Sub-cycle switching of a photonic bandstructure via ultrastrong light-matter coupling

Abstract

Abstract. Phase-locked multi-terahertz transients map out the full photonic bandstructure of a one-dimensional photonic crystal while a 12-fs control pulse activates ultrastrong interaction on a sub-cycle time scale with quantized electronic transitions in semiconductor quantum wells. We trace the build-up dynamics of a large vacuum Rabi splitting and observe an unexpected asymmetric formation of the upper and lower polariton bands. The pronounced flattening of the photonic bands causes a slow-down of the group velocity by one order of magnitude on the time scale of the oscillation period of light.Tailoring optical properties of solids on the sub-wavelength scale has opened exciting possibilities to shape the way light interacts with electronic excitations. Recently, nanostructured optical cavities have been used to reach a new regime of ultrastrong light-matter coupling (USC) where embedded quantum emitters absorb and reemit virtual photons at a rate comparable to the frequency of the bare cavity mode [1][2][3][4]. Nonadiabatic modulation of ultrastrong interaction was predicted to cause unconventional quantum electrodynamical phenomena, such as the release of correlated photon pairs [5]. Experimentally, phase-sensitive multi-THz optoelectronics has been exploited to trace ultrafast activation of USC [2]. First proof-of-principle studies, however, have allowed for a limited control of the dispersion of the photon field only, besides requiring complex coupling geometries.Here, we combine a one-dimensional photonic crystal (PC) with optically switchable intersubband (ISB) resonances of semiconductor quantum wells (QWs) in order to approach full spatial and temporal control of light. Phase-locked multi-terahertz transients trace an asymmetric formation of the lower polariton (LP) and upper polariton (UP) branches on a time scale of the oscillation period of light. A pronounced flattening of the photonic bands causes a slow-down of the group velocity by one order of magnitude [6]. The room-temperature switching device operated in straightforward transmission geometry paves the way towards studies of nonadiabatic quantum electrodynamics (QED) phenomena. Figure 1a depicts the design of the semiconductor device used in our experiments. A 2 µm thick Al 0.95 Ga 0.05 As cladding layer followed by a sequence of 50 GaAs/Al 0.33 Ga 0.67 As QWs is epitaxially grown on an undoped GaAs substrate. The QW region supports a planar waveguide mode. A gold grating, with a period of a = 3.6 or 4.1 µm, is then thermally evaporated on the sample surface. The purpose of this grating is twofold: it enables surface plasmon propagation within the QW region and EPJ Web of Conferences