Broadband femtosecond pulses

Two main techniques are available to perform 2D-IRS, namely a pulsed-Jrequency-domain technique [88] and a time-domain-pulsed-Fourier transform technique [89]. A hybrid method using acousto-optic modulation has also been proposed recently [90]. In the pulsed-frequency-domain experiments, an intense broadband femtosecond pulse is split into a pump pulse, which passes a filter to reduce the band width to sections of typically 10 cm, and an unfiltered probe pulse. Both pulses are focused into the sample within an adjustable time delay. The probe pulse (full bandwidth) measures the spectral changes of the sample after the arrival of the pump pulse. For this purpose, the intensity of the probe light beam is recorded using a spectrometer equipped with a broadband array detector. 

Unlike the typical laser source, the zero-point blackbody field is spectrally white , providing all colours, CO2, that seek out all co - CO2 = coj resonances available in a given sample. Thus all possible Raman lines can be seen with a single incident source at tOp Such multiplex capability is now found in the Class II spectroscopies where broadband excitation is obtained either by using modeless lasers, or a femtosecond pulse, which on first principles must be spectrally broad [32]. Another distinction between a coherent laser source and the blackbody radiation is that the zero-point field is spatially isotropic. By perfonuing the simple wavevector algebra for SR, we find that the scattered radiation is isotropic as well. This concept of spatial incoherence will be used to explain a certain stimulated Raman scattering event in a subsequent section. 

In a most versatile and flexible scheme, all processes are therefore carried out in the same experimental setup what is needed is a broadband laser sonrce, the capability to deliver the femtosecond pulses in situ in the microscope with highest possible intensity, the ability to control the phase to select the desired process, and a multichannel detection unit. In a schematic way, this is shown in Fignre 7.3. 

We performed femtosecond infrared pump-probe experiments, using pulses with a bandwidth of 200 cm"1 (FWHM). A small fraction of the infrared pulses was split off to obtain a broadband probe pulse, which was spectrally dispersed after interaction with the... 

In conclusion, it is worth reiterating that the anomalous absorption effects described here may be manifest in any experiments that employ sufficiently high-intensity broadband radiation. To this extent, anomalies may be observable in experiments not specifically involving USES light. In particular, the continued advances in techniques of laser pulse compression have now resulted in the production of femtosecond pulses only a few optical cycles in duration (Knox et al. 1985 Brito Cruz et al. 1987 Fork et al. 1987) which necessarily have a very broad frequency spread, as the time/energy uncertainty principle shows. Thus, mean-frequency absorption may have a wider role to play in the absorption of femtosecond pulses. If this is correct, it raises further questions over the suitablity of absorption-based techniques for their characterization. 

Optical solitons in fused quartz fibers can be utilized to achieve stable femtosecond pulses in broadband infrared lasers, such as the color-center laser or the Tiisapphire laser. Such a system is called a soliton laser [706-713]. Its experimental realization is shown in Fig. 6.41. 

In the middle of the 1990s, a new type of an ultrafast time-resolved measurement system was developed by Hamm et al. [24] femtosecond infrared pulses over a broadband were dispersed by a polychromator and detected by a multichannel infrared detector. They obtained broadband infrared pulses tunable over a wide frequency range (pulse width 400 fs, spectral bandwidth (FWHM) 65cm ), and divided these pulses into two one was used for probing the sample and the other for reference. The infrared pulses passing through the sample and the reference pulses were detected separately by two MCT array detectors each with 10 pixels. By this system, time-resolved infrared spectra of photoexcited molecules were measured [24]. 

Snee et al. [26] obtained broadband femtosecond infrared pulses (FWHM about 200 cm ) by the difference-frequency generation in LiI03 between the fundamental... 

Figure 12.1 Schematic of the spectroelectrochemistry apparatus at the University of Dlinois. The thin-layer spectroelectrochemical cell (TLE cell) has a 25 p.m thick spacer between the electrode and window to control the electrolyte layer thickness and allow for reproducible refilbng of the gap. The broadband infrared (BBIR) and narrowband visible (NBVIS) pulses used for BB-SFG spectroscopy are generated by a femtosecond laser (see Fig. 12.3). Voltammetric and spectrometric data are acquired simultaneously.

T Advanced Multiphoton and CARS Microspectroscopy with Broadband-Shaped Femtosecond Laser Pulses... 

Another technological breakthrough in optical hber technology, however, allows one to upgrade established 100 fs-class laser systems for broadband applications and even surpass the bandwidth of dedicated short-pulse Ti sapphire lasers. Key to this is the use of novel microstructured optical hbers, which are designed to exhibit extremely high optical nonlinearities. If nanojoule femtosecond laser pulses are launched into such a hber, the combination of different nonlinear optical processes leads to the creation of new frequency components. Therefore, the laser bandwidth can be increased dramatically by orders of magnitude. 

The flash lamps used in archetypal time-resolved techniques produced an intense pulse of short duration and broadband continuous-wave characteristic, and provided information in a millisecond timescale. Gradual improvements, and especially use of laser sources, allow decrease of resolution times to micro-, nano- and femtoseconds [2,10-12],... 

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