Lasers coherence properties

Single atomic ions confined in radio frequency traps and cooled by laser beams (Figure 7.4a) formed the basis for the first proposal of a CNOT quantum gate with an explicit physical system [14]. The first experimental realization of a CNOT quantum gate was in fact demonstrated on a system inspired by this scheme [37]. In this proposal, two internal electronic states of alkaline-earth or transition metal ions (e.g. Ba2+ or Yb3+) define the qubit basis. These states have excellent coherence properties, with T2 and T2 in the range of seconds [15]. Each qubit can be... 

Exploitation of these is possible in LEDs that display coherence properties, in thresholdless laser diodes, and in many other optical, opto-electronic and quantum electronic devices. 

Coherent control Control of the motion of a microscopic object by using the coherent properties of an electromagnetic held. Coherent phase control uses a pair of lasers with long pulse durations and a well-defined relative phase to excite the target by two independent paths. Wave packet control uses tailored ultrashort pulses to prepare a wave packet at a desired position at a given time. 

The question is how radiation from therapeutic lasers and LEDs works at the cellular and organism level and what the optimal light parameters are for different uses of these light sources. It has been suggested [24] that coherent properties of laser fight are not manifested at the molecular level by light interaction with biotissue. Some additional (therapeutic) effects from coherent and polarized radiation... 

New and promising developments can be seen in high-intensity (2-photon) laser photolysis, in the combination of fields and radiation (see Chap. 3), in exploitation of the coherence properties (see Chap. 2), and in the detection of more autocatalytic or autoinductive reaction systems. 

Laser micro/nano drilling and on-site nanoscale measurement utilizing a coherence property of light and the dynamic control on wave-front... 

This property has been used to measure the coherence properties of laser pulses [28]. Because it is readily obtainable by Fourier transform from the... 

The conceptual framework underlying the control of the selectivity of product formation in a chemical reaction using ultrashort pulses rests on the proper choice of the time duration and the delay between the pump and the probe (or dump) step or/and their phase, which is based on the exploitation of the coherence properties of the laser radiation due to quantum mechanical interference effects [56, 57, 59, 60, 271]. During the genesis of this field. 

This constant is essentially the limit of K as gi en by Eq. (5.14), in the case where the two absorbed photons become identical however, the factor rtiniY is replaced by n (n — 1) since the photon annihilation operator acts twice on the same radiation mode. As will be seen below, this difference is ultimately reflected in a dependence on the coherence properties of the laser source, which is uniquely associated with single-beam processes. It is also worth observing that although the first two terms of Eq. (5.13) become identical if the two absorbed photons are deiived from the same beam, inclusion of a factor of 2 in Eq. (6.1) would amount to double-counting the time-ordered diagrams, and is therefore not ap])ropriate. 

Figure 45. Schematic representation of the preparation and detection of rotational coherence in a molecule. The case depicted corresponds to the linearly polarized excitation (polarization vector ,) of a symmetric top molecule in ground-state ro-vibronic level S0v0 J0K0M0) to those rotational levels of the excited vibronic state 15,1 ,) allowed by the rotational selection rules germane to a parallel-type transition moment. The excitation process creates a superposition state of three rotational levels, the coherence properties of which can be probed by time resolving the polarized fluorescence (polarization it) to the manifold of ground-state ro-vibronic levels S0vf JfKfMfy, or by probing with a second, variably time-delayed laser pulse (polarization...

The coherence properties of laser radiation provide an opportunity to exert some external control over intramolecular dynamics. Control over photofragmentation product branching ratios has been achieved in both time and frequency domain experiments. 

Apart from the obvious property of defining pulses within short time intervals, the pulsed laser radiation used in reaction kinetics studies can have additional particular properties (i) high intensity, (ii) high monochromaticity, and (iii) coherence. Depending on the t) e of laser, these properties may be more or less pronounced. For instance, the pulsed CO2 lasers used in IR laser chemistry easily reach intensities between... 

Strategies for achieving intra- and intermolecular selectivity are the subject of a very active freld of current research with many open questions. Under the label coherent control it includes approaches that exploit the coherence properties of laser radiation to control chemical reactions. Figure B2.5.18 summarizes the different schemes of intra- and intermolecular selectivity. 

There is another way of measuring of an optical transition which is not based on a coherent optical effect but also employs a coherence property of the laser, namely, its monochromaticity. In the hole-burning technique, as in the OFID method, a cw dye laser is used to create a hole in the absorption spectrum. When this hole is transient, its width, being determined in the low-intensity limit by 2( 7 ) , may be probed by side-band modulation as first demonstrated by Szabo on ruby. When the hole is permanent, as is the case in photochemical hole-burning, the width may be easily measured by means of a narrow-band excitation spectrum as first performed by de Vries and Wiersma on dimethyl j-tetrazine in durene. ... 

Our present system uses a cw 7.5 mW HeNe laser. As mentioned on our display board, it should be possible to make use of the coherent properties of the laser for speckle work. Unfortunately due to other pressures we have not been able to follow up this line of research but would hope to do so in the future. 

Most of the vibrational and rotational spectra obtained before the second world war were measured using Raman methods. Interest in Raman then declined as infrared and microwave absorption instrumentation developed, but the introduction of visible lasers in the early 1960s has led to dramatic renaissance in Raman spectroscopy. As well as decreasing the acquisition time and increasing the sensitivity of conventional Raman spectra by orders of magnitude, the high power and coherence properties of laser radiation has spawned a host of new nonlinear Raman spectroscopies, some of which can be performed without a... 

The spectral width Aty can be further increased by focusing the laser pulses into a special optical fiber, which consists of a photonic crystal (Fig. 9.88) where by self-phase modulation the spectrum is considerably broadened and extends over one decade (e.g., from 1064 nm to 532 nm) (Fig. 9.89). This corresponds to a frequency span of 300 THz [1327] It was found by interference experiments, that the coherence properties were preserved in this broadened spectmm, i.e. the nonlinear processes in the optical fiber did not destroy the coherence of the original frequency comb. 

One way of studying temporal coherence in laser systems is by measuring photon statistics [224]. In this technique the transient laser emission properties are measured using pulsed excitation and a time-resolved setup [225], The transient emission curve generated by each pulse above the laser threshold intensity is divided into time intervals that are smaller than the emission coherence time. The number of photons is then measured in each time interval and for each pulse, and a photon number histogram is calculated to obtain the probability distribution function (PDF) of the photons for each time interval. Photon statistics is achieved separately for each time interval, and correlation between different time intervals or between different wavelengths of the emission spectrum can be also studied. It is expected that for coherent radiation the Poisson distribution determines the PDF, whereas for noncoherent light... 

In general, present-day optical communication systems utilize an intensity modulation scheme to encode information onto an optical beam. In these schemes, the coherent properties of the laser are not employed. For detection, the detector simply counts the number of incident photons during a given amount of time in order to recover... 

Coherence constitutes one of the most important attributes of laser radiation. Here, we provide a brief summary of the coherence properties of laser radiation, and how they may impact on laser chemistry experiments. 

In the interaction of a coherent laser beam with an ensemble of particles (atoms or molecules), one may treat the individual particles as nearly stationary, because even for a fast atomic/molecular beam the particles move only a few micrometres on the time-scale of the photon interaction. Consequently, if the laser photons are absorbed in the interaction, the coherence properties of the laser radiation are transferred to the particle ensemble. It is this coherence transfer that is exploited in experiments such as the orientation of reagents in chemical reactions, or the probing of intramolecular motion in transition states and orientation of products. 

The book begins with a discussion of the fundamental definitions and concepts of classical spectroscopy, such as thermal radiation, induced and spontaneous emission, radiation power and intensity, transition probabilities, and the interaction of weak and strong electromagnetic (EM) fields with atoms. Since the coherence properties of lasers are important for several spectroscopic techniques, the basic definitions of coherent radiation fields are outlined and the description of coherently excited atomic levels is briefly discussed. 

Many experiments in laser spectroscopy depend on the coherence properties of the radiation and on the coherent excitation of atomic or molecular levels. Some basic ideas about temporal and spatial coherence of optical fields and the density-matrix formalism for the description of coherence in atoms are therefore discussed at the end of this chapter. 

Ketterle W, Miesner H-J. (1997) Coherence properties of Bose condensates and atom lasers. Phys. Rev. A 56 3291-3293. 

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