Pump beam

A RIKES experunent is essentially identical to that of CW CARS, except the probe laser need not be tunable. The probe beam is linearly polarized at 0° (—>), while the polarization of the tunable pump beam is controlled by a linear polarizer and a quarter waveplate. The pump and probe beams, whose frequency difference must match the Raman frequency, are overlapped in the sample (just as in CARS). The strong pump beam propagating tlirough a nonlinear medium induces an anisotropic change in the refractive mdices seen by tlie weaker probe wave, which alters the polarization of a probe beam [96]. The signal field is polarized orthogonally to the probe laser and any altered polarization may be detected as an increase in intensity transmitted tlirough a crossed polarizer. When the pump beam is Imearly polarized at 45° y), contributions... 

Here we consider the response of the system to a monochromatic pump beam at a frequency oi. 

The linear and nonlinear optical responses for this problem are defined by e, 2, e and respectively, as indicated in figure Bl.5.5. In order to detemiine the nonlinear radiation, we need to introduce appropriate pump radiation fields E(m ) and (co2)- If these pump beams are well-collimated, they will give rise to well-collimated radiation emitted tlirough the surface nonlmear response. Because the nonlinear response is present only in a thin layer, phase matching [37] considerations are unimportant and nonlinear emission will be present in both transmitted and reflected directions. 

Flere we model the pump beams associated with fields E(a> ) and (102) as plane waves with wavevectors Jti = and Jt, = feiwiv/fii (wil/r - The directions of tlie reflected and transmitted beams can... 

Figure Bl.5.8 Random distribution of (a) non-chiral adsorbates that gives rise to a surfaee having effeetive oo m-synnnetry (b) ehiral moleeules that gives rise to effeetive oo-synnnetry. (e) SH intensity versus the angle of an analyser for a raeemie (squares) and a non-raeemie (open eireles) monolayer of ehiral moleeules. The pump beam was p-polarized the SH polarization angles of 0° and 90° eorrespond to s- and p-polarization, respeetively. (From [70].)...

The gates referred to above can be created in various ways. For example, suppose that the probe beam goes tlirough the sample, but only half of its physical width (in the sample) is crossed with the pump beam. Now, if we have two photodiodes, one can measure the intensify of the perturbed part of the probe beam, whilst the second measures the unperturbed part as a result of creating spatial gates, the two recorded output signals can be used to measure the... 

A schematic representation of a PR apparatus is shown in Figure 2. In PR a pump beam (laser or other light source) chopped at frequency 2 creates photo-injected electron-hole pairs that modulate the built-in electric field of the semiconductor. The photon energy of the pump beam must be larger than the lowest energy gap of the material. A typical pump beam for measurements at or below room temperature is a 5-mW He-Ne laser. (At elevated temperatures a more powerful pump must be employed.)... 

Commercial versions of PR are available. Other contactless methods of electro-modulation are Electron-Beam Electro-reflectance (EBER) and Contacdess Electroreflectance (CER). In EBER the pump beam of Figure 2 is replaced by a modulated low-energy electron beam (- 200 eV) chopped at about 1 kHz. However, the sample and electron gun must be placed in an ultrahigh vacuum chamber. Contactless electroreflectance uses a capacitor-like arrangement. 

In the continuous wave (CW) experimental setup a sample is constantly illuminated by a probe beam and the steady state change in the transmission is detected (see Fig. 7-1). An argon ion laser has been used to generate the pump beam and the probe beam was from an incandescent lamp (tungsten or others), producing a broad spectrum (0.5 to 5 pm) [6]. Both pump and probe beams are directed onto the sample film and the transmitted probe light is collected, filtered through a monochromator, and detected by a photodetector. Both the pump and the probe... 

The PIA-investigations were carried out under dynamic vacuum (p< 10 5 mbar) and at 77 K with films cast from toluene solution onto KBr substrates. For the dispersive method [29, 30] the globar, the KBr-prism premonochromator, and the grating monochromator of a Perkin Elmer 125 lR-spcctrometer were used in the spectral range of 0.25 to 1.24 eV. The pump beam was chopped mechanically... 

The layout of the experimental set-up is shown in Figure 8-3. The laser source was a Ti sapphire laser system with chirped pulse amplification, which provided 140 fs pulses at 780 nm and 700 pJ energy at a repetition rate of 1 kHz. The excitation pulses at 390 nm were generated by the second harmonic of the fundamental beam in a 1-nun-thick LiB305 crystal. The pump beam was focused to a spot size of 80 pm and the excitation energy density was between 0.3 and 12 ntJ/crn2 per pulse. Pump-... 

After the preamplifier, the beam is expanded to 2 mm, collimated and imaged onto a 1 mm aperture, producing a flat-top intensity profile. A 3-element telescope relays the aperture plane to the amplifier with a collimated 0.5-mm diameter. The telescope contains a spatial filter pinhole. The nominal power levels are 3 mW into the preamp, 500 mW out of the preamp and 200 mW out of the aperture. A 6° angle of incidence bounce beam geometry is utilized in the amplifier cell. The "bounce" foofprinf overlaps with the 4 pump beam fibers, arranged in 2 time sefs of 13 kHz. The pump fibers have f 50-60% fransmission. The amplifier brings the power up to < 20 W at 26 kHz. 

The pump enhanced singly resonant OPO. It consists in an OPO in which the cavity is resonant for the signal and pump beams (Schiller et al., 1999). The basic properties of this device are the same as the SROPO s but with a lower threshold. 

A second way to overcome the high reactivity of carbenes and so permit their direct observation is to conduct an experiment on a very short timescale. In the past five years this approach has been applied to a number of aromatic carbenes. These experiments rely on the rapid photochemical generation of the carbene with a short pulse of light (the pump beam), and the detection of the optical absorption (or emission) of the carbene with a probe beam. These pump-probe experiments can be performed on timescales ranging from picoseconds to milliseconds. They provide an important opportunity absent from the low temperature experiments, namely, the capability of studying chemical reactions of the carbene under normal conditions. Before proceeding to discuss the application of these techniques to aromatic carbenes, a few details illuminating the nature of the data obtained and the limitations of the experiment need to be introduced. 

The pump beam comes from a laser. The necessity of high light intensity in a short time demands this. Exceptions are possible for relatively unreactive intermediates a flash lamp was used in the first direct detection of a carbene (Closs and Rabinow, 1976), but the availability of modern high-power, pulsed uv-lasers has made this approach obsolete. One requirement then is that the precursor to be irradiated absorb at an available laser frequency. For aromatic carbenes, this is not a restrictive requirement. 

The probe beam for picosecond timescale experiments must originate from the same laser as does the pump beam. The time period between creation of... 

Fig. 2.11. Transient anisotropic reflectivity change for Si(001) in the T25 geometry (left) and its continuous wavelet transform (right). Inset in the right panel defines the polarization of the pump beam relative to the crystalline axes. From [47]...

Fig. 3.11. Left spectrum of the tailored pump beam for different spectral separations Aw CC2 — oji between two spectral packets (oJi and Lof). The bottom case is resonant to the E2 phonon frequency l o- Right transient transmittance of GaN excited with the tailored pump pulses, showing the enhancement of coherent oscillation of the E2 phonon for u) — u)2 — Oo- From [28]...

We measured and analyzed the vertical emission from the resonators under pulsed optical pumping. The experimental setup is illustrated in Fig. 12.8a A Ti/sapphire mode-locked laser was used to optically pump the devices at a center wavelength of 980 nm, repetition rate of 76.6 MHz, and pulse duration of approximately 150 fs. A variable attenuator was used to control the pump power. The average pump power and center wavelength were monitored by a wavemeter, through a 50/50 beamsplitter. The pump beam is focused on the back side of the sample with a 50 x objective lens. A 20 x objective lens is used to collect the vertical emission from the sample and to focus it on an IR camera to obtain the NF intensity pattern and to... 

EXAMPLE 2.6 In an OPO system, the pump beam wavelength is set at 355 nm. Assuming that the signal beam is produced at 450 nm, determine the wavelength corresponding to the idler beam. 

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