L. J. Denes

Switchable vanadium oxide films by a sol‐gel process

Thin polycrystalline films of VO2 and V2O3 were deposited on a variety of substrates using a sol‐gel process. The orientation, microstructure, optical constants, and optical and electrical switching behavior are presented. These films exhibited sharp optical switching behavior even on an amorphous substrate such as fused silica. The method yields reproducible results and is amenable to the coating of large substrates and curved surfaces such as mirrors and lenses.

Investigations of glow discharge formation with volume preionization

The discharge formation process has been studied experimentally for CO2 planar TEA laser discharges. Theoretical models are presented which predict the preionization electron and ion densities, the spatiotemporal development of the discharge plasma, discharge voltage, and current waveforms, and the quasisteady operating characteristics of the discharge. The preionization is provided in the experiments by a pulse of ultraviolet radiation. The discharge formation model accounts for cathode photoemission and anode collection of electrons, discharge−circuit interactions, and gaseous ionization processes. The model predicts that photoemission and anode collection can be neglected when strong preionization and moderate overvoltages are used. When photoemission and anode collection are neglected, the discharge formative time is independent of the discharge volume. Calculated and experimental voltage and current waveforms are in very good agreement. The results of the calculations show that the discharge formativ...

Rotational temperature determinations in molecular gas lasers

The small‐signal gain expressions for vibrational‐rotational transitions are examined in detail to determine possible methods of extracting the rotational temperature from experimental gain measurements in molecular gas lasers. Approximate values of Tr can be deduced from the rotational quantum numbers for which the P‐ and R‐branch gains are maximum. Quite accurate values of Tr and the population inversion density (nv′−nv″) can be determined by fitting data to suitably linearized gain relationships, or by performing least‐squares fits of the P‐ and R‐branch experimental data to the full gain expressions. Experimental gain measurements for 15 P‐branch and 12 R‐branch transitions in the 10.4‐μm CO2 band have been performed for pulsed uv‐preionized laser discharges in CO2 : N2 : He=1 : 2 : 3 mixtures at 600 Torr. These data are subjected to the several gain analyses described herein, yielding a rotational temperature of 401±10 °K and an inversion density of (3.77±0.07) ×1017 cm−3 for conditions of maximum ga...

Laser gain characterization of near‐atmospheric CO2:N2:He glows in a planar electrode geometry

Temporally and spatially resolved gain measurements are reported for near‐atmospheric CO2:N2:He glow discharges in a contoured planar electrode configuration. When suitably preionized by rows of electrically stressed dielectric interfaces, the entire interelectrode volume is filled with a uniform glow discharge for large ranges of capacitor energy, voltage, and gas pressure. The 10.59‐μm gain reaches a peak value of 2.8% cm−1 in 2 μsec, and decays exponentially with a 20‐μsec time constant in near‐optimum laser mixtures of 60 Torr CO2, 180 Torr N2, and 60 Torr He. Peak gain is uniform over the transverse dimensions to within 5%. Mathematical expressions are developed which relate the small‐signal gain to the P and R branch rotational quantum numbers in a linear fashion. This analysis shows that for optimized laser conditions the 00°1–10°0 population inversion is [inverted lazy s]1.8×1017 cm−3 with a rotational temperature of [inverted lazy s]400 °K. This corresponds to a maximum 10.6‐μm optical storage de...

Arc suppression in CO2 laser discharges

We have parameterized the times to arc formation in CO2 : N2 : He mixtures using a TEA configuration which produces homogeneous 10‐liter laser discharges. The onset of arcing is suppressed by increasing the helium or decreasing the CO2 concentration. Arcs are also suppressed by adding a low‐ionization‐potential seedant such as tripropylamine. Direct variation of the uv spark intensity which establishes the initial preionization has minimal impact on the arc formation process.

Electrode‐ and preionizer‐geometry effects on TEA laser discharge formation

We show that both the shape of the electrodes and the spatial distribution of the preionization can play an important role in defining the cross‐sectional shape of uv‐preionized self‐sustained glow discharges in CO2 laser mixtures. Studies of a homogeneous 10×10×100 cm3 discharge are described.

Electrode surface field and preionization effects on the spatial distribution of arcs in CO2 laser discharges

We have studied the spatial distribution of arcing as a function of electrode spacing and preionization pattern in CO2 laser discharge mixtures, using circular electrodes which allowed a gap of up to 10 cm and a discharge volume of ∼3 liters. We utilized both cathode‐field enhancement with uniform preionization and apertured preionization with uniform field to define the discharge boundaries. In both cases, we found a threshold value of cathode‐field enhancement below which arcing was uniform and confined to the discharge region and above which arcing became highly nonuniform, with arcs localized to the high‐field region.

Evaluation of collision frequency in high-pressure glow discharges

We have used far infrared absorption techniques to measure the time resolved electron-neutral collision frequency in pulsed high-pressure glow discharges in nitrogen, helium, and CO 2 laser mixtures. The equilibrium collision frequency for a constant applied electric field has also been calculated as a function of E/N , the electric field-to-gas density ratio. The measured and calculated collision frequencies are in good quantitative agreement. The measured temporal variation of the collision frequency arises from the temporal variation of E/N in the discharge.