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

Over the past three decades a new spectroscopic technique with unique possibilities has emerged. Based on coherent and time-resolved detection of the electric field of ultrashort radiation bursts in the far-infrared, this technique has become known as terahertz time-domain spectroscopy (THz-TDS). In this review article the authors describe the technique in its various implementations for static and time-resolved spectroscopy, and illustrate the performance of the technique with recent examples from solid-state physics and physical chemistry as well as aqueous chemistry. Examples from other fields of research, where THz spectroscopic techniques have proven to be useful research tools, and the potential for industrial applications of THz spectroscopic and imaging techniques are discussed.

Terahertz spectroscopy and imaging – Modern techniques and applications

The evolution of the theory of mode-locking over the last three and a half decades is reviewed and some of the salient experiments are discussed in the context of the theory. The paper ends with two-cycle pulses of a mode-locked Ti:sapphire laser.

Mode-locking of lasers

The generation transmission, and detection of extremely rapid electromagnetic pulses have been achieved using fast photoconducting materials as time‐varying Hertzian dipoles. This approach, which has a measured time response of 1.6 ps, overcomes many of the limitations imposed by transmission line structures, and due to its jitter‐free behavior and open geometry is ideally suited for transient electromagnetic measurements of materials.

Picosecond photoconducting Hertzian dipoles

Time-resolved, pulsed terahertz spectroscopy has developed into a powerful tool to study charge carrier dynamics in semiconductors and semiconductor structures over the past decades. Covering the energy range from a few to about 100 meV, terahertz radiation is sensitive to the response of charge quasiparticles, e.g., free carriers, polarons, and excitons. The distinct spectral signatures of these different quasiparticles in the THz range allow their discrimination and characterization using pulsed THz radiation. This frequency region is also well suited for the study of phonon resonances and intraband transitions in low-dimensional systems. Moreover, using a pump-probe scheme, it is possible to monitor the nonequilibrium time evolution of carriers and low-energy excitations with sub-ps time resolution. Being an all-optical technique, terahertz time-domain spectroscopy is contact-free and noninvasive and hence suited to probe the conductivity of, particularly, nanostructured materials that are difficult or impossible to access with other methods. The latest developments in the application of terahertz time-domain spectroscopy to bulk and nanostructured semiconductors are reviewed.

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Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

We report a novel passive mode‐locking technique in which two synchronized counterpropagating pulses interact in a thin, saturable absorber to produce a short pulse. Continuous stable trains of pulses shorter than 0.1 psec are obtained using a ring laser configuration.

Generation of optical pulses shorter than 0.1 psec by colliding pulse mode locking

We show that far-infrared radiation can be generated in the depletion field near semiconductor surfaces via the inverse Franz-Keldysh effect or electric-field-induced optical rectification. This mechanism is conceptually different from those previously proposed and accounts for many recent experimental observations

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Optical rectification at semiconductor surfaces

We report a new technique for passive mode-locking lasers in which a key pulse shortening mechanism is the interaction of two oppositely directed pulses in a thin saturable absorber. Because of the central role played by this interaction or collision, we refer to the process as colliding pulse mode-locking (CPM).

Generation of optical pulses shorter than 0.1 psec by colliding pulse mode-locking

Organic molecules and polymers have long been known to exhibit large values of the third- order susceptibility χ(3). However, in spite of many investigations, the mechanisms governing the nonlinear optical response of these materials are not well understood. Recently, tremendous progress in two related areas has made this problem ripe for further examination. Intense study of polyacetylene and related materials has led to increased understanding of the ground state and elementary excitations of conjugated polymer chains,1 while studies of quasi-2-D inorganic semiconductors, notably GaAs/AIGaAs multiple quantum well structures (MQWSs), have highlighted the importance of reduced dimensionality for optical nonlinearity.2 We have extended the experimental and theoretical approach recently applied in MQWSs to quasi-1-D polydiacetylene with unusual and novel results.3

Benjamin I. Greene

Papers

Generation of optical pulses shorter than 0.1 psec by colliding pulse mode locking

Optical rectification at semiconductor surfaces

Generation of optical pulses shorter than 0.1 psec by colliding pulse mode-locking

Phonon-mediated optical nonlinearity in polydiacetylene

Phonon-mediated optical nonlinearities in polydiacetylene