  We perform transmission spectroscopy on single quantum dots and examine the
effects of a resident carriers spin, the incident laser spot size,
polarization, and power on the experimental contrast. We demonstrate a factor
of 4 improvement in the maximum contrast by using a solid immersion lens to
decrease the spot area. This increase yields a maximum signal to noise ratio of
2000 Hz-1/2, which will allow for MHz detection frequencies. We anticipate that
this improvement will allow further investigation of spectral fluctuation and
open up the feasibility for an all-optical read-out of an electron spin in a
quantum dot.
