We report a method of producing a lateral gradient in the optical properties of anodically etched porous silicon layers. Lateral gradation details of the porous silicon layer are governed by the etch mask pattern involved. Unlike other methods that rely on uneven hole current distribution, we believe that in our method the diffusion of reactive ions in the etchant plays a key role. As an implementation of the proposed method, we demonstrate a linearly graded optical bandpass filter operating at the range by employing a tapered etch window opening. The resultant optical filter exhibited a tuning range with a sharp transmission bandwidth of . Computer simulations indicate that an uneven hole current distribution cannot be the reason for the observed gradient along the taper axis, supporting the view that the diffusion-limited etch process plays the key role.
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