Plasmon photothermal and macroscopic deformation realize fiber taper pulling

Micro-nano optical fiber is a one-dimensional optical waveguide with a diameter close to or less than the guided wave wavelength, with the characteristics of small size, large surface volume ratio, strong evanescent wave transmission characteristics, etc., so that they show high sensitivity, fast response speed and low detection limit to the external environment, and are often used as important tools for studying near-field optics, quantum optics, and high-precision sensing.

At the preparation level, micro-nano fibers can generally be prepared by standard optical fibers (diameter > 100 μm) by flame heating and displacement table stretching, and finally presented as a double-cone structure composed of standard fiber, waist region and taper transition (Figure 1), and the process of preparing such fibers is also called fiber taper pulling.

Figure 1: Preparation of biconical microfiber by flame stretching

In order to draw a standard fiber thin to a micro-nano fiber, the fiber in the deformed area needs to be hard to soft, otherwise direct stretching will cause the fiber to be brittle and break. Therefore, in the process of optical fiber taper pulling, the temperature of the heating zone needs to reach above the glass transition temperature of silica, and the optical fiber changes from glass to viscous flow and then applies tensile force through the displacement table to produce deformation. The key is the control of “heat source” and “heat and pull”.

Corresponding examples include the “flame brush” technique proposed by Birks et al. in 1992, which is to achieve a controlled morphology of fiber cone by controlling the movement of the flame burner relative to the slowly stretching fiber. Subsequent innovations include the use of electric heaters, high-power CO2 laser heating methods, and more to improve airflow disturbances, burner inertia, and temperature-controlled heating.

In addition to the above solutions, to provide the “heat source” and “pull” elements of optical fiber taper, you can also find the answer outside the traditional thinking framework.

Recently, Professor Qiu Min’s research group of Westlake University reported the technology of using the plasmon effect of metal micron sheets and macroscopic precast deformation in the whole optical fiber to provide “heat source” and “tensile force” respectively, and then realize the self-pulling cone of optical fiber. Instead of using large heat sources such as flames and heaters, and displacement stages that provide tensile force from the outside, the compact and vacuum-compatible experimental system can be transferred to a scanning electron microscope to enable in situ observation and in-situ control of the fiber cone pulling process at nanometer resolution, and the system also demonstrates an experimental method for in situ study of photothermal effects based on micro-nano fibers. At the theoretical level, the researchers used the simulation methods of photothermal coupling and structural mechanics to elaborate the generation mechanism of “heat source” and “tensile force” in this optical fiber cone pulling technology.

The results were published in Light: Advanced Manufacturing as “Fibre Tapering Using Plasmonic Microheaters and Deformation-Induced Pull.”

As shown in Figure 2A, the experimental device body of the in-situ cone pulling of the fiber in the electron microscope includes (1) a micron fiber pre-pulled by the traditional flame stretching method; and (2) gold micron sheets transferred to the waist region of the pre-tensioned cone. The standard fiber region of this pre-tensioned cone fiber is further connected to the laser outside the electron microscope by a fiber vacuum flange. In addition, the drawing process can also be carried out in an air environment.

Figure 2: In situ fiber cone pulling experiment in a scanning electron microscope chamber. (A) Schematic diagram of the experimental device (B) Before and after comparison of micron fiber cone pulling (C) Adjust the dynamic cone pulling process controlled by the light source switch.

When a laser is introduced, the evictile field of the microfiber interacts with the gold micron sheet, and surface plasmon modes (SPPs) can be excited in a specific wavelength range, thus exhibiting strong light absorption and photothermal effects. The heat generated in the gold micron wafer is mainly transferred to the microfiber by heat conduction, thus providing the “heat source” necessary to heat the fiber above the glass transition temperature (Tg). After the experiment is completed, the ablative micron sheet can be removed with metal stripping solution or ultrasonically. For another element “tensile force”, during the experiment, the pre-tensile cone fiber is bent into a door-shaped structure and fixed on a rigid substrate, and in the area where the metal sheet is located, the axial stress state of the optical fiber is tensile stress, so that it spontaneously stretches to both sides after thermal softening and forms a double-conical structure (Figure 2B); When the heat source is withdrawn, the micron fiber temperature drops below the glass transition temperature (Tg) and the taper pulling process stops. Due to the compactness and vacuum-compatible nature of the system, this micro-nanoscale cone pulling process can be monitored in real time using SEM (Figure 2C).

Photothermal coupling simulations yield the temperature distribution of a microfiber heated by a plasmon micron sheet, as shown in Figure 3A. Among them, the length of the fiber above the glass transition temperature is in the order of microns, and the size is much smaller than the heating range of flame or conventional heaters. In addition, the researchers found that when using CW light lasers and high-repetition rate (100 kHz) pulsed lasers, the temperature threshold (Tg) can be reached at an average input power of the order of milliwatts; However, when using a low repetition rate pulsed laser, thermal damage to the gold sheet or softening of the fiber cannot be observed due to the longer pulse interval time, and the thermal accumulation between pulses as shown in Figure 3B cannot occur.

Figure 3: Photothermal conversion and heat conduction process of micron fiber-gold sheet. (A) Continuous laser heating, temperature distribution after the system reaches steady state. The effective hot zone is marked as an area where the temperature exceeds the glass transition temperature. (B) Interpulse thermal accumulation formed by heating a supercontinuum laser with a repetition rate of 100 kHz.

In terms of tensile force generation, as shown in Figure 4A, the micron fiber with a zigzag bending shape needs to achieve a bending moment balance between the fixed curved end (section 1) and the free-suspended straight end (section 2), and at the straight end, that is, the position where the micron sheet is placed needs to be in a tensile stress state, which is different from the convex side of the curved end section is tensile and the concave side is compressed. When the straight end is heated to softening temperature due to the photothermal action of the micron sheet, the micron fiber will undergo self-stretching at the straight end and self-recovery at the curved end (the bending radius becomes larger) to release its internal stress (Figure 4B), thereby realizing the self-pulling cone of the fiber without the need for an external stretching system.

Figure 4: Mechanism of tensile force generation in a glyph bending fiber. (A) Meet the stress state resulting from the equilibrium of bending moments in sections 1 and 2. (B) The self-pulling cone process of the optical fiber after being softened by heat at the straight end.

Summary and outlook

This new optical fiber cone pulling technology creatively uses plasmon photothermal effects and macroscopic deformation of materials to provide high temperature and tensile conditions, realizes in-situ optical fiber cone pulling in scanning electron microscopy, and establishes a theoretical model of photo-thermal-force coupling, which provides a model for the in-situ study of light-matter interaction at the micro-nano scale.

Considering the cost and difficulty of practical operation, its innovation and enlightenment in experimental design and theoretical analysis are greater than the practicality of device processing and preparation. In the future, improving the controllability of tensile stress and the adjustability of heat source is the key problem to break through the bottleneck of practicality of this technology.

Related paper information:

(Source: Advanced Manufacturing WeChat public account)
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