Controlling Light with Light

 

Our results:

The figure below shows the transmitted spectral response of our structure. One sees that on-resonance the transmitted power drops by more than 10 dB with respect to that at off-resonance. The losses at off-resonance wavelengths are 3.5 dB, which include the fiber-to-waveguide coupling losses and the propagation losses in the 7-mm-long waveguide.

Quasi-TM transmission spectrum of a single-coupled ring resonator in the absence of optical pump. Inset shows both probe wavelength settings (λ_probe1 = 1535.2 nm and λ_probe2 = 1535.6 nm) used for characterizing the dynamic response of the switch.

 

The temporal responses of the transmitted probe signals are shown below for two distinct probe wavelengths around λ_res1: λ_probe1 = 1535.2 nm (below resonance) and λ_probe2 = 1535.6 nm (on resonance). These probe wavelengths were tuned relative to the ring resonance in order to maximize the modulation depth by setting the transmission without pump to high and low levels, respectively. An important figure-of-merit for switching is the modulation depth (MD), defined as MD = ( I_max - I_min ) / I_max, where I_max and I_min are, respectively, the maximum and minimum transmitted probe optical powers. We measured MD_probe1 = 94% for λ_probe1 and MD_probe2 = 91% for λ_probe2.

 

Temporal responses of the probe signal to the pump excitation, showing transmission for probe wavelengths below resonance (solid line) and on resonance (dotted line).

 

We obtain from the experimental data a wavelength peak shift of  Δλ= -0.36 nm and a relaxation time of t_fc = 450 ps. The wavelength peak shift of the ring resonator corresponds to an effective index change of Δn_eff = -4.8×10^-4, or equivalently to a refractive index change in the silicon core of Δn_Si = –5.2×10^–4. This refractive index change is caused by a free-carrier concentration of ΔN = ΔP = 1.6×10¹⁷ cm^-3. This free carrier concentration generated in the waveguided resonator is proportional to the square of the peak pumping power. We estimate that the optical pulse energy absorbed inside the ring resonator in order to excite such a free-carrier concentration is of only 0.15 pJ. The remaining pump power, necessary for the two-photon absorption effect is scattered from the ring. However, this energy could be recycled by using an add/drop configuration, where an additional waveguide is added symmetrically adjacent to the ring. The losses due to the probe absorption, estimated from free-carrier concentration, are Δα = 6.9 cm^-1, significantly lower than the estimated scattering losses in the ring resonator of α_ring = 33.6 cm^-1. The relative low absorption losses indicate that the observed modulation is due only to a refractive index change and that thermal effects can be neglected. This is of foremost importance for the application of the proposed device as an all-optical gate, enabling near 100% transmission of the data signal once the gate is open.