Fig. 3: Experimental result.

a Absorption-induced transmission: a voltage spike (orange) indicates the electrical breakdown of argon gas to plasma. This breakdown is accompanied by an increase in transmission (green) resulting from plasma’s absorption. b Plasma luminescence (black) which we use to estimate the plasma’s density. Red: assumed exponential rise and decay. c Blue-detuned transmission during plasma ignition shows a dip followed by a peak change governed by absorption followed by a refraction-induced drift that reduces transmission. Blue: experimental results. Line: best fit using Eq. (1). d Red detuned transmission during plasma ignition shows, as expected, absorption-induced effects identical to (c) and refraction-induced effects opposite to (c). e Repeatability and controlled detuning. We can repeat plasma ignition as many times as needed with repeatability as appears in the plot. Note that the slight overall downward slope in transmission is indicating an intentional slow continuous detuning of the laser frequency in respect with the thermally broadened resonance frequency of the cold cavity. In this manner, we can scan to any desired region at the resonance sidebands. b, c were measured simultaneously. The circle size represents the estimated error.