WO2018212677A1 - Variable focus liquid lens - Google Patents

Abstract

The invention relates to the field of optofluidics, adaptive optics and optoelectronics, and can be used for creating adaptive optical systems capable of changing the sign of the focal length and providing optical tuning without using a system of movable lenses. The proposed variable focus liquid lens is characterized by the simplicity of its structure and of the method of controlling the focal length, and is further characterized by a wide range of adjustment of the focal length and the possibility of changing the sign of the focal length. The essence of the invention is that a droplet of a non-volatile liquid with a laser beam-absorbing colourant added thereto is applied to a transparent surface, said droplet forming a contact angle other than zero, and the focal length of the droplet is changed as a result of the deformation of its free surface under the effect of centrifugal thermocapillary forces generated by the thermal action of a laser beam, wherein the value and sign of the focal length are determined by the strength of the beam. The advantages of the proposed variable focus liquid lens are that the liquid used can be any non-volatile liquid, there is no need to prepare a sealed cell or a cell having a complex structure; and the focal length is tuned without contact using a laser beam or focused radiation from any other light source such as, for example, a photodiode, a mercury-vapour lamp or a halogen lamp, it being sufficient that the radiation is absorbed either in the body of the droplet or by the material of the substrate.

Description

 DESCRIPTION OF THE INVENTION

Varifocal Liquid Lens

The alleged invention relates to the field of optofluidics, adaptive optics and optoelectronics and can be used to create an adaptive optical system that changes the sign of the focal length and realigns the optical characteristics without using a system of moving lenses.

 Liquid optical elements are a good alternative to mechanical systems, because they can perform an unlimited number of cycles without any wear, have a high focusing and reorientation speed due to fluidity properties, as well as the ability to update the optical medium by pumping liquid

A known liquid lens [1] is an interface (meniscus) between water and oil, while the water is in a micro reservoir with a transparent bottom and side walls of a thermosensitive hydrogel with the addition of gold nanoparticles that are highly absorbing IR radiation, and the oil layer is on top. A change in the volume of the hydrogel walls caused by IR irradiation leads to a change in the magnitude and sign of the meniscus curvature, and, consequently, the focal length. Despite the wide range of focal length adjustment and the ability to work with both a collecting and a diffusing lens, serious disadvantages of this lens are: the high cost of the materials used; complicated and expensive procedure for manufacturing a cell for liquids using microfabrication methods; a very limited set of working fluids, which should be completely immiscible and have substantially different refractive indices; and also the need to further stabilize the interface between such liquids by hydrophobizing their contact zone.

 In [2], a varifocal lens was proposed in the form of a meniscus of two immiscible liquids (the lower layer is water, the upper layer is 1-bromododecane) filling a cylindrical cell (6 mm in diameter) with transparent upper and lower walls, and an aperture protrusion of 3 mm in diameter, in the area of the meniscus to hold it. The adjustment of the focal length is carried out by changing the magnitude and sign of the meniscus curvature by manually moving the movable piston mounted in the lower part of the cuvette. The disadvantages of this method include the crude, namely manual, mechanism for adjusting the focal length, which, given the presence of a denser upper liquid, can lead to the meniscus leaving the retention zone and, as a consequence, to overturning the two-layer system.

 Known varifocal lens [3], which is a portion of a liquid (deionized water) filling a cylindrical cell with side walls of a piezoelectric and a bottom of a silicone membrane. The free surface of the liquid maintains a convex shape at a resonant frequency of the voltage applied to the piezoelectric transducer, and the focal length varies with the magnitude of the control voltage. The disadvantages of such a lens include the inability to change the sign of the focal length (the liquid surface works only as a collecting lens), and the limitation of its applicability as part of complex devices associated with the generation of spurious vibrations.

A varifocal liquid lens in the form of a sedentary drop of an electrically conductive liquid on a dielectric substrate coated with a thin layer of a dielectric was demonstrated in [4], and a network of control electrodes was mounted between the substrate and the dialectic. The control of the focal length of such a lens is carried out by changing its curvature, in depending on the voltage applied to the electrodes. Significant disadvantages of such a lens include: an extremely small range of focal length adjustment (about 20% of the initial value corresponding to zero voltage), the inability to change the sign of the focal length (a drop works only as a collecting lens), the need to use a wetting film of dielectric fluid to prevent edge hysteresis the wetting angle of the droplet during the adjustment of the focal length. In addition, the use of electrically conductive liquids as the lens material, a limitation is placed on the applicability of such a lens, and a complex electrode system requires the development of a control software package.

 A varifocal liquid lens is known [5], the body of which is formed as a sedentary drop from a layer of a solution of a non-volatile positive tensoactive substance in a volatile solvent using the effect of concentration-capillary convection controlled by a light beam, and the focal length is adjusted by changing the beam power. Despite the small size and wide range of adjustment of the focal length, this lens has the following disadvantages: it is necessary to use a sealed cell to prevent solvent leakage, as well as to heat the lid of the cuvette to prevent condensation of vapors on it, leading to optical interference; restriction on the choice of a pair of working fluids, which are subject to the requirements of mutual solubility, a large difference in surface tension and saturated vapor pressure between the fluids. In addition, this drop only works as a collecting lens.

The technical result of this invention is a significant simplification of the control of the focal length and design of the liquid lens, expanding the range of the focal length in the modes of the collecting and scattering lenses, expanding the range of liquids used to form the body of the lens, and, in general, increase the reliability of the varifocal lens.

 The technical result is achieved by using a droplet of a non-volatile liquid sitting on a transparent substrate with the addition of a dye laser absorbing the control beam, and the change in its focal length is due to deformation of the free surface of the drop caused by centrifugal thermocapillary forces induced by the thermal action of the control laser beam [6-8], while the magnitude and sign of the focal length are determined by the power of the control beam.

The principle of operation of the varifocal fluid lens is illustrated in FIG. 1. Here 1 is a sedentary drop of liquid, 2 is a transparent substrate, 3 is a laser beam. In the absence of laser radiation, a sedentary drop of liquid on a transparent flat substrate is a collecting lens, FIG. 1 (a), the focal length of which is associated with a radius of curvature of its free surface as a ¥ = R / (ni ~ 1) wherein R free surface curvature radius of the droplet, n _g refractive index of the liquid. When you turn on the laser beam directed normally to the base of the drop, the temperature of its free surface in the area of the beam increases, due to absorption of radiation in the volume of the drop, resulting in a local decrease in surface tension. As a result, a centrifugal field of thermocapillary forces arises on the droplet’s free surface, which, due to viscosity, move the liquid from the heated zone to the cold edge of the droplet, thereby causing deformation of its free surface, FIG. 1 (bd). The shape of the free surface and the magnitude of its deformation depends on the power of the control laser beam. A gradual increase in the power of the control laser beam (shown by the axis of the beam power in Fig. 1) causes a sequential change in the free surface of the droplet: flattening of the free surface, leading to an increase in the radius of curvature and, consequently, the focal length of the droplet, FIG. 1 (6), up to the moment when the free surface becomes almost flat, and its focal length tends to infinity, FIG. 1 (c); a change in the sign of curvature due to deformation of the free surface of the droplet in the form of a thermocapillary depression [6-8], accompanied by a decrease in the absolute value of the radius of curvature and focal length, FIG. 1 (y-d). In the first case, the droplet is a varifocal collecting lens, and in the second, a varifocal scattering lens. A decrease in the beam power allows reversibly changing the magnitude and sign of the focal length. Turning off the laser beam leads to relaxation of the deformation of the free surface of the droplet, as a result of which the latter takes its original shape, FIG. 1 (a).

 In FIG. Figure 2 shows the dependence of the focal length of a drop of benzyl alcohol (radius of curvature R = 9.7 mm at P = 0 mW) sitting on a glass slide, depending on the power of the control laser beam (λ = 532 nm, beam diameter d = 0.8 mm). To ensure absorption of laser radiation, benzyl alcohol is slightly colored with a crystalline violet dye. As seen in FIG. 2, with an increase in the P laser from 0 to 40 mW, the focal length F of the drop increases from +18 to +56 mm (3 times). In the range from 40 to 53 mW, a transition state takes place - the free surface of the droplet passes the position R—> oo, and then a thermocapillary depression begins to form in the droplet, the curvature changes its sign and with a further increase in P to 200 mW, F varies from -200 to - 2.7 mm.

In FIG. Figure 3 illustrates the change in the curvature of the droplet surface with grid images obtained using a lens based on a droplet of ethylene glycol on a glass substrate controlled by a laser beam (= 532 nm, d = 0.8 mm): (a) initial position — the laser is turned off, the radius of curvature R = 6 mm; (b) the drop is irradiated with a laser beam P = 85 mW, the positive radius of curvature increases to R = 25 mm; (c) the laser power is P = 125 mW, the droplet surface is concave, and the radius of curvature of the concave portion of the surface is R = -61 mm. Thus, the proposed varifocal liquid lens, characterized by simplicity and reliability, has the following advantages: a wide range of adjustment of the focal length and the ability to change the sign of the focal length; any non-volatile liquid can be used as a working fluid, which forms a sessile drop on the substrate with a non-zero wetting angle; the manufacture of a sealed cell or cell with a complex structure of the inner surface of the walls is not required; focal length control is carried out contactlessly using a laser beam. In addition, any light beam can be used to deform the surface, for example, focused radiation of a photodiode, mercury or halogen lamp, it is enough that the radiation is absorbed either in the droplet volume by adding a dye or by choosing the radiation wavelength absorbed by the liquid itself or by the substrate material.

LITERATURE

 1. Zeng X., Jiang N. Tunable liquid microlens actuated by infrared light-responsive hydrogel. // Applied Physics Letters, 93, 151 101, 2008.

 2. Patra R., Agarwal S., Kondaraju S., Bahga S.S. Membrane-less variable focus liquid lens with manual actuation. // Optics Communications, 389, 74-78, 2017.

3. Feng G. H., Liu J. H. Simple-structured capillary-force-dominated tunable-focus liquid lens based on the higher-order-harmonic resonance of a piezoelectric ring transducer. // Applied Optics, 52 (4), 829-837, 2013.

 4. Krupenkin, T., Yang S., Mach P. Tunable liquid microlens. // Appl. Phys. Lett., 82 (3), 316-318, 2003.

 5. Bezugly B.A., Shepelenok SV., Tarasov O.A. Adaptive optical device based on a liquid lens. // RF patent 2149434, 05.20.2000.

6. Da Costa G., Calatroni J. Transient deformation of liquid surfaces by laser-induced thermocapillarity. // Applied Optics, 18 (2), 233-235, 1979. 7. Helmers H., Witte W. Holographic study of laser-induced liquid surface deformations. // Optics Communications, 49 (1), 21-23, 1984.

 8. Bezugly B.A., Ivanova H.A., Zueva A.Yu. Thermocapillary deformation of a thin liquid layer caused by a laser beam. // PMTF, 42 (3), 130-134, 2001.

Claims

CLAIM Varifocal Liquid Lens A varifocal liquid lens, which is a drop of liquid that absorbs radiation in the volume of a drop and sits on a solid transparent substrate, and the focal length of the drop is rearranged due to a change in the curvature of its free surface due to the thermal effect of the laser beam on the drop, which can be distinguished that the liquid is non-volatile and, in the absence of laser radiation, the liquid drop is a collecting lens, and the curvature of the surface of the drop changes under the influence of thermocapillary forces, are excited thermal action of the laser beam on the droplet, and the magnitude and sign of the curvature and focal length determined by the power of the laser beam.