Georg A. Reider

Determination of the nonlinear optical susceptibility χ(2) of surface layers by sum and difference frequency generation in reflection and transmission

The theoretical investigation of sum and difference frequency generation in thin surface layers with rotational symmetry leads to formulas which connect the generated light intensities to the surface second order nonlinear susceptibility tensor. A maximum of seven tensor components can be determined in the case of lowest symmetry. Measurements in transmission should be especially useful since they allow easy variation of both polarization and angle of incidence. On the other hand, large signal enhancements are expected for total internal reflection geometries. A consistent set of χ(2) tensor components for a thin layer of rhodamine-6G adsorbed on fused silica is found based on data from reflection and transmission measurements.

Observation of rapid exciton-exciton annihilation in monolayer molybdenum disulfide.

Monolayer MoS2 is a direct-gap two-dimensional semiconductor that exhibits strong electron-hole interactions, leading to the formation of stable excitons and trions. Here we report the existence of efficient exciton-exciton annihilation, a four-body interaction, in this material. Exciton-exciton annihilation was identified experimentally in ultrafast transient absorption measurements through the emergence of a decay channel varying quadratically with exciton density. The rate of exciton-exciton annihilation was determined to be (4.3 ± 1.1) × 10(-2) cm(2)/s at room temperature.

Single-shot measurement of terahertz electromagnetic pulses by use of electro-optic sampling

We demonstrate a simple scheme for capturing the temporal waveforms of a freely propagating terahertz electromagnetic transient in a single shot. The method relies on electro-optic sampling in a noncollinear geometry for the terahertz radiation and the visible probe beam, coupled with multichannel detection. The approach provides time resolution that is comparable to that of conventional electro-optic sampling measurements.

Laser ignition of methane–air mixtures at high pressures

Abstract Laser induced plasma ignition of methane–air mixtures at initial pressures up to 4 MPa was investigated experimentally using nanosecond Nd:YAG-laser pulses at 1064, 532 and 355 nm. Variations of the focusing system yielded, independent of the laser wavelength, an optimum focal spot diameter of about 20 μm under the experimental conditions, indicating that small impurity particles provide the seeds for laser plasma generation. The minimum laser pulse energy needed for ignition (ranging from 8 to 15 mJ) decreased with increasing initial pressure and with increasing equivalence ratio in the mixture regime of φ =0.91–0.56. Corresponding threshold intensities ranged from 10 10 to 10 11 W/cm 2 . Evidence for a non-resonant breakdown mechanism was established.

Fast latching type optical switch

A magneto-optical latching switch with a yttrium orthoferrite optical rotator is described. The crystal remains in a given magnetic state for unlimited duration without energy supply. The response time is about 20 ns and the aperture of the optical rotator is 350 μm.

Magnetooptic switch based on domain wall motion in orthoferrites

A prototype of an optical switch based on Faraday rotation in yttrium orthoferrite crystals is developed. The switch is of latching type and has an operating time below 100 ns. Integrated arrays of switches are feasible due to the dimensions of the optical rotator of less than 3 mm/sup 3/.

Efficient femtosecond laser micromachining of bulk 3C-SiC

We demonstrate surface micromachining of bulk 3C silicon carbide (3C-SiC) wafers by employing tightly focused infrared femtosecond laser pulses of energy less than 10 nJ directly from a femtosecond laser oscillator, thus eliminating the need for an amplified system and increasing the micromachining speed by more than four orders of magnitude. In addition, we show that high aspect ratio through-tapered vias can be drilled in 400 µm thick wafers using an amplified femtosecond laser.

Electroelastic effect in alpha quartz

The electroelastic effect of α quartz is investigated by means of high‐resolution measurements of ultrasonic transit times as a function of a superimposed biasing field. Experimental results are reported for 25 different configurations of wave normal, displacement, and electric field vectors. It is shown that these results may consistently by expressed in terms of an effective fifth‐rank tensor with eight independent components.

Magnetization reversal and aftereffect in orthoferrites

A magnetic aftereffect of several μs duration has been discovered in 1.2 mm thick samples of yttrium orthoferrite crystals cut perpendicularly to the optical axis at 1.3 μm wavelength. The aftereffect is associated with nucleation and expansion of oppositely magnetized domains that occur in a monodomain sample after removal of an external magnetic field. Magnetization reversal processes are visualized in a Faraday setup and studied by measuring the temporal dependence of the state of polarization of a transmitted light beam.

Surface studies with optical second-harmonic generation

Abstract This introductory review covers the application of the non-linear optical processes of second-harmonic and sum-frequency generation to the study of surfaces and interfaces. The technique provides an intrinsic surface sensitivity on the scale of a single monolayer for surfaces and interfaces of all centrosymmetric media. The newly developed method is capable of providing a broad range of information on surface and interface properties, such as orientation of molecular adsorbates, relative adsorbate coverage, surface symmetry, electronic and vibrational spectroscopy, and time resolved data. The application of the technique to diverse problems of interface and surface characterization is emphasized.