Suitable labels are at the core of Luminescence and fluorescence imaging and sensing. One of the most exciting, yet also controversial, advances in label technology is the emerging development of quantum dots (QDs)--inorganic nanocrystals with unique optical and chemical properties but complicated surface chemistry--as in vitro and in vivo fluorophores. Here we compare and evaluate the differences in physicochemical properties of common fluorescent labels, focusing on traditional organic dyes and QDs. Our aim is to provide a better understanding of the advantages and limitations of both classes of chromophores, to facilitate label choice and to address future challenges in the rational design and manipulation of QD labels.

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Quantum dots versus organic dyes as fluorescent labels

The rise time of intense radiation determines the maximum field strength atoms can be exposed to before their polarizability dramatically drops due to the detachment of an outer electron. Recent progress in ultrafast optics has allowed the generation of ultraintense light pulses comprising merely a few field oscillation cycles. The arising intensity gradient allows electrons to survive in their bound atomic state up to external field strengths many times higher than the binding Coulomb field and gives rise to ionization rates comparable to the light frequency, resulting in a significant extension of the frontiers of nonlinear optics and (nonrelativistic) high-field physics. Implications include the generation of coherent harmonic radiation up to kiloelectronvolt photon energies and control of the atomic dipole moment on a subfemtosecond $(1{\mathrm{f}\mathrm{s}=10}^{\mathrm{\ensuremath{-}}15}\mathrm{}\mathrm{s})$ time scale. This review presents the landmarks of the 30-odd-year evolution of ultrashort-pulse laser physics and technology culminating in the generation of intense few-cycle light pulses and discusses the impact of these pulses on high-field physics. Particular emphasis is placed on high-order harmonic emission and single subfemtosecond extreme ultraviolet/x-ray pulse generation. These as well as other strong-field processes are governed directly by the electric-field evolution, and hence their full control requires access to the (absolute) phase of the light carrier. We shall discuss routes to its determination and control, which will, for the first time, allow access to the electromagnetic fields in light waves and control of high-field interactions with never-before-achieved precision.

Intense few-cycle laser fields: Frontiers of nonlinear optics

We stabilized the carrier-envelope phase of the pulses emitted by a femtosecond mode-locked laser by using the powerful tools of frequency-domain laser stabilization. We confirmed control of the pulse-to-pulse carrier-envelope phase using temporal cross correlation. This phase stabilization locks the absolute frequencies emitted by the laser, which we used to perform absolute optical frequency measurements that were directly referenced to a stable microwave clock.

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Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis

We review the field of femtosecond pulse shaping, in which Fourier synthesis methods are used to generate nearly arbitrarily shaped ultrafast optical wave forms according to user specification. An emphasis is placed on programmable pulse shaping methods based on the use of spatial light modulators. After outlining the fundamental principles of pulse shaping, we then present a detailed discussion of pulse shaping using several different types of spatial light modulators. Finally, new research directions in pulse shaping, and applications of pulse shaping to optical communications, biomedical optical imaging, high power laser amplifiers, quantum control, and laser-electron beam interactions are reviewed.

Femtosecond pulse shaping using spatial light modulators

Ultrafast lasers, which generate optical pulses in the picosecond and femtosecond range, have progressed over the past decade from complicated and specialized laboratory systems to compact, reliable instruments. Semiconductor lasers for optical pumping and fast optical saturable absorbers, based on either semiconductor devices or the optical nonlinear Kerr effect, have dramatically improved these lasers and opened up new frontiers for applications with extremely short temporal resolution (much smaller than 10 fs), extremely high peak optical intensities (greater than 10 TW/cm2) and extremely fast pulse repetition rates (greater than 100 GHz).

Recent developments in compact ultrafast lasers

We show that pairs of prisms can have negative group-velocity dispersion in the absence of any negative material dispersion. A prism arrangement is described that limits losses to Brewster-surface reflections, avoids transverse displacement of the temporally dispersed rays, permits continuous adjustment of the dispersion through zero, and yields a transmitted beam collinear with the incident beam.

Negative dispersion using pairs of prisms.

A compressor is designed that presents an opposite sign of the dispersion to that of standard two grating compressors. This is achieved by placing a telescope between gratings in order to modify in an adequate manner the phase shift for different wavelengths. The telescope simultaneously provides a high magnification in order to compensate more than 90 km of standard monomode fibers in the 1.6 μm region, yielding compression factors as high as 3000. Analytical expressions for Gaussian beams are found and limitations due to lateral spectral walkoff and telescope pupils are discussed.

3000 times grating compressor with positive group velocity dispersion: Application to fiber compensation in 1.3-1.6 &#181;m region

It is shown that a negative contribution to the group-velocity dispersion always accompanies angular dispersion. We describe structures, such as slabs and prisms, that use this contribution to provide adjustable group-velocity dispersion. We discuss, in particular, considerations regarding incorporation of these structures with a laser resonator. A description of possible submillimeter semiconductor devices is also given.

Negative group-velocity dispersion using refraction

The grating and prism compressors are studied for the case of finite beam size by using the Kirchhoff–Fresnel integral and an amplitude transfer operator for the grating and prism. The results show that, even for well-collimated beams, pulse broadening may occur (mainly because of the wavelength dependence of the lateral walk-off). Methods to avoid the different types of distortions appearing in these systems are discussed.

Grating and prism compressors in the case of finite beam size

A direct measurement of the amplitude and the phase of a femtosecond light pulse is performed for the first time to our knowledge The measurement is made in the frequency domain, and the time dependence of the field can be easily obtained by a Fourier transform The technique relies on a pulse synthesis scheme to unravel the frequency dependence of the phase A mask filters the spectrum, which gives rise to a pulse with a measurable temporal profile related to the frequency dependence of the phase In particular, with a rectangular slit the time delay of the synthesized pulse is the first derivative of the phase with respect to the frequency of the original pulse at the central frequency of the filter The amplitude of the spectrum is obtained from the power spectrum

Oscar E. Martínez

Papers

Negative dispersion using pairs of prisms.

3000 times grating compressor with positive group velocity dispersion: Application to fiber compensation in 1.3-1.6 µm region

Negative group-velocity dispersion using refraction

Grating and prism compressors in the case of finite beam size

Direct determination of the amplitude and the phase of femtosecond light pulses.

Oscar E. Martínez

Papers

Negative dispersion using pairs of prisms.

3000 times grating compressor with positive group velocity dispersion: Application to fiber compensation in 1.3-1.6 &#181;m region

Negative group-velocity dispersion using refraction

Grating and prism compressors in the case of finite beam size

Direct determination of the amplitude and the phase of femtosecond light pulses.

3000 times grating compressor with positive group velocity dispersion: Application to fiber compensation in 1.3-1.6 µm region