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We simulate the early dynamics of enhanced light transmission through a subwavelength metallic slit and find that the amplitude of the transmitted light can be modulated. To understand this novel phenomenon and underlying physics, we develop a new analytical model. The field of each light period is considered as an individual unit. Each field is partially transmitted through the slit as the first subunit. The portion reflected from the exit interface travels a round trip in the slit and then partially exits again as the second subunit. There may be a gap in time between these two subunits. This process repeats so as to produce a subunit train, which is verified by the simulation of an incident sinusoidal pulse of one light period. When the wave units are continuous, the superposition of the trains produces the observed light. While the round-trip time is an integer multiple of the light period, the modulation period is the same. Besides academic importance, this study may be applicable to photonics with short laser pulses.

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