Broadband and Ultrafast Optical Phase Modulation by Colloidal 2D Semiconductors

Ultrathin two-dimensional semiconductors are heavily investigated for applications in opto-electronics, where their strongly excitonic character and remarkable light-matter interactions prime them for diverse applications in light emission and photo-detection. However, many propertes and potential applications remain unexplored to date such as the ability to manipulte the phase of the light field. Often used in interferometric sensors or switches, or detrimental in delicately designed photonic cavities, the ability of a material to substantially change its refractive index upon excitation and hence manipulate optical phase, either electrical or optical, merits deeper investigation.<br/>Measuring optical phase modulation of dilute 2D materials is however not trivial with common ultrafast methods. In this work, we first demonstrate that 2D colloidal CdSe quantum wells, a useful model system, can modulate the phase of light across a broad spectrum using an experimental femtosecond interferometry method. Next, we proceed to develop a toolbox to calculate the time-dependent refractive index of colloidal 2D materials from more widely available broadband transient absorption data using a modified effective medium algorithm. We confirm the interferometry experiments quantitatively and show that the pronounced room temperature excitonic features found in 2D materials result in broadband, ultrafast and sizable phase modulation, even extending sub-band gap to the near-infrared where modulation is associated with well defined intraband transitions.