The properties of NPC structures in strontium tetraborate are analyzed. Different types of NPC structures are revealed that possess different nonlinear properties, and their spectral dependences of frequency conversion efficiency are calculated and compared. Experimental study of these structures is reported for the process of doubling of the second harmonic of fs Ti:S laser. Tuning of generated radiation is obtained in the range 187.5 - 232.5 nm, with extreme insensitivity to the angular orientation of NPC. Behavior of tuning curve along investigated fundamental wave range is similar in all studied samples, but efficiency obtained depends on the type of structure. Conversion efficiency and spectral quality of generated radiation is experimentally shown to grow better when using NPC with improved structure. Prospects of VUV converter on a single NPC are discussed. NPCs of SBO are demonstrated to be useful for autocorrelation diagnostics both in random QPM geometry and in the geometry of nonlinear diffraction from virtual beam.

Using a nonlinear photonic crystal (NPC) of strontium tetraborate, we obtained the second-harmonic generation of femtosecond pulses in the nonlinear diffraction regime with an efficiency of 1.9% that is tunable in the near-UV spectral range, as well as fourth-harmonic generation under the conditions of random quasi-phase-matching with maximum efficiency of 10−5 that is tunable in the far-UV spectral range. The efficiency of second-harmonic generation in strontium tetraborate NPC under the conditions of random quasi-phase-matching was significantly lower. The fourth-harmonic was tunable between 187.5 and 232.5 nm. The previously predicted red shift of spectral components in the generated radiation spectrum upon NPC rotation is demonstrated experimentally.

Random quasi phase matching (RQPM) in nonlinear photonic crystal was investigated for the conversion of both narrowband and broadband fundamental radiation. Angular dependence of RQPM effect was experimentally studied, and agreement with a simple theoretical model was demonstrated. The enhancement factor due to RQPM is shown to be fairly predictable on the basis of known NPC geometry. The spectral dependence of RQPM effect was calculated for a number of NPC structures that are typically produced in SBO. Main features of these dependences will be discussed. The relation between angular and spectral dependences is analysed.

Random (stochastic) quasi-phase matching is demonstrated for broadband radiation in a one-dimensional irregular strontium tetraborate nonlinear photonic crystal. Harmonic generation tunable in the range 187.5 — 215 nm is achieved. The average power of the generated signal is a quadratic function of pump power and reaches 1 μW. In a steady-state model, taking into account sum frequency generation within the spectrum of the input light causes no broadening of the peaks in the spectrum of the generated signal.

Noncollinear frequency doubling in a random nonlinear photonic crystal of strontium tetraborate via nonlinear diffraction from virtual beam is investigated. This effect is shown to be useful for ultrashort pulses characterization in broad spectral range. Nonlinear photonic crystals of strontium tetraborate are a promising medium for ultrafast diagnostics in short-wavelength applications.

Strontium tetraborate (SBO) possesses transparency in the VUV that allows nonlinear generaton to the wavelengths as short as 125 nm. The existence of domain structures consisting of alternating oppositely poled domains is found in as grown crystals of SBO. Domain walls are perpendicular to a crystallographic axis. In this paper non-collinear generation (nonlinear diffraction) of the second and third harmonics of nanosecond Nd:YAG laser is used for the investigation of reciprocal superlattice vectors (RSV) spectrum of the domain structures. When fundamental radiation propagates in the vicinity of direction perpendicular to the domain walls, random quasi phase matching (RQPM) was observed in the process of generation of 266 nm radiation via doubling of the second harmonic of nanosecond Nd:YAG laser. Maximum efficiency of conversion is 0.5% and exceeds that obtained for 2D nonlinear photonic crystal based on strontium barium niobate.