  Thin film optical elements exhibiting translational invariance, and thus
robustness to optical misalignment, are crucial for rapid development of
compact and integrated optical devices. In this letter, we experimentally
demonstrate a beam-shaping element that generates an annular beam by spatially
filtering the fundamental Gaussian mode of a laser beam. The element comprises
of a one-dimensional photonic crystal cavity fabricated using sputtered thin
films. The planar architecture and in-plane symmetry of the element render our
beam-shaping technique translationally invariant. The generated annular beam is
sensitive to the polarization direction and the wavelength of the incident
laser beam. Using this property of the annular beam, we show simultaneous
generation of concentric annular beams of different wavelengths. Our
experimental observations show an excellent agreement with simulation results
performed using finite-difference time-domain method. Such a beam-shaping
element has applications in areas ranging from microscopy and medicine to
semiconductor lithography and manufacturing in microelectronics industry.
