Optically deformable membranes and cantilevers based on azobenzene liquid crystal polymer networks (azo-LCN) are demonstrated in the context of dynamic optical systems. Large modulations in laser beam propagation direction or amplitude directed by laser-induced changes in material shape are demonstrated. These macroscopic shape changes are induced by local changes to the liquid crystalline order induced by photoisomerization processes. We demonstrate herein a number of concepts including the focusing and defocusing action of an azo-LCN membrane, laser beam steering from a bimorph azo-LCN/metal cantilever, and surface initiated bending and blocking of a parallel propagating laser beam. High speed and large angle deformations of an incident or reference beam is demonstrated when coupled into suitable optical architectures. The concepts under discussion appear to be highly practical for a number of applications due to the significant nonlinearity and photosensitivity of azo-LCN materials.
© 2009 OSA
Smart optical systems consisting of light-responsive materials are gaining in significance, particularly for imaging and sensing purposes. Such functionality is typically obtained using materials with nonlinear optical refraction, diffraction, or transmission (absorption). In the case of optical sensors, the required photosensitivity of the detector necessitates the use of optical materials with extremely high optical nonlinearity. Material classes that possess both high optical nonlinearity and large photoinduced changes to their optical properties are rather limited. However, recently developed azobenzene liquid crystals (azo LCs) have shown huge changes of the optical anisotropy, Δn ~0.3 induced with only nanowatt-microwatt power radiation [1–6]. The most notable optical application of azo-based materials is the formation of surface relief gratings in azo-based polymers formed by spatial or polarization holography .
In general, coupling of photoisomerizable molecules into liquid crystalline polymer networks (both glassy and elastomer, “LCN”) amplifies the microscopic photoinduced molecular events into large scale macroscopic outputs. Such macroscopic outputs have primarily been demonstrated as contraction/expansion cycles of thin films or bending of a cantilever. At the core of the photomechanical effect is the chemistry of the LCN, which can be based on side-chain and/or main-chain photoisomerizable mesogens at concentrations ranging from 2 to 100%, cured in either mono- or polydomain orientations. Most often, the cured LCN network is composed of azobenzene liquid crystal monomer. Trans-cis photoisomerization of azobenzene with UV light is well-known to induce changes in the molecular order of LC systems via a decrease in the anisotropic molecular order resulting in the collapse of the LC network and a microscopic volume contraction. Using polarized UV light, the dichroic absorption of azobenzene molecules can be leveraged to induce polarization-controlled effects. Regardless of whether the azo-LCN is composed of a 100% [8,9] or less than 20% [10–12] azobenzene, the key to polarized-UV directed bending is the strong absorption of UV light by azobenzene to generate maximal contractile forces on the exposed surface that accordingly induce a mechanical response.
The uni-directional, UV-driven bending of azo-LCN films and cantilevers often require a second, higher wavelength light source to undo the photomechanical output (cis-trans isomerization) [8,13]. Only by adjusting the orientation of the LCN has bending in the negative direction (away) of the actinic source been demonstrated with UV light [14–16]. This too is a uni-directional effect, as the bending in these systems is now exclusively in the negative direction. By inducing a related, but fundamentally different photochemical event, we have demonstrated bidirectional ( ± 90°) bending of azo-LCN cantilevers . Exposing the azo-LCN materials, many of which are closely related to the systems previously referred to, to blue-green (rather than UV) light results in trans-cis-trans reorientation (also known as the Weigert effect ). This effect is a product of the nearly equivalent absorption of trans and cis in this spectral region and the dichroic absorption of azobenzene, which results a statistical buildup of trans-azobenzene reoriented 90° to the polarization direction of the source laser [17–21]. Blue-green laser directed bending is fast (~180°/s) and the direction of bending (multi-directional) can be controlled by either the power or polarization of the light source.
In the work here, we show that azo-LCN materials may allow the design of optical systems with highly efficient and practical photoresponsive features. These developments inject realism into the prospects of producing optically controlled LCN components whose ability to interact with light can be modulated by incoming radiation. In this paper, we demonstrate the utility of photoresponsive azo-LCN materials as membranes and cantilevers to form dynamic optical elements. This work focuses on demonstrating the basic principles that we hope will be expanded by us and others in the future.
2. Conceptual development
Figure 1 illustrates the basis of the concepts under discussion. The deformation of a photoresponsive polymeric material is utilized to control an incident and actuating laser light. Figure 1 (a) and 1 (b) demonstrates the geometry in which a membrane consisting of azo-LCN material reflects incident laser light. At low laser power density, the azo-LCN material does not deform and the photoresponsive membrane acts as a mirror, directionally reflecting incoming light. As the power density increases, the membrane deforms due to local changes in the molecular order yielding a concave (or convex) structure. This deformed state acts to diffuse the reflected incident laser light. In this scenario, the incident light is also the beam that is being modulated. One might also envision a case where the back surface of the membrane is made reflective by an appropriate coating and a second beam, part of an optical system, is reflected at a specified angle. Irradiation of the front surface could be used to locally deform the front surface causing a change in the direction of a second beam being reflected off the back.
Similar demonstrations can be envisioned if the azo-LCN thin film is cut into a cantilever shape . Figure 1 (c) and 1 (d) schematically demonstrate the use of this bidirectional photodisplacement to either mitigate incident laser light or to redirect incident laser light along an optical pathway. When using blue-green radiation instead of UV, polarization control in the direction of bending opens up alternative control motifs.
These conceptual concepts are based on a reflective geometry due to the strong localized surface behavior of this class of materials. Since deformations are related to surface contraction or expansion due to the generation of cis-isomers and/or reorientation of trans-isomers at the front surface (relative to the back), only a small amount of light penetration is needed. The large absorption coefficient of most azo-based molecules results in loss becoming a key issue in thicker samples. Thus, reflective optics is more advantageous than transmissive optics as the latter require bulk, homogeneous thin film material properties.
3. Nonlinear reflection from an azo-LCN membrane
A 0.66 mm diameter, 488 nm beam from an Ar+ laser was incident on an azo-LCN membrane at 45° (concept developed in Fig. 1 (a) and Fig. 1 (b)). The azo-LCN membrane, composed of a 20-μm thick polydomain azo-LCN obtained by thermal copolymerization of an azo LC monomer and crosslinker (as previously reported [8,17]), was mounted onto a circular aperture with a 12-mm opening diameter. The reflection coefficient at 488 nm was 7.4%. The membrane, as mounted in the 12-mm aperture, is depicted in the photograph inset in Fig. 2 (a) . The 0.66 mm diameter exposure is evident in the central portion of the membrane. Since the membrane is polydomain, the deformation of the membrane can form either a convex (laser-driven surface expansion) or a concave lens (laser-driven surface contraction), Fig. 2 (b). The results presented in Fig. 2 are for a concave lens. The power of the reflected beam was measured with an apertured power meter. The diameter of the aperture was large enough to not obstruct the reflected beam at low power levels when no deformation of the membrane was present (Fig. 1 (a)).
As the power of the incident laser beam increased, the spot size of the reflected beam increased considerably as shown in Fig. 2. The divergence of the beam, the ratio of the beam spot diameter to the distance from the film, increased by more than an order of magnitude as the power is increased from 0.5 mW (63 mW/cm2) to 21 mW (2.6 W/cm2). The power transmitted through the aperture subsequently decreased as shown in Fig. 3 (a) . Above 250 mW/cm2 input power density, a sharp decrease in the measured power density occurs. At power density levels greater than 630 mW/cm2 (Fig. 3 (b)), the magnitude of the beam as detected by the power meter is reduced to extremely small values beyond the sensitivity of the power meter.
The decrease in the reflected power after initial exposure occurs over a period of only a few seconds for sub-mW powers. The time between the maximum in the reflection power and the steady-state reflectivity is inversely proportional to the input power (Fig. 4 ). The evolution of the reflected beam spot profile in time for 7.2 mW (0.9 W/cm2) input power is shown in Fig. 5 and presented in real-time in Media 1. Over the exposure time, the divergence of the beam increases which correspondingly decreases the power. The repeatability of the process was tested by periodically blocking and unblocking the laser beam. The process was fully reversible for even relatively high power levels ~5 mW as shown in Fig. 6 .
4. Nonlinear deflection of light via a photoresponsive cantilever
In this part of the study, polydomain azo-LCN films were cut into rectangular cantilevers (7 mm × 1 mm × 0.02 mm sizes). An Ar+ laser was linearly polarized and provided 135-mW power in a multi-mode regime comprising radiation of 458 nm, 488 nm, and 514 nm wavelengths. The beam was expanded with a negative lens of −8 mm focal length to cover the whole width of the cantilever. A 10 μm-thick silver film was deposited on one side of the cantilever by electrostatic adhesion. A He-Ne laser beam, focused on the silver reflective side using a 150 mm focal length, was used as a probe to characterize the photoinduced cantilever bending as shown in Fig. 7 . The resulting reflection coefficient at 633 nm was 79%.
As previously shown , these polydomain azo-LCN cantilevers bend towards the laser output when the polarization is parallel to the long axis of the cantilever and away from the laser output (reverse direction) when the polarization is orthogonal to the long axis of the cantilever. The attachment of the silver film to one surface affects the bimorph mechanical properties and enables laser bending only away from the source for both pump beam polarizations, although the bending is stronger for the orthogonal polarization.
The deflection angle of the probe beam as a function of the power density is shown in Fig. 8 for a vertically mounted cantilever (orthogonal polarization). The initial position of the reflected probe beam is restored after blocking the pump beam. A maximum deflection angle of 35° is obtained at 0.2 W/cm2 power density.
The dynamics of the beam deflection caused by the cantilever bending is shown in Fig. 9 for 0.14 W/cm2 and 0.22 W/cm2 power density values of the pump beam linearly polarized along the cantilever long axis. Both the bending and restoration (pump beam turned off) processes are fast, reaching ~60°/s and ~30°/s at the intermediate stages of the bending and restoration processes, correspondingly. The effect is equivalently strong for both polarization states of the pump laser beam, parallel and perpendicular to the cantilever axis as shown in Fig. 10 .
Photos of the position of the probe beam reflected from the silver film at different times during the blocking and unblocking of the Ar+ laser beam are shown in Fig. 11 for a vertically mounted cantilever subject to the pump beam with polarization parallel to the cantilever axis. Photos are taken on a screen arranged perpendicular to the incidence direction of the probe beam and the real-time beam steering that occurs is shown in Media 2. The beam is deflected downwards upon unblocking of the Ar+ laser beam and restores its initial position when blocking the Ar+ beam.
5. Tangential propagation
Since the bending of the azo-LCN cantilevers is a result of photoinduced expansion/contraction of a surface with respect to bulk of the material, it can be induced by a laser beam propagating along the surface if the material is close enough in proximity to the beam to be efficiently influenced by it as schematically shown in Fig. 12 (a) and (b) . Laser beams typically have Gaussian distribution of intensity and the photoresponse of the azo-LCN can be expected to be highly critical to the alignment between the beam and the cantilever axis. Cantilever bending in this case can change the propagation properties of the incident beam by deflecting and defocusing it or by blocking its propagation. This is demonstrated in a 7 mm × 1 mm × 0.04 mm sized cantilever as shown in Fig. 12 (c) and (d). The azo-LCN cantilever bends into the beam, thereby blocking it, for a laser beam of 35 mW power propagating parallel to the long axis of a cantilever lying on a surface. The amount of radiation reaching a screen behind the film is greatly reduced as indicating by the visual images shown in Fig. 12 (e) and (f). The bending and unbending to large angles (100°) occurs quickly as shown in Fig. 13 and Media 3.
All these demonstrations enable one to envision various pump-probe or pump-only geometries where a photo-induced change to the local structure yields a controlled dynamic optical change. This is especially true when coupled to high reflectivity thin films as either directly coupled to the azo-LCN materials or as part of a system (i.e. a cantilever pressing on a pure metallic foil). Viability of these concepts hinges upon the development of a technology for producing large area high quality azo-LCN films. The optical actuation properties of such azo-LCN films, as well as films with mirror coated surfaces and micro-opto-mechanical systems based on such films should be the subject of future research. The range of opportunities for modification and application of these concepts beyond what was demonstrated is very large and the study of these in variety optical and component architectures is warranted.
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