Direct numerical simulations were performed to investigate conditions
for the strong effects of photon-assisted quantum transport. Transmiss
ion probabilities of incident electrons as Gaussian wave packets were
calculated for single- and double-barrier structures irradiated by ele
ctromagnetic fields that were focused within a finite region xi(ac). W
hen the period of the ac electromagnetic field is short compared with
the time tau(xi ac), which is the traversal time of the electrons in t
he region of the applied ac field, the transmission probability can be
affected by photon emission and photon absorption. The magnitude of t
hese effects strongly depends on the width xi(ac) and the bound-state
character of the electrons. These dependences can be understood from t
he selection rule, which results from the momentum conservation of the
electron and photon system. Consequently, we have shown that photon-a
ssisted transport over a single barrier is difficult to achieve in two
-dimensional electron-gas devices due to the relatively long screening
length. We have demonstrated, however, that the photon-assisted proce
ss is significantly enhanced in double-barrier devices due to quasibou
nd-to-extended-state transitions.