Infrared vibronic laser oscillators

Tunable, nearly transform-limited femtosecond pulses in the near- and mid-infrared regions are of great interest for a variety of applications, including medicine, telecommunications, time resolved spectroscopy of molecules, studies of intersubband transitions and photoexcitation dynamics in semiconductors. The existing infrared femtosecond technology is based on the developed, but rather complex multistage cascade OPO systems. Compact sources of tunable fs pulses are not only highly desirable for the above application fields, their realization would generally constitute a real breakthrough in the infrared femtosecond technology, eliminating serial pumping stages of OPO`s, based, for example, on AgGaSe2, ZnGeP2, CdSe or GaSe. This project aimes at extending the highly successful ultrashort pulse laser technology based on mirror-dispersion- controlled (MDC) Ti:Sapphire and Cr:LiSAF oscillators to the near- and mid-infrared regions. This task is to be accomplished by combining the accumulated substantial knowledge of novel laser materials, diode-pumping, and mode-locking techniques, based on the implementation of dispersive dielectric mirrors and novel saturable- absorber material systems using semiconductor-doped-silica films. The research will be focused on the development and investigation of two novel laser systems with common methodology: * Directly diode-pumped compact cw tunable and ultrashort pulsed MDC sources around 1.5 microns, based primarily on Cr^4+:YAG and related materials. In particular, it is intended to realize Cr^4+:YAG laser delivering more than 250 mW in the continuous-wave and about 100 mW in the mode-locked regime. By optimization of the mirror dispersion and using newly developed mirrors with exceptionally low losses, shortening of the pulse- duration down to 20 fs is envisaged. Very recently the first directly diode-pumped continuous-wave operation of such a laser, yielding as much as 100 mW of output power, has been realized by the applicant. Studies with Nd:YLF pumping have shown the feasibility of the proposed power scaling approach. * Compact femtosecond sources, operating between 2 and 3 microns, based on the recently developed new class of vibronic Cr^2+-doped ZnSe-type crystals. These crystals are unique in their spectroscopic as well as thermal properties, comparable only to Ti:sapphire. The bandwidth of these crystals allows pulses as short as 20 fs, corresponding to as few optical cycles as in the 7-fs pulses in the 800 nm region. High power (380 mW) broadly tunable continuous-wave laser action of the transversly pumped ZnSe crystal has been very recently demonstrated, proving feasibility of femtosecond Cr^2+:ZnSe lasers. Within the framework of this project and in close relation with the progress in research a lecture course on the principles and applications of solid-state lasers is planned.

The broadband crystalline solid-state lasers ("vibronic lasers") are unique solid-state light sources capable to produce tunable continuous-wave and ultrashort-pulse radiation with duration nowadays of only few optical cycles. Both the tuning ability and femtosecond (10 -15 s) pulses are being increasingly used in science, technology and medicine. So far, the vibronic laser technology was limited to the visible and near-infrared regions. The project resulted in development of a number of novel laser systems, operating in the new wavelength regions further in the infrared. Most importantly, these new lasers are operating at room temperature and can be diode-pumped, meaning that they can be made compact, inexpensive and user-friendly. The wavelength range around 1.5 m is very important for telecommunications and medicine. For the first time the shortest light pulses at 1.5 m could be produced from the directly diode-pumped femtosecond Cr4+:YAG laser. These eye-safe pulses consist out of only five oscillations of the electromagnetic field and produce a broad range of spectral lines, covering the whole transmission window of the optical fibers used in telecommunications. The developed Cr4+:YAG laser has been further implemented for the generation of the ultrabroad spectra (supercontinuum), using the novel art of crystalline photonic fibers. These achievements open up unprecedented perspectives in telecommunications, as well as in medicine (allowing resolution on the level of a single cell through optical coherence tomography) and metrology (as a frequency comb generator). The infrared wavelength range of 2-3m and beyond is of particular interest for chemistry and medicine, since the frequencies involved coincide with frequencies of the internal vibrational motion of many molecules (the "molecular fingerprint" region). Within the project, compact directly diode-pumped tunable laser sources based on Cr2+:ZnSe and Cr2+:ZnS have been developed. They can serve advantageously for (i) remote light detection and ranging (LIDAR) with sensitivity of one part-per-billion in volume of many trace gases and vapours that are important in pollution detection and atmospheric chemistry, (ii) ultrasensitive detection of drugs and explosives with one part-per-trillion sensitivity using photoacoustic or intracavity spectroscopy, (iii) in medical applications like microsurgery, dentistry, keratectomy or non-invasive diagnostics by means of breath analysis. At the latest stage of the project the unique properties of Cr2+:ZnSe and Cr2+:ZnS crystals have led to the rather unexpected discovery of a new art of nanolasers. The latter opens up a new field of research of active ion doped nanophotonics, having fascinating perspectives and practical importance comparable only with that of semiconductor lasers. Along with numerous publications, the outcome of the project includes a recently published book1 , a lecture course on the principles and applications of solid-state lasers, as well as a Habilitation work and a PhD thesis. 1 "Mid-infrared Solid-State Laser Sources", Springer Topics in Applied Physics Reihe, herausgegeben von I. T. Sorokina and K. L. Vodopyanov, (Springer, Berlin, 2003)