Amplified stimulated emission

We expect that our dynamical theory based on the density mauix formalism can be useful in modeling other fundamental and applied problems of nonlinear optics like spectral-hole burning, self-focusing, white light generation, the dynamics of the spectral profiles of amplified stimulated emission etc. The scope is certainly wide for future researh in these areas. 

From these expressions, it becomes clear that operating the chemosensing CP film near its lasing threshold will yield enhancements in fluorescence quenching, thereby enhancing sensitivity. Inspection of Fig. 5.9 shows that optimal sensitivity, at practical pump powers, is obtained when the pump power of the CP is at the onset of amplified stimulated emission (ASE), for the exposed film. Practical implementation of the concept would require rastering of the incident power intensity to locate the pump power that yields the maximum signal response. 

Jasieniak, J., Pacifico, J Signorini, R Chiasera, A Ferrari, M Martucci, A., and Mulvaney, P. (2007) Luminescence and amplified stimulated emission in CdSe-ZnS-nanocrystal-doped Ti02 and Zr02 waveguides. Adv. Fund. Mater., 17, 1654-1662. 

Interaction of an excited-state atom (A ) with a photon stimulates the emission of another photon so that two coherent photons leave the interaction site. Each of these two photons interacts with two other excited-state molecules and stimulates emission of two more photons, giving four photons in ail. A cascade builds, amplifying the first event. Within a few nanoseconds, a laser beam develops. Note that the cascade is unusual in that all of the photons travel coherently in the same direction consequently, very small divergence from parallelism is found in laser beams. 

Figure 10-5. Transient transmission changes AV/Po in PPV for different lime delays between the pump and probe pulse. The pump pulse is a 100 fs laser pulse at 325 nm obtained by frequency doubling ol amplified dye laser pulses, (a) and (b) correspond to different sides of a PPV-film. The spectra in (a) were obtained lor the unoxidized side of the sample while the set of spectra in (b) was measured for the oxidized side of the same sample. The main differences observed are a much lower stimulated emission effect for the oxidized side. The two bottom spectra depict the PL-spectra for comparison. The dashed line indicates the optical absorption (according to Kef. (281).

Now today, we have found about a hundred different masers in space and some lasers. The difference between a maser and a laser is of course only in the wavelength. But there are some astronomical systems where infrared is getting amplified. Now as has been pointed out, amplification in interstellar space doesn t involve resonances, but it does involve stimulated emission. You know, somebody could have seen these interstellar masers in the radio regions of the spectrum many years ago. Anybody who used the radio technology of 1936, and looked up into the sky, could have detected this water frequency. They didn t bother to look, but it was there all the time. So now we know, lasers have been there for billions of years. Masers have been there billions of years. So that s another way we might have discovered them, but we didn t. Now I emphasize this to indicate that we need to search, we mustn t be too confined by what we think is going to work, we ve got to explore. 

The word LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. Similar to the way in which transistor systems are available to generate and amplify electrical signals, with the advent of lasers we have at onr disposal devices that are able to generate and amplify coherent light. 

FIGURE 8.4 If more population is in the lower state than the upper state (left), absorption is stronger than stimulated emission, and radiation is absorbed from an external field. If the upper state is more populated (right), which never happens at equilibrium because of the Boltzmann distribution, an external field will he amplified. The situation on the right will create a laser. 

The above treatment is valid quite generally, even if f2 — fi > huj. when the denominator in (4.17) is negative. Under the same conditions, the absorption coefficient in (4.15) is also negative, and the spontaneous emission rate in (4.17) remains positive. When the absorption coefficient is negative, stimulated emission overcompensates the rate of upward transitions and the 2-level system amplifies the incident light exactly as in a laser. The condition f2 — fi > tko is also known as the lasing condition and is called inversion. 

In contrast to this anti-maser action in formaldehyde, H20 and OH are observed in maser emission. Collisional or radiative pumping is thought to maintain the population inversion between the two levels. As photons pass through the cloud, they are amplified by stimulated emission of radiation. The maser emission of H20 is possibly the most unusual of the observed anomalies, both from an astrophysical and a spectroscopic point of view. This is not the place to discuss details of the various models suggested, we would refer to concentrate on some of the general features observed in the maser emission spectra. 

When spontaneous emission is amplified by stimulated emission, the spectrum is narrowed because the highest amplification occurs at the wavelength where the gain is the highest. Fig. Vll-8 shows a series of emission spectra from the... 

A laser (acronym for light amplification by stimulated emission of radiation) amplifies light in a different region of the electromagnetic spectrum by the same method that the maser amplifies microwaves. 

Spontaneous emission initiates a chain reaction in which a series of stimulated emissions amplifies the number of photons enormously. 

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