Context : 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 .

Aims : 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 .

Methods : We analysed the whole GALACTICNUCLEUS survey , a high-angular resolution ( \sim 0.2 ^ { \prime \prime } ) 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 .

Results : 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 \pm 0.07 between the obtained extinction indices , \alpha _ { JH } = 2.44 \pm 0.05 and \alpha _ { HK _ { s } } = 2.23 \pm 0.05 .
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 \pm 0.03 and A _ { H } / A _ { K _ { s } } = 1.84 \pm 0.03 , consistent with all the regions of the GALACTICNUCLEUS catalogue .

Conclusions :