Tunable continuous-wave lasers

Today we have at our disposal a wide arsenal of different methods for Doppler-free saturation spectroscopy. It appears likely that future progress will come not so much from the development of still further techniques, but rather from an extension of the wavelength range of highly monochromatic tunable continuous wave laser sources, in particular towards the ultraviolet, where many interesting transitions remain unexplored. 

Historically, this has been the most constrained parameter, particularly for confocal laser scanning microscopes that require spatially coherent sources and so have been typically limited to a few discrete excitation wavelengths, traditionally obtained from gas lasers. Convenient tunable continuous wave (c.w.) excitation for wide-held microscopy was widely available from filtered lamp sources but, for time domain FLIM, the only ultrafast light sources covering the visible spectrum were c.w. mode-locked dye lasers before the advent of ultrafast Ti Sapphire lasers. 

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

It ought to be pointed out that the methods of transfer of population and also of polarization between the states are being developed further. One might mention, in particular, the cw all-optical triple resonance (AOTR) method evolved by Stwalley s group [282] and tried out on Na2-The method consists of the use of three tunable single-frequency continuous wave lasers allowing flexible movement of both population and polarization from the thermally populated level to any other level of the ground state. 

In a sample containing a mixture of compounds, individual species may be resonance enhanced at different wavelengths. In some cases it may be possible to measure resonance Raman spectra from individual components in a mixture by selective excitation of specific absorption bands. Moreover, the assignment of resonance-enhanced vibrations provides detailed information about the local symmetry of the species. RRS has been used widely to characterize biological samples in which electronic transitions occur at visible excitation wavelengths, and commercial continuous wave lasers are readily available. Resonance Raman spectroscopic characterization of solid catalysts and adsorbed species has seen limited application. Many catalytic materials are white, but their electronic transitions often occur at ultraviolet wavelengths. With the availability of continuous wave and tunable, pulsed ultraviolet laser sources, we anticipate the application of RRS to catalysts will increase substantially. This expectation has motivated the present review. 

Clays, K., and A. Persoons. 1994. Hyper-Rayleigh scattering in solution with tunable femtosecond continuous-wave laser source. Rev Sci Instrum 65 2190-2194. 

The titanium-sapphire laser is perhaps the ultimate near-infrared laboratory laser for Raman spectroscopy. It is a continuous wave laser that can deliver thousands of milliwatts of laser light and is continuously tunable from below 700 to above 1000 nm. It provides a narrow bandwidth with a high-quality spatial mode. The spontaneous emission from the titanium-sapphire crystal must be filtered from the laser beam. 

Optical Kerr Effect. Another important method used to characterize polymers is the optical Kerr effect (OKE). The optical Kerr effect differs from the quadratic electrooptic effect in that the birefringence effects are induced solely by an optical field (37). In this measurement, an intense linearly polarized pump pulse induces birefringence in the nonlinear sample through an intensity-dependent refractive index change. The sample is placed between crossed polarizers and a weak, typically tunable, continuous wave (cw) probe laser (usually at a different wavelength and polarized at 45° to the pump pulse) overlaps the pumped region. The increased transmission of the probe beam when the pump pulse arrives is proportional to (Xeff), a combination of elements of the tensor. Many... 

Synchronously Pumped Mode-Locked Dye Laser. Organic dyes have proven to possess excellent properties for the generation of ultrashort laser pulses. Numerous approaches have been taken to the mode-locking of a dye laser, including both active and passive techniques. As a result, tunable continuous wave (cw) dye lasers have been successfully mode-locked over the past 25 years [183, 184], and pulse widths on picosecond and femtosecond timescales have been reported by many groups. 

The first far-infrared LMR spectra were recorded by using HCN, H2O and D2O lasers. Since then, optically pumped lasers are usually used with a line-tunable continuous-wave CO2 laser as a light source. By use of these lasers, over 1000 laser frequencies are available in the far-infrared region (30-1200 pm) some 500 have been used in LMR. 

New impetus was given to photomedicine by development of lasers that are compatible with the clinical environment. These include HeNe, Ar ion, mby, and tunable dye lasers operating in the continuous wave (cw) mode. Prior to the advent of lasers in medicine, only the treatment of newborn jaundice, and the appHcation of long wavelength uv irradiation in conjunction with adininistration (or topical appHcation) of psoralen class sensitizers to treatment of skin diseases (86), principally psoriasis, were clinically important phototherapies. 

Additional laser diode technologies recently reported include a continuous-wave rhodamine 700 dye laser with a maximum wavelength output at 758 nm, powered by two laser diodes each operated with two standard AA batteries (RDT E division of the US Naval Command, Control, and Ocean Surveillance Center, San Diego, California), and a tunable laser diode with output laser wavelengths of 650, 780, 850, and 1320 nm (New Focus, Mountain View, California). 

As a method to control wavepackets, alternative to the use of ultra-short pulses, I would like to propose use of frequency-modulated light. Since it is very difficult to obtain a well-controlled pulse shape without any chirp, it is even easier to control the frequency by the electro-optic effect and also by appropriate superposition of several continuous-wave tunable laser light beams. 

Details of the continuous wave, axial flow, line tunable, extended cavity CO2 laser system, and of our alignment procedure, are given in ref. [14]. To access a wavelength near 12.0 /im required operating the laser on the P-branch of the 01 1 — [11 0,03 0]/ hot-band of [22], and using the laser beam co-... 

Table 2. Characteristic properties of tunable dye lasers with different pumping sources (cw = continuous wave)...

IR measurements on doped Si and shown that the subsurface mobile carriers can be probed by their response to an IR near-field with a spatial resolution of 30nm [48]. The group of Havenith presented a scanning near-field infrared microscopy (SNIM) system this is an IR s-SNOM set-up based on a continuous-wave optical parametric oscillator (OPO) as an excitation source with a much wider tunability compared to the usually applied CO2 lasers [49]. With this set-up, a subsurface pattern of implanted gallium ions in a topographically fiat silicon wafer was imaged with a lateral resolution of <30 nm. 

An optical-optical double resonance (OODR) scheme exists, utilizing two continuous-wave (cw), monochromatic, tunable lasers, whereby the rotational quantum numbers of all observed lines may be established without ambiguity, prior knowledge of B-values, trial-and-error searches for consistent combination differences, or redundant confirmation lines that are weak because of level population may be distinguished from those that are weak because of intrinsic linestrength and forbidden transitions may be made to appear with comparable peak intensities but considerably narrower widths than allowed transitions. 

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