Near-field photocurrent measurements with multiwavelength excitation sources are applied to the investigation of a lateral p-n junction grown on patterned GaAs (111)A substrate. In order to probe the internal properties of this device, propagation modes into the sample are utilized retaining high resolution with the contribution of a penetration depth smaller than the aperture diameter. By systematically varying the penetration depth over a wide range up to 900 nm, photocurrent signals due to internal optical response clearly appear. The capability of "tomographic" diagnostics is demonstrated and the slant angle of the p-n interface is determined to be 30±8°.
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