Annular-slot arrays as far-infrared bandpass filters.

Arrays of both annular and square annular slots in a conducting sheet on a dielectric substrate have been fabricated photolithographically. The structures are shown to behave as bandpass filters in the far infrared, with a resonant wavelength slightly larger than the average circumference or perimeter of the slot. The measured far-infrared transmittance of the annular array is approximately 76% of that predicted by theory, while its resonant frequency agrees with theory to within 5%.

Equivalent thin film of a periodic metal grid.

This paper establishes that any thin periodic metal grid on a plane boundary between two semi-infinite media, irradiated normally at any specified wavelength larger than its period, can be represented by an equivalent thin film. Explicit formulas for the parameters of this film are derived, and a numerical example of the equivalence is given.

Thin capacitive meshes on a dielectric boundary: theory and experiment.

A rigorous formulation is used to calculate the transmission properties of a thin, perfectly conducting biperiodic capacitive mesh on a dielectric boundary. The formulation is analogous to the well-known modal method used for inductive meshes, with the modal electric fields replaced by modal currents. Measurements made at submillimeter wavelengths are presented for square capacitive meshes on a crystal quartz substrate (n = 2.1). These measurements are shown to be in good agreement with the theory. The applicability of simple equivalent circuit models is investigated and the variation of the equivalent circuit parameters with the refractive index of the substrate is discussed. A modified expression of Babinet's principle is presented which is valid in the nondiffracting region for thin meshes on a dielectric interface.

Phase shifts in transmission line models of thin periodic metal grids.

This paper describes simple formulas for the phase shifts associated with reflection and transmission of electromagnetic waves by a thin periodic metal grid on a plane boundary between two lossless dielectrics, for the case where the wavelengths on both sides of the boundary are greater than the grid period. The phase shifts are expressed in terms of the zero-order intensity reflectance and transmittance of the grid and are correct even when the grid has loss. They correspond exactly to the phase shifts that can be calculated using the well-known equivalent-circuit models for thin metallic grids but are applicable for any thin nondiffracting layer on a dielectric boundary, for normal incidence.

Carbon dioxide laser tuning through 110 lines in 3 ms for airborne remote sensing.

Rapid tuning of a 3-m long glow discharge excited CO(2) laser is achieved by a rotating polygonal mirror scanning the intracavity beam across a diffraction grating. The system can tune through 110 different rotational lines spanning wavelengths from 9.2 to 11.1 microm in 3 ms with typical peak powers of 100-500 W at burst repetition rates up to 360 Hz and is suitable for airborne remote sensing. The effects of laser output coupler reflectance, burst rate, scan direction, and current modulation are investigated, and the performance of an optimized system for mineral reflectance measurements is presented. A reflection spectrum of a shale obtained with the rapidly tuned laser at a range of 5 m is shown to compare well to data gathered for the same sample using a slowly tuned cw CO(2) laser.

Strip gratings on dielectric substrates as output couplers for submillimeter lasers.

This paper describes the use and advantages of metallic strip gratings on dielectric substrates as output couplers for both optically pumped and discharge-excited submillimeter lasers. Formulas are presented for the calculation of transmittance and loss of such couplers, taking account of loss in the strip grating as well as loss and multiple reflections in the substrate. Included are expressions for the phase shifts on reflection and transmission by an arbitrary lossy grid on a plane boundary between two dielectrics according to a transmission-line model that is applicable for wavelengths in both dielectrics longer than the grid period. In relation to these phase shifts attention is drawn to an important sign convention. The theory is shown to agree well with measured transmittance of a typical device between 500 and 1600 GHz as well as spot measurements at 891 (337-microm HCN laser), 1540, and 1578 GHz (195- and 190-microm DCN laser). Finally, the theory is used to design a low-loss coupler for the low-gain 119-microm line of discharge excited H2O.

Scaling laws for cw 337-microm HCN waveguide lasers.

The gain and the saturation intensity of optimized cw 337-microm lasers are shown empirically to vary inversely as tube diameter. An expression is then derived for the power of a waveguide laser as a function of geometrical parameters, losses, and coupling. For optimized coupling the power is a strong function of tube diameter with a well defined maximum. Optimum diameter depends on tube length and losses only. The results agree well with the measured powers of three waveguide lasers delivering 30 mW, 100 mW, and 170 mW from discharges of 1-m, 2-m, and 3-m lengths. Such lasers are competitive with optically pumped submillimeter lasers.