New applications for the intricate spatial manipulation of optical fields through diffractive optics are emerging every day. Such applications include the optical trapping and manipulation of cold atoms, quantum entanglement of photons for various applications, data transmission through orbital angular multiplexing and material processing. In the field of cold atom trapping, far off resonant traps are essential for studying condensates in a wide variety of varying geometries. However, owing to the limited damage thresholds of a majority of optical fibers delivering the laser light to the atoms, high efficiency mode conversion from the output of the fiber to the desired optical potential is obligatory for deep optical traps.
In instances where dynamic phase modulation is not required, the volume hologram is a superior alternative to a spatial light modulator as it can achieve greater total diffraction efficiencies and does not suffer from the pixilation losses of a spatial light modulator.
Sebastien Tempone-Wiltshire and colleagues at Monash University in Australia developed a method in which the photopolymer Bayfol HX is used together with a spatial light modulator for the generation of volume holograms of arbitrary complex optical fields. Their work alleviates the problems associated with spatial light modulators, requires only a single spatial light modulator for the hologram forming process and utilizes easily accessible materials. Their work is now published in Optics Express.
The researchers utilized a phase only, twisted nematic liquid crystal display as their spatial light modulator, with its limited phase shift being overcome by utilizing the phase only complex valued spatial filter.
A laser light of 532 nm wavelength was initially split by a non-polarizing beam splitter cube to produce object and reference beams, which were then passed through a series of polarizers and half-waveplates. The beams were then spatially filtered and then magnified. The object beam then diffracted off the spatial light modulator, passing through a pair of lenses which allowed for the selection of the desired mode through spatially filtering in the Fourier plane. The object beam was then magnified to the same size as the reference, and then channeled at an angle onto the photopolymer film.
In the hologram recording process, one beam was directed onto a camera which resulted in the beams diffracting into each other resulting in a grating, that served as a check for the over modulation of the refractive index. The object beam was then blocked and the hologram played back with the reference beam which allowed the research team to ascertain the quality of the hologram. A hologram of a Hermite-Gaussian mode, which is useful for cold atom trapping, was recorded and used to trap a Bose-Einstein condensate. The generation of optical modes with greater angular momentum per photon was achieved with the use of a Laguerre-Gaussian mode which overcomes the limitations of the spatial light modulator.
It was also observed that the overall efficiency of the recorded hologram was approximately 50%, which indicated that the holograms could attain the highest mode conversion efficiency limit for a single, phase only hologram.
The technique developed here created high fidelity, cost-efficient, high efficiency holographic optical elements for ultracold-atom trapping by the use of a spatial light modulator and Bayfol HX photopolymer, as well as the generation of optical modes with greater angular momentum by the use of the spatial light modulator.
T.W. Sebastien, S. Johnstone, K. Helmerson. High efficiency, low cost holographic optical elements for ultracold atom trapping. Optics Express volume 25 (2017) page 296-304.
School of Physics and Astronomy, Monash University, Victoria 3800, Australia.Go To Optics Express