Marko Zgonik

University of Ljubljana · Department of Physics

The terahertz range of the electromagnetic spectrum reveals important insights when studying material properties. We present an ultra‐broadband terahertz time‐domain spectroscopy system based on state‐of‐the‐art, high‐stability organic nonlinear optical crystals used both as THz‐wave generator and detector. In the transmission geometry, we achieve...

As an alternative to electrically injected diodes, UV light emission can be obtained via second harmonic generation (SHG). In weakly birefringent materials such as aluminum nitride (AlN), the phase matching of the driving and second harmonic waves can be achieved by the quasi-phase-matching (QPM) technique, where the polarity of the material is per...

Second-harmonic generation was studied in III-metal-polar GaN films grown on sapphire substrates by metalorganic chemical vapor deposition and formed into ridge waveguides. Broadband near-IR femtosecond pulses of an optical parametric amplifier system were injected by end-fire coupling and the nonlinear response was measured while tuning the centra...

Aluminium nitride (AlN) is a wide bandgap semiconductor with an intrinsic band-to-band absorption edge at ~200 nm wavelength. Therefore, the material is promising for optical use in the ultraviolet region. With a wurtzite-type, noncentrosymmetric crystal structure, AlN is also interesting for second order nonlinear optics and electro-optics. As gro...

Bulk aluminium nitride single crystals grown by physical vapour transport were characterised for their nonlinear optical properties. The two independent nonzero optical nonlinear coefficients were determined by the Maker Fringe method, with a LiNbO3 plate as a reference. At a wavelength of 1030 nm, the measured coefficient |d33| = (4.3 ± 0.3) pm V−...

Periodically poled AlN thin films with submicron domain widths were fabricated for nonlinear applications in the UV-VIS region. A procedure utilizing metalorganic chemical vapor deposition growth of AlN in combination with laser interference lithography was developed for making a nanoscale lateral polarity structure (LPS) with domain size down to 6...

AlN waveguides on sapphire substrates were fabricated from AlN films grown by metalorganic chemical vapor deposition. By tuning the wavelength of the pump light we demonstrated a second harmonic generation in the UV at 306, 331, and 356 nm using the d33 coefficient and modal dispersion phase matching (MDPM). A theoretical model for MDPM with two- a...

We report on measurements of the refractive indices and the absorption in bulk single crystals of aluminum nitride, in the region from 1 to 8 THz. The birefringence is approximately 0.2 and is larger than in the optical frequency range. Both indices exhibit normal dispersion with no pronounced absorption resonances. Optical power loss coefficients...

Dispersion of the extraordinary and ordinary refractive indices of Al- and N-polar AlN waveguides is measured by multiple angle-of-incidence and spectroscopic ellipsometry techniques. The polarity-controlled AlN layers are grown by metal–organic chemical vapor deposition on (0001)- sapphire substrates. Taking into consideration the different surfac...

We present a compact, room temperature, and narrowband terahertz source, based on difference-frequency generation in the organic nonlinear optical crystals OH1 (2-[3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene]malononitrile). The system employs a specific dual-wavelength infrared laser that emits coaxial, synchronous 10-ns pulses of similar...

We present a compact, room temperature, and narrowband terahertz source, based on difference-frequency generation in the organic nonlinear optical crystals OH1 (2-[3-(4-hydroxystyryl)-5, 5-dimethylcyclohex-2-enylidene] malononitrile). The system employs a specific dual-wavelength infrared laser that emits coaxial, synchronous 10-ns pulses of simila...

We demonstrate efficient generation of narrowband (

Optical waveguides of III-metal-polar and N-polar AlGaN are grown on sapphire substrates in order to test their use in integrated optics. The dispersion of the ordinary and extraordinary indices of refraction for films with aluminum mole fraction between 0.0 and 0.30 at four discrete wavelengths has been determined by the prism coupling method. The...

We present a tunable THz-source based on difference frequency generation in novel highly nonlinear organic materials (DSTMS, OH1) with a THz output power of about 16 μW and less than 100 GHz bandwidth.

We compare the oscillation thresholds and slope efficiencies in 10 ns pulsed eyesafe OPOs based on MgO-doped PPSLT and monolithic KTP. The optimal combination of low threshold and high slope efficiency is achieved with PPSLT.

We present a detailed theoretical and experimental study of the development of the EM field in pulsed, passively Q-switched solid-state lasers. We solve the rate equations separately for the gain medium and a passive Q-switch, thus accounting for the beam propagation and profile variations between them. Our model enables us to follow the pulse form...

The field of three-dimensional optical storage has lately witnessed a tremendous increase of interest from both the scientific and the industrial communities. Volume holographic storage was proposed by van Heerden [1] shortly after the introduction of the laser, and interest in this technology has continued at varying degrees of intensity since. Ho...

This study aims to fabricate optical planar waveguide structures by H+ ion implantation and direct electron beam patterning. Waveguide losses of ~20 dB are measured by light scattering detection. A reduction of the refractive index of the organic crystal is observed which is due to reduced polarizability of some molecules.

We report for the first time to our knowledge optical waveguiding in an organic crystalline waveguide produced by ion implantation. Using H⁺ ions a refractive index barrier suitable for waveguiding has been realized in the highly nonlinear optical organic crystal 4-N, N-dimethylamino-4’-N’-methyl-stilbazolium tosylate (DAST). The refractive index c...

Oxygen octahedra ferroelectrics are important materials because of their electrooptic and nonlinear-optic properties [1]. In this group, the perovskites BaTiO3 and KNbO3 crystals are especially promising materials. These two crystals are interesting for nonlinear-optical applications because of their large value of the spontaneous polarization at r...

In plate-like materials the smallest volume of holograms is achieved by confocally focusing the reference beam. With nonoverlapping records the information content in page-oriented monochromic recording scales as the square root of the plate thickness.

Optical waveguiding in the highly nonlinear optical crystal DAST has been demonstrated. Planar waveguides have been realized by H+ ion implantation whilst fs ablation has been used to pattern channel waveguide structures.

In this chapter, we summarized the effects of the various anisotropic material properties on light diffraction, space-charge field formation, and photorefractive two-wave mixing interactions. These properties include the birefringence, the dielectric constants, the dichroism, the electro-optic constants, the elastic and photoelastic constants, the...

Photostructuring by photobleaching and femtosecond laser ablation are promising techniques tor surface structuring with one processing step. We report our results of applying these two different structuring techniques to the highly nonlinear optical crystal DAST, where standard photolithography can not be easily applied.

In this chapter the results of scanning probe microscopic (SPM) investigations near ferroelectric (ferroic)—paraelectric phase transitions are presented. Submicroscopic investigations of domains near the transition temperature are of fundamental importance both for the basic understanding of the phase transitions itself but also for devices operati...

Photobleaching of organic optical materials can be used to structure the surface layer for integrated optics applications. Linear optical properties and absorption were studied in 4-N, N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST) organic crystals within the absorption band from 260–700 nm in order to determine the depth-range of photob...

We investigate two-color holography using Tb doped near-stoichiometric LiNbO3 . We present the measurements of the hologram strength and sensitivity dependences on the wavelength and intensity of the writing near-IR laser light and on the gating UV light intensity. The diffraction efficiency of the holograms is satisfactory when the writing light w...

We investigate two-color holography using Tb doped near-stoichiometric LiNbO3. We present the measurements of the hologram strength and sensitivity dependences on the wavelength and intensity of the writing near-IR laser light and on the gating UV light intensity. The diffraction efficiency of the holograms is satisfactory when the writing light wa...

The wavelength dependence of the space-charge field as induced in near-stoichiometric LiTaO3 crystals is investigated by measuring the bulk photogalvanic effect and the photoconductivity. LiTaO3 crystals of composition nearer to stoichiometry exhibit a pronounced reduction of the saturated space-charge field, which results from a larger increase in...

The complete set of self-consistent parameters of nominally undoped LiNbO3 crystals of congruent composition that describe the electro-optic, piezoelectric, elasto-optic, elastic, and dielectric response has been determined by numerically evaluating available measurements. The parameters were determined at room temperature and consist of the low-fr...

We report on the investigations of electrically switchable polymer dispersed liquid crystal gratings. Diffraction properties, grating anisotropy, and temperature dependencies were analyzed to obtain information on the liquid crystal orientation inside the ellipsoidal droplets and the distribution of the droplets in a polymer host. We compare the sa...

We report on the investigations of electrically switchable polymer dispersed liquid crystal gratings. Diffraction properties, grating anisotropy, and temperature dependencies were analyzed to obtain information on the liquid crystal orientation inside the ellipsoidal droplets and the distribution of the droplets in a polymer host. We compare the sa...

The polarization and wavelength dependence of photogalvanic current and photoconductivity has been studied for LiTaO3. Two different wavelengths (532 nm green and 350 nm UV lights) have been used. In order to study the optical damage, a focused beam of 532 nm green laser was used.

We investigate physical properties of polymer dispersed liquid crystal transmission gratings which influence the light diffraction efficiency behavior. Scanning electron microscopy is used to determine the orientation and shape of nanometer-sized liquid crystal droplets in a polymer host. Dependence of diffraction efficiency on light polarization s...

Optical second harmonic generation was measured across the ferroelectric phase transition in PbHPO4 under an applied electric field. The authors show that linear coupling of nonlinear susceptibility to the order parameter describes well the temperature and electric field dependence of d111. This indicates simultaneous reorientation of heavy ions an...

Light-induced changes in optical properties such as refractive index and absorption coefficients are always related to material excitation, i.e. deviation from (thermal) equilibrium [4.1]. Examples of material excitations are an increase in the number of conducting free electrons, the phonon density or the number of electrons occupying excited stat...

The field of three-dimensional optical storage has lately witnessed a tremendous increase of interest from both the scientific and the industrial communities. Volume holographic storage was proposed by van Heerden [5.1] shortly after the introduction of the laser and interest in this technology has continued at varying degrees of intensity since. H...

We present experimental and theoretical results of photorefractive one-dimensional self-focusing in an Fe-doped KNbO 3 crystal. The self-focusing is investigated by Z-scan experiments with and without an applied electric field. We observe the strongest focusing effect for a ratio I ˆ / I D ≈ 6 of the beam peak intensity to the so-called dark intens...

We describe an efficient photorefractive characterization method for new materials. The usual two beam coupling experiment in co-propagating geometry is not sensitive to the effective trap density in highly doped crystals. We present experiments in a counter-propagating geometry which are able to determine the effective trap density also for this c...

We give experimental and theoretical evidence that two-wave mixing light amplification in electro-optic and photoconductive materials is dramatically influenced by an anisotropic photoexcitation cross section of mobile charge carriers. An enhancement of the two-wave mixing gain is predicted and an increase of 90% is observed in a dichroic Ni doped...

LiNbO ₃ single crystals with a composition close to stoichiometry are grown by using a double crucible Czochralski method equipped with an automatic powder supply system. The photorefractive properties are compared with those of LiNbO ₃ crystals grown from the congruent composition melt. Two beam coupling experiments show...

Grating formation in pure KNbO3 crystals is observed after short pulse uv illumination which allows interband photoexcitation. The time resolved diffraction of 70 ps pulses at 532 nm on a nanosecond time scale is measured in geometries where an electro-optic contribution is significant and in geometries where it is forbidden by crystal symmetry. Di...

We show that high-temperature reduction in a CO–CO2 atmosphere increases the photorefractive sensitivity of KNbO3:Rh at 860 nm by 4 orders of magnitude compared with that of the as-grown crystal. The effective trap density is increased by a factor of 3, and the photoconductivity by a factor of 30, and the photorefractive response at a grating spaci...

The photorefractive properties of stoichiometric LiNbO3 crystals with a small number of defect densities grown by the double-crucible Czochralski method are investigated and compared with the defect densities of commercially available congruent Fe-doped LiNbO3 crystals. Two-wave-mixing experiments show that novel stoichiometric crystals exhibit lar...

We study the mechanism of operation of a ring-cavity self-pumped phase-conjugate mirror and focus our interest on the influence of reflection gratings on the response time and the reflectivity of the mirror. Our ring-cavity self-pumped phase-conjugate mirror is constructed around an Fe-doped KNbO 3 crystal. In the analysis we consider six possible...

We report observations of photorefractive grating fixing in potassium niobate at room temperature by means of ferroelectric domains. A grating is recorded while the crystal is simultaneously depoled. By repoling the crystal, the presence of a latent grating is revealed. This fixed grating diffracts light with an efficiency which increases when an e...

We determine the anisotropy of the mobility of photoexcited charge carriers in single crystals of KNbO3 and BaTiO3 by means of a purely optical method. In orthorhombic KNbO3 the mobility anisotropy is measured for both electrons and holes giving mua/muc = 1.05+/-0.06, mub/muc = 2.9+/-0.3 for electrons and mua/muc = 1.15+/-0.09, mub/muc = 1.9+/-0.2...

The coupled wave theory of Kogelnik [H. Kogelnik, Bell Syst. Tech. J. 48, 2909 (1969)] is extended to the case of moderately absorbing thick anisotropic materials with grating vector and medium boundaries arbitrarily oriented with respect to the main axes of the optical indicatrix. Dielectric and absorption modulation with common grating vector and...

Grating formation in pure KNbO3 crystals is observed after short pulse uv illumination which allows interband photoexcitation. The time resolved diffraction of 70 ps pulses at 532 nm on a nanosecond time scale is measured in geometries where an electro-optic contribution is significant and in geometries where it is forbidden by crystal symmetry. Di...

We have measured, for the first time, the low frequency (undamped) electro-optic and piezoelectric coefficients in Ce: BaTiO3 crystals using interferometric techniques. When the piezoelectric contribution to the phase change by a beam passing through the crystals is ruled out, the undamped electro-optic coefficients of the poled Ce: BaTiO3 single c...

We have measured,for the first time,the low frequency (unclamped)electro-optic and piezoelectric coefficients in Ce:BaTiO3 crystals using interferometric techniques.When the piezoelectric contribution to the phase change by a beam passing through the crystals is ruled out,the unclamped electro-optic coefficients of the poled Ce:BaTiO3 single crysta...

The photorefractive sensitivity of potassium niobate crystals doped with Ce, Co, Cu, Fe, Mn, Ni, and Rh and double-doped with Mn and Rh is investigated over an extended spectral range. We present experimental evidence on extrinsic properties important for the photorefractive effect, such as absorption and effective trap density. Photorefractive gra...

Summary form only given. Photorefractive holograms can be fixed in ferroelectric crystals by a structure of 180/spl deg/ ferroelectric domains, which electrically compensate the photorefractive space charge (domain fixing). Exactly how and where the domains are formed or distorted such that the photorefractive space-charge is compensated is not kno...

Self-aligning of optical links and good wavefront matching in heterodyne detection can be achieved through the use of phase conjugation. We demonstrate an optical self-aligning link between two mutually incoherent laser sources using photorefractive Fe:KNbO3 as a double incoherent phase conjugate mirror. The performance of a heterodyne detection sy...

Efficient optical pumping of impurity levels in BaTiO 3 is achieved using low intensity ultraviolet (UV) exposure. The resulting large photoinduced absorption in the visible is assessed in the context of interband charge transfer. An induced absorption of up to 12 cm^-1 is observed at 633 nm. The effect, which takes place close to the UV‐...

In a crystal without inversion symmetry there exist two-step indirect contributions to third-order nonlinear optical processes (cascading). Contributions to optical four-wave mixing occur through optical rectification and linear electro-optic effects. In contrast to cascading by second-harmonic generation, which has to satisfy strict phase-matching...

In order to achieve large refractive index nonlinearities one can take advantage of combined processes of second-order nonlinear optical effects. This large effective nonlinear refractive index n2 can be obtained in noncentrosymmetric media through (a) second-harmonic generation and difference-frequency mixing and/or (b) optical rectification and t...

Interband photoexcitation in photorefractive ABO 3 perovskites has only recently attracted the attention of researchers. Band to band excitation of electrons can lead to fast photorefractive response times [1] and, through trapping, to the sensitisation of impurity levels in the band gap. Strong effects are observed that lead to cw photorefractive...

It is known that the optical properties of oxide crystals, such as BaTiO 3 , LiNbO 3 and KNbO 3 , are strongly influenced by the presence of impurity levels in the bandgap of the material. The distribution and population of these levels have been the focus of much recent research into the tailoring of photorefractive materials to respond to near in...

Intensity thresholding in a self-pumped phase conjugate mirror with a ring cavity is investigated in order to use it for an intensity thresholder in all optical data processing and associative memory system. The rise time and the saturated value of the phase conjugate reflectivity of the mirror depend on the incident intensity. Using these dependen...

Spatially modulated electric fields can be induced optically in some ferroelectric materials. We review some of our data obtained for crystals of BaTiO3, which indicate that such photo-induced fields may influence antiparallel domain patterns in such a way that the space charge becomes compensated by the bound charge created by domain walls. This l...

We show that the three enigmatic lobes or petals observed in the far-field fanning pattern of photorefractive BaTiO3 crystals are correctly predicted by considering the piezoelectric and photoelastic contributions of crystal deformations to the photorefractive nonlinearity.

We relate the linear electro‐optic coefficients to the quadratic polarization‐optic and the elasto‐optic tensors by analyzing the dielectric phenomena driven by the spontaneous and induced polarization as well as the mechanical effects given by the deformation of the crystals. We determine the temperature dependence and the wavelength dispersion of...

Recently determined complete sets of materials parameters describing the dielectric, elastic, piezoelectric, elasto-optic, and electro-optic properties of BaTiO3 and KNbO3 crystals at room temperature are used to calculate the effective electro-optic coefficients and dielectric constants required for describing photorefractive phenomena. We show a...

Photorefractive crystals are those in which the refractive index can be electro-optically varied by an internal, light-induced space-charge field. Many photorefractive crystals are also ferroelectric, which means that they posess a spontaneous polarization that can be reversed by an electric field. The same light-induced space-charge field that alt...

Characterization and optimization of KNbO3 crystals for photorefractive applications over an extended spectral range is described. Intrinsic properties are used to describe the refractive index response to space-charge field gratings. Extrinsic properties that are important for the photorefractive effect, such as absorption, photoconductivity, char...