light forces (original) (raw)
Definition: forces associated with absorption, reflection or scattering of light
Categories: 
quantum photonics, 
methods
Related: radiation pressureoptical tweezerslaser coolingLasers Attract Dust to Cavity Mirrors
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Contents
What are Light Forces?
Photons carry not only energy but also momentum. When a particle absorbs light or changes its direction (for example, through refraction or scattering), momentum is transferred between the light and the particle. This exchange of momentum exerts a force on the particle. Such light forces are responsible, for example, for the tails of comets, which always point away from the Sun because sunlight pushes small dust particles outward.
The concept of light exerting force was first proposed by the astronomer Johannes Kepler in the early 17th century. James Clerk Maxwell, the founder of electromagnetic theory, later formalized the concept through his electromagnetic theory. The effect was experimentally confirmed by the Russian physicist Pyotr Lebedev in 1900 [1].
Radiation Pressure and the Scattering Force
In its simplest form, a light force is described as radiation pressure, which acts in the direction of light propagation. When a photon is absorbed or scattered by a particle, it imparts its momentum ($â„Ź k$). Repeated absorption and re-emission events produce a net force along the beam direction, known as the scattering force (or radiation-pressure force).
This is a non-conservative (dissipative) force: It depends on the rate of photon absorption and emission. Because it depends on the particle's velocity, it plays a key role in laser cooling, where it serves to slow and dampen atomic motion.
Gradient Force
Not all light forces push in the direction of light propagation. When light intensity varies spatially, as in a tightly focused laser beam (for example, a Gaussian beam), the electric field intensity gradient induces a dipole interaction with polarizable particles. This produces the gradient force, which pulls or pushes the particle toward regions of higher or lower optical intensity, depending on the sign of the detuning between the laser frequency and an atomic resonance:
- For red detuning (laser frequency below resonance), the particle is attracted toward higher intensity.
- For blue detuning (laser frequency above resonance), the particle is repelled from higher intensity regions.
The gradient force is conservative (assuming constant beam intensity), as it can be derived from a potential energy function associated with the dipole interaction. It dominates in optical traps such as optical tweezers, where particles are confined near the beam focus.
Combined Action: Optical Trapping
In most practical situations, both forces act together:
- The gradient force provides spatial confinement (restoring force toward the focus).
- The scattering force provides a steady push along the beam direction but can also introduce damping through photon scattering.
Careful balancing of these effects enables stable three-dimensional trapping of microscopic particles, atoms, or molecules.
Applications and Occurrences
Light forces are relevant in many different situations:
- Astronomy: They explain the formation of comet tails, as suggested by Kepler in 1619.
- Trapping and manipulation: Used in optical tweezers, levitating traps, and optical clocks to confine and control particles.
- Photon thrusters: In proposed spacecraft control systems, intracavity light forces could adjust the relative positions of satellites without propellant, using only electrical power from photovoltaic cells.
- Isotope separation: Light forces can enable isotope-selective manipulation when many photon-scattering events enhance frequency selectivity [6].
- Precision measurement: In high-sensitivity interferometers for gravitational-wave detection, unwanted light forces (radiation pressure noise) can disturb the suspended test masses.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What are light forces?
Light forces arise from the momentum carried by photons. When light is absorbed, scattered, or has its direction changed by a particle, momentum is transferred, exerting a physical force on the particle.
What is the difference between the scattering force and the gradient force?
The scattering force, or radiation pressure, pushes a particle in the direction of light propagation due to photon absorption and re-emission. The gradient force pulls a particle towards or pushes it away from regions of high light intensity, depending on the light's frequency relative to a resonance.
How do optical tweezers trap particles?
Optical tweezers use a tightly focused laser beam to create a strong intensity gradient. This generates a gradient force that confines a particle near the beam's focus, while the scattering force provides a weaker push that must be overcome for stable trapping.
Which force is conservative and which is non-conservative?
The gradient force is conservative and can be described by a potential energy landscape. The scattering force is non-conservative (dissipative) because it involves the irreversible processes of photon absorption and spontaneous emission.
Where are light forces relevant?
Light forces are used in optical tweezers for manipulating microscopic particles, in laser cooling of atoms for optical clocks, and are considered for propellant-less photon thrusters. They also explain the direction of comet tails and can cause noise in gravitational-wave detectors.
Bibliography
| [1] | P. Lebedev, “Untersuchungen über die Druckkräfte des Lichtes”, Annalen der Physik 311 (11), 433 (1901) |
|---|---|
| [2] | A. Ashkin, “Atomic-beam deflection by resonance-radiation pressure”, Phys. Rev. Lett. 25 (19), 1321 (1970); doi:10.1103/PhysRevLett.25.1321 |
| [3] | T. W. Hänsch and A. L. Schawlow, “Cooling of gases with laser radiation”, Opt. Commun. 13, 68 (1975); doi:10.1016/0030-4018(75)90159-5 |
| [4] | D. J. Wineland and W. M. Itano, “Laser cooling of atoms”, Phys. Rev. A 20 (4), 1521 (1979); doi:10.1103/PhysRevA.20.1521 |
| [5] | J. Ye et al., “Trapping of single atoms in cavity QED”, Phys. Rev. Lett. 83 (24), 4987 (1999); doi:10.1103/PhysRevLett.83.4987 |
| [6] | C. Y. Chen et al., “Ultrasensitive isotope trace analyses with a magneto-optical trap”, Science 286 (5442), 1139 (1999); doi:10.1126/science.286.5442.1139 |
| [7] | C. Savage, “Introduction to light forces, atom cooling, and atom trapping” (1995), https://arxiv.org/abs/atom-ph/9510004 |
| [8] | D. Van Thourhout and J. Roels, “Optomechanical device actuation through the optical gradient force” (review article), Nature Photon. 4, 211 (2010); doi:10.1038/nphoton.2010.72 |
| [9] | H. Li et al., “Optical pulling forces and their applications”, Advances in Optics and Photonics 12 (2), 288 (2020); doi:10.1364/AOP.378390 |
| [10] | A. A. R. Neves and C. L. Lenz, “Analytical calculation of optical forces on spherical particles in optical tweezers: tutorial”, J. Opt. Soc. Am. B 36 (6), 1525 (2019); doi:10.1364/JOSAB.36.001525 |
| [11] | H. Li et al., “Optical pulling forces and their applications”, Advances in Optics and Photonics 12 (2), 288 (2020); doi:10.1364/AOP.378390 |
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