Formation of domain structure by electron beam irradiation in congruent lithium niobate covered by surface dielectric layer with planar and channel waveguides produced by Soft Proton Exchange (SPE) process has been studied. Formation of domains with arbitrary shapes as a result of discrete switching has been revealed. The fact was attributed to ineffective screening of depolarization field in the crystals with a surface layer modified by SPE process. The dependences of the domain sizes on the dose and the distance between irradiated areas have been revealed. Finally, we have demonstrated that electron beam irradiation of lithium niobate crystals with surface resist layer can produce high quality periodical domain patterns after channel waveguide fabrication. Second harmonic generation with normalized nonlinear conversion efficiency up to 48%/(W cm2) has been achieved in such waveguides.

III-nitrides provide a versatile platform for nonlinear photonics. In this work, we explore a new promising configuration - composite waveguides containing GaN and AlN layers with inverted polarity, i.e., having opposite signs of the χ(2) nonlinear coefficient. This configuration allows us to address the limiting problem of the mode overlap for nonlinear interactions. Our modelling predicts a significant improvement in the conversion efficiency. We confirm our theoretical prediction with the experimental demonstration of second harmonic generation with an efficiency of 4%W-1cm-2 using a simple ridge waveguide. This efficiency is an order of magnitude higher compared to the previously reported results for III-nitride waveguides. Further improvement, reaching a theoretical efficiency of 30%W-1cm-2, can be achieved by reducing propagation losses.

We present the study of domain growth in congruent LiNbO3 crystals with planar soft proton exchanged waveguides (CLN SPE). The spatial distribution of H ions was measured by confocal Raman microscopy. The growth of elongated irregular shape domains followed by slow domain widening was obtained. Domain approaching led to a change in the growth directions. Domain widening represented formation of submicron domains with depth of tens microns in front of the domain walls. Low threshold field is attributed to the surface composition gradient. The suppression of hexagonal domain growth is related to existence of the surface dielectric layer.

The periodical domain structure with period 2 mu m was created in 1-mm-thick MgO doped lithium niobate single crystals by electron-beam irradiation. The second harmonic generation (SHG) around 370 nm was obtained during pumping the crystals by 100 mW continuous wave tunable Ti:Sapphire laser. The normalized efficiency was 0.05%/W. The dependence of the SHG power on the pump beam position has indicated that the periodical domain structure depth is about 300 mu m. The obtained results have demonstrated that the electron-beam poling can be used for creation of the short pitch periodical domain structures for light frequency conversion.

The domain growth during polarization reversal with small round liquid electrodes has been studied in 0.5-mm-thick 36 degrees Y-cut congruent lithium niobate, which is of practical interest for high average power nonlinear frequency conversion of ultra-short laser pulses. Studying the polarization reversal in constant field, we analyzed the time dependence of the switched domains fraction to obtain the domain nucleation probability and determine the activation field equal to 42 kV/mm. Studying the polarization reversal in rising field, we observed linear field dependence of the forward domain growth velocity with a threshold of 30.3 kV/mm.

We have studied the effects of proton source acidity and exchange duration at T=330°C. Surface SHG has shown that middle acidity is of great interest.

Despite their attractive features, integrated optical devices based on Congruent-melted Lithium Niobate (CLN) suffer from Photo-Refractive Damage (PRD). This light-induced refractive-index change hampers the use of CLN when high-power densities are in play, a typical regime in integrated optics. In bulk devices, the resistance to PRD can be largely improved by doping the lithium-niobate substrates with magnesium oxide. However, the fabrication of waveguides on MgO-doped substrates is not as straightforward as on CLN and either the resistance to PRD is strongly reduced by the waveguide fabrication process (as it happens in Ti-indiffused waveguides) or the nonlinear conversion efficiency is lowered (as it occurs in annealed-proton exchange). Here, we fabricate waveguides starting from MgO-doped substrates using the Soft-Proton Exchange (SPE) technique and we show that this combination represents a promising alternative. We demonstrate that, with a small adaptation of the exchange parameters, SPE allows producing MgO-doped LN refractive-index profiles almost identical to those produced in CLN without reducing the nonlinearity in the substrate. We also prove that the SPE does not affect substantially the resistance to PRD characteristics of MgO-doped substrates. Therefore, we think that SPE is the right recipe to outperform standard techniques and to fabricate robust and efficient waveguides for high-intensity-beam confinement.

Introduction: Lithium Tantalate (LT) has a shorter cut-off wavelength than Lithium Niobate (LN) which makes the material attractive for UV generation through nonlinear processes. While congruent LT (CLT) suffers strongly from photorefractive damage, MgOdoped LT (MgO:LT) is an interesting alternative as it was shown on LN that MgO reduces the photorefractive effects. On the other hand, fabricating highly confining, low-loss optical waveguides on LT while preserving its nonlinear properties is still an issue, as it was previously shown that the index variation obtained using proton exchange is one order of magnitude lower than that on LN and that direct Proton Exchange (PE) erases both nonlinear coefficient and periodic domains organization as on LN.

The domain structure evolution has been studied in congruent lithium niobate crystals with surface layers modified by three different proton exchange techniques. The significant decrease of the nucleation threshold field and qualitative change of domain rays nucleation and growth have been revealed. The formation of a broad domain boundary and dendrite domain structure as a result of nanodomains merging in front of the moving rays has been demonstrated. The obtained effects have been attributed to features of the domain kinetics induced by built-in electric field and retardation of the bulk screening of depolarization field.

The strong dependence of the domain kinetics on the applied field and the thickness of the surface layers produced by proton exchange has been revealed in congruent lithium niobate. The correlated nucleation leads to formation of self-assembled structures consisting of isolated domains. Formation of the nanodomains in front of the domain wall results in its continuous motion. The pronounced self-organization effect leads to formation of broad domain boundaries and ensembles of isolated nanodomains consisting of nanodomain chains and nets. The obtained effects were attributed to highly non-equilibrium switching conditions caused by retardation of the bulk screening of depolarization field.

In this paper, we present the material characterization that has been made to explain the poor nonlinear performance observed in certain channel waveguides produced by soft proton exchange in periodically poled congruent lithium niobate (PPLN) crystals. The study was performed using complementary methods of domain visualization: piezoelectric force microscopy and confocal Raman microscopy. It has been shown that the waveguide fabrication process can induce the formation of the structure of needle like nanodomains, which can be responsible for the degradation of the nonlinear response of the waveguides created in PPLN crystals.

In this paper we present the study of nano domains formation during the fabrication of channel waveguides produced by Soft Proton Exchange (SPE) in periodically poled congruent lithium niobate (PPLN) crystal. The study was performed using complementary experimental methods such as Piezo Force Microscopy (PFM) and Confocal Raman Microscopy (CRM). It shows that the waveguide fabrication process induces the formation of needle like nano domains on the Z oriented surface that may be responsible for the poor nonlinear behavior of the waveguides.

Formation and evolution of the bands with nanodomain structure in front of the moving domain wall were studied in proton exchanged LiNbO3 in field range. The width of the nanodomain bands grew with field increasing. For switching in constant field three types of domain structure evolution were studied: 1) continuous growth of hexagon domains at 21.0 kV/mm, 2) formation and growth of wide domain wall at 21.5 kV/mm, 3) growth of micro- and nanodomain ensembles at 22.0 kV/mm. Discrete switching being the origin of obtained effects is caused by retardation of depolarization field screening in crystal with dielectric surface layer.

Formation of abnormal domain shapes was studied in single crystalline LiNbO3 with surface layer modified by proton exchange. It has been shown that the isolated domain shape is very sensitive to the electric field: 1) three-rayed stars and concave polygons appear in low field, 2) hexagonal domains form in moderate field, 3) oriented domain rays grow in high field. The transformation from concave to convex polygon and the fast growth of narrow domain rays in front of the growing domain wall were investigated. The obtained abnormal behavior was attributed to non-effective external screening of depolarization field caused by artificial dielectric layer.

The abnormal domain kinetics in congruent lithium niobate single crystals caused by surface modification using proton exchange was studied experimentally. The dependence of domain kinetics on polarity of the growing domains at the surface with proton exchange was revealed. The abnormal smooth motion of irregularly shaped domain walls with finger-like features and correlated nucleation was observed for growth of domains with Z(+) at surface with proton exchange. Such behavior drastically differs from jump-like motion of plane domain walls oriented along Y directions observed both in conventional lithium niobate and for growth of domains with Z(-) at surface with proton exchange.

We study Erbium doped LiNbO3 waveguides for their suitability to store quantum information encoded into single photons, as required for a quantum repeater. Specifically, we perform stimulated photon-echo experiments for storage and readout of (classical) light pulses, which yield important information about the storage of single photon states. Furthermore, we investigate Stark-shift based line-shifts of the 1.53 μm transition, as required for a highly efficient quantum state storage protocol. Our findings demonstrate the potential of Er:LiNbO3 waveguides for quantum state storage. Introduction The last years have seen a remarkable advance of experimental quantum communication, in particular of quantum cryptography that promises informationtheoretic secure communication [1]. Yet, many problems still have to be overcome before a quantum secured communication network will be available. A major challenge concerns the increase of the transmission distance, which is, among others, limited through the combination of absorption in the transmission channel and detector noise. In contrast to classical telecommunications, a direct amplification of a single-photon quantum state is impossible. The concept of a quantum repeater [2] is based on several key elements, which are the creation and distribution of entangled pairs of photons over sub-sections of the complete link, swapping to extend the entanglement over the whole channel, and quantum memories that allow increasing the efficiency of the overall process. The entanglement can then be used for any kind of quantum communication task, for instance for quantum cryptography. Many schemes for storage of non-classical light have been proposed. However, only a few experiments can be mentioned in the context of quantum memory [3], and efficient, reversible transfer of quantum information between different species has not yet been accomplished. An original protocol for a quantum, as well as classical, memory in solid state material is based on controlled reversible inhomogeneous broadening (CRIB) of a single atomic absorption line [4]. It requires an atomic ensemble with a large optical depth and a long optical coherence time. It also relies on the possibilities to prepare, through optical pumping, a narrow absorption line on a nonabsorbing background, to broaden this line in a controlled and reversible way, and to apply a position dependent phase shift. Erbium doped crystalline waveguides are interesting candidates for the realization of CRIB, as interaction lengths of many centimeters can be achieved, allowing for large absorption even at low doping concenration. In addition, the 1.53 μm, I15/2→I13/2 transition in Erbium can feature coherence times in the ms range [5], and is well matched to standard telecommunication fibre, which allows future interfacing of such a memory with the standard telecommunication fibre network. In this paper we report on the storage, recall and measurement of classical optical pulses via stimulated photon echoes [6], which provides an important test-bed for future quantum state storage based on CRIB. Furthermore, we present investigations of the linear dc-Stark effect for controlled broadening [7]. The experiments took advantage of Er doped LiNbO3 crystals with waveguiding structures, cooled to a temperature of around 3.5 Kelvin. While LiNbO3 waveguides are extensively used in integrated optics, these are, to the best of our knowledge, the first studies in the context of all optical data storage. Photon-echo based storage of classical light pulses A common approach to storage and retrieval of light is based on three-pulse photon echo (3PE), also known as stimulated photon echo [6]. In this process a first, strong, optical write pulse excites the medium, creating an atomic coherence. The data pulses, a sequence of pulses encoding the information to be stored, are sent into the medium some time after the write pulse, which transfer the coherence into a frequency-dependent population grating in the ground and excited states. In order to retrieve the information, a third, strong, read pulse is used, which scatters off the grating and causes a photon echo to be emitted a time after the read pulse, which is equal to the time separation between write and data pulse [8]. If certain conditions for excitation energy and absorption depths are met, the echo is, to a high degree, an amplitude and phase replica of the stored data pulses. Now, consider a data field consisting of two pulses (D1 and D2) with an amplitude ratio R and relative phase φ (see Fig. 1). The 3PEs appear at times te = tr + tDi tw (i = 1, 2), where tr is the arrival time of the readout, tDi the arrival time of data pulse Di (i = 1, 2), and tw the arrival time of the write pulse. The echoes will thus be dt = tD2 tD1 apart. Because the efficiency of the 3PE is at best a few percent [9], much of the frequency-dependent population grating is preserved in the atomic ensemble after the read pulse. Therefore more echoes can be produced by sending in several read pulses. In our experiment, two subsequent read pulses were used to produce two copies of the data pulse. If we chose the distance between the read pulses to be dt, the same as the distance between the two data pulses D1 and D2, the echo of the second data pulse read out by the first read pulse (D2|R1), and the echo of the first data pulse read out by the second read pulse (D1|R2), will overlap and interfere (Fig. 1 c). The intensity of the echo in the central time interval (time-bin) is thus controlled by the phases of the write, the data and the read pulses, i.e. α1, α2/3, and α4/5, respectively. This is true provided that the phase coherence or amplitude ratio is not perturbed partially or totally during storage and retrieval. The sequence of two data pulses above is related to what is known in quantum communication as a timebin qubit [10]. A time-bin qubit is a coherent superposition of a photon being in two time-bins, separated by a time difference long compared to the coherence time of the photon. It can be written as: |ψ〉= c0 |1,0〉+ c1e |0,1〉, (1) where |1,0〉 (|0,1〉) represents a photon in the first (the second, respectively) time-bin, and φ=α2-α3 denotes their relative phase. In the present experiment, classical (strong) coherent pulses were used, with width smaller than the temporal spacing dt between the two pulses; the state of light is thus described by a Poisson distribution of photons (n~10), each of which is in the state described by Eq.1. Note that one could also describe our experiment as a setup containing two interferometers, as used for phase-coding quantum cryptography [1]: one interferometer prepares the time-bin qubits, i.e. here our two data pulses, while the second allows the projection measurement, i.e. our two read pulses. Let us now describe the experimental setup. The output from an external-cavity cw diode laser was gated by a combined phase and intensity modulator, followed by an intensity modulator (both fibre optic). The first modulator was used to create the five excitation pulses and to apply phase shifts to some of the pulses, depending on the particular experiment. The second modulator was synchronized to the first one and used to improve the peak-to-background intensity ratio. The pulses were then amplified by an Erbium Doped Fiber Amplifier (EDFA). In order to obtain a good background suppression (>70 dB), the EDFA was followed by an acousto-optical modulator, which opened only for the series of pulses and suppressed light for all other times. The resulting pulses had durations of tpulse=15 ns, with peak powers of around 5 mW for the write pulses, and around 1 mW for the other pulses. The first data pulse was created at tD1 = 0.6μs, the time between the data pulses was typically dt = 60ns and the read-out pulses were delayed with respect to the data pulses by 1 to 2 μs. The light was then coupled into an Er-doped LiNbO3 crystal (with waveguide, see below), which was cooled to 3.4 K by means of a pulse tube cooler. A magnetic field of about 0.2 Tesla was applied parallel to the C3 axis. This reduces decoherence due to spectral diffusion [11], resulting in a coherence time T2 of about 6 μs. Finally, the photon echoes were detected by a fast photo detector and displayed on an oscilloscope. The clock frequency was of 30 Hz, which ensured that all atomic excitations (featurig radiative a lifetime of 10 ms) had decayed between two subsequent storage/recall sequences. The z-cut LiNbO3 crystal was Erbium doped over a length of 10 mm by indiffusion of an evaporated 8 μm thick Er-layer at 1130°C for 150 h, leading to a Gaussian concentration profile of 8,2 μm 1/e penetration depth and 3.6x10cm surface concentration. The guiding channel was fabricated by indiffusion of a 7 μm wide, 98 nm thick Titanium-stripe at 1060°C for 8.5 h, resulting in a mono-mode guide with a mode size of 4.5x 3 μm FWHM intensity distribution [12]. The light was injected and collected with stanFig. 1: Illustration of the sequence of pulses for the interference of photon echoes. The data is read out twice and the phase between the data (or read) pulses is changed to produce interference in the central time bin. -100 -50 0 50 100 destructive interference constructive interference Ec ho In te ns ity (a rb .u ni ts ) time (ns) -2 -1 0 1 2 3 4 5 6 7 0.0 PE Ar ea (a rb .u ni ts )