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 delves into the basics of laser cooling, explores different cooling techniques, and explains how laser coolers function.
Laser cooling encompasses various strategies designed to decrease the temperature of atomic and molecular samples close to absolute zero. These techniques take advantage of the phenomenon where an atom or molecule changes its momentum upon re-emitting a photon, leading to a decline in its overall kinetic energy.
In a particle collection, the thermodynamic temperature correlates with the distribution of particle velocities. Thus, a more uniform velocity distribution among particles implies a lower temperature.
Laser cooling methods leverage atomic spectroscopy alongside the mechanical effects of light to achieve a narrower velocity distribution in particle groups, thereby cooling them.
Doppler cooling stands out among the various techniques employed in laser cooling; its practicality often leads to it being equated with laser cooling itself.
Other laser cooling techniques 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 utilization of the Zeeman slower.
Doppler cooling is used to trap and reduce atom motion, effectively cooling substances. A stationary atom does not experience a shift in laser light and therefore does not absorb the photon.
An atom moving away perceives the light as red-shifted and does not absorb the photon. However, an atom moving towards the laser finds the light blue-shifted and absorbs it, slowing down. The absorption excites the atom, elevating an electron to a higher state, and upon re-emission of a photon, which occurs in a random direction, there is no net momentum change over numerous emissions.
The technique's core principle is that photons interacting with an atom are mainly unaffected by it. The atom is transparent to most photon frequencies, resonating only at very narrow frequency bands. Interactions occur at specific colors, unlike the mixed colors of white light.
When a resonant photon engages with the atom, it's briefly absorbed. The atom re-emits the photon in random directions, which does not significantly affect the thermal energy of atoms, contrary to the common belief about lasers increasing thermal energy.
If nearly motionless and carefully controlled to frequency, an atom remains unresponsive to most laser frequencies, reacting only at certain electromagnetic frequencies. When matched with these frequencies, the atom absorbs a photon, gains its momentum, and moves with the photon. After a short while, the atom emits a photon, typically in a random direction, potentially losing gained momentum. If the re-emitted photon travels oppositely, more momentum is acquired in that direction.
To maintain momentum, this causes the atom's velocity to double. Often, re-emitted photons impart a sideways thrust. Laser frequency is adjusted by using monochromatic light, slightly below a resonant frequency. Initially, frequency well under the resonant frequency passes through. Eventually, moving atoms absorb photons and slow, while atoms moving away remain unaffected.
On a velocity graph, faster right-moving atoms are dots on the right, and left-moving ones are dots on the left. A narrow band on the left represents velocities absorbing photons from the left laser. Within this band, each collision slows atoms based on the photon’s momentum. Adjusting the laser frequency narrows the interaction band, causing a velocity shift towards zero, achieving the cold state.
Sisyphus cooling achieves temperatures beneath the Doppler cooling limit by employing two counter-propagating laser beams with orthogonal polarizations. When atoms navigate the potential landscape generated by these standing wave lasers, they lose kinetic energy. Optical pumping transitions them to lower energy states, reducing total atomic energy.
The principle involves polarization gradients created by counter-propagating lasers with orthogonal polarizations that lead to a standing wave. This gradient alternates between circularly polarized light, varying over λ/2, and mirrors about the y-z plane. Depending on position, these beams can create a π/2 phase difference. Polarization frees into circular when there's no phase difference, transitioning to linear. In these gradient areas, ellipticity appears.
This method cools tightly bound atoms and ions beyond the Doppler limit, approaching ground states. It's mainly applied to atoms intensely trapped, reaching the lowest motion energy level.
Cold and trapped, atoms resemble quantum mechanical harmonic oscillators. If spontaneous decay rates are lower than atom vibrational frequencies, these systems divide into internal levels, each with vibrational states.
Effective laser cooling requires aligning laser frequency to the red sideband. Spontaneous emissions occur at the carrier frequency, cooling the ion by one vibrational level when recoil energy is negligible to vibrational quantum energy. Efficient resolved sideband cooling begins at low initial temperatures.
Typically, particles first reach the Doppler limit, then undergo several sideband cycles. Subsequently, measurements or state manipulations occur. The narrow quadrupole transition for cooling connects ground states to a long-lived state, essential for cooling efficiency.
Raman cooling, an atomic physics technique, achieves temperatures beneath Doppler cooling limits using optical methods. This surpasses Doppler cooling's constraints by employing optical molasses or superimposed optical lattices for heightened atomic temperature control.
Raman cooling uses twin lasers to transition atoms between hyperfine states. The first excites the atom below the actual transition frequency to a virtual state, while the second de-excites it back. The beams align their frequency differences to the transition between hyperfine states, enabling effective cooling.
Raman sideband cooling starts with atoms in a magneto-optical trap, intensifying an optical lattice to trap a significant atom fraction. Each lattice site is akin to a harmonic trap if lattice lasers have ample power. Initially, atoms are in an excited harmonic level, but the goal is transitioning to the ground state at each site.
For a two-level atom (F=1), threefold degenerate m-states (m=-1, 0, 1) require degeneracy lifting via magnetic fields and Zeeman splitting. Zeeman splitting must align with energy spacing between harmonic potential levels.
Raman processes allow transitions to states with decreased magnetic moments and vibrational states. Post-transfer, atoms in the lowest lattice vibrational state, pumped optically to the m=1 state, do not further change vibrationally if temperature is low compared to pump frequency, facilitating cooling. Adjust laser power and timing for effective vibrational state transfer.
Cooling enables higher atom densities at low temperatures. Although photon recoil complicates the process, operating in the Lamb-Dicke regime mitigates it.
Gray molasses, a sub-Doppler laser cooling method, merges Sisyphus cooling with a dark state, untouched by resonant lasers, for experiments in ultra-cold atomic physics with poor-resolved species, like lithium and potassium isotopes. It achieves temperatures under the Doppler limit.
Unlike magneto-optical traps, gray molasses, limited to atom slowing but not trapping, efficiently cools but operates only for milliseconds before trapping and extra cooling. It relies on two-photon and Raman transitions between hyperfine-split ground states, mediated through excitation.
Bright and dark states, ground state orthogonal superpositions, interact via dipole-driven transitions. Bright states lead to excited states; dark states, accessible from excited states through spontaneous emission, oscillate due to fluctuating momentum-dependent state coupling.
The polarization gradient creates a sinusoidal bright state-potential. Atoms lose kinetic energy towards potential maxima, corresponding to electric dipole-transition enabling circular polarizations. Atoms transition optically pumped back to dark state-excited cycles, creating Sisyphus-like cooling. Cold atoms enter and exit cycles through bright, excited to dark states.
Through coherent optical nonlinearity, a medium becomes transparent over a narrow spectral range centered on an absorption line, allowing "slow light." It involves quantum interference permitting light passage through otherwise opaque mediums and utilises two highly coherent optical fields interacting with three distinct quantum states.
Configuring the probe field close to resonance reveals transparency and significant dispersion. The principle relies on atomic state transition probability amplitude interference. Optimizing cooling requires three atom-level systems: ground, excited, and intermediate states.
Ensuring dipole coupling between states, coupling lasers excite states, while weak cooling lasers induce transitions. Transparency cooling ensues when ground-stable state differences equate carrier transition energies. Alignment to Fano-like feature-dark resonance enhances cooling overother methods through larger excitation probability ratios.
Developed to explain cooling below Doppler limitations, polarization gradient cooling achieves microkelvin to sub-microkelvin temperatures. It generates spatially varying gradients through superposed orthogonal polarization light beams, creating varying polarization states.
Orthogonal linear polarizations oscillate between linear and circular over half-wavelength spatial periods, while circular ones rotate linearly along the propagation axis, yielding similar cooling yet differing mechanisms.
In linear polarizations, cooling arises from periodic atomic ground state shifts leading to Sisyphus processes. Circular ones engender Zeeman level imbalances due to motion-induced polarization differences, impacting radiation-induced atomic motion resistance, reaching recoil limits. However, its narrow capture range necessitates prior Doppler cooling despite enabling lower temperatures.
Implemented using a 3D optical setup, it involves six lasers perpendicular to an atomic ensemble, each counterbalanced, generating orthogonal light polarization. Laser frequency detunes from ground-excited state transitions. To avoid irrelevant state transitions, a re-pumping laser returns atoms to ground states.
Reduction and detuning follow Doppler cooling. Using the time-of-flight method, laser shutdown permits ensemble expansions measured by probe beams after delays to track expansion rates, indicative of atomic temperature.
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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