This article will take an in-depth look at laser coolers and laser cooling.
The article will bring more detail on topics such as:
This chapter will cover the concept of laser cooling, various cooling methods, and the operational principles of laser coolers.
Laser cooling encompasses a range of techniques used to reduce the temperature of atomic and molecular samples to near absolute zero. These methods exploit the principle that when an atom or molecule re-emits a photon— a particle of light—its momentum changes, resulting in a reduction of its overall kinetic energy.
In a collection of particles, the thermodynamic temperature is related to the variance in their velocities. This implies that a more uniform velocity distribution among the particles corresponds to a lower temperature.
Laser cooling techniques integrate atomic spectroscopy with the mechanical effects of light to narrow the velocity distribution of a group of particles, thereby achieving cooling.
Laser cooling employs various methods, with Doppler cooling being one of the most widely used. In fact, Doppler cooling is often considered synonymous with laser cooling due to its prevalence in practical applications.
Additional methods of laser cooling include Sisyphus cooling, resolved sideband cooling, Raman sideband cooling, velocity selective coherent population trapping (VSCPT), gray molasses, cavity-mediated cooling, polarization gradient cooling, anti-Stokes cooling in solids, electromagnetically induced transparency (EIT) cooling, and the use of the Zeeman slower.
This mechanism is used to trap and slow the motion of atoms to cool a substance. Essentially, a stationary atom does not experience any shift in the laser light and thus does not absorb the photon.
For an atom moving away from the laser, the light appears red-shifted, causing it not to absorb the photon. Conversely, an atom moving towards the laser perceives the light as blue-shifted and absorbs the photon, which slows it down. Absorption excites the atom, elevating an electron to a higher quantum state. When the atom re-emits a photon, the direction of this photon is random, resulting in no net change in momentum over many photons.
The core principle behind this process is that most photons interacting with a particular atom remain almost completely unaffected by it. The atom is nearly transparent to most photon frequencies or colors. Only a small number of photons, which resonate with the atom within very narrow frequency bands, will interact with it. This interaction occurs at specific colors rather than a mixture of colors like white light.
When one of these resonant photons approaches the atom, it is briefly absorbed. The atom then re-emits a photon in a random and unpredictable direction. Contrary to the common belief that lasers increase the thermal energy of matter, this process does not significantly impact the thermal energy of individual atoms.
If the atom is nearly motionless, or "cold," and the laser's frequency is carefully controlled, most frequencies will not affect the atom, making it effectively invisible to them. Only specific electromagnetic frequencies will interact with the atom. When the laser frequency matches these frequencies, the atom absorbs a photon, transitioning to an excited electronic state and gaining the photon's momentum. Consequently, the atom drifts in the direction of the photon’s movement.
After a short period, the atom emits a photon in a random direction as it returns to a lower electronic state. Typically, the atom loses the momentum it gained, potentially becoming motionless again. If the emitted photon travels in the opposite direction, the atom must provide additional momentum in that direction, effectively gaining more momentum towards the original photon’s direction.
To conserve momentum, this results in the atom accelerating to double its initial velocity. Often, the emitted photon moves in another direction, giving the atom a sideways thrust. The laser's frequency can be adjusted by positioning the laser to use monochromatic light slightly below one of the atom’s resonant frequencies. At this frequency, the laser does not directly affect the atom’s state.
If the laser is directed towards the atoms, the Doppler Effect increases its frequency. At a specific velocity, this frequency will align with the resonant frequency of the atoms, causing them to begin absorbing photons. Initially, a laser frequency well below the resonant frequency will pass through most atoms without interaction.
Atoms moving rapidly towards the laser will absorb the photons, slowing down until they become transparent again. Conversely, atoms moving away from the laser remain transparent to its photons. On a velocity graph, atoms moving rapidly to the right will appear as stationary dots far right, and those moving to the left will appear as stationary dots far left.
A narrow band on the left side of the graph represents the speeds at which atoms start absorbing photons from the left laser. Atoms within this band interact with the laser. When a photon from the left laser collides with an atom in this band, the atom slows down by an amount corresponding to the photon’s momentum. If the emitted photon moves in the opposite direction, the atom’s velocity adjusts accordingly. As the laser frequency increases, the boundary of this interaction band contracts, causing all velocity points on the graph to shift towards zero. This narrowing of the velocity distribution defines the 'cold' state.
Sisyphus cooling is a technique for laser cooling that enables reaching temperatures below the Doppler cooling limit. This method involves shining two counter-propagating laser beams with orthogonal polarizations onto a sample of atoms. As atoms move through the potential landscape created by the standing wave of these lasers, they lose kinetic energy. When they reach the points of maximum potential, optical pumping transitions them back to a lower energy state, thereby reducing the total energy of the atom.
Sisyphus cooling operates on the principle that two counter-propagating lasers with orthogonal polarizations create a standing wave with a polarization gradient between left-handed and right-handed circularly polarized light along the wave. This gradient varies over a distance of λ/2 and mirrors itself about the y-z plane. In certain positions, the counter-propagating beams can have a phase difference of π/2. The polarization is generally circular, but when there is no phase difference, it becomes linear. In intermediate regions, the superimposed fields exhibit ellipticity in their polarization gradient. Effective cooling requires the dissipation or loss of energy.
This laser cooling technique enables the cooling of tightly bound atoms and ions to temperatures beyond the Doppler cooling limit, potentially reaching the ground state of these atoms and ions. It is generally used to cool atoms that are strongly trapped to the lowest energy state of their motion.
When an atom is both cold and trapped, it can be approximated as a quantum mechanical harmonic oscillator. If the rate of spontaneous decay is much less than the vibrational frequency of the trapped atom, the energy levels of the system can be distinguished into internal levels, each corresponding to a series of vibrational states.
For effective laser cooling, the frequency of the laser beam must be adjusted to the red sideband. When this is done, spontaneous emission will primarily occur at the carrier frequency, provided that the recoil energy of the atom is negligible compared to the vibrational quantum energy. The result is that the ion is cooled by one vibrational energy level. To achieve efficient resolved sideband cooling, the process must begin at sufficiently low temperatures (ǹ).
Typically, the particle is first cooled to the Doppler limit. Following this, several sideband cooling cycles are applied. Finally, either a measurement is taken or the state is manipulated. The narrow quadrupole transition used for cooling connects the ground state to a long-lived state, which must be pumped out to achieve optimal cooling efficiency.
Raman cooling is a technique in atomic physics used for sub-recoil cooling of atoms through optical methods, achieving temperatures below the limits of Doppler cooling. Unlike Doppler cooling, which is constrained by the recoil energy of a photon absorbed by an atom, Raman cooling can be performed using simple optical molasses or by superimposing an optical lattice. This method allows for more precise control over atomic temperatures, surpassing the limitations of traditional Doppler cooling.
In Raman cooling, two laser beams are used to induce a transition between two hyperfine states of an atom. The first beam excites the atom to a virtual excited state, with its frequency set slightly below the actual transition frequency. The second beam then de-excites the atom back to the hyperfine state. The frequency difference between the two beams precisely matches the transition frequency between the hyperfine levels, facilitating effective cooling.
This technique begins with atoms confined in a magneto-optical trap. An optical lattice is then gradually intensified so that a significant fraction of the atoms become trapped. Each site in the lattice can be modeled as a harmonic trap, provided the lattice lasers are sufficiently powerful. Initially, the atoms are not in their ground state but rather in an excited level of the harmonic oscillator. The goal of Raman sideband cooling is to bring the atoms to the ground state of the harmonic potential at each lattice site.
Consider a two-level atom with a ground state quantum number F=1, which is threefold degenerate with m=-1, 0, and 1. The degeneracy of these m states is lifted by applying a magnetic field through the Zeeman effect. The magnetic field is adjusted so that the Zeeman splitting between m=-1 and m=0, and between m=0 and m=1, matches the energy spacing between the two levels in the harmonic potential created by the lattice.
Using Raman processes, an atom can be transferred to a state where its magnetic moment decreases by one and its vibrational state also decreases by one. After this transfer, atoms in the lowest vibrational state of the lattice potential are optically pumped to the m=1 state.
During the pumping process, the atom may not change its vibrational state further if the temperature of the atoms is sufficiently low compared to the frequency of the pumping beam. This results in the atom ending up in a low vibrational state, thus achieving cooling. To ensure effective transfer to the lower vibrational state, laser parameters such as power and timing must be carefully tuned for optimal results.
These parameters vary for different vibrational states because the strength of coupling depends on the vibrational level. This cooling technique enables the attainment of a high density of atoms at low temperatures using optical methods alone. Although photon recoil from the transition can complicate the process, this issue can be mitigated by operating in the Lamb-Dicke regime.
Gray molasses is a method of sub-Doppler laser cooling that combines principles from Sisyphus cooling with a dark state, which is not addressed by the resonant lasers. This technique is typically employed in ultra-cold atomic physics experiments with atomic species that have poorly resolved hyperfine structures, such as isotopes of lithium and potassium. Gray molasses is used to achieve temperatures below the Doppler limit.
Gray molasses can only slow atoms but not trap them, unlike a magneto-optical trap, which combines a molasses force with a confining force. Due to this, gray molasses is efficient as a cooling mechanism but operates only for milliseconds before further cooling and trapping stages are applied. The cooling mechanism of gray molasses relies on two-photon and Raman-type transitions between hyperfine-split ground states, mediated by an excited state.
In this technique, bright and dark states are orthogonal superpositions of the ground states. The bright state couples to the excited state via dipole transitions driven by the laser, while the dark state is accessible from the excited state via spontaneous emission. Both states evolve into each other, as neither is an eigenstate of the kinetic energy operator.
The evolution of these states is proportional to the external momentum of the atom. The polarization gradients in the molasses beam create a sinusoidal potential energy landscape for the bright state. Atoms lose kinetic energy by traveling to the maxima of this potential energy landscape.
These potential energy maxima correspond to circular polarizations that facilitate electric dipole transitions to the excited state. Optical pumping transitions atoms in the excited state to the dark state, which then evolves back to the bright state, restarting the cycle. The repeated cycles from bright to excited to dark states result in Sisyphus-like cooling in the bright state, further selecting the coldest atoms to enter the dark state and escape the cycle.
This method involves a coherent optical nonlinearity that makes a medium transparent over a narrow spectral range centered around an absorption line. By creating significant dispersion within this transparency window, it allows for the phenomenon known as slow light. This is achieved through a quantum interference effect that permits light to pass through an otherwise opaque atomic medium. The process of electromagnetically induced transparency involves two highly coherent optical fields, like lasers, which are carefully tuned to interact with three distinct quantum states of the material.
The probe field is adjusted to a resonance position close to a different transition. If the coupling field is properly configured, it will generate a transparency window within the spectrum, which can then be detected by the probe. This principle of electromagnetically induced transparency relies on the destructive interference of the transition probability amplitude between atomic states. For effective cooling, a system with three atomic levels must be considered: a ground state, an excited state, and an intermediate stable state.
The excited state is dipole-coupled to both the stable and ground states. An intense coupling laser excites the stable state, while a weaker cooling laser drives transitions from the ground state to the excited state. Electromagnetically induced transparency cooling occurs when the difference in energy between the ground state and the stable state matches the energy difference involved in the carrier transition, aligning with the dark resonance of a Fano-like feature.
To optimize cooling, the red sideband should align with the peak of the Fano-like feature, while the blue sideband should be positioned in a region with low excitation probability. The cooling limit is reduced compared to Doppler or sideband cooling due to the larger ratio of excitation probabilities, assuming similar cooling rates for each method.
This technique, employed in the laser cooling of atoms, was developed to account for observations of cooling beyond the limits of the Doppler Effect. While Doppler cooling can achieve temperatures in the range of hundreds of microkelvins, polarization gradient cooling can reduce temperatures to a few microkelvins or even lower. This method creates a spatially varying polarization gradient through the superposition of two counter-propagating light beams with orthogonal polarizations. The nature of the gradient depends on the type of polarization used.
When orthogonal linear polarizations are utilized, the resulting polarization oscillates between linear and circular forms over a spatial period of half a wavelength. Alternatively, using orthogonal circular polarizations produces a rotating linear polarization along the axis of propagation. Although both configurations yield similar cooling outcomes, the underlying physical mechanisms differ.
In the case of orthogonal linear polarization, the polarization gradient introduces periodic light shifts in the atomic ground state, facilitating the Sisyphus cooling process. On the other hand, orthogonal circular polarization creates a motion-induced imbalance in the Zeeman levels due to the rotating polarization, which affects the radiation pressure opposing the atom's motion. Both configurations achieve sub-Doppler cooling and can reach the recoil limit. However, since polarization gradient cooling has a narrower capture range compared to Doppler cooling, pre-cooling of the atomic gas is necessary, even though it can achieve lower temperatures.
Polarization gradient cooling is typically implemented using a three-dimensional optical arrangement. This setup involves three pairs of laser beams arranged perpendicularly around an atomic ensemble located at the center. Each pair of beams has orthogonal polarizations relative to its counterpart. The laser frequency is deliberately detuned from a specific transition between the ground and excited states of the atom.
Since the cooling process involves multiple transitions, special attention is required to ensure that atoms do not transition out of the relevant states. To address this, a secondary re-pumping laser is used to return any atom that exits these states back to the ground state. Prior to applying polarization gradient cooling, the atoms must be pre-cooled using the same optical setup via Doppler cooling.
For effective polarization gradient cooling, it is necessary to reduce the laser intensity if Doppler cooling has already been performed. Additionally, the detuning must be increased. The atomic temperature can be measured using the time-of-flight technique. In this method, the laser beams are abruptly turned off, allowing the atomic ensemble to expand. After a predetermined delay, a probe beam is activated to measure the spatial distribution of the expanding ensemble. By observing the expansion at various time intervals, the rate of expansion can be determined.
This chapter will cover various laser cooling devices and the different types of chillers available.
Various instruments used for laser cooling include:
This device is used in quantum physics to reduce the temperature of an atomic beam from room temperature or higher to a few Kelvin. Initially, the atoms enter with an average velocity of several hundred meters per second and a velocity spread in the order of a few meters per second. By the time they exit the slower, their speed is reduced to around ten meters per second with a significantly smaller spread.
The Zeeman slower operates on principles akin to Doppler cooling. In this setup, a two-level atom can be cooled by a laser. When the atom moves in a certain direction and interacts with a laser beam tuned to its transition frequency, it is likely to absorb a photon.
When an atom absorbs a photon, it receives a momentum boost in accordance with the principles of momentum conservation, elevating it to an excited state. This excited state is transient, and the atom eventually returns to its ground state via spontaneous emission.
While the re-emission of photons can occur, potentially increasing the atom's speed, the direction of these emitted photons is random. The initial absorption process, however, reduces the atom's speed in a specific direction, as the absorbed photon comes from a precisely directed source. In contrast, the emission process does not affect the atom's speed because the direction of emitted photons is random.
This vacuum pump operates by sputtering a metal surface to achieve high levels of vacuum. Under optimal conditions, these ion pumps can reach pressures as low as 10-11 mbar. The process involves ionizing the gas within the vessel and applying a high electrical potential, typically between 3 to 7 kV. This potential accelerates the ions towards a solid electrode, where they are collected and removed.
The sputtering of tiny particles from the electrode into the chamber facilitates the trapping of gases. This is achieved through chemical reactions that occur between the highly reactive sputtered material and the gases present on the chamber's surface.
Chillers are categorized based on several factors, including the method used to release absorbed heat, the specific function of the design, and the type of compressor employed.
The centrifugal chiller makes use of compression so as to convert kinetic energy into static energy. This leads to the pressure and temperature of the refrigerant increasing. The refrigerant is pulled in and compressed using impeller blades.
Laser chillers are used to cool laser systems and equipment.
Laser chillers are essential for maintaining the optimal wavelength, ensuring that lasers operate at their highest efficiency.
Carbon dioxide lasers, ion lasers, and high-power exciters require precision and accuracy, which are maintained through a chiller water cooling system.
In an absorption chiller, the refrigerant is converted into vapor by a generator that utilizes hot water or steam.
The vapor flows to the condenser, where it is condensed and then directed to the absorber. In the absorber, the vapor is taken up by a solution, which then re-condenses, generating heat in the process.
This chiller works by extracting heat from water and releasing it into the surrounding air. It is typically used in environments where heat discharge does not pose an issue. The evaporator absorbs heat from the circulating chilled water, and the refrigerant, after condensing in the condenser, dissipates the heat into the air.
This chapter will explore the various applications of laser coolers and the process of laser cooling.
Laser cooling is employed in high-resolution spectroscopic measurements, such as frequency standards in optical clocks, which utilize ultra-cold atoms or ions to eliminate Doppler broadening.
Additionally, laser cooling is crucial in ultra-precise gravitational field measurements, which are applied in gravitational physics and oil field exploration. This exploration relies on the Doppler shift of free-falling cooled atoms and Bloch oscillations.
Laser cooling also finds application in lithography, enabling the precise creation of controlled structures using cold atomic beams. Initially, laser cooling generates ultra-cold atoms for quantum physics experiments conducted near absolute zero, where quantum phenomena like Bose-Einstein condensation can be observed. Additional uses include vibration-free optical refrigeration and radiation-balanced solid-state lasers.
The word laser stands for Light Amplification by Stimulated Emission of Radiation, which occurs when electrons in optical materials like glass, crystals, or gases absorb energy from an electrical current. This energy causes the electrons to become excited, moving from a low energy state to a higher one, thus producing electromagnetic radiation.
In regular light, wavelengths vary in direction and length, with each wavelength representing a different color, resulting in what appears to the naked eye as white light. Conversely, a laser beam consists of coherent light, where all the wavelengths align and travel in the same direction, with their peaks and valleys synchronized. This focused light generates substantial heat energy.
When a laser beam is directed onto a surface, its light energy transforms into heat, which can cut, melt, or burn the material. The heat generated must be dissipated to maintain the laser's efficiency and prevent damage. Industrial lasers use various cooling methods, including chillers, to maintain a consistent temperature.
Laser chillers are specifically designed to remove excess heat from the laser process, helping to maintain the laser's wavelength stability and ensuring the beam quality. High-powered lasers typically require water-cooled chillers, while lower-powered lasers may be sufficiently cooled with fans or other cooling mechanisms.
When selecting a laser chiller, it's crucial to choose one that is compatible with the specific type of laser in use. Laser chillers are commonly used with:
Cold plates play a crucial role in laser cooling systems and are typically used in conjunction with recirculating chillers. They come in various designs, including tubed cold plates and aluminum vacuum-brazed versions.
Cold plates are often installed on lasers to receive cooling fluids supplied by a chiller. The heated liquid from the cooling process is then routed back to the chiller. Additionally, in some designs, the cold plate may function as the electrodes within the laser system.
Thermoelectric coolers (TECs) operate based on the Peltier effect, which involves generating a heat flux at the junction of two different materials when a direct current (DC) is applied. In a TEC, positive and negative semiconductors are arranged parallel in the thermal pathway and in series in the electrical pathway. When a voltage is applied, electrons transport heat from one side to the other, cooling one side while heating the opposite side. Fans then dissipate the accumulated heat into the surrounding air.
A TEC assembly can be directly attached to a cold plate or used to cool a refrigerant liquid. Typically, TECs are suited for applications requiring cooling capacities of up to 400 watts.
Vapor compression chillers operate by circulating a refrigerant through a sequence of components: an evaporator, compressor, condenser, and expansion valve. This method is effective for cooling high power loads while consuming relatively little energy. In laser systems, vapor compression chillers can be adapted by adjusting the evaporator cycle, whereas other methods might involve running the refrigerant directly through the cold plate.
The compressor-based vapor liquid chiller is a widely utilized cooling technique for high-powered lasers. These chillers are capable of managing cooling loads up to 10 kW and are compatible with various types of lasers.
The miniature rotary compression system is now a popular choice for laser cooling applications. Advances in rotary compressor technology have enabled the creation of compact systems capable of handling heat loads up to 100 watts. This smaller, more efficient design has become widely adopted for various laser cooling needs.
Miniature rotary compressor chillers offer portability, precise temperature control, and a cooling capacity ranging from 3 kW to 140 kW. They operate effectively within a temperature range of 20°F to 80°F.
In a direct expansion chiller system, refrigerant circulates directly through the cold plate, driven by the compressor. This approach eliminates the necessity for a separate water cooling loop, simplifying the overall system design. By removing the need for an additional coolant loop, this method provides precise temperature control through the isothermal phase change of the refrigerant as it passes over the cold plate.
Direct expansion systems have the typical components of a compressor, condenser, expansion valve, and evaporator, which absorbs the heat directly. They are a compact cooling solution with a miniature rotary compressor. The advantage of the system is the precision at which it controls the temperature. Direct expansion cooling systems are designed for high heat flux applications such as laser cutting and burning.
In general, laser cooling refers to the use of dissipative light forces so as to reduce the random motion of contained atoms and also reduce the temperature of small particles like atoms and ions. The temperature attained can result in different magnitudes all depending on the type of mechanism used.
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