Abstract: We investigate generation of nonclassical photon states using nonlinear cavities. Our goal is to develop single photon sources which are needed e.g. in information processing, quantum computing and fundamental quantum optical experiments. We have recently shown that antibunched photons (i.e. nonclassical light) can be obtained from nonlinear cavities with multi-photon absorption [Häyrynen et al., Nonlinear laser cavities as nonclassical light sources, submitted (2012)]. The antibunching in nonlinear cavities occurs because the energy pumping raises the probability of states having n>0 photons but simultaneously the second order absorption in the nonlinear element causes an additional decay for states having n>1 photons relative to the single photon state. Therefore, light fields which are essentially superpositions of zero photon and one photon states are generated. These results suggest that a nonclassical light emitter could be realized by coupling a nonlinear absorber to a light source like a light emitting diode (LED) or by adding a nonlinear absorbing layer into a laser cavity. The nonlinearity can be achieved e.g. by replacing one of the cavity mirrors with a two-photon saturable absorber mirror [Thoen et al., Two-photon absorption in semiconductor saturable absorber mirrors, Applied Physics Letters, vol. 74, p. 3927 (1999)] or a nonlinear mirror [Schirmer et al., Nonlinear mirror based on two-photon absorption, J. Opt. Soc. Am. B, vol. 14 p. 2865 (1997)]. This type of a setup is technologically attractive since it potentially provides room temperature realization of photon antibunching with essentially standard optoelectronic materials and processing techniques. We maximize the probability of the single photon state by optimizing the strengths of linear losses, nonlinear absorption and photon emission. Furthermore, we investigate the effect of exciting the photon emitter with time dependent current pulses to provide single-photon-on-demand sources.