Ultrashort pulse chirp measurement via transverse second-harmonic generation in strontium barium niobate crystal

Abstract

Pulse compression in dispersive Strontium Barium Niobate (SBN) crystal with a random size and distribution of the antiparallel orientated nonlinear domains is observed via transverse second harmonic generation. The dependence of the transverse width of the second harmonic trace along the propagation direction allows for the determination of the initial chirp and duration of pulses in the femtosecond regime. This technique permits a real-time analysis of the pulse evolution and facilitates fast in-situ correction of pulse chirp acquired in the propagation through an optical system. Ultrashort laser pulses, with their variety of peak powers and durations, are becoming an important tool in an increasing number of applications in the fields of technology (materials processing, etc.) biomedical sciences and basic research in general. As the pulses become shorter, the dispersion effects that modify pulse properties during propagation through optical materials become increasingly relevant so a precise characterization of the pulse properties is needed. During the last decade different techniques have been implemented and extensively adopted [1][2][3][4] either for partial or/and complete temporal characterization of the pulses. They include for instance, Frequency-Resolved Optical Gating (based on the concept of optical gating) or Spectral Phase Interferometry for Direct Electric-field Reconstruction, (based on the concept of spectral interferometry) or even for 3D characterization (STARFISH, SEA Tadpole) [5][6]. Although a detailed field reconstruction is needed in particular applications and thus fully justifies the use of these advanced techniques, there are still many situations requiring just a partial characterization of the pulse. Hence simple and cost-effective methods of pulse measurement and characterization are always of potential interest.Typical as-grown ferroelectric crystals, such as Strontium Barium Niobate (SBN), exhibit a random-sized distribution of needele-like oppositely oriented ferroelectric domains all aligned parallel to the optical axis (z axis). A shematic representation of such domains is shown in Figure 1a. While the reversed orientation of domains corresponds to inversion of sign of the quadratic susceptibility, the refractive index of these crystals remains practically homogeneous [7]. Such crystals provide phase matching for frequency conversion processes over wide angular and frequency bandwidths without need for angular or temperature tuning as it is usual in typically used homogeneous nonlinear crystals [8][9][10]. Phase matching is obtained thanks to the continuous set of reciprocal lattice vectors, G, arising from the random size and distribution of the nonlinear domains, which entails a random distribution of the nonlinearity sign. These lattice vectors, with different modulus and orientation, lie in the xy plane. As a result, planar second harmonic emission is observed when the input fundamental beam propagates in the direction perpendicular to the optical axis. This effect consti...

Single-shot d-scan technique for ultrashort laser pulse characterization using transverse second-harmonic generation in random nonlinear crystals

“…The retrieved pulses have a duration of 46 fs (FWHM). The SBN dispersive properties obtained from the self-calibrating algorithm are GVD = 430 fs 2 /mm, in very good agreement with the expected value of GVD = 439.5 fs 2 /mm at 840 nm [18], and the value of the TOD is quite small, with no noticeable effect on the pulse propagation, as observed in a previous work [15]. In order to validate these results, we changed the input pulse spectral phase by introducing known amounts of dispersion and recovered its values from the d-scan measurements.…”

“…These unique emission properties make possible the detection of the SHG signal in the direction transverse to pulse propagation. This configuration has been recently used in transverse auto- [15] and cross- [16] correlation schemes, where the nonlinear signal was recorded directly from the top surface of the crystal, following the evolution of the TSHG signal along the length of the crystal, which is not possible in any other conventional autoor cross-correlation scheme. While transverse autocorrelation cannot provide direct information about the spectral phase, TSHG is well-suited to d-scan since the medium simultaneously performs the two functions needed for the technique.…”

“…While transverse autocorrelation cannot provide direct information about the spectral phase, TSHG is well-suited to d-scan since the medium simultaneously performs the two functions needed for the technique. First, since phase matching is no longer an issue, the TSHG crystals can be long [some millimeters (mm) or centimeters (cm) along the beam propagation direction] [15]. Therefore, the material dispersion along the beam propagation can be used as an effective dispersion scanning element.…”

“…Because of the SBN crystal dimensions and very favorable optical characteristics, namely its large phase matching spectral and transparency window, as well as high dispersion in the 800-900 nm spectral region [15], this experimental setup, including a 1 cm SBN crystal, is able to measure pulses with spectra corresponding to Fourier-limited durations up to 60 fs in this region. In addition, the application range may be extended towards longer Fourier-limited pulse durations by simply increasing the medium length, and, hence, the amount of dispersion that can be applied to the pulse.…”

Theory of noncollinear frequency doubling of transform limited pulses in non-steady-state regime

“…The non-collinear interaction is preferable for autoand crosscorrelation measurements of femtosecond pulses because of the background free auto-and crosscorrelation traces [5][6][7][8][9][10]. Recently disordered nonlinear media were successfully used for these purposes [11][12][13][14]. Despite the numerous studies on non-collinear SHG [5,11,15], the theory though lacks consistency.…”