Positive Refractive Index Change Research Articles

Field-induced optical effect in a five-step asymmetric coupled quantum well with modified potential

A five-step asymmetric coupled quantum well (FACQW) is demonstrated and analyzed for its field-induced optical effect. A strong combined exciton absorption peak is caused by e1hh2 and e2hh1 transitions for the TE mode and e1lh2 and e2lh1 for the TM mode at a small negative applied electric field, while two separated exciton absorption peaks appear at positive applied electric field. The large change of the exciton absorption strength produces large positive refractive index change whose value is larger by over one order of magnitude compared to that in a rectangular quantum well (RQW) when the operation wavelength is away from the absorption edge. A tensile-strained FACQW is employed to improve the polarization dependence. The difference of the refractive index change for TE and TM modes is under 2% while that in the unstrained FACQW is larger than 50%. A modified FACQW structure is also studied for a negative refractive index change. The strong combined exciton absorption peak without a red shift is observed in the measurement of photocurrent spectra of the FACQW at room temperature. The absorption properties of the FACQW under high applied electric field are also observed and analyzed. The measured results are in good agreement with bur numerical analysis. Finally, ridge-waveguide Mach-Zehnder modulators have been fabricated by using the FACQW and RQW structures. The half-wave voltage of the FACQW modulator is as low as 3 V while that of the RQW modulator is 8 V. The result proves that the FACQW structure indeed has larger refractive index change than that of the RQW structure. These properties obtained with the FACQW family have a great potential for application to ultrafast and low-voltage optical modulators and switches.

Analysis of X-Intersecting Waveguide Switches with a Large Branching Angles Ranging from 2° to 12°

This is a study on the analysis of the X-junction waveguide switches with a large branching angle ranging from 2° to 12°. The characteristics of the modeled are simulated by using the beam propagation method. We will show that, for an X-intersecting waveguide with a branching angle more than 2°, the eigenmodes interference remains and prevails even up to 12°. This produces a high unbiased crosstalk. In the analysis of the modulation efficiency of the switch, it is revealed that a large positive refractive index change will produce higher modulation efficiency than a negative refractive index change because of optical field re-distribution. In designing an X-intersecting waveguide switch using the coupled mode theory, a -40 dB crosstalk can be achieved for device smaller than 2 µ m. The interference X-intersecting waveguide switches will be more suitable for large scale switching arrays.

Theoretical and experimental studies of electric‐field‐induced refractive index change in quantum‐film, ‐wire, and ‐box structures

AbstractIn this article we report theoretical as well as experimental studies of the electric‐field‐induced refractive index change in InGaAs/InP quantum‐well structures, viz., quantum‐film, quantum‐wire, and quantum‐box structures. The refractive index change, easily measured using a Mach‐Zehnder interferometer setup, was around 1%, 4%, and 7% in quantum film, quantum wire, and quantum box, respectively, in the longer‐wavelength region corresponding to the positive refractive index change peak. Moreover, we will discuss that the refractive index change dependency on the polarization of incident light in a quantum film can be controlled by introducing suitable tensile strain in it. It was found that for a well width of 11 nm, 0.3% tensile strain should be induced to obtain polarization‐independent refractive index change. © 1994 John Wiley & Sons, Inc.