In the photoluminescence spectra of a two-dimensional electron gas in the fractional quantum Hall regime we observe for the first time the states at filling factor =4/ 5, 5/7, 4/11, and 3/8 as clear minima in the emission peak intensity or area. The first three states are described as interacting composite fermions in fractional quantum Hall regime. The minimum in the intensity at =3/ 8, which is not explained within this picture, can be an evidence of a suppression of the screening of the coulomb interaction among the effective quasiparticles involved in this intriguing state.Several magnetotransport and magneto-optical studies have been realized on the two-dimensional electron gas ͑2DEG͒ created in a modulation-doped semiconductor quantum well, allowing the observation of the integer quantum Hall effect ͑IQHE͒ and fractional quantum Hall effect ͑FQHE͒. 1-4 Various theories, that envisage the formation of Landau levels ͑LLs͒ and composite fermions ͑CFs͒, have been proposed to explain these phenomena. 1-5 Many body interactions rule the arrangement of electrons in two dimensions placed in a high magnetic field, leading them to form highly correlated states. 1 The presence of residual disorder is a fundamental ingredient for the observation of these quantum effects. However, FQHE at fractional filling factors that cannot be explained as IQHE of CFs has been observed in transport experiments, 2 and not all the features observed in the photoluminescence ͑PL͒ experiments are well understood but they keep on producing sources of research and discussion. 6 In this work we report the first evidence in optical emission experiments of CFs in fractional quantum Hall regime at filling factors =4/ 5, 5/7, 4/11, and of the exotic state =3/ 8. In particular we analyze the intensity and the area of the 2DEG PL emission as a function of the magnetic field B, for filling factors down to 2/7. Using a model based on the CFs picture 5 and the coulomb screening of the 2DEG ͑Ref. 7͒ we explain the structures observed in the spectra at several fractional .The experiments were performed on a 20-nm-thick single sided modulation-doped GaAs/AlGaAs single quantum well with carrier density n = 1.8ϫ 10 11 cm −2 and mobility = 1.6ϫ 10 6 cm 2 V −1 s −1 measured at 1.5 K. Indium contacts were placed on the sample to allow simultaneous transport and optical experiments. The sample was placed in a dilution fridge and was excited at 1.748 eV with the light of a Ti:sapphire laser. We present the PL measurements at the temperatures of 590 and 35 mK, with laser excitation power lower than 400 W.In Fig. 1 we show the PL spectra at 590 mK as a function of B and we observe that the intensity of the emission, deriving from the recombination of the 2DEG electrons in their first Landau level L 0 with photocreated valence holes in their first LL, varies markedly as the magnetic field increases. For filling factors greater than 2, i.e., B smaller than 3.8 T ͑B =2 ͒, we observe the emission from the higher LLs, which are at higher energy with respect to the L ...