Editing Radiation pressure (section)

== Laser applications of radiation pressure ==

=== Optical tweezers ===
{{Main|Optical tweezers}}
[[Laser]]s can be used as a source of monochromatic light with wavelength <math>\lambda</math>. With a set of lenses, one can focus the laser beam to a point that is <math>\lambda</math> in diameter (or <math>r = \lambda/2</math>).

The radiation pressure of a ''P'' = 30&nbsp;mW laser with ''λ'' = 1064&nbsp;nm can therefore be computed as follows.

Area:
<math display="block">A = \pi\left(\frac{\lambda}{2}\right)^2 \approx 10^{-12} \text{ m}^2,</math>

force:
<math display="block">F = \frac{P}{c} = \frac{30 \text{ mW}} {299792458 \text{ m/s}} \approx 10^{-10} \text{ N},</math>

pressure:
<math display="block">p = \frac{F}{A} \approx \frac{10^{-10} \text{ N}} {10^{-12} \text{ m}^2} = 100 \text{ Pa}.</math>

This is used to trap or levitate particles in [[optical tweezers]].

=== Light–matter interactions ===
[[File:Cavity Optomechanics.png|thumb|242x242px|In this optomechanical cavity, light is trapped and enhanced between two mirrors. One of the mirrors is attached to a spring and can move. The radiation pressure force of the light circulating in the cavity can damp or amplify the oscillation of the mirror on the spring.]]
{{Main|Cavity optomechanics|Laser cooling}}
The reflection of a laser pulse from the surface of an elastic solid can give rise to various types of elastic waves that propagate inside the solid or liquid. In other words, the light can excite and/or amplify motion of, and in, materials. This is the subject of study in the field of optomechanics. The weakest waves are generally those that are generated by the radiation pressure acting during the reflection of the light. Such light-pressure-induced elastic waves have for example observed inside an ultrahigh-reflectivity [[dielectric mirror]].<ref>{{cite journal |last1=Požar |first1=T. |last2=Možina |first2=J. |title=Measurement of Elastic Waves Induced by the Reflection of Light. |journal=Physical Review Letters |volume=111 |issue=18 |page=185501 |doi=10.1103/Physrevlett.111.185501|pmid=24237537 |year=2013 |bibcode=2013PhRvL.111r5501P }}</ref> These waves are the most basic fingerprint of a light-solid matter interaction on the macroscopic scale.<ref>{{cite journal |last1=Požar |first1=T. |last2=Laloš |first2=J. |last3=Babnik |first3=A. |last4=Petkovšek |first4=R. |last5=Bethune-Waddell |first5=M. |last6=Chau |first6=K. J. |last7=Lukasievicz |first7=G. V. B. |last8=Astrath |first8=N. G. C. |title=Isolated detection of elastic waves driven by the momentum of light |journal=Nature Communications |volume=9 |issue=1 |page=3340 |doi=10.1038/s41467-018-05706-3|pmid=30131489 |pmc=6105914 |year=2018 |bibcode=2018NatCo...9.3340P }}</ref> In the field of ''cavity'' optomechanics, light is trapped and resonantly enhanced in [[Optical cavity|optical cavities]], for example between mirrors. This serves the purpose of gravely enhancing the [[Intensity (physics)|power]] of the light, and the radiation pressure it can exert on objects and materials. Optical control (that is, manipulation of the motion) of a plethora of objects has been realized: from kilometers long beams (such as in the [[LIGO|LIGO interferometer]])<ref>{{Cite news|last=Johnston|first=Hamish|date=10 Dec 2019|title=Quantum squeezing boosts performance of LIGO and Virgo gravitational-wave detectors|work=PhysicsWorld|url=https://physicsworld.com/a/quantum-squeezing-boosts-performance-of-ligo-and-virgo-gravitational-wave-detectors/}}</ref> to clouds of atoms,<ref>{{Cite journal |last1=Schreppler|first1=Sydney|last2=Spethmann|first2=Nicolas|last3=Brahms|first3=Nathan|last4=Botter|first4=Thierry|last5=Barrios|first5=Maryrose|last6=Stamper-Kurn|first6=Dan M.|date=2014-06-27|title=Optically measuring force near the standard quantum limit|url=https://www.science.org/doi/10.1126/science.1249850 |journal=Science |language=en |volume=344 |issue=6191 |pages=1486–1489|doi=10.1126/science.1249850|issn=0036-8075|pmid=24970079|arxiv=1312.4896|bibcode=2014Sci...344.1486S|s2cid=206554506}}</ref> and from [[Microfabrication|micro-engineered]] [[trampoline]]s<ref>{{Cite journal |last1=Kleckner | first1=Dustin | last2=Marshall|first2=William|last3=de Dood|first3=Michiel J. A.|last4=Dinyari|first4=Khodadad Nima | last5=Pors|first5=Bart-Jan|last6=Irvine|first6=William T. M.|last7=Bouwmeester|first7=Dirk|date=2006-05-02|title=High Finesse Opto-Mechanical Cavity with a Movable Thirty-Micron-Size Mirror|journal=Physical Review Letters |volume=96 |issue=17 |pages=173901|doi=10.1103/PhysRevLett.96.173901|pmid=16712296|bibcode=2006PhRvL..96q3901K|hdl=1887/65506 |s2cid=1801710 |hdl-access=free}}</ref> to [[superfluids]].<ref>{{Cite journal|last1=Harris|first1=G. I.| last2=McAuslan|first2=D. L.|last3=Sheridan|first3=E.|last4=Sachkou|first4=Y.|last5=Baker|first5=C.|last6=Bowen|first6=W. P. | date=2016|title=Laser cooling and control of excitations in superfluid helium|url=https://www.nature.com/articles/nphys3714 | journal=Nature Physics| language=en |volume=12 | issue=8| pages=788–793| doi=10.1038/nphys3714| issn=1745-2481| arxiv=1506.04542|bibcode=2016NatPh..12..788H|s2cid=118135792}}</ref><ref>{{Cite journal| last1=Kashkanova|first1=A. D.| last2=Shkarin|first2=A. B.| last3=Brown|first3=C. D.| last4=Flowers-Jacobs|first4=N. E.| last5=Childress|first5=L.| last6=Hoch|first6=S. W.| last7=Hohmann|first7=L.| last8=Ott|first8=K.| last9=Reichel|first9=J.| last10=Harris|first10=J. G. E.|date=2017|title=Superfluid Brillouin optomechanics|url=https://www.nature.com/articles/nphys3900 | journal=Nature Physics | language=en| volume=13| issue=1| pages=74–79| doi=10.1038/nphys3900| issn=1745-2481| arxiv=1602.05640|bibcode=2017NatPh..13...74K|s2cid=10880961}}</ref>
[[File:Cavity-optomechanical-spring-sensing-of-single-molecules-ncomms12311-s2.ogv|thumb|upright=1.5|In this optomechanical system, the radiation pressure force is leveraged to detect a single [[Protein|protein molecule]]. Laser light interacts with a [[Microsphere|glass sphere]]: the radiation pressure force causes it to vibrate. The presence of a single molecule on the sphere disturbs that (thermal) vibration, and the disturbance in the sphere's motion can be detected in the [[Harmonic oscillator#Sinusoidal driving force|oscillator spectrum]] on the left.<ref>{{Cite journal | last1=Yu|first1=Wenyan | last2=Jiang|first2=Wei C. | last3=Lin|first3=Qiang | last4=Lu|first4=Tao | date=2016-07-27 | title=Cavity optomechanical spring sensing of single molecules|journal=Nature Communications | language=en| volume=7| issue=1| page=12311| doi=10.1038/ncomms12311| pmid=27460277| pmc=4974467| arxiv=1504.03727| bibcode=2016NatCo...712311Y| issn=2041-1723| doi-access=free}}</ref> ]]

Opposite to exciting or amplifying motion, light can also damp the motion of objects. [[Laser cooling]] is a method of cooling materials very close to [[absolute zero]] by converting some of material's motional energy into light. [[Kinetic energy]] and [[thermal energy]] of the material are synonyms here, because they represent the energy associated with [[Brownian motion]] of the material. Atoms traveling towards a laser light source perceive a [[doppler effect]] tuned to the absorption frequency of the target element. The radiation pressure on the atom slows movement in a particular direction until the Doppler effect moves out of the frequency range of the element, causing an overall cooling effect.<ref name=":0">{{Cite journal|last1=Aspelmeyer|first1=Markus|last2=Kippenberg|first2=Tobias J.|last3=Marquardt|first3=Florian| date=2014-12-30| title=Cavity optomechanics|journal=Reviews of Modern Physics| volume=86| issue=4| pages=1391–1452| doi=10.1103/RevModPhys.86.1391| arxiv=1303.0733|bibcode=2014RvMP...86.1391A|s2cid=119252645}}</ref>

An other active research area of laser–matter interaction is the radiation pressure acceleration of ions or protons from thin–foil targets.<ref>{{Cite journal|last1=Meinhold|first1=Tim Arniko|last2=Kumar|first2=Naveen|date=December 2021| title=Radiation pressure acceleration of protons from structured thin-foil targets|journal=Journal of Plasma Physics| language=en|volume=87|issue=6|pages=905870607|doi=10.1017/S0022377821001070|bibcode=2021JPlPh..87f9007M|s2cid=244636880| issn=0022-3778|doi-access=free|arxiv=2111.14087}}</ref> High ion energy beams can be generated for medical applications (for example in ion beam therapy<ref>{{Cite journal|last1=Malka|first1=Victor|last2=Fritzler|first2=Sven|last3=Lefebvre|first3=Erik| last4=d'Humières|first4=Emmanuel|last5=Ferrand|first5=Régis|last6=Grillon|first6=Georges|last7=Albaret|first7=Claude| last8=Meyroneinc|first8=Samuel|last9=Chambaret|first9=Jean-Paul|last10=Antonetti|first10=Andre| last11=Hulin|first11=Danièle| date=2004-05-27|title=Practicability of protontherapy using compact laser systems| url=http://doi.wiley.com/10.1118/1.1747751|journal=Medical Physics| language=en| volume=31| issue=6| pages=1587–1592 | doi=10.1118/1.1747751|pmid=15259663|bibcode=2004MedPh..31.1587M}}</ref>) by the radiation pressure of short laser pulses on ultra-thin foils.