Colloquium: Vladimir Pervak - Advanced Multilayer Coatings for Femtosecond and Attosecond Physics

Abstract(s):
The goal of generating short laser pulses down to the limit set by a single wave cycle of light has been pursued ever since the invention of lasers. Laser pulses consisted of only a small number of wave cycles allow more efficient exploitation of nonlinear optical effects with implications as striking as the generation of single sub-femtosecond light pulses. The controlled superposition of light frequencies extending over more than one octave together with carrier-envelope phase control pave the way for shaping the sub-cycle evolution of light fields in laser pulses.

Dr. V. Pervak will provide an overview of dispersive multilayer optics. Dispersive mirror (DM) offers high reflectivity and controlled group delay dispersion (GDD) over some 1.5 octaves spanning ultraviolet to near infrared frequencies. Nowadays, we cannot imagine ultrashort pulses being obtained without dispersive multilayer optics. A DM is a dispersive optical interference coating usually designed by optimizing the initial multilayer design. A DM is characterized by a certain value of the group delay (GD) or GDD. GD is the first derivative of the phase shift with respect to the angular frequency: where φis the phase-shift obtained on reflection or transmission, ωis the angular frequency. GDD is the second derivative of the phase shift with respect to the angular frequency or is first derivative of the GD:

The short pulse penetrating through dispersive medium becomes longer due to the introduced GD, as illustrated in Fig. 1. By introducing an inverse GD with a DM, the pulse can be compressed to its original pulse duration. In general, the GDD of mirror should compensate material dispersion (through which the initially short pulse passes) or the (nonlinear) chirp of the pulse so that the residual dispersion oscillations are acceptably small in all of the relevant spectral range.

The DM is one of the key elements of most ultrafast (femtosecond) lasers. Whilst being able to provide control of phase over unprecedented bandwidths and high efficiency, the DM technology suffers from unavoidable spectral oscillations of the phase. Reflection from the top layer of a multilayer structure introduces oscillations to the GDD curve due to interference between waves reflected from the top layer and waves which have penetrated and have been reflected from deeper. These oscillations may adversely affect the quality of the femtosecond laser pulses which are being controlled with DMs. Usually during design optimization the residual fluctuations drop down to a low level. The GDD oscillations can broaden the pulse and lead to energy transfer from the initial single pulse to satellites. The period of the ripples in the spectral domain determines the position of the satellite in the temporal domain, and the amplitude of these oscillations determines the amount of energy which is transferred to the satellites.

The manufacture of DMs can be as challenging as their design, as DMs are extremely sensitive to a discrepancy in layer thickness. In most cases, magnetron-sputtering and ion-sputtering technologies provide sufficient precision of the layer thickness control. Modern sputtering technology can provide sub-nm precision in controlling the layer thickness. Some applications such as highly dispersive mirrors require angstrom precision. The extreme sensitivity of the DM can then be overcome by applying a special, robust design algorithm.

The result of the 20-year evolution of the design and fabrication of dispersive multilayers now allows the development of structures with low loss and high dispersion over a wide spectral range, and permits compression down to the theoretical limit of the pulse duration.

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