Since Arthur Ashkin first demonstrated the ability to optically levitate and trap particles, optical tweezers and optical trapping have been applied in the physical, chemical, biological, material, and atmospheric sciences. Optically trapped microparticles in air are more likely to be affected by external disturbances, such as vibration or airflow, than those in liquid, which makes them difficult to trap in air. Recently, technology for optical trapping in air was developed. The gradient force generated by a high-focus laser and the photophoretic force resulting from thermal processes play dominant roles in the optical trapping of particles in air. When the particles are trapped, their physical and chemical properties can be studied using spectroscopic techniques. In this paper, the principles and experimental devices of the optical trapping of airborne particles are introduced, and the applications, progress, and challenges of optical trapping and laser spectroscopy are reviewed.

When a photon interacts with a particle, the partial momentum of the photon is transferred to the particle, which forms the scattering and gradient forces, where the gradient force is used to trap the particle. For r?λ (particle radius, r; laser wavelength, λ), the ray optics model can be used to calculate the two forces (Fig.1). For r?λ, the Rayleigh scattering model is often used. In addition, the absorbing particles will also be trapped by the photophoretic force, which results from thermal processes.

A single-Gaussian-beam trap using a tightly focused single beam can trap a particle in three dimensions. It employs a high numerical aperture (NA) objective, which provides a strong gradient force at low laser power. However, the single-Gaussian-beam trap has a very short working distance, which limits its compatibility with other measuring techniques. The two counter-propagating beams can balance the scattering force and retain the gradient force so that the dual-Gaussian-beam trap can obtain a longer working distance (Fig.2). A single-beam photophoretic trap uses only a single laser beam that contains low intensity regions to trap absorbing particles. For instance, a hollow beam, usually formed using axicons, has the advantages of a simple configuration and long working distance (Fig.3). A dual-hollow-beam trap has stronger trapping robustness than a single-hollow-beam trap, and the number and size of trapping particles can be controlled by adjusting the distance between the two focal points (Fig.4). However, the two foci must be aligned with each other at a precision of sub-micrometers. Fortunately, confocal-beam traps integrate the simplicity of single-beam traps and the robustness of dual-beam traps (Fig.5). Among the above optical traps, none were able to trap both transparent and absorbing particles until the universal optical trap was developed (Fig.6). In our experiments, particles were trapped with different arrangements using different shape laser beams (Fig.7), and we realized a variety of particles trapped by dual-hollow-beam and dual-Gaussian-beam traps (Fig.8).

The combination of optical trapping and spectroscopic measurements can be used to investigate the physical and chemical properties of airborne particles. Optical trapping Raman spectroscopy (OT-RS) is mainly used to study droplets; therein, the size and refractive index of aerosol droplets can be obtained from the stimulated Raman spectroscopy (Fig.9). Because the spontaneous Raman scattering intensity is weak, a higher power laser is required (Fig.10). By optimizing the slit setting, the problem of signal superposition from different positions of the droplets can be eliminated, and a high spatial resolution can be obtained (Fig.11).

Compared to OT-RS of droplets, fewer studies have been reported on OT-RS of solid particles. Most solid airborne particles have arbitrary size, composition, and morphology, which introduce challenges in the repeatability of experiments. In 2012, OT-RS of carbon nanotubes was investigated for the first time using a dual-hollow-beam trap (Fig.12). Researchers have improved the trapping robustness of solid airborne particles using a variety of means and have realized OT-RS detection and rapid identification of various oxides and bioaerosols. Combined with an imaging system, it can also monitor changes in particle size and morphology .

Stable trapping enables us to measure the temporal evolution processes of airborne particles in situ for a sufficiently long time. For example, the hydration and dehydration of trapped particles, the reactions of particles with the ambient atmosphere, and photochemical reactions can be investigated with OT-RS. In addition to Raman spectroscopy, optical trapping can also be combined with other laser spectroscopic techniques, such as cavity ringdown spectroscopy (Fig.14) and laser-induced breakdown spectroscopy (Fig.15).

At present, research on optically trapped airborne particles is still in its infancy. Although a variety of methods, such as OT-RS, have been developed to retrieve fundamental information from airborne particles in their native states, there are still many problems in practical applications, such as weak spectral signals, complex trapping forces, and inappropriate particle introduction methods.

In recent years, the optical trapping and spectroscopic measurement of airborne particles have been improved. In this review, optical trapping forces are briefly introduced. Diverse optical configurations used in the optical trapping of airborne particles are discussed, and the configuration simplicity and trapping robustness are evaluated. Optical trapping combined with spectroscopic techniques can characterize the physicochemical properties of a single airborne particle in its native state, and the study of heterogeneous chemical reactions under controlled environments can be realized with high temporal and spatial resolution ability. However, owing to the limitation of the trapping force, most particles reported to date are approximately 1?50 μm. It is hoped that with the development of optical trapping, there will be more research involving single nanoparticles, and the on-site monitoring of environmental particles can be realized in combination with real-time sampling apparatus.