31
dBm, high power rating of more than 47 dBm, and a low noise figure of 3.4 dB.
