L. F. Johnson

Coherent Emission from Rare Earth Ions in Electro‐optic Crystals

We report the observation of coherent emission from three rare‐earth ions in electro‐optic crystals. Oscillation is observed from Nd3+ ions in Bi4Ge3O12, LiNbO3, and Ba0.75Ca0.25Nb26O at room temperature, and from Tm3+ and Ho3+ ions in LiNbO3 at 77°K. Second‐harmonic radiation is obtained from LiNbO3:Tm3+ and electric‐field modulation of threshold is observed for certain crystals of LiNbO3:Nd3+. The latter effect is believed to arise from crystal strain. The propagation angle for phase‐matched second‐harmonic generation is calculated to lie at 43° to the c axis for both LiNbO3:Tm3+ and LiNbO3:Ho3+ at 77°K. Phasematched second‐harmonic generation from LiNbO3:Nd3+ is not possible, since the maser line is an extraordinary wave. However, the polarization of the maser line in LiNbO3:Nd3+ is appropriate for phasematched parametric frequency conversion in the near infrared. Attempts to observe such radiation were not successful due to the limited power available in the fundamental.

Infrared‐to‐Visible Conversion by Rare‐Earth Ions in Crystals

An analysis of infrared conversion to visible and ultraviolet radiation by Yb3+–Er3+, Yb3+–Ho3+, and Yb3+–Tm3+ ions in crystals is presented. The expression for the visible power output in the presence of back transfer from the active ion to the energy‐transferring ion (Yb3+) is given and the relationship between the intermediate state transfer and back transfer coefficients for maximum output is found. If this relationship is satisfied the visible output in the presence of back transfer is equal to the maximum output with no back transfer. The behavior of the power output when the transfer and back transfer coefficients depart significantly from this optimum condition is examined and the parameters which govern the power output under these conditions are determined. The analysis is applied to Yb3+–Er3+, Yb3+–Ho3+, and Yb3+–Tm3+ ions in BaYF5 and BaY2F8. Measurements on the brightest composition of Yb3+–Er3+ ions in BaYF5 indicate that the advantage of a long‐lived intermediate state is diminished by back...

Optical Maser Characteristics of Rare‐Earth Ions in Crystals

Several divalent and trivalent rare‐earth ions incorporated in various host crystals have been found to exhibit stimulated emission in the near infrared. A report is presented describing some of the basic characteristics of these materials: absorption and fluorescence spectra, energy level diagrams, optical maser wavelengths and operating temperatures, and thresholds for stimulated emission. Recent observations on the CaWO4:Nd3+ optical maser in continuous operation are also described.

Generation of periodic surface corrugations.

The generation of periodic surface corrugations by ion-beam milling and chemical etching of grating relief patterns in photoresist is analyzed. A general treatment is developed for gratings of any desired period on substrates of arbitrary reflectivity, but particular emphasis is given to the generation of gratings with deep grooves and fine periods (Lambda < 3000 A) on GaAs. Analysis of the intensity distribution in photoresist for both p- and s-polarized incident beams reveals that the standing waves generated by reflection from the substrate are diminished for p-polarized beams, but the existence of a displaced grating for certain ranges of substrate reflectivity and angle of incidence severely limits groove depth in resist. The requirements are given for the establishment of an intensity maximum at the photoresist-substrate interface, a condition desired for subsequent chemical etching. It is shown further that the alternative use of a quarterwave intermediate oxide layer to achieve this condition on GaAs results in a lower limit being imposed on grating period. Constantintensity contours approximating the groove profiles in resist demonstrate that an imbalance in incident beam intensity may lead to severing of the resist stripes, and the dependence of this phenomenon on substrate reflectivity is determined. For beams of equal intensity, a similar phenomenon occurs with increasing reflectivity of the substrate. The transfer of a grating relief pattern to the substrate by ion-beam milling is treated by considering the erosion profiles produced by ion bombardment. This analysis is used to examine the influence of milling geometry on the depth and shape of the groove. Although the ion-beam milling rate of GaAs is several times greater than AZ-1350 photoresist, it is shown that the groove aspect ratio (depth/period) in GaAs can be no more than about 1.2, a figure that is obtained, surprisingly, by milling at the angle of maximum removal rate of photoresist. For a metal substrate, the groove aspect ratio decreases with increasing grating period. For gratings produced by chemical etch, the problem of the weakly exposed layer of resist adjacent to the substrate is solved by using a combination of ion-beam milling and chemical etching. Using a preferential chemical etch with a sufficiently slow etch rate, gratings with well-defined planar features, a period ~2500 A, and a groove aspect ratio >0.6 have been produced on GaAs.

Phonon‐Terminated Coherent Emission from V2+ Ions in MgF2

Infrared fluorescence and phonon‐terminated stimulated emission from V2+ ions in MgF2 is reported. Fluorescence is produced in transitions from the first excited state 4T2(4F) to the ground state 4A2(4F). Stimulated emission occurs in the vibronic sideband at 1.1217 μ (8915 cm−1) and lies 1166 cm−1 from the no‐phonon line. A gain curve calculated from the optical spectra predicts an oscillation frequency in reasonable agreement with the observed frequency, with a cross section for stimulated emission of 8×10−21 cm2. From the effect of excited state absorption on the gain profile it is concluded that the excited state absorption cross section at the maser frequency is <3×10−21 cm2.

Optical Maser Characteristics of Nd3+ in CaF2

Stimulated emission was observed at 77 deg K. Lamp intensity variation with time, including fluorescence and stimulated emission and scattered light at 1.046 gamma at 77 and 95% above the threshold energy, was measured. Data are presented in graphical form. (L.N.N.)

Evolution of grating profiles under ion-beam erosion

The faceting phenomenon of ion-beam erosion is used to determine erosion profiles of grating relief patterns on surfaces. The orientation of the planar features formed on an eroding mask are defined in terms of the erosion geometry angles ø and gamma the angle of maximum mask erosion rate over the profile. The geometry required to produce a single facet inclined at any prescribed angle is determined. Expressions are given for the facets generated in the substrate under erosion transverse to the grating stripes, taking into account shadowing effects. The results are illustrated by constructing profiles produced in GaAs by erosion of relief patterns in photoresist and examining the influence of the physical parameters of the mask, divergence of the ion beam, and selection of a different substrate. Profiles observed experimentally confirm the analysis but reveal striking modifications caused by redeposition. These modifications, however, do not involve significant buildup of sputtered material. In contrast to the transverse geometry, significant buildup of redeposited material is observed under erosion parallel to the grating lines. It is shown that buildup does not occur on surfaces that undergo erosion and redeposition simultaneously. Finally, it is shown that even uniformly eroding materials should exhibit facet formation and that the analysis is valid for all materials, regardless of the dependence of erosion rate on angle of incidence.

Ion polishing with the aid of a planarizing film.

An ion polishing technique employing a planarizing film in conjunction with ion-beam erosion at the planarizing angle was used to remove surface irregularities associated with conventional polishing of optical surfaces. By maintaining a planar surface throughout the erosion process, nonuniform erosion resulting from faceting, redeposition, and ion reflection is eliminated. Smooth surfaces on fused quartz are obtained by erosion of a planarizing film of photoresist at an angle of 60 degrees. The method is applicable to a wide variety of materials and may be useful for removing the surface roughness limitation on the laser-induced damage threshold of optical surfaces.

Optical Maser Characteristics of Nd3+ in SrMoO4

Stimulated emission was observed at 77 deg K. Lamp intensity variation with time, including fluorescence and stimulated emission and scattered light at 1.0463 mu at 9 and 16% above the threshold energy, was measured. Data are presented in graphical form. (L.N.N.)

Optical Studies of Ho3+ Ions in YGaG and YIG

Optical absorption and emission spectra are used to determine the energy levels of the 5I8 ground state and 5I7 excited state of Ho3+ in diamagnetic yttrium gallium garnet (YGaG) and ferrimagnetic yttrium iron garnet (YIG). The results for YGaG are consistent with the D2 site symmetry of the dodecahedral site in the garnet structure. Energy levels in YIG that are separated by site inequivalency are identified from spectra observed as the Fe3+ magnetization is rotated in the (110) plane. Stimulated emission is observed from Ho3+ in YGaG at 2.086 and 2.114 μ and from Ho3+ in YIG at 2.086, 2.089, and 2.107 μ. Each of the lines in YIG is composed of two superimposed electronic transitions. The exchange field in YIG makes possible magnetic tuning of coherent emission over a range of 70 A. This paper is a brief description of work that will be treated in full elsewhere.