  The characterisation of the extinction curve in the near infrared (NIR) is
fundamental to analyse the structure and stellar population of the Galactic
centre (GC), whose analysis is hampered by the extreme interstellar extinction
($A_V\sim 30$ mag) that varies on arc-second scales. Recent studies indicate
that the behaviour of the extinction curve might be more complex than
previously assumed, pointing towards a variation of the extinction curve as a
function of wavelength. We aim at analysing the variations of the extinction
index, $\alpha$, with wavelength, line-of-sight, and absolute extinction,
extending previous analysis to a larger area of the innermost regions of the
Galaxy. We analysed the whole GALACTICNUCLEUS survey, a high-angular resolution
($\sim 0.2''$) $JHK_s$ NIR survey specially designed to observe the GC in
unprecedented detail. It covers a region of $\sim 6000$\,pc$^2$, comprising
fields in the nuclear stellar disc, the inner bulge, and the transition region
between them. We applied two independent methods based on red clump (RC) stars
to constrain the extinction curve and analysed its variation superseding
previous studies. We used more than 165,000 RC stars and increased
significantly the size of the regions analysed to confirm that the extinction
curve varies with the wavelength. We estimated a difference $\Delta\alpha =
0.21\pm0.07$ between the obtained extinction indices, $\alpha_{JH}=2.44\pm0.05$
and $\alpha_{HK_s} = 2.23\pm0.05$. We also concluded that there is no
significant variation of the extinction curve with wavelength, with the
line-of-sight or the absolute extinction. Finally, we computed the ratios
between extinctions, $A_J/A_H = 1.87\pm0.03$ and $A_{H}/A_{K_s} = 1.84\pm0.03$,
consistent with all the regions of the GALACTICNUCLEUS catalogue.
