  We introduce a grating assisted tunneling scheme for tunable synthetic
magnetic fields in photonic lattices, which can be implemented at optical
frequencies in optically induced one- and two-dimensional dielectric photonic
lattices. We demonstrate a conical diffraction pattern in particular
realization of these lattices which possess Dirac points in $k$-space, as a
signature of the synthetic magnetic fields. The two-dimensional photonic
lattice with grating assisted tunneling constitutes the realization of the
Harper-Hofstadter Hamiltonian.
