Photothermal Optical Beam Steering Using Large Deformation Multi-Layer Thin Film Structures

Abstract

Photothermal actuation of microstructures remains an active area of research for microsystems that demand electrically isolated, remote, on-chip manipulation. In this study, large-deformation structures constructed from thin films traditional to microsystems were explored through both simulation and experiment as a rudimentary means to both steer and shape an incident light beam through photothermal actuation. A series of unit step infrared laser exposures were applied at increasing power levels to both uniformly symmetric and deliberately asymmetric absorptive structures with the intent of characterizing the photothermal tilt response. The results indicate that a small angle (<4° at ~74 W/cm2) mechanical tilt can be instantiated through central placement of an infrared beam, although directional control appears highly sensitive to initial beam placement. Greater responsivity (up to ~9° mechanical tilt at ~54 W/cm2) and gross directional control was demonstrated with an asymmetrical absorptive design, although this response was accompanied by a large amount (~5–10°) of mechanical tilt burn-in and drift. Rigorous device cycling remains to be explored, but the results suggest that these structures, and those similar in construction, can be further matured to achieve controllable photoactuation suitable for optical beam control or other applications.

Large Tilt-Angle Photothermal Optical Beam Steering Using Micropatterned CNT-Based Infrared Absorber on Multilayer Thin Film Structures

“…The devices characterized in this work are shown in Figure 1a-b and include a variant with and without the CNT infrared absorber in order to enable direct experimental comparison of photothermal actuation performance. The layout and geometry is identical to the structures previously examined in [3], expect these structures only have a single "arm" highly deformed out of plane upon removal of the sacrificial layer causing the central plate to be initially tilted 21° (see Figure 1c). As in prior work, the deformation of this arm is a result of the alternating tensile stress gold and compressively stressed silicon nitride films on the surface of the polysilicon structure which effectively creates a series of Sshaped deformation profiles that elevate that portion of the structure.…”

“…The mechanical tilt (i.e. beam steering response) was measured by observing the reflection of a second low power visible laser (λ = 632 nm, spot size = 200 [3]) with a CNT/nanodiamond/PMMA blend (inspired by [5]) on bottom arm µm diameter) off of the Au coated central plate onto a screen (distance = 11.1 cm) with a high speed camera (KronTech Chronos 1.4 -6456 fps). Two separate test series were performed on each of the two devices, one with a CNT coating and one without.…”

“…The device fabrication process used the same core methods outlined in Refs [3,4] with the PECVD nitride layer (1µm thick) directly post-processed onto PolyMUMPS TM foundry produced die. Film thicknesses are summarized in Figure 2.…”

“…Light induced mechanical actuation has long been of interest for autonomous microsystems that require electrical isolation, often due to cost, complexity, or energy limitations. Some compelling examples manipulate the incident radiation itself by modulating the transmitted power ( [1], [2]) or by altering the optical phase characteristics ( [3], [4]). Our team has been interested in developing devices that when illuminated with a strong optical source can preferentially steer a portion of the incident radiation off-axis in a tunable fashion to large angles.…”

“…We previously reported up to 9° of photothermally induced steady-state mechanical tilt at ~54 W/cm 2 irradiance with large deformation multilayer film structures that resemble the shape of a tetragonal pyramid [3]. These structures were constructed from selectively patterned regions of gold and silicon nitride films on polysilicon and exploit the residual stress inherent within these films to elevate the structure out of plane upon removal of the underlying sacrificial layer.…”