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.

/pdf/quantum-dots-versus-organic-dyes-as-fluorescent-labels-50cbzj94q6.pdf

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.

/pdf/carrier-envelope-phase-control-of-femtosecond-mode-locked-4lkf4vexna.pdf

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 demonstrate that, by changing the altitude and the azimuth incident angles on the gratings of the conventional grating-pair compressor used in chirped-pulse amplification, an extra degree of freedom is added. This results in a continuous adjustment of second-, third-, and fourth-order dispersions, which allows one to compensate for those dispersions that originated in the expansor or in the amplifier medium as a result of material dispersion or self-phase modulation, even with small out-of-plane tilts of the expansor and compressor. Analytical calculations of the high-order dispersions introduced by this compressor and examples for a pulse with a central wavelength at 800 nm are presented.

Quartic phase compensation with a standard grating compressor.

Thermal expansion of the sample or tip in a laser-assisted scanning tunneling microscope (STM) junction can obstruct the study of other phenomena related with STM–light interactions, when photoinduced currents are analyzed. In this article, we show that the thermal contribution is proportional to the average tunneling current and that this can be used to distinguish it from other contributions. Simultaneous tunneling current–voltage and photoinduced current–voltage curves are recorded for highly oriented pyrolitic graphite and gold samples with a Pt tip. We have done the measurements for two different polarizations of the incident beam. We show that the I–V curves can be used to discriminate between different mechanisms that appear, producing photoinduced currents.

Spectroscopic response of photoinduced currents in a laser-assisted scanning tunneling microscope

Frequency domain phase measurement of ultrashort light pulses. Effect of noise

We have demonstrated that the reproducibility of electron beam pulses generated by a high power, cold cathode glow discharge is greatly improved by adding a small continuous keep-alive discharge current. A current of the order of 200 μA was found to limit the shot to shot current variation to within 1.5%. This stabilization in turn reduces by an order of magnitude the fluctuations of the energy density deposited on the target, demonstrating a reliable energy source for surface treatment.

/pdf/stabilization-of-a-cold-cathode-electron-beam-glow-discharge-dc3azbsr26.pdf

Stabilization of a cold cathode electron beam glow discharge for surface treatment

A focus error signal resulting from the photothermically-induced expansion is measured in a sample of material under analysis. A laser is disposed as a periodically modulated heating source which is directed to the sample and a device for focus error measuring which is directed to de surface being heated. A device measuring focus error generates a signal representative of the displacement of the surface of material in perpendicular direction due to the expansion produced by the periodic heating, which is filtered, either analogically or digitally, to discriminate the displacement component at the frequency in which it was modulated or at any other related frequency, such any harmonic or a sum with any other modulation. The focus error signal, appropriately calibrated, gives a precise and sensitive measure of the magnitude the expansion. Said magnitude and its dependence with the modulation frequency allows the determination of physical properties such as the thermal expansion or thermal diffusivity coefficient, the thickness of a coating film or the absorption coefficient of the light from the heating beam. By varying the wave length of the directed radiation it is possible to determine the absorption spectrum of the sample even for very small sized particles in which the fraction of absorbed energy is very little.

Oscar E. Martínez

Papers

Quartic phase compensation with a standard grating compressor.

Spectroscopic response of photoinduced currents in a laser-assisted scanning tunneling microscope

Frequency domain phase measurement of ultrashort light pulses. Effect of noise

Stabilization of a cold cathode electron beam glow discharge for surface treatment

Method and apparatus for determining the thermal expansion of a material