“Continuum” or “white” light generation is a technique commonly used for spectroscopic purposes to produce a very broad spectrum of light. The technique relies on non-linear optics, and produces an optical spectrum which covers the all visible range and beyond starting from a light pulse initially spectrally much narrower (a few tens of nanometers or less). Conventionally, white light continua used for u1ltrafast broadband spectroscopy are produced with an amplified laser system, by focusing an intense femtosecond pulse (1 microJ typically) into some bulk, transparent medium (water, sapphire, etc…). In that case, the non-linear process causing the spectral broadening is mainly self phase modulation. Such white light sources are used by several teams at DON in various ultrafast spectroscopy experiments.
Non linear optics is observed with enhanced efficiency in fibers as compared to bulk materials, since the power of the guided light remains concentrated over a short area (inducing high intensities) all over the propagation trough the fiber. In addition, photonic crystal fibers are microstructured fibers made of silica, in which the dispersion and birefringence properties can be tuned by the details of the design of the microstructures. Hence specific non linear processes can be enhanced. White light continuum generation is one major application of these fibers: when a few-nJ femtosecond pulse of light is injected in such a fiber, the pulse broadens from a few tens of nanometers to several hundreds . Picosecond and nanosecond light sources work equally well, although different non-linear processes are involved, but the continuum needs a longer distance of to develop fully, so that several-m long fibers are currently used. Apart from PCFs, tapered fibers are non-structured mono-mode fibers which have been stretched so as to reduce the core diameter to or below the wavelength of the injected pulse. In any case, the output spectrum spreads over the all visible spectral range (“white light”) and even further. In the femtosecond regime, self-phase modulation and four-wave mixing are the typical non-linear mechanisms which are involved in this phenomenon. In addition, when the fiber design is such that the incident pulse propagates in the anomalous dispersion regime, solitonic propagation phenomena are involved which further extend the spectra towards near-UV and near-IR, producing what is called a “supercontinuum” (SC) of light. These supercontinua find many applications, one of the most famous being the implementation of stabilized “frequency combs” which revolutionized optical frequency measurements for metrology purposes .
At DON the BIODYN team has built a novel set-up for ultrafast spectroscopy with broadband detection based on a PCF. A non-amplified laser source (Ti:Sapphire oscillator by KML) is used to inject a few-nJ of a 40-fs pulse into a sub-cm short piece of a birerefringent PCF produced at XLIM, Limoges (France). The spectro-temporal distribution of the SC pulse is obtained by time-gated two-photon absorption in a semi-conducting ZnS plate of thickness 0.1 mm. It is displayed on Fig. 1 with artificial-color scale, along with the spectral power distribution of the SC. We show that an ultrabroad spectrum ranging from 450 nm to beyond 1.1 micron is delivered as a single pulse, the overall duration of which is about 300 fs. The novelty is to observe a single pulse (thanks to the shortness of the fiber) with such a broad spectrum. Another novelty is to use that SC for ultrafast spectroscopy with a non-amplified laser system. Ultrafast transient spectroscopy of (bio)-molecules in solution is routinely performed with this system, with broadband detection and excellent sensitivity and stability