Calculated signal intensities versus different detection frequencies with (a) two-color probe lasers of 1200 nm+800 nm and 800 nm+533.3 nm (frequency ratio ω 2 / ω 1 = 3 / 2 ); (b) two-color probe lasers of 1600 nm+400 nm and 800 nm+200 nm (frequency ratio ω 2 / ω 1 = 4 ).