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Here, in particular, is where advances in optical science are the rate-limiting step in advancing the understanding of chemistry. A further step could be direct visualization of the nuclear dynamics during such reactions by means of x-ray spectroscopy, and optics would play an enabling role in that as well.

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The first femtosecond x-ray pulses have recently been produced by Thomson scattering of femtosecond laser pulses by an accelerated electron beam. Efforts to apply these pulses to questions of microstructural dynamics are under way. Other possible applications include the study of protein folding using time-resolved x-ray scattering.

The current rapid advance in femtosecond technology is made possible by recently invented methods for ultrashort pulse generation with solid-state lasers. Early pioneering work with dye lasers opened the femtosecond time domain to science and laid many of the foundations for present technological advances. Solid-state technology now brings greater reliability, shorter pulses, and higher powers to scientific applications.


It also opens the door for more widespread commercial application by making femtosecond sources more efficient, more compact, and less expensive. Each is important for a somewhat different type of application. What they have in common, which sets them apart from previous technologies, is that they are no longer simply determined by the properties of basic materials. They can be optimized artificially, engineered, and enhanced by synthetic design techniques. Opportunity still exists for the discovery of new enabling. The first particle accelerators, developed in the s, explored the properties of the atomic nucleus.

The accelerator is basically a transformer, in which electrons gain energy as they traverse the voltage applied across the structure. A key breakthrough in accelerator design was the use of voltage at radio frequencies in the cyclotron to accelerate electrons. This was followed by the use of microwave frequencies in a linear accelerator to reach electron energies of billions of electron volts.

Today most accelerators operating in the world are driven by microwave radiation.

Techniques, Materials and Devices

Conceptual designs for the Next Linear Collider, based on high-frequency microwave radiation, call for a km linear collider with GeV colliding particle energy. Livingston studied the evolution of particle accelerator technology and noted that each new step toward high-energy involved a new design approach or a new source of high peak power electromagnetic radiation. One new approach to accelerator technology is to use the laser to generate future tera-electron volt TeV accelerators.

Recent success in experimental laser acceleration has been achieved by a group at the University of California, Los Angeles, using a plasma structure Joshi and Corkum, However, not all researchers think that this approach will be practical. Laser-driven TeV accelerators, if constructed, would allow probing of fundamental particles that were formed in the earliest few moments at the birth of the universe.

Why the universe is formed of matter and why we are here are questions that could be studied with the next generation of high-energy particle accelerators. Today, terawatt 10 12 W peak power lasers operate on a table top. In the future, these lasers would operate at high-efficiency and high average power to drive kilometer-long TeV linear accelerators. Research into the optical materials for these ultrafast lasers and into devices for femtosecond pulse manipulation continues to play a crucial role in the development of this emerging technology.

New laser crystals for different wavelengths, improved nonlinear optical materials, novel active and nonlinear fiber devices, and a better understanding of light-matter interactions are all still needed to nurture the technology's success and widen its impact. To date, the principal beneficiaries of developments in femtosecond technology have been scientific applications, but the prospect of compact, practical, cost-effective femtosecond laser sources is now creating opportunities for a variety of mainstream commercial applications.

Some take direct advantage of the ultrahigh-speed aspect of the technology. Others derive benefit from related aspects such as broad spectral width, high focused peak intensities, phase coherence, improved. Optical communication systems have already benefited from advances in ultrafast optics, and they remain a fertile domain for new applications based on ultrashort pulses, such as soliton transmission, all-optical switching and networking, wideband wavelength-division multiplexing WDM , ultrahigh-rate data processing, and clock distribution. Femtosecond laser sources with high peak and average power open the possibility of using a laser to accelerate electrons in place of the microwave source traditionally used for this application.

Laser sources with higher peak power and shorter wavelengths Box 7.

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If successfully developed over the next two decades, the laser-driven particle accelerator may open new avenues for studies of the physics of fundamental constituents of matter at the TeV energy scale Figure 7. Measurement technologies such as device and circuit testing, optical ranging, surface monitoring, and microscopy can take advantage of the peak powers and low temporal coherence of femtosecond pulses as well as their short durations.

Low-coherence confocal microscopy and two-photon microscopy are examples of rapidly developing applications, as are biomedical imaging by optical coherence tomography and laser surgery. The feasibility of combining ultrafast microscopy with the measurement of ultrafast optical response will expand the list of possible applications. A particularly novel and unexpected application of femtosecond light pulses is the generation of terahertz radiation, or T-rays.

T-rays in the wavelength range of 0. Over the last 35 years, peak power has increased by 11 orders of magnitude. Courtesy of P. Pronko, University of Michigan. The average acceleration gradient is calculated to be 0.

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Nuss, Lucent Technologies. Successful development of this and other emerging applications will depend on the availability of cheap and versatile optical pulse power supplies. The availability of such supplies can also be expected to stimulate still other applications, as yet unforeseen. Femtosecond optics and sources offer an opportunity for dramatic impact on science and technology. Agencies should focus attention on this opportunity and encourage innovative work in this cutting-edge field. Lasers are now essential to the national economy, enabling applications that extend from CD-ROM to the fiber information network; from the processing and fabrication of semiconductors to the cutting of cloth, plastic, and industrial materials; and from laser vision correction to theraputic medicine.

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Laser sources have made a transition from the gas discharge tubes associated with the ubiquitous red helium-neon lasers used in supermarket scanners and the blue-green argon ion lasers familiar in laser light shows to solid-state laser diodes LDs and advanced solid-state lasers. This transition to the solid-state will have a profound impact on the future growth of laser markets. Laser diodes and solid-state lasers can be produced at low cost and operate at high electrical efficiency with high reliability.

One consequence of the transition from discharge tubes to the solid-state is lower production costs because of the increasing volume of production. Moore's law for the exponential increase in the number of transistors on a wafer, a consequence of the lithographic production techniques applied to silicon, is for the first time applicable to lasers.

We can expect that laser diodes and solid-state lasers will be produced at ever-increasing volume and ever-decreasing cost as the markets continue to expand. This in turn will lead to a continued growth of many new laser applications. Lasers diodes based on the semiconductor gallium arsenide GaAs , an extension of silicon technology, are widely used 30 years after the invention of the laser.

GaAs laser diodes operate from the red to the near-infrared IR at power levels from milliwatts to multiple watts. The GaAs material technology is more complex than silicon-based material technology, but lithography-based planar-processing technology developed for the silicon industry is applicable to GaAs laser diode production, so laser diodes can be manufactured in large volumes at low cost.

The cost of laser diode sources decreases rapidly with production volume. This low cost, along with reliable operation and small size a grain of sand is larger than the diode laser source , opens many new markets for laser diode devices. Applications of diode lasers include data storage, optical displays Box 7. Of course, the laser source is a critical and enabling element in a larger system.

The growth and market size of laser-based systems continue to track the growth and market size of computer-based systems with a delay of about 15 years. Research into organic-based LEDs is now under way in many laboratories. Organic LEDs, manufactured from polymer films that are patterned and electrically excited directly, offer the promise of very low-cost optical displays that are like paper in flexibility, cost, and ease of use.

Progress in research toward brighter, longer-life, organic-based LEDs continues to show promise Kido et al. Porous silicon emits light that is broadband in nature and rather weak in output power. Nevertheless, silicon-based LEDs offer promise as inexpensive sources of light for speciality applications such as alphanumeric indicators directly on the silicon circuit based electronic component Collins et al.

Laser diodes based on GaAs are the key to the future growth of the laser markets. GaAs refers to not just one material system but a complex set of material systems based on extensions of GaAs. The progress in wavelength extension of diode lasers from the near IR to the blue depends critically on the development and understanding of the III-V semiconductor material systems. Today, commercially available laser diode products operate from the red to the near-infrared.

Power levels range from 10 mW for reading compact disks to more than 20 W for pumping solid-state lasers. The laser diode is more efficient and can operate at higher output power than the light-emitting diode LED; Box 7. Further, the laser diode has a coherent output beam that can be focused into an optical fiber or onto the surface of a CD. Like LEDs, laser diodes are being improved and extended in wavelength. Today, the critical area of research is to extend the laser diode wavelength into the blue.

The first realization of a blue laser diode based on GaN gallium nitride , a material closely related to GaAs, was announced by the Japanese company Nichia in The GaN material system is not as well developed as that of GaAs. It took 10 years from the first demonstration of a room-temperature GaAs red laser diode to the introduction of commercial laser diode products. A similar time might be required before blue laser diodes are available commercially. When introduced as a product, the blue laser diode will open major application areas including information storage and optical displays.

Other applications include fluorescence microscopy for biological and medical applications. There is significant economic leverage for the development of blue laser diodes for application to the optical storage systems of the future.

Today companies in the United States lead the world in the manufacture of high-power LD sources. High-power laser diodes are used directly for medical and industrial applications and to pump advanced solid-state lasers. The leading producers of laser diodes for information storage, such as CD players, are located in Japan as is the first company to demonstrate the GaN blue laser diode. LEDs are less efficient and less powerful than laser diodes but are also much less expensive. LEDs are familiar as indicator lights in electronics and as red brake lights in some automobiles. However, they will soon be found in signal lights, road signs, exit signs, and lighting displays see Chapter 3.

The market for LEDs is larger than the laser market and is growing rapidly because of the small size, long operating life, high visual brightness, and low cost of LEDs. As blue LEDs become commercially available, three-color displays will become possible and will add to the growth of the multibillion-dollar LED market.

Applications for blue LEDs include color displays, specialty lighting, and medicine and biology where blue LEDs induce fluorescence in molecules for detection. For example, an array of blue LEDs has been used for research on the treatment of jaundice in newborn babies.

Visible laser diodes are used for information storage, bar-code readers, and laser alignment in construction. High-power laser diodes are used for materials processing, soldering, xerographic image generation, and medical applications. Laser diodes coupled to optical fibers enable direct processing of materials through cutting, welding, and annealing.

High-power laser diode arrays are also widely used today to pump advanced solid-state laser sources. Advanced solid-state lasers play a unique, significant, and growing role in scientific and industrial applications.