Case study: Lasers
Lasers provide the archetypal example of how a discovery in basic physics led to an invention, several decades later, that was unpredictably world-changing.
What are lasers?
Lasers are devices that emit narrow beams of intense electromagnetic radiation (light). The term laser originated as an acronym for “light amplification by stimulated emission of radiation”. A laser beam has the special property that the light waves emitted are all in step with one another – coherent – and usually of one wavelength, or colour. There are many different kinds of lasers, from giant installations emitting powerful pulses of high-energy radiation, such as X-rays, to tiny devices etched onto semiconductor chips producing infrared light.
Many different kinds of material can be made to “lase” – such as gases, crystalline solids, glasses and polymers – and which one is used depends on the application. Some lasers are designed to emit a continuous beam while others can spit out rapid pulses of light that are ultra-short. The wavelengths of light generated by certain types of laser can even be “tuned” for specific applications, making them extremely versatile.
Lasers offer a way of generating, controlling and directing intense light in remarkable ways, yet when they were first invented, physicists were not sure what they could be used for – they were famously described as a “solution looking for a problem”. In fact, although the first laser was constructed in the 1950s, practical applications did not appear until a couple of decades later – as is often the case in science. Since then, thanks to research activity in both university physics departments and companies, including those in the UK, lasers have become ubiquitous and are central to many technologies that are used in manufacturing, communications, medicine and entertainment.
Today, lasers are key tools in manipulating and communicating information (in CD and DVD players, supermarket barcode readers and broadband telecommunications), in measurement (surveying and environmental studies), chemical analysis (of foods, medical specimens and materials) and, increasingly, in transforming materials (welding, cutting and etching, printing, and surgery).
Research into lasers continues apace – new types of laser are being developed with a variety of characteristics and potential applications. In some cases, the result is a cheaper, more compact portable device designed for a specific use, or a more powerful laser used to generate power, for instance. UK university physics departments are at the forefront of many of these areas. In particular, physicists in the Central Laser Facility (CLF) at the Rutherford Appleton Laboratory develop novel high-powered laser systems and make them available for both pure and applied research.
The laser would never have been developed without a profound understanding of an area of fundamental physics – quantum theory. The principle behind the laser goes back to the world’s most famous physicist, Albert Einstein, who in 1917 proposed a theory of stimulated light emission. Einstein had previously shown that light was composed of tiny packets of wave energy called photons (the wavelength depending on the energy).
He theorised that if the atoms that make up a material are given excess energy and so emit photons, these photons could stimulate nearby atoms to emit further photons, creating a cascade effect. All the photons would have the same energy and wavelength and move off in the same direction.
However, it was not until 40 years later that physicists were able to convert this idea into a practical laser. The principle is that the “active” material has first to be pumped with energy from another light source or an electrical current. The resulting stimulated light emission is then amplified by bouncing the light back and forth through the lasing material in a mirrored cavity, so stimulating more emission, before it escapes through a transparent mirror section as a laser beam. A device that amplified microwaves was constructed in 1953 by Charles Townes and colleagues at Columbia University. Townes shared a Nobel Prize in Physics in 1964 with Nikolay Basov and Aleksandr Prochorov of the Lebedev Institute in Moscow (who independently also demonstrated what came to be called a maser).
The next few years saw a race to build the first visible light laser. Theodore Maiman at Hughes Research Laboratories in California pipped Townes and his team at the post when he built the first working laser in 1960 using ruby as a lasing medium – although who should be credited for the laser’s invention was then hotly contested.
Initially the laser concept was not taken very seriously, nevertheless the 1960s saw a huge expansion in laser research including the development of high-power gas lasers, chemical lasers and semiconductor lasers. However, they were still rather specialised research tools. By the 1970s, semiconductor lasers that worked at room temperature had been developed and this led to the advent of the compact disc (CD).
Without the discovery of lasers, the entire fundamental field of cold atoms would never have opened up. Research in this field has led to the award of several Nobel Prizes in Physics, including the discovery of Bose–Einstein condensates (BEC). BEC has opened the door to a host of applications such as atom lasers, improved atomic clocks and quantum computers.
Today, semiconductor diode lasers are the most common type, found in industry, commerce and the home.
- Information Technology
The largest application of lasers is in optical storage devices (e.g. CD and DVD players), in which a focused beam from a semiconductor laser, less than 1 mm wide, scans and reads the disc surface. Other everyday uses include barcode readers, laser printers and laser pointers. Over the past 25 years the publishing and newsprint industries have been revolutionised by the use of lasers, which have replaced traditional “hot metal” printing.
The second largest application is in fibre-optic communications. Broadband depends on the transmission of light pulses alongoptical fibres, which are generated and relayed via lasers. This is made possible by fibre amplifiers, invented in the UK, which are an important component in long-distance fibre links.
Lasers can deliver concentrated energy in the form of fine controllable light beams, so physicians soon took advantage of them to perform micro-surgery, which involves less pain and scarring, lower blood loss and shorter recuperation time in hospital. Laser beams delivered via flexible optical fibres allow surgeons to reach inside the gut, for example, and seal a bleeding ulcer. One of the most publicised uses of lasers is in eye surgery to treat disease and, increasingly, improve bad eyesight.
Lasers can deliver enough power to heat and melt metal joints, and so are used for welding, as well as for cutting. When controlled by a computer, a laser can cut complex designs into a material such as wood or paper, as is increasingly being seen in furniture and other home goods.
- Measurement and analysis
Lasers have long been used by the military for range-finding, but now even estate agents employ laser tape measures. Because lasers can be tailored to produce specific wavelengths, they are used to analyse chemical and physical structure, and so are used in factory quality control and to monitor environmental pollutants remotely. Lasers can be used for a type of measurement called interferometry which can measure tiny changes in distance.
- Scientific research
Virtually every university science department in the UK relies on lasers for some aspect of its research programmes – they have become indispensible research tools. Without lasers, many recent discoveries would never have been made, which illustrates the synergic relationship between developments in physics and other fields. Lasers interact with matter at the quantum level in very specific ways and so are important probes in research. They can be used to follow chemical reactions and elucidate structure at the atomic and molecular scale. Increasingly, life scientists are employing lasers in new types of microscopy designed to highlight cellular structures.
Physicists are continually developing new lasers and many UK teams are involved in these projects. These include nanoscale devices that emit light and that are expected to find use in chemical and biological sensors on “lab-on-a-chip” devices. The University of St Andrews, for example, has developed laser optical tweezers to manipulate biological cells to contribute to the burgeoning area of biophotonics. Several UK research groups are developing a new semiconductor laser called the quantum cascade laser, which promises to be an excellent source of terahertz radiation (between infrared and microwaves) now being introduced for national security screening. New laser technology will also play a role in developing the all-optical computer.
Researchers at the universities of Bath and Southampton pioneered a type of laser based on micro-structured optical fibres, which can produce light across the entire visible spectrum. Fibre lasers can be made to emit low-power light, allowing physicists to manipulate single photons. These are needed for fundamental experiments aiming to explore strategies underpinning the developing concept of quantum computing, which would allow the processing of unbelievable amounts of data, and also quantum cryptography, which offers an ultra-secure means of transmitting data.
Fibre lasers may also provide the next generation of very high power devices, producing X-rays for many kinds of enabling research, particularly in the life sciences. The European X-ray free electron laser (XFEL), a large facility being constructed in Germany, is expected to offer X-rays at intensities not achieved before, and the UK is supporting this project. The UK’s CLF also hopes to host the world’s most powerful laser set-up, HiPER, which could demonstrate nuclear fusion as a potential clean, renewable source of energy.
Looking further ahead, researchers are undertaking nuclear physics research that could eventually lead to a gamma-ray laser to store nuclear energy, while exploitation of the atom laser might produce a whole gamut of new, and probably unanticipated, applications for the future.
Lasers have become a multi-billion dollar industry. Even by 1994, the US National Academy of Sciences estimated that the economic impact of laser technology was $100 bn a year. In 2004, excluding diode lasers, about 131 000 lasers were sold worldwide with a value of $2.19 bn, and about 733 million diode lasers valued at $3.2 bn. In 2008, the rapidly growing fibre-laser market alone was worth $240 m and is expected to reach $500 m by 2011.
Lasers are one of the most important enabling technologies to have been developed in the past 50 years and it is difficult to evaluate their impact. Lasers not only drive the modern information economy, allowing data to be transferred quickly across the internet and to be stored economically and efficiently, but they are also an essential research tool without which modern science, technology and medicine would not progress.