It is finally time to build our first optical tweezers. In this Chapter, we shall do just that: we will build the optical tweezers shown in Fig. 8.1 proceeding step-by-step and explaining all the details and tricks. In a nutshell, a simple optical tweezers is a laser beam coupled to an optical microscope, which can be either commercial or homemade. For completeness, we will first proceed to realise a homemade microscope. Then, we will couple it to a laser beam appropriately prepared. Along the way we will explain all the necessary tricks and tips and we will also present some basic information on how to achieve steerable and multiple optical tweezers. By the end of this Chapter, we will have built the optical tweezers shown in Fig. 8.1, apart from the position detection part that will be explained in Chapter 9, and we will be able to trap and manipulate microscopic particles. In the following Chapters, we will explain how to acquire and analyse optical trapping data [Chapter 9], how to measure nanoscopic forces and torques [Chapter 10], how to realise more complex optical trapping configurations using holographic optical tweezers [Chapter 11] and alternative trapping geometries [Chapter 12].
* This chapter was written together with Giuseppe Pesce.
8.1 The right location
8.2 Inverted microscope construction
8.2.1 Objectives
8.2.2 Illumination schemes
8.3 Sample preparation
8.4 Optical beam alignment
8.4.1 Lasers
8.4.2 Lenses
8.4.3 Mirrors
8.4.4 Filters
8.4.5 Polarisation control
8.5 Optical trapping and manipulation
8.5.1 Steerable optical tweezers
8.6 Alternative set-ups
Problems
References

Chapter 8 — Building an Optical Tweezers*
Figure 8.1 — Homemade optical tweezers
Figure 8.2 — Homemade inverted microscope
Figure 8.4 — Köhler illumination
Figure 8.5 — Contrast enhancement techniques
Figure 8.6 — Sample preparation
Figure 8.7 — Wavelength dependence of water absorption and photodamage to biological samples
Figure 8.8 — Alignment of the laser beam to generate an optical tweezers
Figure 8.9 — Beam alignment technique
Figure 8.10 — Back-scattered light patterns from a focused beam
Figure 8.12 — Action of lenses illustrated with ray diagrams