Figure 1

Measured signal vs the two-photon detuning $\Delta$ and the power-law fit. Inset (a): zoom-in of the measured signal over the range $\pm 300\mathrm{Hz}$. Inset (b): an illustration of the one-dimensional beam experiment. Returning atoms contribute to the total intensity irrespective of the location of their reentry point thus effectively reducing the dimensionality to one.Reuse & Permissions

Figure 2

Measured signal for the $1d$ beam configuration (triangles), and for the wide area beam (circles) vs the complex detuning size $|s|$ in units of ${\Gamma }_0/2$. The lines are theory with a single fit parameter—the critical exponent. We find $\beta =0.56\pm 0.01$ over two decades in frequency and ${\beta }^{\prime }=0.97\pm 0.01$ over two decades.Reuse & Permissions

Figure 3

Map of critical exponent $\beta$ calculated from the exact theory [Eq. (5)] as a function of the diffusion broadening ${\Gamma }_D$ and the power broadening ${\Gamma }_p$ in units of ${\Gamma }_0$. We extract $\beta$ from the calculated spectrum using linear fit, with $\Delta$ in the range $\pm 8\mathrm{kHz}$. The linearity of the fit significantly increases towards zero and infinity. When ${\Gamma }_p\to \infty$ at a fixed ${\Gamma }_D$, $\beta \to 0$, signifying flat spectrum over the fit range. When the beam size tends to infinity, (${\Gamma }_D\to 0$) at a fixed ${\Gamma }_p$, the line shape becomes a Lorentzian, and $\beta \to 1$. The universal regime is where $\beta \to 0.5$. The triangle (circle) indicates the position where we performed our $1\mathrm{\text{−}}d$ (wide-area) measurements. The expected slope according to this prediction is 0.52, compared to the measured $0.56\pm 0.01$. A similar calculation for the wide area beam yields 0.99, compared to the measured $0.97\pm 0.01$.Reuse & Permissions