1. Introduction

Over the past decade, femtosecond laser micromachining in bulk transparent materials has emerged as a powerful tool for enabling a variety of three dimensional (3D) functional components, such as optical waveguides [1,2], microfluidic channels [3,4], and embedded microelectrodes [5]. For both microfluidic and micro-optical applications, fused silica can be an ideal substrate material due to its excellent physical and chemical properties. Moreover, by virtue of its unique ability to simultaneously alter the chemical and optical properties inside the glass, femtosecond laser direct writing offer a direct approach to incorporate micro-optical and microfluidic structures in a monolithic substrate [6–10].

On the other hand, liquid crystals (LCs), due to their liquid like behavior, large birefringence, and extreme sensitivity to external field and surface interactions, offer great flexibility for enabling tunable and reconfigurable photonic devices [11–14]. Conventionally, LCs are sandwiched between two glass plates coated with polyimide films as alignment layers and successive indium-tin-oxide thin films as driving electrodes. However, although the sandwiched LC structures are well-established and widely used in LC display, the production of integratable 3D LC devices remains a challenge. In fact, an individual LC microcell can hardly be achieved by traditional process, because of the associated difficulties in fabrication of miniaturized alignment layers and driving electrodes [15].

In our previous work [16], we have developed a LC alignment technology based on femtosecond-laser-induced surface nanoripples [17]. In particular, we demonstrate that uniform nanoripples can be produced on the sidewalls of the micro-cell vertically embedded in fused silica glass by femtosecond laser direct ablation. This is because that when the LC molecules are filled into the micro-cells, the LC molecules will rotate by themselves toward the direction in parallel with that of the surface nanoripples to minimize their elastic deformation energy [18]. The direction of nanoripples can be easily controlled by changing the polarization direction of the femtosecond laser, enabling microscopic homogeneous LC alignment with different azimuthal angles in the laser ablated microcell. This technique provides a fast and direct method to fabricate LC microcell embedded in transparent materials.

In this contribution, we demonstrate a fully integrated LC electro-optic light modulator fabricated in fused silica by femtosecond laser direct writing. The light modulator consists of a LC microcell, four embedded microelectrodes and a buried optical waveguide. The symmetrically embedded arc-shaped microelectrodes could give rise to nearly uniform electric field in the LC cell, which enable flexible control of the polarization of light passing through the waveguide by controlling the orientation of the LC molecules.

2. Experimental setup

The optical waveguides, microelectrodes and LC cell were fabricated on a femtosecond laser micromachining platform, which consisted of a femtosecond laser source (Coherent, Inc., emits 800 nm, 40 fs pulses with a spectral bandwidth of ~30 nm at a 1-kHz repetition rate), an imaging system, a computer-controlled XYZ translation stage, beam control devices and delivery optics, where the Gaussian laser beam was trimmed by a 5 mm diameter aperture for ensuring a high beam quality. The laser pulses energy was adjusted by a combination of half-wave plate and polarizer and a set of neutral density filters. An achromatic half-wave plate was used to change the polarization direction of initially linearly polarized femtosecond laser. Commercially available fused silica with a size of 10 × 5 × 3 mm was used as the substrate. The sample was mounted on a computer-controlled XYZ stage with a translation resolution of 1 μm. Commercial nematic LC 5CB (Shijiazhuang Yongsheng Huatsing Liquid Crystal Co. Ltd.) was used. The surface morphologies of the formed nanoripples on the inner wall of microcell were characterized by a scanning electron microscopy (SEM, FEI Sirion 200).

3. Working mechanism

Figures 1(a) and (b) present the structure of a LC phase modulator, which is composed of four embedded microelectrodes, an embedded liquid crystal cell filled with positive uniaxial nematic liquid crystal 5CB, and a buried optical waveguide across the cell. Due to the intrinsic optical anisotropic of well-aligned LC molecules, the LC cell behaves as a phase retardation plate. As a result, light passing through the LC cell would experience a phase delay between the ordinary and extraordinary components given by

 

Fig. 1 (a) 3D schematic diagram of a LC modulator in fused silica (b) Top-view optical micrograph of the fabricated device

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4. Fabrication process

Figure 2(a) presents a schematic diagram of fabrication of LC microcell. A linearly polarized laser beam was focused on the fused silica substrate by a 20 × microscope objective (NA = 0.45). The dimensions of microcell were defined by femtosecond laser ablation using a line-by-line and layer-by-layer approach. For example, a U-shaped groove with a width of ~20 μm, a length of ~80 μm and a depth of ~70 μm was fabricated by inscribing a volume consisting of a matrix of 10 × 20 (transverse × vertical) lines at a scan speed of 10 μm/s and a pulse energy of ~3 μJ, and the transverse and vertical intervals of adjacent lines were chosen to be 2 μm and 3 μm, respectively. As we have shown before, by rotating the polarization of the ablation laser in the horizontal plane, we are able to control the orientation of the nanoripples on the inner wall [16]. When the laser polarization direction E is in parallel with the writing direction S, vertical nanoripples could be induced on the inner wall of microcell, as shown in Fig. 2(b). The results from SEM observation show that the induced nanoripples have uniform periodicity across the whole inner wall, and the average period of the nanoripples is ~170 nm. However, due to the micro-explosion effect and the debris deposition on the surface, the amplitude of nanoripples ranges from few tens to several hundreds of nanometers. For achieving uniform LC alignment in the microcell, it is critical to achieve uniform nanoripples in large areas, which could be improved by choosing suitable machining parameters, such as pulse energy, spacing between two consecutive lines, scan speed, etc.

 

Fig. 2 (a) Schematic diagram of femtosecond laser direct writing of embedded microcell. The laser propagation direction (k), the writing direction (S), and the laser polarization direction (E) are indicated. (b) SEM image of inner walls of the fabricated micro-cells.

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After femtosecond laser ablation, the substrate was rinsed in an ultrasonic bath with ethanol to remove debris inside microcell. Next, the microcell was filled with the nematic LC 5CB through a pre-prepared microchannel (see Fig. 1(b)) at 50 °C (above the clear point of 35.4 °C). After cooling to the room temperature (~25 °C), the LC molecules could spontaneously orient themselves along the nanoripples on the inner wall of microcell. More details about LC alignment in the microcells can be found elsewhere [16].

To create a uniform electric filed across the LC microcell and meanwhile allow light to pass through the microcell, a set of embedded arc-shaped metal electrodes were directly fabricated in the fused silica. The fabrication process of embedded electrodes mainly consists of two steps: (1) femtosecond-laser-assisted chemical etching and (2) femtosecond-laser-assisted eletroless plating. The 3D geometries of microelectrodes were defined by femtosecond laser modification [20], as shown in Fig. 3(a).The radii of inner and outer arc of each electrode are 100 μm and 130 μm, respectively, with an arc angle of 45°. All electrodes have the same depth of 100 μm to achieve uniform electric field along the direction perpendicular to the sample surface. After the laser exposure, the sample is immersed in an ultrasonic bath with a 5M KOH aqueous solution for 3 hours, and then the hollow mcirogrooves embedded in fused silica could be achieved, as shown in Fig. 3(b).

 

Fig. 3 (a)-(c) Schematic diagrams of fabrication process of embedded microelectrodes. (d) 2D numeric simulation of the electric field producd by the microelectrodes.

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The femtosecond-laser-assisted eletroless plating mainly consists of three steps [5], including formation of AgNO₃ films in the microgrooves; scanning of the laser beam on the bottom of microgrooves for producing Ag seeds; and electroless copper plating. After undergoing the electroless plating for 20 hours, the microgrooves were filled with copper, as shown in Fig. 3(c). In some cases, over-deposition of copper would occur, resulting in formation of undesirable copper films on the surface of substrate surrounding the embedded microelectrodes, which can be easily removed by an additional mechanical polishing process.

The direction of electric field in the plane parallel with substrate surface could be controlled by applying different voltages to the electrodes [21]. Typically, a homogeneous electric field around the center of the area encircled by the electrodes can be created by applying same electric voltages between the left and right electrodes, as illustrated in Fig. 3(d) (simulation result).

The optical waveguide was fabricated by a single scanning perpendicular to the laser propagation direction. In order to obtain a symmetrical cross section for the written waveguide, the femtosecond laser beam was shaped by passing through a silt with a width of ~500 μm placed before the objective lens [22]. The slit was oriented in a direction parallel to that of the sample translation. The shaped beam was then focused by a 20 × microscope objective (NA = 0.45) into the substrate at depth of 50 μm below the substrate surface. The scan speed and the pulse energy were set at 10 μm/s and 800 nJ, respectively.

5. Demonstration of light modulation

The experimental setup for charaterization of the fabricated liquid crystal electro-optic phase modulator is illustrated in Fig. 4(a). A 633 nm He-Ne laser was coupled by a 20 × microscope objective into the optical waveguide, and the near- field intensity distribution of the exit of optical waveguide was projected by a 20 × microscope objective onto a CCD camera.

 

Fig. 4 (a) Schematic diagram of experimental setup for the electro-optic phase modulation. P1and P2: polarizers, OBJ: objective, (b) and (c) Near-field intensity distributions at the exit of optical waveguide for “on” and “off” states, respectively.

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Here, the nematic LC 5CB preferentially aligned along the Z direction on the inner walls. The orientations of the crossed polarizers P1 and P2 were adjusted to 45° with respect to the orientation direction of the nematics (See Fig. 4(a)), so that the incident light had equal components along both the ordinary and extraordinary axes of the crystal. An alternating current source (1 kHz) was used to create a rapidly changing electric field along the X direction. When the applied voltage was 160 V(rms), corresponds to an electric filed of 0.89 Vμm⁻¹, the Freedericksz transition occurs and the LC molecules began to rotate toward the orientation of external field. The phase delay between the lights of two polarization directions passing through the LC cell was measured using transmitted light through two crossed polarizers P1 and P2. When the voltage was further increased to above 245 V(rms), which corresponds to an electric filed of 1.36 Vμm⁻¹, the transmitted intensity of the waveguide was close to zero, which indicates that most of the nematics were rotated into the field direction except those anchored at the surface. Figures 4(b) and 4(c) show the near-field intensity distributions of exiting beams from optical waveguide for “on” and “off” states, respectively, and the extinction ratio was measured to be ~21 dB. One can see some fringes formed around the waveguides in the images in Figs. 4(b) and 4(c), which could be attributed to interference of the lights reflected between the front and back surfaces of the LC cell.

6. Conclusion

We demonstrate a fully integrated LC light modulator in a fused silica substrate with a true 3D configuration, which is fabricated by femtosecond laser micromachining in a one continue process. The fabricated LC cell enables homogeneous alignment of LC molecules owing to the femtosecond-laser-induced nanoripples on the inner wall of LC cell, and the embedded microelectrodes give rise to a homogeneous electric field in the LC cell which enable dynamic orientation of the LC molecules. The ultimate size of the microcell will be determined by the size of the beam passing through the LC cell, which should be slightly larger than the size of the waveguides written in the glass by femtosecond laser. Thus, with further reductions of the sizes of both the LC microcell and the embedded electrodes, we expect that integrated LC photonic devices of smaller footprints and lower driven voltages are achievable. Besides controlling the orientation of LC molecules, the uniform electric field provided by the 3D microelectrodes can also be used for steering biomolecules or living cells in a femtosecond laser fabricated microfluidic system [23].