In general, FDTD uses pulse as source signal, which is more efficient than a sinusoidal wave. Please refer this post:
Ansys Insight: FDTD 为什么采用脉冲光源激励而不采用正弦波光源
you can set a relatively narrow spectrum (longer pulse), and only record the single frequency in the frequency-domain monitor.
If you really want a single wavelengh injection, you can set a cw source:
- FDTD CW source
but you will not be able to use frequency domain monitor, since the Fourier transform requires the signal to decay completely.
In summary, you can simply use a relatively narrow spectrum to get the monochromatic result in CW, steady state from DFT monitor.