Diffraction in optics is the main wave process which sets the limiting size for the localization of an optical field in its scattering from an obstacle or its passage through a focusing system. For a collecting lens with a round aperture the diffraction limit for the focusing of radiation with a plane wavefront is roughly one wavelength. This classical diffraction limit was obtained under the assumption that after the lens the light wave freely propagates to the focal region through a distance

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... uch longer than the wavelength, i.e. that the electromagnetic field diffraction is considered in the far-field zone. Meanwhile, there exist the near-and transition-field zones of diffraction near objects of different physical nature illuminated by optical radiation. In these zones the light field has a more complicated spatial structure and may be localized tighter than prescribed by the classical treatment. In particular, this effect underlies the operation of near-field microscopy devices and the subwavelength field focusing in the excitation of plasmon polaritons at the surface of metallic objects. One more way of obtaining extreme light field localization is the use of symmetric transparent dielectric mesoscale objects (microspheres, cylinders, ellipsoids) as optical focusers, i.e. objects whose transverse dimensions exceed the scattered light wavelength by no more than an order of magnitude. When a light wave is incident on such an object, in the near-field diffraction zone in the geometrical shadow region there forms a narrow light structure termed a photon Nanojet (PNJ) [1]. This PNJ may be close to diffraction-limited in the transverse dimension (of the order of a wavelength) and may persist almost without variation in shape for a distance equal to several radiation wavelengths in the medium. The PNJ is nothing more than the region of external focus of the light wave diffracted by a transparent particle. However, owing to the strong near-field object effect this wave structure has dimensional characteristics untypical for an ordinary collecting lens. The region of light focusing by a micrometer-sized particle stretches anomalously along the direction of radiation incidence to acquire the shape of a light jet produced by the interference of the waves transmitted through and refracted by the particle. The problems of enhancing the longitudinal and transverse localization of the optical field in a PNJ were considered in several works with the corresponding variations in particle size and refractive index, internal structure (gradient inhomogeneous refractive index) and shape or with the use of resonator properties of transparent microspheres. In the latter case, by way of exciting the natural structural (morphological) resonance of a dielectric sphere [2] by incident radiation it is possible to obtain an anomalously narrow PNJ near the particle due to the decaying eigenmode field 'leaking' through the surface. The resonance excitation of internal optical field in a particle is an intricate technical problem by itself and may require either a precision matching of the microsphere radius to the incident radiation wavelength or the use of a spectrally tunable radiation source for a given size of the available particle. Here, we proposed the use of an ultrashort (femtosecond) laser pulse, which is initially spectrally broad, for the resonance excitation of the optical field of a microsphere. It turns out that the scattering of such a pulse by the particle almost always results in the resonance excitation of the internal optical field, when the eigenfrequencies of one or several high-Q resonance modes of the particle fall within the central part of the initial radiation spectrum. The PNJ formed under this pulsed irradiation scenario also turns out to be transient and generally comprises non-resonant and resonant time phases [3] . In this report we review the problem of PNJ control and present our recent results concerning the temporal dynamics of a PNJ emerging in the scattering of ultrashort laser radiation by a transparent spherical microparticle. Using numerical simulations we show that the late phases of PNJ development are characterized by a tight subdiffraction spatial localization of the optical field near the particle.