Spintronics and opto-spintronics promise non-volatile information processing, storage and communication with a minimum inter-task lag time. However, the lack of non-magnetic semiconductors that allows efficient spin generation at room temperature has so far limited its development. Here, we designed and implemented a remote defect spin-filtering mechanism in the nanostructure of GaNAs/QD which amplifies the InAs quantum-dot (QD) electrons spin polarization through the adjacent tunnelling-coupled GaNAs room-temperature spin amplifier. By such construction, we manage to achieve over 90% QD electron spin polarization at/above room temperature. Furthermore, we show that the room-temperature spin amplifier has an inherent spin nonlinearity. In GaNAs/QD, we showcase the spin higher-order harmonic generation by converting the low-frequency modulation of the excitation polarization to the higher-frequency oscillation of the QD emission intensity and polarization. The nonlinear spin response in such all-semiconductor nanostructure is expected to be faster than 1 GHz and can be explored for novel nonlinear spintronic applications.