All-optical active THz metasurfaces for ultrafast polarization switching and dynamic beam splitting

Miniaturized ultrafast switchable optical components with an extremely compact size and a high-speed response will be the core of next-generation all-optical devices instead of traditional integrated circuits, which are approaching the bottleneck of Moore’s Law. Metasurfaces have emerged as fascinating subwavelength flat optical components and devices for light focusing and holography applications. However, these devices exhibit a severe limitation due to their natural passive response. Here we introduce an active hybrid metasurface integrated with patterned semiconductor inclusions for all-optical active control of terahertz waves. Ultrafast modulation of polarization states and the beam splitting ratio are experimentally demonstrated on a time scale of 667 ps. This scheme of hybrid metasurfaces could also be extended to the design of various free-space all-optical active devices, such as varifocal planar lenses, switchable vector beam generators, and components for holography in ultrafast imaging, display, and high-fidelity terahertz wireless communication systems.

Angle-resolved measurements. The passive measurement of the angle-resolved spectra was performed using a fiber laser integrated terahertz time-domain system (Menlo system) consisting of a pair of photoconductive antenna transmitter and receiver.
The incident terahertz beam was normal to the sample surface, and the transmission signals were recorded by rotating the angle of the receiver around the sample from -80° to +80° using steps of 1°. A wire-grid polarizer was inserted in front of the receiver as a polarization analyzer. The time-domain signals were transformed to the frequency domain and normalized to the reference using

Supplementary Note S2: Saturation of photoconductivity
There will be a saturation of photoconductivity at larger pump fluences, which limits the modulation depth, due to two reasons: 1. Charged carrier density increases and thus carrier-carrier scattering mechanism becomes more pronounced, which reduces the charge carrier mobility and photoconductivity; 2. At high pump fluence, the conduction band will be gradually filled, which shifts the absorption edge according to the Burstein-Moss effect. Therefore, the absorption coefficient of silicon epilayer reduces, which thus limits the increasing of photoconductivity.

Supplementary Note S4: Jones Matrix and the understanding of its elements
In the h-SRR metasurfaces, the structures are embedded on substrate with all the materials being linear and reciprocal. With a plane wave propagating in the positive z direction, the electric field is described by is the wave vector and the complex amplitude ix and iy are the states of polarization. The transmission field is described as A generalized Jones calculus (T matrix) could be adopted for a coherent plane wave, instead of the Mueller calculus that is necessary for incoherent light, to connect the 15 incidence and transmission. The relation is: x

Supplementary Note S6: Normal transmission of co-polarized component
Since the radiation phase of co-polarized component shows independent on the geometrical size as well as the spatial rotation of h-SRR, we observe an almost constant phase value around 0° (using phase response of unit cell #1 as reference) as shown in supplementary Figure S7b. The radiation amplitude of each unit cell also reveals a stable value in a broad frequency band as shown in Figure S7c. Therefore, no extra momentum is introduced for the co-polarized component, and thus the light still propagates in the normal direction in the entire frequency regime of interest as shown in supplementary Figure S7d.

Supplementary Note S7: Experimental limitation
For the experimental characterization, we had to perform measurements by using different terahertz setups. For the passive angle-resolved measurements, we employed the fiber laser integrated terahertz time-domain system whose transmitter and receiver are flexible to be moved in free space. By inserting a wire-grid polarizer in front of receiver, we were able to selectively capture the co-or cross-polarized components by rotating the aligned polarizer at 0° or 90°. The sample was set at the center of rotating circle of receiver, and far-field time-domain transmission signals were measured for angles ranging from -80° to +80° with a step of 1°. The sample might not be accurately resided in the center of the rotating receiver circle, which would introduce systematic errors. Two limitations exist in the fiber laser system: 1. Since the transmitter and receiver are pumped by a fiber coupled oscillation beam, the pulse energy is too low to excite the free carriers in the hybrid metasurfaces. Therefore, experiments of ultrafast dynamics, ultrafast switch of polarization states, and active modulation of beam splitting ratio cannot be done by using this setup. 2. Since the cross-polarized component is dispersed in angular space, we have to collect the frequency-dependent intensity spectrum in an indirect way as the shaded area shown in Figure 5a.
For the active measurements which require a coherent high power source as pump, we have to utilize another terahertz setup, ZnTe crystal based TDS system. The generation of terahertz radiation is based on the nonlinear effect of ZnTe crystal with a high power amplified laser beam in this system. A coherent amplified laser beam was split as a pump source for the dynamic measurements. However, only free-space laser beam was available for this setup, and rigorous alignment was required for the optical setup.
Therefore, we could only perform the normal transmission measurements in this stationary system. In this context, it is not possible to capture the direct evidence of cross-polarized intensity modulation for the beam splitter with external pump, but an indirect evidence of the correlated co-polarized intensity modulation was experimentally captured.

Supplementary Note S8: Photoconductivity
The frequency-resolved photoconductivity of unpatterned silicon on sapphire substrate was measured by using the OPTP method at different pump fluences. On the basis of the free carrier dynamics of silicon epilayer as shown in Figure 2c, the pump beam was set ~30 ps ahead of the terahertz pulse so that terahertz pulse captures the maximal accumulation of photo-induced carriers when reaching the sample surface. First, the static transmission spectrum of SOS substrate without pump was measured as a reference (electric field amplitude,   0 Et ). Then we moved to the dynamic measurements at different pump fluences, and obtained the respective terahertz time-