optical physics Atomic, molecular, and optical physics (AMO) is the study of matter-matter and light-matter interactions; at the scale of one or a few atoms and energy scales around several electron volts. The three areas are closely interrelated. AMO theor ...

, laser detuning is the tuning of a

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...

to a

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from '' angular frequency''. Frequency is measured in hertz (Hz) which is ...

that is slightly off from a

quantum system Quantum mechanics is a fundamental Scientific theory, theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including qua ...

resonant frequency Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillati ...

. When used as a noun, the laser detuning is the difference between the resonance

of the system and the laser's optical frequency (or

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...

). Lasers tuned to a frequency below the resonant frequency are called ''red-detuned'', and lasers tuned above resonance are called ''blue-detuned''.

Illustration

Consider a system with a resonance frequency

\omega_0

in the optical frequency range of the

electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from ...

, i.e. with frequency of a few THz to a few PHz, or equivalently with a wavelength in the range of 10 nm to 100 μm. If this system is excited by a laser with a frequency

\omega_L

close to this value, the laser detuning is then defined as:

\Delta \omega\  \overset \ \omega_L - \omega_0

The most common examples of such resonant systems in the optical frequency range are optical cavities (free-space, fiber or microcavities),

atoms Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas ...

, and dielectrics or

semiconductors A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...

. The laser detuning is important for a resonant system such as a cavity because it determines the phase (modulo 2π) acquired by the laser field per roundtrip. This is important for linear optical processes such as interference and scattering, and extremely important for

nonlinear optical Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in ''nonlinear media'', that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typica ...

processes because it affects the phase-matching condition.

Applications

Laser cooling of atoms

Lasers can be detuned in the lab frame so that they are

Doppler shift The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who ...

ed to the resonant frequency in a moving system, which allows lasers to affect only atoms moving at a specific speed or in a specific direction and makes laser detuning a central tool of

laser cooling Laser cooling includes a number of techniques in which atoms, molecules, and small mechanical systems are cooled, often approaching temperatures near absolute zero. Laser cooling techniques rely on the fact that when an object (usually an atom) ...

and

magneto-optical trap A magneto-optical trap (MOT) is an apparatus which uses laser cooling and a spatially-varying magnetic field to create a trap which can produce samples of cold, trapped, neutral atoms. Temperatures achieved in a MOT can be as low as several micro ...

Optomechanics

Similar to the laser cooling of atoms, the sign of the detuning plays an important part in

Optomechanical Optomechanics is the manufacture and maintenance of optical parts and devices. This includes the design and manufacture of hardware used to hold and align elements in optical systems, such as: * Optical tables, breadboards, and rails * Mirro ...

applications. In the red detuned regime, the optomechanical system undergoes cooling and coherent energy transfer between the light and the mechanical mode (a " beam splitter"). In the blue-detuned regime, it undergoes heating, mechanical amplification and possibly squeezing and entanglement. The on-resonance case when the laser detuning is zero, can be used for very sensitive detection of mechanical motion, such as used in LIGO.

References

{{optics-stub Detuning