The paper describes a technique to visualize subbands in semiconductor superlattices, which consists of a photon counting method to measure extremely weak photoluminescence (PL) and an image processing using a personal computer for visualization. The technique has been applied to measure the electric-field dependence of the subbands for GaAs/AlAs superlattices with layer thicknesses of 6.8 nm/1.7 nm (sample 1), 6.2 nm/3.4 nm (sample 2) and 6.0 nm/5.0 nm (sample 3), respectively. The field effects of the subbands such as the resonance and Stark-ladder transition are clearly visualized from the PL images. We observed the PL emission between the second Γ(Γ
2) and heavy hole (hh
1) states due to the resonance between the ground Γ(Γ
1) and Γ
2 states, and the PL emission between the ground X (X
1) and hh
1 states due to the resonance between X
1 and Γ
2 states. Furthermore, we observed the PLs from the Stark-ladder transitions between the Γ
1(+1) and hh
1 states and between the X
1(+1/2) and hh
1 states. This technique is useful for studying electronic states in semiconductor superlattices.
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