2.2 The Minitweezers experimental setup

Most of the optical tweezers setups are based on modified commercial microscopes to which a laser beam is introduced in the optical path to produce an optical trap [79]. Minitweezers is the name given by its inventor, Steve Smith, to the experimental setup used in this work [16,17,18]. This setup is a miniaturized evolution of previous instruments and achieves better stability and resolution in the measurements. Its compactness allows to hang it from the ceiling and isolate the instrument to prevent that vibrations and air flows affect the experiments. Minitweezers measure force by conservation of light momentum according to the ideas developed in Section 2.1.4. Therefore, the instrument was designed not only to produce an optical trap, but also to collect all the light emerging from the interaction between the laser light and the trapped particle. It requires the use of two microscope objectives: one to focus the laser beam and the other one to work as a condenser that collects the outcoming light. Thus, Minitweezers look like two microscopes facing each other with a common focal point. The most relevant innovation, though, is the steering of the laser beam, which is performed using a device called wiggler. The wiggler redirects the laser beam and repositions the optical trap by using piezoelectric crystals that provide high reliability and stability. The optical system is enclosed in the so-called head (see Figure 2.9). The control of the instrument and the measurement of signal is performed with customized electronic boards (the electronic controller) that use PIC microcontrollers. The experimentalist also interacts with the instrument with a Mac computer (also known as the host computer), which communicates with the electronic boards through a USB port. The following sections describe the parts of the experimental setup.

Figure 2.9: General scheme. The experiments are performed in the head, which contains all the optics. The lasers are powered by the laser controller. The electronic controller communicates the head and the host computer. The user interacts with the experiment by means of the host computer. The TV screen allows to visualize the experiment.

JM Huguet 2014-02-12