Revisiting the Photon-Drag Effect in Metal Films

The photon-drag effect, the rectified current in a medium induced by conservation of momentum of absorbed or redirected light, is a unique probe of the detailed mechanisms underlying radiation pressure. We revisit this effect in gold, a canonical Drude metal. We discover that the signal for $p$-polarized illumination in ambient air is affected in both sign and magnitude by adsorbed molecules, opening previous measurements for reinterpretation. Further, we show that the intrinsic sign of the photon-drag effect is contrary to the prevailing intuitive model of direct momentum transfer to free electrons.

Figure 1

(a),(b) Schematic illustration of the experiment for illumination at the (a) free space-metal or (b) glass-metal interface. “$+$” refers to the amplifier input. Plane of metal: $x\text{−}y$; plane of incidence: $x\text{−}z$; $\theta$ rotation axis: $y$. (c) Device layout showing Au waveguide: light yellow; Ohmic contact area: dark yellow; illuminated area: red. (d) Representative measured waveforms for $s$- and $p$-polarized illumination at the free space-metal interface with $\theta =+45°$ in ambient air.

Figure 2

Dependence on angle of incidence for $s$ polarization (a),(b) and $p$ polarization (c)–(e). (a) $V_f^s(\theta )/E_p$, (b) $V_g^s(\theta )/E_p$, (c) $V_f^p(\theta )/E_p$, and (d),(e) $V_g^p(\theta )/E_p$ in ambient air and vacuum, along with ${\xi }_j^i{p}_{\parallel }(\theta )/E_p$ fits. (e) Enlarged view of experimental data in (d) around $+{\theta }_{\mathrm{SPP}}$.

Figure 3

(a) ${\xi }_f^s$ vs $1/d$ for three Au thicknesses, along with a linear fit ($R^2=0.991$). (Inset) Diagram of measurement circuit. Dashed box represents amplifier and oscilloscope. (b) $V_g^s(\theta )/E_p$ for illumination with photon energies of 0.95 and 0.83 eV, along with ${\xi }_g^s{p}_{\parallel }(\theta )/E_p$ fits. Extracted $\xi (0.95\mathrm{eV})=(4.6\pm 0.23)\mathrm{GV}/(\mathrm{N}\mathrm{s})$ and $\xi (0.83\mathrm{eV})=(5.2\pm 0.26)\mathrm{GV}/(\mathrm{N}\mathrm{s})$, where the uncertainty includes the 95% confidence interval and systematic errors.

Figure 4

$V_f^p(-45°)/E_p$ vs time during environmental procedures: evacuation alone, evacuation with UVC illumination, evacuation with Joule heating (5 min during which the device was disconnected), and venting with ambient air.