Lasers: emitting in the visible

In the typical setup, excitation light is provided by a pulsed (e.g., nanosecond) laser (emitting in the visible range, e.g., at 532 nm, if Mb is investigated), while the probe is delivered by a continuous-wave (cw) laser. The two beams are spatially overlapped in the sample, and the temporal changes in the optical properties (such as optical absorption or frequency shift) that follow the passage of the pump pulse are registered by a detector with short response time (relative to time scale of the processes monitored), such as a fast photodiode. 

Most of the early gas lasers emitted in the visible region. Continuous-wave (CW) lasers such as Ar+ (351.1-514.5 nm), Kr+ (337.4-676.4 nm), and He-Ne (632.8 nm) are now commonly used for Raman spectroscopy. More recently, pulsed lasers such as Nd YAG, diode, and excimer lasers have been used for time-resolved and ultraviolet (UV) resonance Raman spectroscopy. 

Raman spectroscopy is the other vibrational spectroscopic method often considered complementary to infrared spectroscopy. It makes use of a microscope equipped with a monochromatic source provided by a laser emitting in the visible spectrum. This light is absorbed by the molecules of the sample, which in response emit a diffraction spectrum in the mid-infrared. The same peaks can be found as in infrared spectroscopy but with different intensities. In particular, the z (CO) vibration peaks of the carbonyls are in this case of low intensity... 

Solid state lasers are especially of interest for laser ablation. The laser medium is a crystal or a glass, doped with a transition metal. The medium is pumped optically by flash-lamps (discharges of 100-1000 J over a few ms) or continuously with a tungsten - halogen lamp. The resonator may be the space between two flat mirrors, or an ellipsoid, with the laser rod at one focus and the flash-lamp at the other. The ruby laser emits in the visible region (694.3 nm). It is thermally very robust, but... 

In the past, LIBS has been primarily used to analyze one or a few elements, mostly metals [68-70], More recently, with the advent of high-resolution, broadband spectrometers, the capability of LIBS to identify compounds could be realized. Every element on the periodic Table has atomic emission lines that emit in the visible spectrum. A broadband spectrometer allows one to capture all of the elements in the sample interrogated by the laser-generated plasma, provided they are present in sufficient abundance. Instead... 

This technique has emerged and developed within the last 15 years or so. A Nd YAG near infrared laser is normally used that emits its energy at 1064 nm (i.e. in the NIR) this is a much higher wavelength than in conventional Raman spectroscopy (which uses lasers that emit in the visible region). The major advantage of this is that there is a reduced (but not totally eliminated) problem of fluorescence, that is, fewer samples fluoresce. A potential disadvantage may be a reduction in the sensitivity of the detector used. 

Figure C2.I6.2 shows the gap-lattiee constant plots for the III-V nitrides. These compoimds can have either the wurtzite or zineblende struetures, with the wurtzite polytype having the most interesting device applications. The large gaps of these materials make them particularly useful in the preparation of LEDs and diode lasers emitting in the blue part of the visible speetrum. Unlike the smaller-gap III-V compoimds illustrated in figure C2.I6.3 single crystals of the nitride binaries of AIN, GaN and InN can be prepared only in very small sizes, too small for epitaxial growth of device structures. Substrate materials such as sapphire and SiC are used instead.

The so-called peak power delivered by a pulsed laser is often far greater than that for a continuous one. Whereas many substances absorb radiation in the ultraviolet and infrared regions of the electromagnetic spectrum, relatively few substances are colored. Therefore, a laser that emits only visible light will not be as generally useful as one that emits in the ultraviolet or infrared ends of the spectrum. Further, witli a visible-band laser, colored substances absorb more or less energy depending on the color. Thus two identical polymer samples, one dyed red and one blue, would desorb and ionize with very different efficiencies. 

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