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

RESUMEN Se presenta el desarrollo de un novedoso sistema que permite el mapeo con alta resolucion espacial de la difusividad termica en funcion de la temperatura hasta 1800K. El sistema se basa en una tecnica fototermica recientemente desarrollada por el grupo, consistente en la medicion con un laser de prueba de la curvatura inducida por el calentamiento con un laser modulado. La utilizacion de tecnologia de fibras opticas provee una alta robustez al equipo. Un horno para microscopio permite realizar rampas a velocidad controlada para el estudio de transiciones de fase y la medicion de la difusividad termica en funcion de la temperatura. Se pueden establecer mesetas para hacer barridos espaciales con resolucion microscopica para el estudio de la evolucion de las fases o hacer tratamientos termicos in situ para estudiar luego la distribucion espacial de las fases presentes. En este trabajo se presenta un ejemplo, donde la senal es colectada para un punto fijo y para una dada frecuencia de modulacion, en funcion de la temperatura de la muestra. Se disena para ello un sistema de correccion de enfoque que compensa la deriva termica generada por la expansion termica del sistema. El desplazamiento lateral es corregido utilizando un algoritmo de correlacion y la imagen de la camara. Este dispositivo fue utilizado para determinar la transicion de fase de una muestra testigo. Estas capacidades se muestran a partir de la determinacion de la energia de activacion de la transformacion de fase amorfo-cristal de una aleacion de Fe-B-Si.

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Microscopía fototérmica para el estudio de transformaciones de fase

A technique has been developed for the determination of the phase transition temperature of very small samples or thin films and coatings. The technique relies on the determination of changes in the heat capacity of the surface by resonant piezoelectric detection of the photoacoustic signal. The sample is illuminated by means of a low power diode laser modulated at the resonant frequency of a piezoelectric detector. The detector senses the acoustic signal generated by the expansion of the surface that has been slightly heated (about 1 K) by the laser. The signal is monitored as the sample is heated at an established rate, and the phase transition temperature is determined by the change in the amplitude and phase of the signal. As the penetration of the light in metals is very small (well below 1 μm), the technique is particularly suitable for the study of surface coatings and thin films.

Testing stability of surfaces and coatings by resonant photoacoustic phase transition detection

Group IV nanostructures have attracted a great deal of attention because of their potential applications in optoelectronics and nanodevices. Raman spectroscopy has been extensively used to characterize nanostructures since it provides non destructive information about their size, by the adequate modeling of the phonon confinement effect. The Raman spectrum is also sensitive to other factors, as stress and temperature, which can mix with the size effects borrowing the interpretation of the Raman spectrum. We present herein an analysis of the Raman spectra obtained for Si and SiGe nanowires; the influence of the excitation conditions and the heat dissipation media are discussed in order to optimize the experimental conditions for reliable spectra acquisition and interpretation. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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Si and SixGe1-x NWs studied by Raman spectroscopy

Properties of orientation-patterned GaAs crystals studied by cathodoluminescence spectroscopy

Intrinsic defects are generated by e-irrradiation, and plastic deformation in ZnO crystals. Spectrally resolved cathodoluminescence (CL) experiments permit the analysis of the optical signature of those defects. We present herein a CL analysis of ZnO crystals irradiated with high energy electrons, and plastically deformed by Vickers indentation. Spectral changes around 3.3 eV and in the deep level emission are observed in the irradiated samples. These changes are compared to those observed around the extended defects introduced by Vickers indentation, which present a similar signature to the e-irradiated samples, suggesting relation between the defects generated by plastic deformation and e-irradiation. Zn vacancies seem to be the dominant defects generated in both processes (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Oscar E. Martínez

Papers

Microscopía fototérmica para el estudio de transformaciones de fase

Testing stability of surfaces and coatings by resonant photoacoustic phase transition detection

Si and SixGe1-x NWs studied by Raman spectroscopy

Properties of orientation-patterned GaAs crystals studied by cathodoluminescence spectroscopy

Cathodoluminescence study of e-irradiated and plastically deformed ZnO crystals