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 present a new method for obtaining the phase of an ultra-short light pulse in the frequency domain. This technique is based on measuring the logarithmic derivative of the cross correlation of the pulse with a frequency-filtered version of it. It only requires sweeping the center frequency of the filter, while the time delay of the correlator is held fixed for measuring the group delay of the frequency components of the pulse. It is, to our knowledge, the first one-dimensional method to recover directly the phase information without resorting to a deconvolution or a procedure involving two Fourier transforms. Since it directly measures the delay operator we have called this technique direct group delay operator measurement. We present the theory, experimental results and a brief discussion of the best choice for the fixed setting of the time delay in order to improve the signal to noise ratio in this technique.

Direct method for phase and amplitude determination of ultra-short light pulses

A novel passive method is described that provides a linear time measurement of picosecond and subpicosecond light pulses in a single shot. The method consists of the measurement, by a phase-contrast technique, of the phase shift introduced to the signal by a pump pulse propagating transversally in a medium presenting an optical Kerr nonlinearity. By adequate spatial filtering the temporal resolution may be brought below that otherwise established by the pump pulse.

Linear measurement of ultrashort light pulses by phase-contrast optical sweep

Stabilized subpicosecond pulse compression due to multiple-order stimulated Raman scattering (A)

A photoacoustic technique for the measurement of the amount of residual dirt on cold-rolled steel plates is presented. A pulsed laser beam impinges on the moving plate and produces the ablation of the dirt, emitting an acoustic signal. The amplitude of this signal is adopted as the measurement of the residual dirt. This method has distinct advantages over the standard procedure based on partially removing the dirt with an adhesive tape.

On-line photoacoustic measurement of residual dirt on steel plates

We present gSUPPOSe, a novel, to the best of our knowledge, gradient-based implementation of the SUPPOSe algorithm that we have developed for the localization of single emitters. We study the performance of gSUPPOSe and compressed sensing STORM (CS-STORM) on simulations of single-molecule localization microscopy (SMLM) images at different fluorophore densities and in a wide range of signal-to-noise ratio conditions. We also study the combination of these methods with prior image denoising by means of a deep convolutional network. Our results show that gSUPPOSe can address the localization of multiple overlapping emitters even at a low number of acquired photons, outperforming CS-STORM in our quantitative analysis and having better computational times. We also demonstrate that image denoising greatly improves CS-STORM, showing the potential of deep learning enhanced localization on existing SMLM algorithms. The software developed in this work is available as open source Python libraries.

Oscar E. Martínez

Papers

Direct method for phase and amplitude determination of ultra-short light pulses

Linear measurement of ultrashort light pulses by phase-contrast optical sweep

Stabilized subpicosecond pulse compression due to multiple-order stimulated Raman scattering (A)

On-line photoacoustic measurement of residual dirt on steel plates

Combining deep learning with SUPPOSe and compressed sensing for SNR-enhanced localization of overlapping emitters.