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practical, photon, absorption, phonon, valence, vector, schematic, diagram, variation, technological, interest, radiative, coefficient, potential.

1. Combine the appropriate words from the two columns to obtain terms. Translate them:

TEXT 5B SEMICONDUCTOR LASERS

Introduction. Semiconductor lasers consists of injection lasers, where a p-n junction or heterojunction is used to inject excess carriers into the active region, optically pum­ped lasers, where an external light source produces excess car­riers, and electron-beam pumped lasers, which use high energy electrons to produce excess carriers. Injection lasers, which are the most practical devices, are discussed at length⁷ in this review.

Operating principles. In this section, we review a few of the key concepts concerning laser action in semiconductors. Extensive theoretical treatments of this subject can be found elsewhere⁸.

Direct and indirect bandgap semiconductors. In direct bandgap semiconductors (the only ones in which sti­mulated emission has been observed), both photon emission and absorption can occur without the need for a phonon to conserve momentum. This is because the lowest conduction band minimum and the highest valence band maximum are at the same vector (k) in the Brillouin zone⁹. Figure I shows the schematic diagram of electron energy vs. k in a semiconductor, such as GaAs, where the smallest bandgap energy E_g = E_c – E_v at k = [000].

In indirect bandgap semiconductors the conduction band minimum and valence band maximum are not at the same k value. Hence, photon emission and absorption require the participation of phonons to conserve momentum. A schematic diagram of an in­direct bandgap semiconductor such as GaP or AlAs is shown in Figure 2. In these semiconductors the lowest-lying conduction band minima are along k= [100].

Lasing in indirect bandgap semiconductors is improbable because the lowest-energy band-to-band transition probabilities are much smaller than in direct semiconductors. Thus, the ra­diative lifetime is long. Because of the relatively long life­time of electrons in the indirect minima, there is time for nonradiative recombination processes to occur, thus yielding low internal quantum efficiency. Furthermore, the stimulated recombination rate is related to the band-to-band absorption coefficient. Since the coefficient is lower for indirect than direct transitions, the potential laser gain is correspondingly reduced.

2200 п.зн.

Words to be learnt:

key concepts – основные понятия;

to treat – трактовать, рассматривать;

treatment – трактовка;

to yield – производить, приносить результаты, давать.

Exercises

1. Read the following nouns. Say which verbs they are derived from:

conductor, semiconductor, conduction, conductivity; injection, emission, absorption, participation, composition, treatment, transition, recombination.

1. Match synonyms:

absorb, conserve, combine, occur, consume, watch, mix, observe, preserve, happen.

1. Translate the adjectives below paying attention to the negative prefixes:

direct – indirect, efficient – inefficient, convenient – inconvenient; probable – improbable, practical – impractical, possible – impossible; radiative – nonradiative, nuclear – nonnuclear, con­ducting – nonconducting; continuous – disсontinuous; fortunate – unfortunate; comfortable – uncomfortable.

1. Translate the following word combinations avoiding prepositions:

внешний источник света, лазеры с накачкой электронным пучком, излучение и поглощение фотонов, зона проводимости, вероятность перехода, переход с уровня на уровень, вероятность перехода с уровня на уровень, коэффициент поглощения.

1. Which of the statements below are true according to Text 5B:

1. In injection lasers high energy electrons are used to produce excess carriers. 2) In indirect band gap semiconductors both photon emission and absorption can occur without the need for a phonon to conserve momentum. 3) Lasing in indirect bandgap semiconductors is improbable becau­se the lowest energy band-to-band transition probabilities are much smaller than in direct semiconductors. 4) In direct bandgap semiconductors the conduction band minimum and valence band maximum are not at the same k value.

1. Translate the sentences below paying attention to the functions of the words which are underlined:

1) The total amount of radiation absorbed from broadband pump sources clearly increases with ion concentration in a given size host crystal. 2) The growth of the density modulation gives in­creasing coherence to the scattering process resulting in a growing scattered wave, which in turn increases the density modulation still further. 3) Laser diodes’ degradation manifests itself primarily in an increase in threshold current although other parameters may also change. 4) Increasing the peak-power output of a laser is constrained by the optical damage¹⁰ properties of the laser medium itself or of the optical materials required to make the laser operate. 5) Laser - pumped glass oscillators provided wavelength versatility¹¹ because of their wide fluorescence bandwidth. 6) Provided the velocity dis­tribution of the electrons in the beam is carefully selected, the radiation emitted by each electron adds coherently to the radiation from other electrons in the beam.

1. Translate the sentences below with a special attention to the Verbals:

   1. If the electron velocity is close to the speed of light, long wavelength imposed fields can be used to build FELs operating in the visible region of the spectrum. 2) As large-scale commercial applications of lasers become more numerous and mature¹², additional cost scaling models and data bases are sure to become available in the field. 3) By varying the composition of a semiconductor diode it is possible to adjust the wavelength of its spectral gain peak. 4) The purpose of these dye absorption curves is to assist the user in selecting the laser pump source which will most effectively pump the dye la­ser. 5) Several molecular lasers should be mentioned when dis­cussing tunable lasers. 6) When placed in a suitable cavity¹³, the device (FEL) will radiate coherently. 7) After the discovery of the dye laser by Sorokin and Landkard, numerous reports followed, most of them detailing the study of various classes of fluorescent organic materials.

2. Answer the questions about Texts 5A and 5B:

   1. What are the classes of laser sources? 2) What are the types of gas lasers? 3) What excitation methods are used to pump gas la­sers? 4) What are the types and methods of pumping liquid lasers? 5) What excitation techniques are used to pump lasers utilizing dielectric insulators? 6) In what way are excess car­riers produced in semiconductor lasers (injection lasers, optically pumped lasers, electron - beam pumped lasers)? 7) What is the difference between direct bandgap semiconductors and indirect bandgap semiconductors? 8) In what type of semicon­ductors do photon emission and absorption require the partici­pation of phonons to conserve momentum? Why? 9) Why is lasing in indirect bandgap semiconductors improbable?

3. Write an abstract of Texts 5A and 5B.

4. Use Table 2 and Figures 1 and 2 to talk about:

1. Types of lasers; b) Direct bandgap semiconductors vs. indirect bandgap semiconductors.

1. Read Text 5C without a dictionary (time limit – 4 minutes) and answer the questions that follow:

a) Каковы преимущества стекла перед кристаллическими материалами?

b) О каком недостатке стекла упоминается в этом тексте?

TEXT 5C GLASS LASERS

Lasers made from vitreous¹⁴ and crystalline materials comprise the two classes of solid state lasers. Their different material properties are complementary for use in lasers. Because of their lower cross sections, glass lasers store energy well and thus make good short pulse lasers and amplifiers. On the other hand, crystalline materials are better for cw oscillators and amplifiers because of their higher gain and good thermal conductivity.

Glass has advantages over crystalline materials. It can be саst¹⁵ in a variety of forms and sizes, from small fibers to meter–sized pieces. Tremendous flexibility in choosing glass and laser properties is afforded by the ability to vary the glass composition over very large ranges. Glass is also relatively inexpensive because of the shorter time required for its manufacture and the use of inexpensive chemical compo­nents. Further, large pieces of laser glass can be made with ex­cellent homogeneity, uniformly distributed rare earth concentrations, low birefringence, and can be finished¹⁶ easily, even in large sizes. The only major drawback of glass is its low thermal conductivity, which limits its appli­cability in high average power systems.

1200 п.зн.

1. Translate Text 5D in writing using a dictionary (time limit – 40 minutes):

TEXT 5D X - RAY LASERS

Research toward advancing lasing to the X-ray spectral re­gions is in an early and progressive state.

The challenge of inventing and developing X-ray lasers may be approached by a) adapting familiar X-ray sources to lasing action; b) extending proven ion laser processes progressively toward shorter wavelengths, perhaps through isoelectronic extrapolation; c) discovering new pumping and emission processes more appro­priate to the task.

With potential applications in the vacuum-ultraviolet spec­tral region seemingly limited as compared to those for the penetrating X-ray region, early thoughts were directed toward making the big leap to the X-ray and perhaps γ-ray regions. Formidable pumping problems were projected. Meanwhile advance­ments into the ultraviolet regions, accompanied by rising uses and interests as specific devices have emerged, seem to indicate that the more reasonable approach is the continued systematic advance toward shorter wavelengths. Indeed, over the past 12 years the so-called short-wavelength “barrier” has been pushed from 200 nm into the vacuum region - first near 100 nm, and presently it appears that 60 nm has been reached. These advances have been achieved both with cavities and in the amplified spontaneous emission (ASE) single pass mode, where the latter requires considerably higher gain.

1400 п.зн

SUPPLEMENTARY READING

Irnee D'Haenens dies; assisted Maiman in building the first laser

January 4, 2008, Los Angeles, CA--Irnee D'Haenens, a physicist who assisted Ted Maiman in making the first laser at Hughes Research Laboratory (Malibu, CA) in 1960, died December 24; he was 73. The two were the only people present when a little ruby rod emitted the world's first pulse of laser light on May 16, 1960. Later, D'Haenens called the laser "a solution looking for a problem," a joke that became common in the early years of the laser era as developers sought laser applications.

Born in Mishawaka, Indiana, the son of a service-station operator, D'Haenens spent his entire professional career at Hughes, starting while he was earning a masters degree from the University of Southern California. He received a Hughes doctoral fellowship and earned his PhD from the University of Notre Dame in 1966. As a member of the technical staff at Hughes, he worked on semiconductor physics, microwave technology, and spectroscopy as well as lasers before retiring in 1989. A long-time Hughes colleague, David Pepper, recalled D'Haenens as "as a wise and learned uncle who helped me travel along my path in life," whose first priority was always his family. He is survived by his wife Shirley, four children, 19 grandchildren, and three great-grandchildren.

(http://www.laserfocusworld.com)

New camera on Subaru Telescope may directly observe exoplanets

The Subaru Telescope, located on the summit of Mauna Kea, is dedicated to exploring the cosmos, gaining a deeper and more thorough understanding of everything that surrounds us. With an 8.2-meter mirror and a suite of sophisticated instruments, astronomers at the Subaru Telescope explore nearby stars looking for planetary systems. A giant step towards this goal was made recently with the "first-light" inauguration of a new state-of-the-art camera.

Subaru uses eight innovative cameras and spectrographs optimized for various astronomical investigations in optical and near-infrared wavelengths. On the night of December 3, 2007, the High Contrast Instrument for Adaptive Optics (HiCIAO) camera was brought to life. The HiCIAO is a technologically adaptable system that will replace the infrared Coronagraphic Imager with Adaptive Optics (CIAO) unit in operation since April 2000. Both systems are designed to block out the harsh direct light from a star, so that nearby faint objects such as planets can be viewed. The new system benefits from a contrast improvement of ten to 100 times, allowing astronomers glimpses into regions never explored.

A further advantage of the HiCIAO camera is that it will be used in concert with an adaptive optics (AO) system that was recently significantly upgraded, which, in turn, increased the clarity of Subaru's vision by a factor of ten, opening up more of the night sky to observing. The new AO system uses 188 actuators behind a deformable mirror to remove atmospheric distortion, allowing the Subaru Telescope to observe close to its theoretical performance limits. In addition, a laser guide-star system was installed to enable observations of tiny regions of sky without bright stars to steady the AO system on.

The HiCIAO system, initiated in 2004, was developed by a team of scientists and engineers from the Subaru Telescope, National Astronomical Observatory of Japan, and the University of Hawaii's Institute for Astronomy. Dr. Ryuji Suzuki, a Subaru astronomer leading the HiCIAO project, says "the unique instrument was primarily designed for the direct detection of extrasolar planets and disks." The system's design allows for high-contrast coronagraphic techniques in three observing modes: direct imaging, polarization differential imaging, and spectral differential imaging. HiCIAO directly detects and characterizes young extrasolar planets and brown dwarfs, sub-stellar objects that occupy the mass range between that of large gas giant planets (e.g. Jupiter), and the lowest mass stars. With the aid of the laser guide-star AO system, HiCIAO targets dim objects including young stars, protostars, and star-forming regions.

HiCIAO is also extremely useful for detecting faint dust disks around nearby stars, and for studying small-scale and inner disk structures and dust grain properties, both of which lead to a clearer understanding of extra-solar planetary systems and their evolutionary processes. Dr. Suzuki reports that "although we already know of more than 250 extrasolar planets, they have all proven their existence indirectly by the Doppler or transit method. Because the direct imaging of an extrasolar planet has never been done, if it happens, that will be exciting." Subaru Telescope may be the first to directly observe a planet outside our solar system. (http://www.laserfocusworld.com)

Terminology

1. ground state – основное состояние системы; steady state – стационарный режим;

2. self - terminated operation – пичковый режим (в отличие от стационарного);

3. relaxation time – время релаксации (жизни);

4. mode – мода, тип колебаний; mode-locking –синхронизация мод;

5. to store – хранить, запасать, накапливать; storage – память, накопление;

6. Q- switching – модуляция добротности;

7. cavity dumping – затухающие колебания ;

8. curve –кривая линия; gain curve – контур усиления;

9. performance – работа, интенсивность работы, рабочие характеристики;

10. to saturate – насыщать; saturation flux – поток насыщения,

Preliminary exercises:

1. Read and translate without a dictionary:

thermal, system, stimulate, integrate, intense, alternately, nominal, radioactivity, signal, combination, diode, orange, rhodamine, collectively.

1. Translate the word-combinations that follow:

medium - laser medium, gain medium, pulse-pumped laser medium, energy-storage medium; level - laser level, ground level, lower laser level, upper laser level, three-level laser system, upper laser level relaxation time, higher-lying pump level; density - average power density, population inversion density, input (output) power density.

1. Find equivalent phrases either in Text 6A or in the right-hand column:

1. Read Text 6A and answer the following questions:

1. От чего зависит режим действия лазера?

2. При каких условиях лазер работает в стационарном режиме?

TEXT 6A PROPERTIES OF SOME IMPORTANT LASERS

Характеристики

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