In reproductive medicine little progress has been achieved regarding the clinical management of patients with a reduced ovarian reserve or poor ovarian response (POR) to stimulation with exogenous gonadotropins -a frustrating experience for clinicians as well as patients. Despite the efforts to optimize the definition of this subgroup of patients, the existing POR criteria unfortunately comprise a heterogeneous population and, importantly, do not offer any recommendations for clinical handling. Recently, the POSEIDON group ( Patient- Oriented Strategies Encompassing Individualize D Oocyte Number) proposed a new stratification of assisted reproductive technology (ART) in patients with a reduced ovarian reserve or unexpected inappropriate ovarian response to exogenous gonadotropins. In brief, four subgroups have been suggested based on quantitative and qualitative parameters, namely, i. Age and the expected aneuploidy rate; ii. Ovarian biomarkers (i.e. antral follicle count [AFC] and anti-Müllerian hormone [AMH]), and iii. Ovarian response - provided a previous stimulation cycle was performed. The new classification introduces a more nuanced picture of the “low prognosis patient” in ART, using clinically relevant criteria to guide the physician to most optimally manage this group of patients. The POSEIDON group also introduced a new measure for successful ART treatment, namely, the ability to retrieve the number of oocytes needed for the specific patient to obtain at least one euploid embryo for transfer. This feature represents a pragmatic endpoint to clinicians and enables the development of prediction models aiming to reduce the time-to-pregnancy (TTP). Consequently, the POSEIDON stratification should not be applied for retrospective analyses having live birth rate (LBR) as endpoint. Such an approach would fail as the attribution of patients to each Poseidon group is related to specific requirements and could only be made prospectively. On the other hand, any prospective approach (i.e. RCT) should be performed separately in each specific group.
We generate conical second-harmonic radiation by transverse excitation of a two-dimensional annular periodically-poled nonlinear photonic structure with a fundamental Gaussian beam. We show that these conical waves are the far-field images of the Bessel beams generated in a crystal by parametric frequency conversion assisted by nonlinear Bragg diffraction.PACS numbers: 42.65.Ky, 42.25.Fx The second-harmonic generation, i.e. conversion of two photons of the fundamental wave into a single photon at twice the frequency, belongs to one of the most intensively studied nonlinear effects [1]. Such a process takes place in quadratic optical media, i.e. nonlinear media without the center of inversion. It is well established that the efficiency of this as well as other similar parametric processes depends critically on the so-called phase-matching condition which in birefringent crystals is commonly achieved with the temperature or angle tuning. In media with weak birefringence, an efficient frequency conversion is achieved by the quasi-phase matching (QPM) [2]. The QPM technique involves periodic modulation of the second-order nonlinearity of the material. In the ferroelectric crystals such as LiNbO 3 or LiTiO 3 , this can be easily realized by spatially periodic poling [3]. In this way one can create one-or twodimensional nonlinear structures, the so-called χ (2) photonic crystals [4,5]. Depending on the symmetry and form of the poling pattern the efficient second-harmonic generation (SHG) can be realized for waves propagating in particular spatial directions [6]. Typical geometries involve, in one spatial dimension, single or multi-period rectangular pattern, and, in two spatial dimensions, nonlinear photonic structures of hexagonal or square symmetries.In this Letter, we report on the generation of conical second-harmonic waves by excitation of a twodimensional annular χ (2) nonlinear photonic fabricated in Stoichiometric Lithium Tantalte (SLT) crystal with a single Gaussian beam. We show that the observed rings of the second-harmonic radiation actually represent the far field of the Bessel beams generated in the crystal via the multi-order nonlinear Bragg diffraction.We consider a two-dimensional nonlinear photonic structure with circular periodic array of ferroelectric domains with a constant linear refractive index. In general, such two-dimensional QPM photonic structures have been explored exclusively in the longitudinal geometry [4,5,7,8] of the second-harmonic generation when the fundamental beam propagates perpendicular to the domains boundaries of periodically varying second-order nonlinearity. Here, we explore a novel transverse geometry when the fundamental light beam propagates along the axis of the structure. The front facet of the annular poled Stoichiometric Lithium Tantalate sample used in our experiments with clearly visible domain boundaries is shown in Fig. 1(a) [for technical details, see [7]]. It is a Z-cut, L = 0.49 mm thick slab with the QPM period of 7.5 µm and duty factor varying inside the...
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