From Lasers to Masers

Dr Juna Sathian

Department of Mathematics Physics Electrical Engineering at Northumbria University

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https://www.northumbria.ac.uk/about-us/our-staff/s/juna-sathian/

JUNA.SATHIAN@NORTHUMBRIA.AC.UK

Lasers are used everywhere from CD players to industrial cutting and laser eye surgery. But before the laser there was the maser, which produces microwaves instead of light. However, due to its extreme cooling requirement the maser never became as widespread as the laser. In this lecture, Dr Sathian briefly explored the history of masers and lasers, followed by recent developments in solid-state laser technology (in the rare gemstone alexandrite), and the newly invented room-temperature maser (in the more-familiar diamond).

The following are notes from the on-line lecture. Even though I could stop the video and go back over things there are likely to be mistakes because I haven’t heard things correctly or not understood them. I hope Dr Sathian, and my readers will forgive any mistakes and let me know what I got wrong.

Biography

Dr Sathian did her Batchelor and Master’s degrees in India, where she did a project on lasers.

She received her PhD in Nonlinear Optics and Laser Physics from Queensland University of Technology, Australia in Sep 2013. Her thesis entitled “Investigation of amplitude modulation contamination in electro-optic modulators”, was solving one of the serious problems of electro-optic modulator devices, a known issue in the LIGO gravitational wave detector system.

She joined Imperial College London, Department of Materials as a Postdoctoral Research Associate in 2014 where she was a key researcher and co-developer of world’s first room-temperature continuous wave maser (in diamond), which has been patented and published in Nature.

She also worked at the Department of Physics, Imperial College London on a project funded by Innovate-UK. The project developed novel precision wavelength-tunable diode-pumped Alexandrite laser technology in collaboration with M Squared Lasers, a premier scientific UK laser company.

She is an Associate Fellow of Higher Education Academy (AFHEA). She is an Honorary Senior Lecturer at Imperial College London and currently a research collaborator to the Maser Group at the Department of Materials and Photonics Group at the Department of Physics.

Dr Juna Sathian joined Northumbria University in July 2019 as a Senior Lecturer in the Department of Mathematics, Physics & Electrical Engineering.

Electromagnetic spectrum

Dr Sathian’s research has involved parts of the electromagnetic spectrum.

https://en.wikipedia.org/wiki/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 below one hertz to above 1025 hertz, corresponding to wavelengths from thousands of kilometres down to a fraction of the size of an atomic nucleus. This frequency range is divided into separate bands, and the electromagnetic waves within each frequency band are called by different names; beginning at the low frequency (long wavelength) end of the spectrum these are: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays at the high-frequency (short wavelength) end. The electromagnetic waves in each of these bands have different characteristics, such as how they are produced, how they interact with matter, and their practical applications. The limit for long wavelengths is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length. Gamma rays, X-rays, and high ultraviolet are classified as ionizing radiation as their photons have enough energy to ionize atoms, causing chemical reactions.

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Electromagnetic waves are transverse waves

https://en.wikipedia.org/wiki/Transverse_wave

In physics, a transverse wave is a wave that vibrates perpendicular to the direction of energy transfer or propagation direction

http://www.docbrown.info/ephysics/wavesintro.htm

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https://upload.wikimedia.org/wikipedia/commons/4/4c/Electromagneticwave3D.gif

Electromagnetic waves are made up of two transverse oscillations (electric and magnetic fields) that are right angles to each other and both of these are at right angles to the propagation direction.

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The parts of the spectrum that have been involved in Dr Sathian’s work are the visible spectrum, microwaves and the near Infra-red. She builds devices that use these wavelengths. Applications of the devices include space technology, medical technology and some other medical applications.

As mentioned earlier, Dr Sathian’s PhD was involved with the LIGO instrumentation, particularly analysing the background noise occurring in the apparatus.

https://en.wikipedia.org/wiki/LIGO

The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. Two large observatories were built in the United States with the aim of detecting gravitational waves by laser interferometry. These observatories use mirrors spaced four kilometres apart which are capable of detecting a change of less than one ten-thousandth the charge diameter of a proton.

LIGO operates two gravitational wave observatories in unison: the LIGO Livingston Observatory in Livingston, Louisiana, and the LIGO Hanford Observatory, on the DOE Hanford Site, located near Richland, Washington. These sites are separated by 3,002 kilometres straight line distance through the earth, but 3,030 kilometres over the surface. Since gravitational waves are expected to travel at the speed of light, this distance corresponds to a difference in gravitational wave arrival times of up to ten milliseconds. Through the use of trilateration, the difference in arrival times helps to determine the source of the wave, especially when a third similar instrument like Virgo, located at an even greater distance in Europe, is added.

Each observatory supports an L-shaped ultra-high vacuum system, measuring 4 kilometres on each side. Up to five interferometers can be set up in each vacuum system.

The LIGO Livingston Observatory houses one laser interferometer in the primary configuration. This interferometer was successfully upgraded in 2004 with an active vibration isolation system based on hydraulic actuators providing a factor of 10 isolation in the 0.1–5 Hz band. Seismic vibration in this band is chiefly due to microseismic waves and anthropogenic sources (traffic, logging, etc.).

The LIGO Hanford Observatory houses one interferometer, almost identical to the one at the Livingston Observatory. During the Initial and Enhanced LIGO phases, a half-length interferometer operated in parallel with the main interferometer. For this 2 km interferometer, the Fabry–Pérot arm cavities had the same optical finesse, and, thus, half the storage time as the 4 km interferometers. With half the storage time, the theoretical strain sensitivity was as good as the full-length interferometers above 200 Hz but only half as good at low frequencies. During the same era, Hanford retained its original passive seismic isolation system due to limited geologic activity in Southeastern Washington.

A gravitational wave is an invisible (yet incredibly fast) ripple in space. Gravitational waves travel at the speed of light (3 x 108m/s) These waves squeeze and stretch anything in their path as they pass by.

https://www.ligo.caltech.edu/

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Simplified operation of a gravitational wave observatory

Figure 1: A beamsplitter (green line) splits coherent light (from the white box) into two beams which reflect off the mirrors (cyan oblongs); only one outgoing and reflected beam in each arm is shown, and separated for clarity. The reflected beams recombine and an interference pattern is detected (purple circle).

Figure 2: A gravitational wave passing over the left arm (yellow) changes its length and thus the interference pattern.

https://spaceplace.nasa.gov/gravitational-waves/en/

https://spaceplace.nasa.gov/gravitational-waves/en/gravitational-wave-detection.en.gif

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When a gravitational wave passes by Earth, it squeezes and stretches space. LIGO can detect this squeezing and stretching. Each LIGO observatory has two “arms” that are each more than 2 miles (4 kilometres) long. A passing gravitational wave causes the length of the arms to change slightly. The observatory uses lasers, mirrors, and extremely sensitive instruments to detect these tiny changes.

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Above left: Aerial view of LIGO facility in Livingston. Above right: Aerial view of LIGO facility in Hansford

What causes gravitational waves?

The most powerful gravitational waves are created when objects move at very high speeds. Some examples of events that could cause a gravitational wave are:

when a star explodes asymmetrically (called a supernova)

when two big stars orbit each other

when two black holes orbit each other and merge

https://youtu.be/zLAmF0H-FTM

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Gravitational waves squeeze and stretch anything in their path as they pass by.

https://www.jpl.nasa.gov/edu/news/2016/3/23/modeling-gravitational-waves/

https://en.wikipedia.org/wiki/First_observation_of_gravitational_waves

The first direct observation of gravitational waves was made on 14 September 2015 and was announced by the LIGO and Virgo collaborations on 11 February 2016. Previously, gravitational waves had been inferred only indirectly, via their effect on the timing of pulsars in binary star systems. The waveform, detected by both LIGO observatories, matched the predictions of general relativity for a gravitational wave emanating from the inward spiral and merger of a pair of black holes of around 36 and 29 solar masses and the subsequent “ringdown” of the single resulting black hole. The signal was named GW150914 (from “Gravitational Wave” and the date of observation 2015-09-14). It was also the first observation of a binary black hole merger, demonstrating both the existence of binary stellar-mass black hole systems and the fact that such mergers could occur within the current age of the universe.

This merger actually happened 13 billion years ago

https://upload.wikimedia.org/wikipedia/commons/transcoded/8/85/Warped_Space_and_Time_Around_Colliding_Black_Holes_%28Courtesy_Caltech-MIT-LIGO_Laboratory%2C_produced_by_SXS_project%29.webm/Warped_Space_and_Time_Around_Colliding_Black_Holes_%28Courtesy_Caltech-MIT-LIGO_Laboratory%2C_produced_by_SXS_project%29.webm.480p.vp9.webm

https://upload.wikimedia.org/wikipedia/commons/transcoded/a/a4/BBH_gravitational_lensing_of_gw150914.webm/BBH_gravitational_lensing_of_gw150914.webm.480p.vp9.webm

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Simulation of merging black holes radiating gravitational waves

https://www.nobelprize.org/prizes/physics/2017/press-release/

The Royal Swedish Academy of Sciences decided to award the Nobel Prize in Physics 2017 with one half to

Rainer Weiss
LIGO/VIRGO Collaboration

and the other half jointly to

Barry C. Barish
LIGO/VIRGO Collaboration

and

Kip S. Thorne
LIGO/VIRGO Collaboration

“for decisive contributions to the LIGO detector and the observation of gravitational waves”

https://en.wikipedia.org/wiki/Rainer_Weiss

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Rainer “Rai” Weiss (born September 29, 1932) is an American physicist, known for his contributions in gravitational physics and astrophysics. He is a professor of physics emeritus at MIT and an adjunct professor at LSU. He is best known for inventing the laser interferometric technique which is the basic operation of LIGO. He was Chair of the COBE Science Working Group.

https://en.wikipedia.org/wiki/Kip_Thorne

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Kip Stephen Thorne (born June 1, 1940) is an American theoretical physicist known for his contributions in gravitational physics and astrophysics.

https://en.wikipedia.org/wiki/Barry_Barish

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Barry Clark Barish (born January 27, 1936) is an American experimental physicist and Nobel Laureate. He is a Linde Professor of Physics, emeritus at California Institute of Technology and a leading expert on gravitational waves.

https://en.wikipedia.org/wiki/Gravitational-wave_observatory

There are other observatories around the world besides LIGO.

Japan is building KAGRA

https://en.wikipedia.org/wiki/KAGRA

The Kamioka Gravitational Wave Detector (KAGRA), formerly the Large Scale Cryogenic Gravitational Wave Telescope (LCGT), is a project of the gravitational wave studies group at the Institute for Cosmic Ray Research (ICRR) of the University of Tokyo. It is Asia’s first gravitational wave observatory, the first in the world built underground, and the first whose detector uses cryogenic mirrors. The design calls for an operational sensitivity equal to, or greater than, LIGO.

https://en.wikipedia.org/wiki/Indian_Initiative_in_Gravitational-wave_Observations

INDIGO or IndIGO (Indian Initiative in Gravitational-wave Observations) is a consortium of Indian gravitational-wave physicists. This is an initiative to set up advanced experimental facilities for a multi-institutional observatory project in gravitational-wave astronomy located near Aundha Nagnath, Hingoli District, Maharashtra.

https://en.wikipedia.org/wiki/AIGO

The Australian International Gravitational Observatory (AIGO) is a research facility located near Gingin, north of Perth in Western Australia. It is part of a worldwide effort to directly detect gravitational waves.

Post PhD work on MASERs and LASERs

At Imperial College, London, Dr Sathian was part of a group working the world’s first room-temperature continuous MASER and the World’s first unidirectional continuous Alexandrite ring LASER.

https://www.imperial.ac.uk/

https://en.wikipedia.org/wiki/Imperial_College_London

Imperial College London (legally Imperial College of Science, Technology and Medicine) is a public research university in London.

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Above left: the world’s first room-temperature continuous MASER. Above right: World’s first unidirectional continuous Alexandrite ring LASER.

The Alexandrite ring LASER emits near Infra-red radiation. Visible light is used to produce an invisible electromagnetic wave.

MASERSs produce microwaves and these have uses in communication, LIDAR and medicine.

https://en.wikipedia.org/wiki/Lidar

Lidar (also LIDAR, LiDAR, and LADAR) is a method for measuring distances (ranging) by illuminating the target with laser light and measuring the reflection with a sensor. Differences in laser return times and wavelengths can then be used to make digital 3-D representations of the target. It has terrestrial, airborne, and mobile applications.

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The MASER research at Imperial College involved several different research groups.

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LASERs are used everywhere including in CD/DVD players and LASER eye surgery.

LASERs amplify light and MASERs amplify microwaves. Surprisingly MASERs were invented before LASERs.

Questions and answers part 1

1) We’ve known about gravitational waves for a long time. Did we have to wait until we got the technology, i.e., lasers, in order to detect gravitational waves?

LIGO needed very little background noise so it took time and still requires work to produce the equipment needed.

The set-up is being improved to increase precision all the time in order to measure the tiny ripples.

A project that Dr Sathian is working on is to reduce the effect of background noise on one of LIGO’s arms.

There is always room to make measurements more precise.

Back to the talk

The principle behind MASERs and LASRS are the same.

A century ago Einstein sparked the notion of the laser – Physics World

The fundamental idea behind this mainstay of modern life was published in 1917 by Albert Einstein. But blink and you’ll miss it in his seminal paper, “The quantum theory of radiation”, published in German in Physikalische Zeitschrift 18 121. Einstein is trying to work out what Max Planck’s “quantum hypothesis” – that the energy of an oscillator must take discrete values equal to some integer multiple of the oscillation frequency times a constant h – implies for the way light interacts with matter.

Maser – Wikipedia

A maser (an acronym for microwave amplification by stimulated emission of radiation) is a device that produces coherent electromagnetic waves through amplification by stimulated emission. The first maser was built by Charles H. Townes, James P. Gordon, and Herbert J. Zeiger at Columbia University in 1953 using ammonia.

https://en.wikipedia.org/wiki/Charles_H._Townes (below left)

Charles Hard Townes (July 28, 1915 – January 27, 2015) was an American physicist. Townes worked on the theory and application of the maser, for which he obtained the fundamental patent, and other work in quantum electronics associated with both maser and laser devices.

James P. Gordon – Wikipedia (below centre)

James Power Gordon (March 20, 1928 – June 21, 2013) was an American physicist known for his work in the fields of optics and quantum electronics.

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Herbert Zeiger – Wikipedia (above right)

Herbert J. Zeiger (b. 16 March 1925 in the Bronx, New York City, United States; d. 14 January 2011) was an American physicist and co-developer of the first maser.

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First prototype ammonia maser and inventor Charles H. Townes. The ammonia nozzle is at left in the box, the four brass rods at centre is the quadrupole state selector, and the resonant cavity is at right. The 24 GHz microwaves exit through the vertical waveguide Townes is adjusting. At bottom are the vacuum pumps.

It was huge because it needed cryogenic temperatures and very big magnets to work.

The brass box is the vacuum chamber through which the ammonia ions travel. The ammonia gas nozzle is at left. The four rods at centre are the quadrupole filter which filters out the lower state ammonia molecules, leaving a population inversion. In the resonant cavity at right, stimulated emission of microwaves by the molecules excites standing waves in the cavity, which pass out through the vertical output waveguide. The devices at bottom are vacuum pumps which evacuated the box. Masers are used as the timekeeping elements in atomic clocks, and as extremely low-noise microwave amplifiers in radio telescopes. They are also used in deep space communications.

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When atoms have been induced into an excited energy state, they can amplify radiation at a frequency particular to the element or molecule used as the masing medium (similar to what occurs in the lasing medium in a laser).

By putting such an amplifying medium in a resonant cavity, feedback is created that can produce coherent radiation.

In 2012, a research team from the National Physical Laboratory and Imperial College London developed a solid-state maser that operated at room temperature by using optically pumped, organic pentacene-doped p-Terphenyl as the amplifier medium. It produced pulses of maser emission lasting for a few hundred microseconds. It also didn’t need the large magnets. So, it took about fifty years to get a room temperature operated MASER

In 2018, a research team from Imperial College London and University College London demonstrated continuous-wave maser oscillation using synthetic diamonds containing nitrogen-vacancy defects. Mass production of MASERs could now happen and compact systems could be built. Dr Sathian is interested in using MASERs for deep space communication.

The early MASERs didn’t take off because they couldn’t be mass produced owing to their size. Initially improvements stopped dead and researchers moved to the LASER(an optical MASER).

Laser – Wikipedia

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term “laser” originated as an acronym for “light amplification by stimulated emission of radiation”. The first laser (ruby) was built in 1960 by Theodore H. Maiman at Hughes Research Laboratories, based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow.

Theodore Maiman – Wikipedia (below left)

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Theodore Harold Maiman (July 11, 1927 – May 5, 2007) was an American engineer and physicist who is widely credited with the invention of the laser

https://en.wikipedia.org/wiki/Arthur_Leonard_Schawlow (above right)

Arthur Leonard Schawlow (May 5, 1921 – April 28, 1999) was an American physicist and co-inventor of the laser with Charles Townes.

Principle of the MASER and LASER

Amplification of light by stimulate emission of radiation

The apparatus consists of a cavity, masing material medium and a pump source.

Laser

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A laser has a crystal placed between two mirrors. Light is “pumped” into the crystal. One of the mirrors is highly reflecting, it can reflect 100% of the light hitting it. The other reflects 99%. This means that 1% of the light can escape here. A stream of light in and bunches of photons form the output i.e., the laser beam

https://upload.wikimedia.org/wikipedia/commons/transcoded/5/54/Laser%2C_quantum_principle.ogv/Laser%2C_quantum_principle.ogv.480p.vp9.webm

https://en.wikipedia.org/wiki/Population_inversion

In science, specifically statistical mechanics, a population inversion occurs while a system (such as a group of atoms or molecules) exists in a state in which more members of the system are in higher, excited states than in lower, unexcited energy states. It is called an “inversion” because in many familiar and commonly encountered physical systems, this is not possible. This concept is of fundamental importance in laser science because the production of a population inversion is a necessary step in the workings of a standard laser.

If an atom is already in the excited state, it may be agitated by the passage of a photon that has a frequency ν21 corresponding to the energy gap ΔE of the excited state to ground state transition. In this case, the excited atom relaxes to the ground state, and it produces a second photon of frequency ν21. The original photon is not absorbed by the atom, and so the result is two photons of the same frequency. This process is known as stimulated emission.

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A diagram of stimulated emission.

An excited atom “oscillates” and one of the consequences of this oscillation is that it encourages electrons to decay to the lowest energy state. When this happens due to the presence of the electromagnetic field from a photon, a photon is released in the same phase and direction as the “stimulating” photon, and is called stimulated emission.

Maser

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The MASER set up is very similar to the LASER however it uses a microwave cavity instead of mirrors. A cooper antenna carries the output.

Crystals are use to generate the waves in both LASERs and MASERs.

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During the early 1960s, the Jet Propulsion Laboratory developed a maser to provide ultra-low-noise amplification of S-band microwave signals received from deep space probes. This maser used deeply refrigerated helium to chill the amplifier down to a temperature of four kelvin. Amplification was achieved by exciting a ruby comb with a 12.0 gigahertz klystron. In the early years, it took days to chill and remove the impurities from the hydrogen lines. Refrigeration was a two-stage process with a large Linde unit on the ground, and a crosshead compressor within the antenna. The final injection was at 21 MPa through a 150 μm micrometre-adjustable entry to the chamber. The whole system noise temperature looking at cold sky (2.7 kelvins in the microwave band) was 17 kelvins. This gave such a low noise figure that the Mariner IV space probe could send still pictures from Mars back to the Earth even though the output power of its radio transmitter was only 15 watts, and hence the total signal power received was only −169 decibels with respect to a milliwatt (dBm).

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https://aip.scitation.org/doi/abs/10.1063/1.1735412?journalCode=jap

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Cryogenics. “The killer”

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© 1960 The American Institute of Physics.

https://en.wikipedia.org/wiki/Ruby_laser

A ruby laser is a solid-state laser that uses a synthetic ruby crystal as its gain medium. The first working laser was a ruby laser made by Theodore H. “Ted” Maiman at Hughes Research Laboratories on May 16, 1960.

Ruby lasers produce pulses of coherent visible light at a wavelength of 694.3 nm, which is a deep red colour. Typical ruby laser pulse lengths are on the order of a millisecond.

In 1958, after the inventor of the maser, Charles Townes, and his colleague, Arthur Schawlow, published an article in the Physical Review regarding the idea of optical masers, the race to build a working model began. Ruby had been used successfully in masers, so it was a first choice as a possible medium. While attending a conference in 1959, Maiman listened to a speech given by Schawlow, describing the use of ruby as a lasing medium. Schawlow stated that pink ruby, having a lowest energy-state that was too close to the ground-state, would require too much pumping energy for laser operation, suggesting red ruby as a possible alternative. Maiman, having worked with ruby for many years, and having written a paper on ruby fluorescence, felt that Schawlow was being “too pessimistic.” His measurements indicated that the lowest energy level of pink ruby could at least be partially depleted by pumping with a very intense light source, and, since ruby was readily available, he decided to try it anyway.

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https://www.nature.com/articles/488285a

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https://www.nature.com/articles/nature11339

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A pentacene crystal is an organic crystal that is processed in a lab.

https://en.wikipedia.org/wiki/Pentacene

Pentacene is a polycyclic aromatic hydrocarbon consisting of five linearly-fused benzene rings. This highly conjugated compound is an organic semiconductor. The compound generates excitons upon absorption of ultra-violet (UV) or visible light; this makes it very sensitive to oxidation. For this reason, this compound, which is a purple powder, slowly degrades upon exposure to air and light.

In 2012, pentacene-doped p-terphenyl was shown to be effective as the amplifier medium for a room-temperature maser

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The image below shows a scanning tunnelling microscopy image of pentacene molecules on nickel.

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As well as pentacene synthetic diamonds with a specific defect can be used in MASERs. When light reaches the defect there is a condition within the energy level that produces the MASER radiation.

Pulsed pentacene MASER 1.45 GHz in 2012

The maser is based on the principle of stimulated emission proposed by Albert Einstein in 1917. When atoms have been induced into an excited energy state, they can amplify radiation at a frequency particular to the element or molecule used as the masing medium (similar to what occurs in the lasing medium in a laser).

By putting such an amplifying medium in a resonant cavity, feedback is created that can produce coherent radiation.

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https://www.london-nano.com/research/characterization-pentacene-based-room-temperature-maser

https://www.nature.com/articles/srep41836

So, what is the big deal with the new MASER?

Maiman was working on producing a MASER but the cryogenic temperatures necessary were a problem and he ended up producing the LASER.

So as mentioned previously the original MASERS needed a magnetic field and cryogenic temperatures so they were bulky, costly and just too difficult to make.

There was no prospect at all of mass production (if there was it would have been done 60 years ago).

The pentacene MASER doesn’t need cooling or a magnetic field.

It can be miniaturised and mass produced

Dr Sathian’s PostDoc years was spent on improving this MASER.

The peculiarity of pentacene was that “pumping” a laser beam into it produced microwaves. However, there was a problem. The output was pulsed, which limited most of the applications.

Most applications needed a continuous stream of radiation.

The crystal was changed to synthetic diamond in 2018.

https://en.wikipedia.org/wiki/Synthetic_diamond

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Synthetic diamonds of various colours grown by the high-pressure high-temperature technique

Synthetic diamond (also referred to as laboratory-grown diamond, laboratory-created diamond, or cultured diamond) is a diamond made of the same material as natural diamonds: pure carbon, crystallized in an isotropic 3D form. Synthetic diamonds are different from both natural diamond, which is created by geological processes, and diamond simulant, which is made of non-diamond material.

The early research of diamond synthesis in the U.S., Sweden and the Soviet Union yielded the discovery of the CVD diamond (chemical vapor deposition).

https://en.wikipedia.org/wiki/Chemical_vapor_deposition

Chemical vapour deposition (CVD) is a vacuum deposition method used to produce high quality, high-performance, solid materials.

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The diamond fluoresces (the red bit in the above image)

https://www.nature.com/nature/volumes/555/issues/7697

https://www.nature.com/articles/d41586-018-03215-3

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https://www.nature.com/articles/nature25970

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Continuous diamond MASER at 9.2GHz and more

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https://www.nature.com/articles/d41586-018-03430-y

The maser goes mainstream: Diamond microwave lasers

https://www.youtube.com/watch?v=S_W7ebTEdqo

The diamond makes the MASER a continuous microwave producer.

Dr Sathian used her expertise on LASERs on the MASER.

Questions and answers part 2

1) Why do we need to fire a laser through the crystal to produce the microwaves? What is it about the crystal?

A-level physics students know about the ground state and energy levels.

https://www.a-levelphysicstutor.com/quantphys-energy-levels.php

In an atom, electrons around a central nucleus can only have particular energy values. These discrete values are termed ‘energy levels’.

In a diagram they are represented by horizontal lines, with the lowest level (the ground state) at the bottom and the highest level (ionisation) at the top.

The levels are a consequence of the wave nature of electrons, as described by quantum mechanics. Around an atom an electron exists as a particular standing wave, with the number of nodes and antinodes dictated by the quantum number ‘n’ .

As an example, take the simplest atom, the hydrogen atom.

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The levels are dictated by concentric wave states of an exact number of electron half wavelengths.

A ‘standing wave’ (or stationary wave) is the interference between two waves of the same wavelength and speed, travelling in opposite directions.

In the example there are 6 maxima and 6 minima. So, the number of half wavelengths is 12.

Therefore, the electron state illustrated has n = 12.

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In the energy level diagram (below), energies have to be measured relative to one another. The ground state is the lowest level and ionisation (the excited state) is the highest. So, you would expect the ground state be zero. However, that is not the case.

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We are measuring potential energy. This is set to zero at infinite distance from the atomic nucleus or molecule. The state is that of an ionised or free electron.

Relative to this state, all the bound electron states have negative potential energy and are measured below it (because they have less energy).

Electron energies are expressed in electronvolts.

An electronvolt (eV) is the kinetic energy acquired by an electron, when accelerated through a p.d of 1 volt. Using E = QV the energy is given by e x 1 = 1.6 x 10-19 Joules.

Quantum numbers are given to each energy level. The ground state is n = 1, with the n numbers increasing by ‘1’ for each level. The highest energy level, where energy is zero, has n = ∞.

Energy level changes

When radiant energy is absorbed by a bound electron it is excited into a higher energy level. Conversely, when an electron falls to a lower level, radiant energy is given out.

Example: an electron moves down from level 3 to level 1 and gives out a photon of light of frequency ν.

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substituting for ν from c = νλ and making λ the subject (c is the speed of light in a vacuum),

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The synthetic diamond has a ground state and an excited state, with energy levels in between. These energy levels are metastable.

https://en.wikipedia.org/wiki/Metastability

In physics, metastability is a stable state of a dynamical system other than the system’s state of least energy.

Some atomic energy levels are metastable. Transitions from metastable excited levels are typically forbidden. This means that any transitions from this level are relatively unlikely to occur. In a sense, an electron that happens to find itself in a metastable configuration is trapped there. Of course, since transitions from a metastable state are not impossible (merely less likely), the electron will eventually decay to a less energetic state.

This slow-decay property of a metastable state is apparent in phosphorescence, the kind of photoluminescence seen in glow-in-the-dark toys that can be charged by first being exposed to bright light. Whereas spontaneous emission in atoms has a typical timescale on the order of 10−8 seconds, the decay of metastable states can typically take milliseconds to minutes, and so light emitted in phosphorescence is usually both weak and long-lasting.

With the synthetic diamond the metastable state splits into three energy states

https://www.nature.com/articles/s41598-019-42323-6

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A simple energy level diagram for the NV− colour centre including a typical emission spectrum.

This metastable state splitting into three states is a peculiarity of this diamond crystal. Transitions between two of these three states gives the microwave emission with specific frequencies. This peculiarity of energy level pumping laser light causes electrons to move up energy levels. When the electrons fall down between these transitions gives the MASER emission.

There is a lot of experimental support for this to happen.

The process is related to the defects in the crystal. Nitrogen vacancies are introduced into the crystal.

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Two of the carbon atoms are removed from the lattice. One is replaced with a nitrogen atom and the other is left vacant. This defect centre produces the microwave emission.

2) What will more precise measurements allow you to find out about gravitational waves?

Gravitational waves are very tiny. So, if the equipment isn’t stable and is affected by the atmosphere you won’t be able to detect them.

The intensities of the signals can vary so you need to have very sensitive equipment to pick up those signals. So, you won’t miss the tiny signals passing through.

Ripples can come from different phenomena such as two black holes colliding or even the Big Bang itself.

https://en.wikipedia.org/wiki/Big_Bang

The Big Bang theory is a cosmological model of the observable universe from the earliest known periods through its subsequent large-scale evolution.

Ripples can have different intensities.

Back to the talk

Applications of MASERs

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Dr Sathian is particularly interested in the use of MASERs for deep-space communications

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The researchers are very good at getting information from A to B, even in challenging circumstances, and you can’t get more challenging than in space.

Using microwave communication, you can get information from the surface of Mars to Earth.

Consider the spacecraft in the outer Solar System trying to send information with a signal power of 10 watts. By the time it reaches Earth the power has dropped to 10-10W. The signal needs to be amplified without amplifying the background noise.

That’s where the MASER comes in. The MASER can amplify the tiny signal without amplifying the background noise.

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The above is an image of the Mars Rover. The background noise means it isn’t as clear as it could be.

https://en.wikipedia.org/wiki/Mars_rover

A Mars rover is a motor vehicle that travels across the surface of the planet Mars. The noise is electromagnetic.

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We are all familiar with sound noise. As a retired teacher I experienced a lot of it.

Noise also affects electrical equipment. Phones can be affected.

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Interference needs to be solved. There is a close link between the economic growth of a nation and the structure with respect to microwave communication.

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The problem is the signal to noise ratio

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You want to be able to retrieve the signal from the noise and clean it up. Amplifying the desired signal without amplifying or adding noise.

Why is the MASER not used everywhere? What is the hindrance?

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So, the MASER couldn’t be used in 1954 because it was too bulky and expensive.

The MASER created by the Imperial tram is stable, small and has more uses.

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The diamond and pentacene MASERs can be as small as 1cm and the research team are aiming to get down to a mm scale and added to a circuit board. It will have many applications.

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Using a MASER gave fantastic images of the Martian Rover.

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It could also give fantastic pictures of tiny grains of Martian terrain

Including tiny MASERs on a circuit board would enable increased biochemistry, medical investigations etc. and disease diagnosis besides the astronomical research.

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The star trek tricorder could soon be with us !!!!!!!!!!!!!!!!

Dr Sathian is continuing with her MASER, LASER as well as Luminescent Concentrator research at Northumbria

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https://en.wikipedia.org/wiki/Chrysoberyl

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The mineral or gemstone chrysoberyl is an aluminate of beryllium with the formula BeAl2O4. Alexandrite is one of the varieties.

https://www.alexandrite.net/chapters/chapter7/the-alexandrite-laser.html

https://en.wikipedia.org/wiki/Luminescent_solar_concentrator

A luminescent solar concentrator (LSC) is a device for concentrating radiation, solar radiation in particular, to produce electricity. Luminescent solar concentrators operate on the principle of collecting radiation over a large area, converting it by luminescence (specifically by fluorescence) and directing the generated radiation into a relatively small output target.

https://www.researchgate.net/publication/228759565_The_luminescent_concentrator_illuminated

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Schematic 3D view of a luminescent concentrator.AM 1.5 light is incident from the top. The light is absorbed by a luminescent particle. The luminescence from the particle is randomly emitted. Part of the emission falls within the escape cone (determined by the angle (a)) and is lost from the luminescent concentrator at the surfaces (1). The other part of the luminescence is guided to the Si cell by total internal reflection (2)

Dr Sathian uses her experience with LASERs and MASERs in her teaching

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Space communication; studying material science including crystals; LIDAR; Medical applications.

Dr Sathian can apply her expertise in a lot area and enables her to collaborate with a lot of other scientists. Coming up with new ideas and inventions. The beauty of photonics research. Broadly speaking she is an optics and photonics scientist.

Optics careers

Optics is the branch of physics that studies the behaviour and properties of light. Optics professionals tend to have a strong background in physics and mathematics.

Some exciting jobs in optics and photonics include:

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Some physics careers

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Questions and answers part 3

1) How do MASERs reduce the noise?

A normal amplifier distorts the signal.

When the tiny signal goes into the amplifier it is amplified but it is distorted by the problems in the amplifier.

If the signal is fed into the MASER it will amplify and produce a clear signal without distortion because of the property of the MASER crystal.

The principle of simulation emission produces much less noise.

2) MASER amplifier is under development in order to make it a more compact device.

The continuous wave room temperature MASER will have many applications when fully developed.

3) Why did the pentacene produce a pulse signal while the diamond produced a continuous signal.

Its about the energy levels and decay rates. How long can the atom stay in the excited state before it decays back to its ground state.

With pentacene the decay rate isn’t so great. With the diamond the electrons can stay in the excited state for a long time. The performance depends on the decay rate from the higher energy levels. This makes a huge difference.

4) Will you be able to produce LASERs and MASERs with different energy levels/bands and spectra?

The continuous diamond MASERs could do this. Applying a magnetic field to the diamond can split the energy levels differently.

You can fine tune the magnetic field to produce spectra with different frequencies.

5) Do you think there are more materials that could be better for LASERs and MASERs that haven’t been discovered yet.

Yes, and at the moment Dr Sathian is working on a patent so she didn’t want to discuss it.

Her research group will be exploring more materials that have the same property as diamond and materials that will produce different wavelengths.

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