Рубрика:
18
signal reaches … certain threshold, … gate opens and lets … incoming
electrical flow from … larger power supply go through.
2. … job of … transistor is, therefore, to amplify or attenuate … incoming
electrical signal depending on … controlling, lower-intensity current.
3. … molecules can thus emit … light in … process called … stimulated
emission: when bombarded with … photons of … correct energy, … electrons
within … molecule can reach … excitation state and then come back to normal
by releasing new, … identical photons.
Text 5
Exercise 1. Make adjectives with negative meaning using prefixes (un-, in-, im-,
dis, non-) from the given ones and translate them:
Efficient, limit, appropriate, possible, altered, usual, important, useful, real, like,
clear, known, exceptional.
Exercise 2. Fill to or with, them make sentences.
Accustomed … sth, acquainted …sb/sth, bear…sb, coincide… sth, differ … sb
(=disagree), engaged … sb, friendly… a cause, patient…sb, unequal…sth.
Part 2
What is "an appropriate medium"? Molecules can emit light through a process
called stimulated emission — the same working principle of lasers: an electron, after
being perturbed by a photon with the proper energy, emits a second photon with the
same phase, frequency, polarization, and direction of travel as the original. However,
obtaining stimulated emission from a molecule with good efficiency, "is tricky from
the experimental point of view," Sandoghdar points out. "In fact, stimulated emission
in molecules is usually an inefficient process, because light is not strongly focused
and single molecules do not have a broad cross-section. By going to low temperature
in order to increase the molecular cross-section and by focusing light to diffraction
limit, we achieved the higher efficiency needed to perform our experiment."
In order to show the single molecule optical transistor at work, two laser sources are
needed. The first laser pumps the electrons of the molecule in an appropriate energy
state; a second laser sources then stimulates the secondary emission of photons from
the molecule. In these terms, the first laser works like the control current in a
transistor. It can, therefore, decide when to open or to close the molecular gate, i.e.
when the molecule only absorbs the photons of the second laser or when it emits
more by stimulated emission. "Three actions are, therefore, possible on the second
beam, i.e. to attenuate it up to ten percent, leave it unaltered or amplify it up to one
percent." Even though this efficiency might sound very low, Diederik S. Wiersma,
leading the group of Optics of Complex Systems at LENS (European Laboratory for
Non-linear Spectroscopy in Florence, Italy), explains that "you need to compare this
result to other non-linear effects where usually not more than one photon out of a
million is affected. The efficiency achieved in this work is incredibly high since you
can control nearly all the photons of a light beam this way."
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