Complex beam sculpting with tunable acoustic gradient index lenses Euan McLeod and Craig B. Arnold Department of Mechanical & Aerospace Engineering, Princeton University, Princeton, NJ ABSTRACT Spatial Light Modulators (SLMs) have been successfully used for beam sculpting in the area of optical ma- nipulation, however in some applications their associated pixelation, slow switching speeds, and incident power limitations can be undesirable. An alternative device that overcomes these problems is the Tunable Acoustic Gradient index (TAG) lens. This device uses acoustically induced density and refractive index variations within a fluid to spatially phase modulate a transmitted laser beam. The acoustic waves within the fluid are generated via a piezoelectric transducer. When driven with a frequency-modulated signal, arbitrary optical phase modulation patterns can be generated at regular time intervals. The resulting sculpted beam is best observed using a pulsed laser synchronized to the frequency-modulated signal of the TAG lens. As this device is purely analog, there is no pixelation in the phase modulation pattern. Also, because the only major requirement on the fluid is that it be transparent, it is possible to select fluids with high damage thresholds and high viscosities. High damage thresholds allow the TAG lens to be used in high power applications that would be unsuitable for an SLM. High viscosities provide fast damping of transient density variations and increase switching speeds between patterns. Discussion here will be limited to axially symmetric beam sculpting, however the results can be generalized to asymmetric cases. Keywords: TAG Lens, Beam Sculpting, Acoustics, Phase Modulation 1. INTRODUCTION Laser beam shaping has lately become of interest especially in the fields of optical manipulation and atom optics. In optical micromanipulation alone, the resulting complex beams have been used in many applications. A few examples include particle sorting and cytometry, 1, 2 microfluidic pumps, 3 the study of 2D colloidal phase transformations, 4, 5 and the assembly of of microscopic structures. 6 In atom optics, uses for complex beams include trapping atoms in an optical bottle beam 7 and transporting cold atoms. 8 Currently, the demand for adaptive optics in the fields of optical manipulation and atom optics has been met primarily using using spatial light modulators (SLMs). 9, 10 However, SLMs do have some limitations. First, they are digital devices, and as a result their output is pixellated, which can degrade beam quality. Second, they have limited refresh rates. Typical spatial light modulators use nematic liquid crystal arrays. Reorienting the liquid crystals is a relatively slow process that limits frame rates to around 100Hz. 11 There do exist ferroelectric SLMs that can operate at rates above 1kHz, however these devices are only binary spatial light modulators; that is, each pixel can only impart one of two given phase delays on the wavefront. 11 Third, SLMs have incident laser power limitations due do the delicate nature of the liquid crystals. An alternative method for adaptive optics is the liquid-filled tunable acoustic gradient index (TAG) lens. 12–14 This method can be thought of as an analog counterpart to the currently digital adaptive optics. TAG lenses do not have the same limitations as SLMs, and therefore may be more suitable in some applications. First, TAG lenses are analog, so there are no pixellation effects. Second, they have fast switching speeds, as high as 20 kHz, although for optimum pattern quality, refresh rates closer to 1 kHz were used. 13 Switching speed can be improved by filling the TAG lens with fluids having a greater viscosity and speed of sound. Third, because a wide range of filling fluids are available, it is possible to design the TAG lens to withstand very large laser powers. Further author information: E-mail: emcleod@princeton.edu 1 Appeared In Complex Light and Optical Forces, eds. D.L. Andrews, E. J. Galvez, G. Nienhuis, Vol. 6483, p. 64830I SPIE, 2007