Semiconductor lasers excitation

Several of the polymethine dyes absorb in the red, far-red, and near-infrared and fluoresce efficienlly as well in this spectroscopic region. Functional derivatives of these may provide excellent fluorescent labels for semiconductor laser excitation. Several research groups are currently actively involved in the synthesis and development of large polyunsaturated dye molecules that are excited and show luminescence in the red and near-infrared. This promises to be one of the most exciting areas of luminescence spectroscopy for the foreseeable future. 

The most useful direct modulation teclmique is the current gain switching of semiconductor laser devices. This technique is unique to semiconductor sources, nearly all other lasers are modulated externally. In tliese devices tire excitation current of tire laser is modulated, resulting in modulated gain and tlierefore modulated output power. A detailed analysis of tliis process is found in [27]. Simply put, an oscillating current of tire fonn... 

The light source for excitation of Nd YAG lasers may be a pulsed flashlamp for pulsed operation, a continuous-arc lamp for continuous operation, or a semiconductor laser diode, for either pulsed or continuous operation. The use of semiconductor laser diodes as the pump source for sohd-state lasers became common in the early 1990s. A variety of commercial diode-pumped lasers are available. One possible configuration is shown in Figure 8. The output of the diode is adjusted by composition and temperature to be near 810 nm, ie, near the peak of the neodymium absorption. The diode lasers are themselves relatively efficient and the output is absorbed better by the Nd YAG than the light from flashlamps or arc lamps. Thus diode-pumped sohd-state lasers have much higher efficiency than conventionally pumped devices. Correspondingly, there is less heat to remove. Thus diode-pumped sohd-state lasers represent a laser class that is much more compact and efficient than eadier devices. 

The introduction and diversification of genetically encoded fluorescent proteins (FPs) [1] and the expansion of available biological fluorophores have propelled biomedical fluorescent imaging forward into new era of development [2], Particular excitement surrounds the advances in microscopy, for example, inexpensive time-correlated single photon counting (TCSPC) cards for desktop computers that do away with the need for expensive and complex racks of equipment and compact infrared femtosecond pulse length semiconductor lasers, like the Mai Tai, mode locked titanium sapphire laser from Spectra physics, or the similar Chameleon manufactured by Coherent, Inc., that enable multiphoton excitation. 

In his article mainly mode-locked tunable dye lasers are discussed. Giant pulse ruby lasers (3 nsec pulse halfwidth) have been successfully used to probe electron densities as a function of time in a rapidly expanding plasma 22). The electron lifetime in the conduction band can be determined with nanosecond semiconductor lasers. By absorption of the laser pulse the electrons in the semiconductor probe are excited into the conduction band, resulting in a definite conductivity. The mean lifetime is obtained by measuring the decrease of conductivity with time 26). 

When the electric field causes conduction electrons to move across the p-/i junction, the resulting situation is one in which the population in the conduction band is greater than the thermal equilibrium population. An excess of electrons in an excited state is an essential feature of lasers, and several semiconductor lasers are based on the p-/i junction. The best known of these is the gallium arsenide laser. 

Applications have been reported for photoelectrochemical experiments, for example, splitting of water [11], local generation of photoelectrodes by spatially selective laser excitation [12], and steady-state electrochemiluminescence at a band electrode array [13,14]. Band electrodes prepared from very thin films approaching molecular dimensions have been used to assess the limits of theory describing electrode kinetics at ultramicroelectrodes [9]. Spectroelectrochemical applications have been extensively reviewed [1], In an intriguing approach, thin, discontinuous metal films have been prepared on a transparent semiconductor substrate they are essentially transparent under conditions in which a continuous metal film containing the same quantity of metal would be expected to substantially absorb [15]. 

Fox and her coworkers observed transient spectra attributable to radical cations upon laser excitation of powdered semiconductor in the presence of olefins (28-31). 

Most fundamental studies of the optical gain in semiconductor laser materials and structures are based on the stripe excitation method used to measure the optical gain [13], This relies on optical pumping and a variation of the length of the excited zone. Great care must be taken in order to avoid saturation effects. 

Since there are only few - and expensive - tunable lasers available for the infrared range, they are not popular for routine applications. However, there are instances where CO2 lasers (with a high efficiency) are used to excite emission spectra (Belz et al., 1987). Semiconductor lasers have been developed for monitoring atmospheric trace gases (Grisar et al., 1987). 

The methods range from simple, inexpensive absorption spectroscopy to sophisticated tunable-laser-excited fluorescence and ionization spectroscopies. AAS has been used routinely for uranium and thorium determinations (see for example Pollard et al., 1986). The technique is based on the measurement of absorption of light by the sample. The incident light is normally the emission spectrum of the element of interest, generated in a hollow-cathode lamp. For isotopes with a shorter half life than and Th, this requires construction of a hollow-cathode lamp with significant quantities of radioactive material. Measurement of technetium has been demonstrated in this way by Pollard et al. (1986). Lawrenz and Niemax (1989) have demonstrated that tunable lasers can be used to replace hollow-cathode lamps. This avoids the safety problems involved in the construction and use of active hollow-cathode lamps. Tunable semiconductor lasers were used as these are low-cost devices. They do not, however, provide complete coverage of the spectral range useful for AAS and the method has, so far, only been demonstrated for a few elements, none of which were radionuclides. 

It was shown that for semiconductor lasers based on InGaN/GaN heterostructures, at optical excitation by the laser pulses of the femtosecond duration ( 150fs) the laser threshold exceed 2 0 GW/cm. It is caused most probably by non-steady excitation conditions. FWHM of the laser spectra alters from 13 to 25 nm. Amplification occurs in a wide interval of 60-80 nm and the gain at maximum reaches 85-153 cm . 

Abstract We have achieved selective gas sensing based on different size semiconductor nanocrystals incorporated into rationally selected polymer matrices. From the high-throughput screening experiments, we have found that when CdSe nanocrystals of different size (2.8 and 5.6nm diameter) were incorporated into different types of polymer fdms, the photoluminescence (PL) response patterns upon laser excitation at 407-nm and exposure to polar and nonpolar solvent vapors were dependent on the nature of polymer. We analyzed the spectral PL response from both sizes of CdSe nanocrystals using multivariate analysis tools. Results of this multivariate analysis demonstrate that a single film with different size CdSe nanocrystals serves as a selective sensor. The stability of PL response to vapors was evaluated upon 16h of continuous exposure to laser excitation. 

In this paper, we report on the development of InN as a potential narrow bandgap semiconductor source of THz radiation nnder 1550 nm fs or CW laser excitation The... 

More recently time-resolved techniques have been applied for studying photocarrier dynamics at the semiconductor-liquid interface. One of the main motivations is that such studies can lead to an estimation of the rate at which photo-induced charge carriers can be transferred from the semiconductor to a redox acceptor in the solution. This method is of great interest because rate constants for the transfer of photocarriers cannot be obtained from current-potential curves as in the case of majority carrier transfer (Section 7.3.5). The main aim is a detailed understanding of the carrier dynamics in the presence of surface states. The different recombination and transfer processes can be quantitatively analyzed by time-resolved photoluminescence emitted from the semiconductor following excitation by picosecond laser pulse. Two examples are shown in Fig. 7.60 [82, 83]. 

Finally in this section, Richter et a/.studied NO desorption from Si(lll) for pulsed laser excitation at wavelengths between 355 and 1 907 nm.81 The wavelength dependence was consistent with desorption resulting from excitation of intrinsic surface states of the semiconductor. This was supported by the dependence of... 

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