Tip-enhanced CARS

The nonlinear polarization of tip-enhanced CARS (TE-CARS) is expressed by... 

FIGURE 10.10 An experimental system of tip-enhanced CARS microscopy. See the text for detail. ND nentral-density filter, P polarizer, DM dichroic mirror, BE beam expander, BS beam splitter, APD avalanche photo diode. 

Fig. 16.11 Tip-enhanced CARS images of the DNA network obtained at (a) on-resonant frequency (1,337 cm ) and (b) off-resonant frequency (1,278 cm- ), (c) Cross-sectional line profiles at the position indicated by the solid arrows. The scaimed area is 1,000 nm x 800 nm (Reprinted from [106], Copyright 2004, with permission from American Physical Society)...

CARS spectroscopy utilizes three incident fields including a pump field (coi), a Stokes field (CO2 C02nonlinear polarization at cOcars = 2c0i — CO2. When coi — CO2 coincides with one of the molecular-vibration frequencies of a given sample, the anti-Stokes Raman signal is resonantly generated [22, 23]. We induce the CARS polarization by the tip-enhanced field at the metallic tip end of the nanometric scale. 

In our tip-enhanced near-field CARS microscopy, two mode-locked pulsed lasers (pulse duration 5ps, spectral width 4cm ) were used for excitation of CARS polarization [21]. The sample was a DNA network nanostructure of poly(dA-dT)-poly(dA-dT) [24]. The frequency difference of the two excitation lasers (cOi — CO2) was set at 1337 cm, corresponding to the ring stretching mode of diazole. After the on-resonant imaging, CO2 was changed such that the frequency difference corresponded to none of the Raman-active vibration of the sample ( off-resonant ). The CARS images at the on- and off- resonant frequencies are illustrated in Figure 2.8a and b, respectively. 

FIGURE 10.9 (a) Spatial conhnement of the excitation efficiency of higher-order nonlinear effects. The hrst-order fnnction represents the distrihntion of the tip-enhanced held intensity as shown in Figures 10.2 and 10.3. (h) Energy diagram of the CARS process. 

Other methods include tip-enhanced Raman using 20- to 30-nm diameter Au or Ag tips, polarized Raman, stimulated Raman, micro-Raman spectroscopy, and coherent anti-Stokes Raman spectroscopy (CARS), where two laser beams are combined to generate an anti-Stokes beam, and so on. 

The small cross sections in multiphoton processes are of course a weakness of nonlinear spectroscopy. Especially in microscopy, this problem becomes serious because of the small volume of a sample. By the use of the signal enhancement techniques, however, the disadvantage can be turned into an advantage of back-ground-free selective measurements. For example, the combined use of HRS with the plasmonic enhancement provides us a chemical imaging with nanoscale spatial resolution when a laser-illuminated metal tip is located adjacent to a sample surface, signal enhancement is locally induced near the tip. This spatial resolution is expected to overcome optical diffraction limit. Such tip-enhanced spectroscopy has already been reported in conventional CARS [15]. 

A series of tests were conducted in the furnace to compare CARS temperature measurements with those acquired with a suction pyrometer [84]. A suction pyrometer is an intrusive probe to measure gas temperature in a flame. The principle of the device is to insert the water cooled probe to the measurement point and draw furnace gases over a thermocouple located at the tip of the probe in an enclosure shielded from flame radiation. The gases are drawn at sufficient velocity to enhance the convective heat transfer to the thermocouple bead. Typically, the flow rate of furnace gases through the probe tip is increased until the thermocouple temperature no longer increases. At this point the thermocouple is assumed to measure the true gas temperature. The disturbance by such a measurement technique can be considerable in the near burner region where chemical reactions are occurring and there is heat release. 

---