Optical waveguides in LiNbO3 formed by ion implantation

Abstract

Ion implantation in LiNbO₃ can be used to modify the refractive index. The change in indices, n ₀ n _e results from the damage formed by energy deposited by the (dE/dx)_nuclear collisions between the ions and the lattice and is independent of the ion species. A saturation change in index of some ?7% occurs after a deposition of 10²³ keV cm^?3 at 300K, greater changes of ?9 % occur with implants at 77K. Annealing studies indicate the optical absorption formed during irradiation is removed below 200°C whereas the index changes exist up to 400°C. For optical waveguide production a negative change in the index is not ideal as the damaged layer cannot directly act as the region of optical confinement. However the (dE/dx)_electronic term is unimportant and so we have been able to form optical waveguides by ion implantation with light energetic ions (e.g. MeV He⁺ ions) because the damaged layer is then formed beneath an unchanged high index surface layer. This retains the desirable electrooptic properties of the single crystal LiNbO₃. The experimentally observed waveguide modes are in accord with our theoretical predictions of the refractive index profiles.     

Assessment of damage ranges by measurements of the refractive index in ion implanted LiNbO3 and LiTaO3

Ion implantation in LiNbO₃ and LiTaO₃ produces radiation damage by nuclear collisions. The amorphisation of the lattice reduces the refractive index of the material. In the case of fast ion bombardment with helium this damage layer is buried below the surface. The refractive index profile which then exists is suitable for an optical waveguide on the surface which supports several modes. Analysis of the refractive index profile yields the damage distribution in the crystal and this in turn can be compared with theoretical estimates of the damage production and ion ranges.

Our analyses show that in the high energy range from 0.5 to 2.0 MeV the depth of the damage is predominantly controlled by the electronic stopping.     

Light-induced refractive index changes in LiNbO3:Ti waveguides

Holographic measurements of light-induced refractive index changes in LiNbO₃Ti waveguides are reported. The results demonstrate that Fe²⁺ centers are stabilized against oxidation in the region of large Ti concentration near the surface thus increasing the sensitivity to optical damage considerably. This undesired effect may be largely abolished by additional in-diffusion of protons.     

Nonlinear optical waveguides fabricated in Mg-doped LiNbO3 by swift heavy ion irradiation: anomalous photorefractive damage behavior

Using swift heavy fluorine ion irradiation, we have successfully fabricated optical waveguides in Mg-doped LiNbO₃ substrates. A systematic characterization of these structures has been carried out including refractive index profiles, propagation losses, nonlinear coefficients, and, specially, photorefractive optical damage. Step-like refractive index profiles with Δn _e ≈ 0.1 and Δn _o ≈ 0.2, propagation losses lower than 0.5 dB/cm and high nonlinear optical coefficients similar to those of the substrate have been obtained. Unexpectedly, the photorefractive damage is only moderately reduced with regard to the one presented in congruent LiNbO₃ waveguides. Specifically, light intensity damage thresholds I _th are only a factor 2 higher at RT and a factor 4 at 90 °C with regard to undoped waveguides. At this latter temperature, a remarkably high I _th = 30.000 W/cm² is reached. A final discussion on the observed anomalous optical damage behavior induced by swift heavy ion irradiation is also included.     

Electron beam evaporated LaF3 thin films prepared by different temperatures and deposition rates

LaF₃ thin films were prepared by electron beam evaporation with different temperatures and deposition rates. Microstructure properties including crystalline structure and surface roughness were investigated by X-ray diffraction (XRD) and optical profilograph. X-ray photoelectron spectroscopy (XPS) was employed to study the chemical composition of the films. Optical properties (transmittance and refractive index) and laser induce damage threshold (LIDT) at 355 nm of the films were also characterized. The effects of deposition rate and substrate temperature on microstructure, optical properties and LIDT of LaF₃ thin films were discussed, respectively.     

Subluminal and superluminal propagation in Er<Superscript>3+</Superscript> doped fiber Bragg grating

The method to pump the FBG written into an Er³⁺-doped optical fiber is proposed to decrease or increase the group velocity of a probing pulse based on the fact that a pump-induced process changes the refractive index and dispersion associated with the ⁴I_15/2-⁴I_13/2 transition in Er³⁺-doped optical fiber. The system equations are derived. The group velocity modification is numerically demonstrated and discussed with the effects of an optical pump power, fiber Bragg grating length, doping concentration of Er³⁺ ions, and modulation amplitude of the grating.     

Origins of waveguiding in femtosecond laser-structured LiNbO<Subscript>3</Subscript>

Femtosecond laser-induced structural changes in LiNbO₃ are studied. Depending on the laser processing parameters two different types of modification are identified and their origin is discussed. Both types of modification can be described within the framework of induced lattice defects. For strong material damage a refractive index increase can be obtained due to the induced stress field. By appropriate tailoring of this stress field thermally stable and highly symmetric waveguides can be obtained well suited for nonlinear integrated-optical applications. PACS 61.80.Ba; 77.84.Dy; 42.65.Re; 42.82.Et     

Laser induced memory bits in photorefractive LiNbO3 and LiTaO3

We study experimentally the formation of refractive index voxels (volume elements) in photorefractive LiNbO₃ and LiTaO₃ crystals illuminated with high irradiance femtosecond laser pulses. We used 150 fs pulses at 800 nm wavelength (energy 6–50 nJ) tightly focused inside the crystals in a single shot regime. This resulted in a formation of a micrometer size region of elevated refractive index, which may be used as memory bits in information storage/retrieval application. The maximum refractive index change of 5×10⁻⁴ was recorded in undoped LiNbO₃ at an average light intensity of ∼TW/cm² that is close to the breakdown threshold. A simple setup for photorefractive recording and in situ monitoring of the refractive index changes has been proposed. M. Sūdžius leaves from: the Institute of Materials Science and Applied Research of Vilnius University, Lithuania.     

Band structure,spontaneous polarization and index of refraction above and below the curie point of BaTiO3 and KNbO3

The spontaneous polarization influence on the refractive index of BaTiO₃ and KNbO₃ perovskites is analysed from their band structure. The coupling effects between the conduction band edge and the spontaneous polarization are expressed through the Bloch orbital interactions and allow to interpret the presence of a precursor polarization in the paraelectric phase and the refractive index experimental data.     

Photochromic behaviour of doped Bi4Ge3O12 single crystal scintillators

The changes induced by ultraviolet (UV) illumination on the optical absorption and electron paramagnetic resonance (EPR) spectra of Bi₄Ge₃O₁₂ single crystals, doped either with Fe (and Gd) or Mn, have been followed at room temperature (RT). In both crystals several overlapping optical absorption bands develop under UV illumination, covering from 0.7 eV up to the band edge of the matrix. The optical damage can be bleached by heating the samples above RT or by illumination with visible light. Although these optical changes temporarily correlate with the variation of the Gd³⁺ and Mn²⁺ concentrations, it has been concluded that other defects are present and partially responsible for the optical damage.     

Nonlinear Refractive Index of Er3+-Doped Fiber and Its Application to Nonlinear Fiber Coupler

A large change in the refractive index originating in the transition absorption of erbium ions is induced in an Er³⁺-doped fiber. It is shown that the Er³⁺-doped fiber exhibits the nonlinear refractive index of n₂=7.4 × 10⁻¹⁵ m²/W, which is 10⁵−10⁶ times larger than that caused by the optical Kerr effect in a usual silica fiber at the pumping, wavelength of 514.5 nm. The large nonlinear refractive index occurred in a fiber directional coupler fabricated with the Er³⁺-doped fiber optically changes the coupling condition. The coupling ratio changes up to 0.16 by increasing the pumping power up to 0.7 mW.     

Observation of isotropic magnetic contribution to the refractive index of ABF3-type cubic crystals

Isotropic magnetic contribution to the refractive index of cubic crystals was observed by studying temperature dependences of the index in perovskite-type crystals KMnF₃, KCoF₃, KNiF₃ and KMgF₃. At low temperature magnetic contribution in KNiF₃ is $\text{δn}{}_{\mathrm{m}}\text{?}\text{3.3 × 10}{}^{\mathrm{?3}}$. Magnetic contribution was found to exist well above the temperature of magnetic ordering due to short-range ordering effects.     

Photoconductivity and photovoltaic behaviour of LiNbO3 and LiNbO3 waveguides at high optical intensities

The photoconductivity and photovoltaic currents of pure LiNbO₃ and proton exchanged waveguides in LiNbO₃ have been measured as a function of the optical intensity up to about several kW/cm² by the use of surface electrodes. For pure LiNbO₃ the observed dependences are a simple extrapolation of the well known low intensity behaviour. The photoconductivity of proton exchanged waveguides is considerably increased compared with pure LiNbO₃ and the curves are strongly nonlinear in the high intensity region. These results can explain, at least qualitatively, the previously observed characteristic time and intensity dependence of light-induced refractive index changes in this type of waveguides. Both the time and temperature behaviour of the dark conductivity of all proton exchanged waveguides give strong evidence of ionic charge transport in the proton exchanged region.     

Optical waveguide with homogeneous refractive index profile in LiNbO3 by double-low-energy Oxygen ion implantation

We report on the formation of the planar waveguide by 550 keV O ion followed by 250 keV O ion implantation in lithium niobate (LiNbO₃), at fluences of 6 × 10¹⁴ ions/cm² and 3 × 10¹⁴ ions/cm², respectively. The Rutherford backscattering/channeling spectra have shown the atomic displacements in the damage region before and after annealing. A broad and nearly homogeneous damage layer has been formed by double-energy ion implantation after annealing. Both the dark mode spectra and the data of refractive index profile verified that the extraordinary refractive index was enhanced in the ion implanted region of LiNbO₃. A homogeneous near-field intensity profile was obtained by double-low-energy ion implantation. There is a reasonable agreement between the simulated modal intensity profile and the experimental data. The estimated propagation loss is about 0.5 dB/cm.     

Metastable phases in HxLi1-xTaO3 waveguide layers and pure LiTaO3

The formation of high-temperature phases in low-doped H:LiTaO₃ waveguide layers in Z-cut LiTaO₃ has been observed both by refractive-index and IR-spectra measurements. This permits us to correlate the index jumps to the changes of the OH bonds in the crystal lattice. Reversible phase transitions were detected in the temperature interval T=50–200 °C over a wide range of hydrogen content including as-grown LiTaO₃. The high-temperature phases are metastable close to room temperature. This was demonstrated by tracing the time evolution of the refractive-index change. It was shown that the high-temperature phases are responsible for the long-term refractive-index instabilities in both H:LiTaO₃ waveguides and virgin LiTaO₃. Received: 7 May 2001 / Revised version: 10 August 2001 / Published online: 30 October 2001     

Heavy ion irradiation-induced phase transformation in polycrystalline Dy2O3

Radiation damage effects in polycrystalline pellets of the rare earth sesquioxide Dy₂O₃ irradiated with 300?keV Kr²⁺ ions were studied by combining grazing incidence X-ray diffraction (GIXRD) with transmission electron microscopy (TEM). Radiation damage was introduced using 300?keV Kr²⁺ ions to fluences up to 1?×?10²⁰?Kr?m^?2 at cryogenic temperature. GIXRD and cross-sectional TEM observations revealed that the crystal structure of the irradiated Dy₂O₃ transformed from a cubic, so-called C-type rare earth sesquioxide structure to a monoclinic, B-type rare earth sesquioxide structure upon ion irradiation. In addition, TEM and high-resolution electron microscopy (HREM) indicated that the transformed surface Dy₂O₃ layer adopts an epitaxial orientation relationship with the substrate Dy₂O₃.     

Cu-diffused layers in LiNbO3 for reversible holographic storage

Some holographic recording characteristics of Cu-diffused layers fabricated in Rh-doped LiNbO₃ have been examined in comparison with those fabricated in undoped lithium niobate.The diffusion layers, whose thickness depends on the diffusion temperature, are found to be enormously susceptible to optically-induced refractive index changes and to be able to attain a high diffraction efficiency. A distinguished difference is found between the persistence of holograms stored in Rh-doped and undoped LiNbO₃.     

Defect structure and optical damage resistance of Zn:Fe:LiNbO<Subscript>3</Subscript> crystals

A series of Zn:Fe:LiNbO₃ crystals were prepared by the Czochralski technique with 0.015 wt. % Fe₂O₃ content and various concentrations of ZnO. The ultraviolet-visible absorption spectra and the infrared absorption spectra of the Zn:Fe:LiNbO₃ crystals were detected in order to investigate their defect structure. Their optical damage resistance was characterized by the photoinduced birefringence change and transmission facula distortion method. The optical damage resistance of the Zn:Fe:LiNbO₃ crystals increases remarkably when the concentration of ZnO is over its threshold concentration (more than 6.0 mol. %). The effects of defects on the optical damage resistance of the Zn:Fe:LiNbO₃ crystals are discussed in detail. Received: 25 October 2002 / Revised version: 6 January 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +86-451/2300-926, E-mail: zzxxhhdoctor@sina.com     

Damage profiles in LiNbO3 by low-energy H+ implantation

Low-energy 120 and 150 keV H⁺ was implanted in z-cut LiNbO₃ at room temperature. The fluence of H⁺ is 5?×?10¹⁶ ions/cm². The damage profiles in LiNbO₃ induced by implantation were investigated using Rutherford backscattering/channelling. The damage profiles were extracted using the channelling results. The experimental damage profiles in LiNbO₃ were analyzed and compared to the simulated results from TRIM. The results show a good consistency between experimental and simulated results. The present results are useful for the fabrication of H-implanted waveguides of oxide crystals, especially LiNbO₃.     

Charge transport processes in LiNbO3:Fe at high intensity laser pulses

Light-induced refractive index changes in LiNbO₃:Fe crystals are investigated at high light intensities (>10⁹ Wm^–2). Holographic gratings are recorded and erased with frequency-doubled pulses of a Q-switched Nd:YAG laser. We find new intensity dependent contributions to the holographic sensitivity, to the photoconductivity, and to the saturation value of refractive index change. Light-induced absorption changes are also detected. These results indicate that the Fe²⁺/Fe³⁺ charge transport model, well established for low intensities, has to be modified for high intensities by assuming additional centers which trap and supply electrons.