NBS

. ... .......:::.. ' .. . '

. Reference booR"not tFBe

taken from the library. - ,q

atonal Bureau of Standards

Research on Laser Standards and Materials

At the National Bureau of Standards

K**T ° F

*<">CAU Of '

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

: ;
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March

3, 1901. Today, in addition to serving as the Nation's central measurement

laboratory, the Bureau is a principal focal point in the Federal Government

for assuring maximum application of the physical and engineering sciences to

the advancement of technology in industry and commerce. To this end the

Bureau conducts research and provides central national services in three broad

program areas and provides central national services in a fourth. These are:

(1) basic measurements and standards, (2) materials measurements and

standards, (3) technological measurements and standards, and (4) transfer of

technology.

The Bureau comprises the Institute for Basic Standards, the Institute for

Materials Research, the Institute for Applied Technology, and the Center for

Radiation Research.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement,

coordinates that system with the measurement systems of other nations, and

furnishes essential services leading to accurate and uniform physical

measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of an Office of Standard Reference Data and

a group of divisions organized by the following areas of science and

engineering:

Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic

Physics—Cryogenics 2 —Radio Physics 2 —Radio Engineering 2 —Astrophysics 2 —

Time and Frequency. 2

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to

methods, standards of measurement, and data needed by industry, commerce,

educational institutions, and government. The Institute also provides

advisory and research services to other government agencies. The Institute

consists of an Office of Standard Reference Materials and a group of divisions

organized by the following areas of materials research:

Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Physical

Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides for the creation of appropriate

opportunities for the use and application of technology within the Federal

Government and within the civilian sector of American industry. The primary

functions of the Institute may be broadly classified as programs relating to

technological measurements and standards and techniques for the transfer of

technology. The Institute consists of a Clearinghouse for Scientific and

Technical Information, 3 a Center for Computer Sciences and Technology, and a group of technical divisions and offices organized by the following fields of

technology:

Building Research—Electronic Instrumentation—Technical Analysis—Product

Evaluation—Invention and Innovation—Weights and Measures—Engineering

Standards—Vehicle Systems Research.

THE CENTER FOR RADIATION RESEARCH engages in research, measurement, and

application of radiation to the solution of Bureau mission problems and the

problems of other agencies and institutions. The Center for Radiation

Research consists of the following divisions:

Reactor Radiation—Linac Radiation—Applied Radiation—Nuclear Radiation.

Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise

noted; mailing address Washington, D.C. 20234. 2 Located at Boulder, Colorado 80302.

3 Located at 5285 Port Royal Road, Springfield, Virginia 22151.
UNITED STATES DEPARTMENT OF COMMERCE

C. R. Smith, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

NBS TECHNICAL NOTE 449

ISSUED JUNE 1968

Research on Laser Standards and Materials

at the National Bureau of Standards

Karl G. Kessler

Institute for Basic Standards

Elio Passaglia

Institute for Materials Research

and

Nicholas N. Winogradoff

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C, 20402

Price 30 cents.
CONTENTS

Page

1. Introduction 1

2. Laser Performance Evaluation 1 2.1. Laser Energy and Power Measurement (D) D.A. Jennings 1 2.2. Laser Pulse Measurements (D) D. McSparron 2 2.3. C.W. Power Measurements (D) D. McSparron 2 2.4. Extension of the Radiometric Techniques to Other Wavelengths (D) D. McSparron 3 2.5. C.W. Laser Power Measurements (D) D.A. Jennings 3 2.6. C0„ Laser Power Measurements (D) L.J. Schoen 4 2.7. Laser Power and Energy Measuring Techniques (D) N.N. Winogradoff 4 2.8. Far and Near Field Studies (D) M.M. Birky 5 2.9. Mode Structure, Near and Far Field Patterns of C0„ and Other Long Wavelength

Lasers (D) N.N. Winogradoff 6

3. Materials 6 3.1. Bulk Optical Properties of Laser Materials (M) G.W. Cleek 6 3.2. Characterization Standards for Laser Materials (M) J.L. Torgesen 10

3.3. Semiconductor Laser Materials and Laser Device Studies (M) N.N. Winogradoff 11 3.4. Optical Evaluation of Laser Rods (D) F. Rosberry 12 3.5. Magnetic Resonance Studies of Laser Materials (M) Te-Tse Chang 18

3.6. Laser Rod Holography (D) K.D. Mielenz and A. Funkhouser 18

3.7. Crystal Characterization Studies by Optical Means (M) R.F. Blunt and M.I. Cohen .... 18 3.8. Liquid Lasers-Light Scattering in Liquids (D) R.D. Mountain 22 3.9. Laser Transitions in HCN and DCN (D) D.R. Lide, A.G. Maki and L.J. Schoen 22

4. General 23 4.1. HCN Laser Frequency Measurement (D) D.A. Jennings 23 4.2. Giant Pulse Mode Control (D) D.A. Jennings 23 4.3. The Pressure Dependence of the (3s„-2p, ) 6328A Transition in Neon (D) M.M. Birky. ... 24 4.4. Fluorescence and Double Resonance Phenomena in C09 (D) A.M. Ronn 24 4.5. Crystal Defect Studies Using Magnetic Resonance (M) Te-Tse Chang 25 4.6. The Measurement of Optical Frequencies and a Redetermination of the Velocity of Light

by Microwave Modulation of Laser Beams (D) Z. Bay and G.G. Luther 30

11
Research on Laser Standards and Materials at the National Bureau of Standards^

Edited by

Karl G. Kessler

Elio Passaglia

and

Nicholas N. Winogradoff

A brief report is presented on research in the areas of laser standards and laser materials now in progress at the National Bureau of Standards. Included are

the measurement of the power and energy output of lasers, their near and far field

patterns and their optical properties. The physical and chemical properties of laser materials are studied in an attempt to relate such properties to the operating characteristics of lasers.

Key words: Holography, laser energy, laser materials, laser power, laser radiometry,

laser standards, laser transitions, modulation of lasers, near and far field patterns,

optical properties.

1. Introduction

With support from the Advanced Research Projects Agency, the National Bureau of Standards is carrying out

a program of research on the development of standards for laser performance, and of research on laser

materials. The standards aspect of the research is aimed at developing standard methods for the measure- ment of energy, power and near-and far-field patterns, as well as to provide a central facility for the calibration of energy measuring devices. The research on laser materials is concerned with their characterization both as optical elements (i.e., with problems of optical quality, etc.) and as materials (i.e.,

with problems of composition and other relevant physical properties). Work is, at present, concerned

primarily with ruby and neodymium glass. Some work is also carried out on gases and semiconductor laser materials and on passive non-linear optical materials.

The reviews reported below are necessarily brief summaries addressed to the general reader.

For the benefit of readers who would like more detailed technical information, the reports generally

include bibliographical references and relevant publications originating from NBS work. The latter

being designated with an asterisk.

t The support for part of the work reported herein comes from three sources, namely, ARPA Materials Office, ARPA Project Defender, and Air Force Avionics Laboratory. In the report the funding source for a given

project is indicated after the title by an (M) (Materials Office) or a (D) (for both Defender and AF Avionics Laboratory).

2. Laser Performance Evaluation

2.1. Laser Energy and Power Measurement (D)

D.A. Jennings

Radio Standards Laboratory, Boulder, Colorado 80302 Institute for Basic Standards

The objective of this task was to develop reliable measurement methods for determining the power and

energy of pulsed lasers at wavelengths of 694.3 and 1060.0 nm and up to 100 J per pulse.

A system, permitting the comparison of our carefully calibrated liquid calorimeter with other energy

measuring devices has been set up, and facilities for such comparisons up to 100 J per pulse have been

established.

We estimated that this comparison system has a precision of ± TU.

Some of the progress during the past year (partially supported by NBS Project No. 2510150) consisted of:

(1) The accurate evaluation of low reflectivity beam splitters by means of interchangeable

calorimeters. (This has increased the energy measurement range of pulsed ruby laser

*1 Jennings, D.A., IEEE Trans. IM-15, 1966, p. 161.

1
energy from « 10 J to « 100 J.)

(2) The design and construction of two "optical waveguide" calorimeters for ruby and Nd

glass laser energy output measurements from « 1 to m 50 joules.

(3) The construction and evaluation of a hollow gold and silver ball calorimeter. This

device was found to be unsatisfactory because of the large rates of heat losses from

the spherical surfaces.

(4) The evaluation, as a function of beam size and energy, of ten calorimeters from four

companies and three Government agencies. This survey showed sources of disagreement.

(5) Partial evaluation of four additional NBS liquid cell calorimeters for thermodynamic

properties and accuracy.

2.2. Laser Pulse Measurements (D)

D. McSparron

Metrology Division, Washington, D.C. 20234 Institute for Basic Standards

Since calorimetric techniques preclude the measurement of pulse shape, the work in this Division was

concentrated on the development of standard radiometric techniques for measuring pulse shape and the energy in individual laser pulses by using fast phototube systems. In this system, the laser flash

is attenuated to a level which can be handled directly by the photo-emissive detector by diffuse scattering from MgO screens.

Cross-correlation of this technique with energy measurement by means of calorimetric devices called for

the use of highly stabilized ruby and neodymium lasers systems. Such a system with 20 joule, 2 pps capability was purchased and checked out.

Since initial comparison of the radiometrically based phototube system and the calorimetric techniques

showed a discrepancy of some 97„, an improved calorimeter, having higher thermal stability, low mass and

reduced heat losses was built.

Further comparison between the two techniques showed agreement to within 4%.

*1McSparron, D.A., Douglas, C.A., and Badger, H.L., N.B.S. Technical Note 418, November 1967.

2.3. C.W. Power Measurements (D)

D. McSparron

Metrology Division, Washington, D.C. 20234 Institute for Basic Standards

The direct applicability of conventional radiometric techniques to the measurement of CW laser power was confirmed experimentally with an 80 milliwatt He-Ne laser. A power meter consisting of a vacuum phototube, diffusing glass entrance window and 7.5 cm diameter MgO coated entrance sphere, was constructed.

This meter was calibrated by 4 standard radiometric techniques:

(1) A filter radiometer calibrated with an NBS lamp standard of spectral irradiance. (2) A prism

spectroradiometer similarly calibrated. (3) A radiation thermopile calibrated with an NBS lamp

standard of total irradiance, and (4) an absolute radiometer calibrated by electrical heating. The results of the direct substitution calibrations of the power meter were as shown in Table I.
Table I

Laser Power Meter

Incident Power Calibration

(milliwatts) (LiA/W)

39.4 2.080

40.1 2.045

82.5 2.095 31.1 2.068

Calibrating

Instrumentation

Filter Radiometer

Prism Spectroradiometer

Radiation Thermopile

Absolute Radiometer

Although this work involved a particular He-Ne laser, operated at a particular wavelength, 6328A, the techniques used are applicable to a much larger class of calibration problems. Provided that the incident laser radiation is not so intense as to physically damage the optical components involved, the filter and prism spectroradiometers should be limited only by the wavelength region for which standards

currently exist, 0.25 to 2.5 microns, and the radiation thermopile and the absolute radiometer should be limited only by the wavelength region for which their response can be assumed to be spectrally flat.

2.4. Extension of the Radiometric Techniques to Other Wavelengths (D)

D. McSparron

Metrology Division, Washington, D.C. 20234 Institute for Basic Standards

The principle of wavelength independence of black body absorbers makes this type of structure particularly attractive for radiation measurement.

Although several such black body thermopiles in the form of conical cavities incorporating absorptive

losses in multiple reflections have been designed, they were particularly vulnerable to radiation damage.

In view of this, a modification of the technique, involving the attenuation of the laser beam by diffuse

scattering from MgO screens prior to incidence onto a large area thermopile was developed.

Exposure of such a system, with a thermopile time constant of 3 seconds, to a laser source pulsed at

1 pps resulted in a "steady state" signal S, •

By prior calibration against a standard source, S, may be related to a definite power incident on the thermopile. Since the rate of pulsing was known, the energy content per pulse could be determined.

Thus, if the pulse train reaching the thermopile wa.s then terminated by a shutter and the thermopile

allowed to return to equilibrium at room temperature, the exposure of the thermocouple to another single

pulse resulted in a ballistic signal S„ from the thermopile and this signal corresponded to the above energy per pulse.

Although this technique gave results which agreed to within about 3% of our phototube system, its

application was limited to pulses whose shapes closely conformed to that used in the calibration of the device

.

2.5. C.W. Laser Power Measurement (D)

D.A. Jennings

Radio Standards Laboratory, Boulder, Colorado 80302 Institute for Basic Standards

The objective of C.W. laser power measurement, (project 2510458) was to develop methods to measure C.W. laser power in the 1 to 10 watt range in the 400 to 1000 nm region of the spectrum, and to use these methods to compare other power measuring devices.

The initial approach was two-fold:

(1) To increase the power of the existing lasers (Ar and YAG-Nd) to the one watt level.

(2) To develop power measurement techniques using pulsed Ruby laser calorimeters (energy measuring devices) as integrators with timed shutters.

Measurements already made suggest that we should be able to achieve a ± 5% precision and accuracy.
We will be working very closely with Dr. Knecht at Wright-Patterson Field (Air Force) on this project.

2.6. C0„ Laser Power Measurements (D)

L.J. Schoen

Atomic Physics Division, Washington, D.C. 20234 Institute for Basic Standards

• Considerable consideration has been given to the design of a power measuring device for C.W. infrared

lasers. Initial experiments will be carried out on two C0„ lasers having power outputs in the range

5 to 75 watts. It is expected that when the final design is settled upon it will be possible to scale

it up or otherwise modify the design so that measurements may be made on more powerful lasers.

Several tentative designs have been considered, but the final design has not yet been settled upon. We

plan to use a calorimetric type of measurement based on some type of steady state differential temperature measurement. To this end there has been considerable consultation with various calorimetry experts

here at the Bureau of Standards.

The calorimeter will be filled with an absorbing gas so that the device need not be constructed in the form of a blackbody. Preliminary experiments indicate that SF, is a suitable absorbing gas. Calibration

of the device will involve passing a known electrical current through a heater coil of known resistance

in such a manner as to duplicate the power received from the laser beam.

2.7. Laser Power and Energy Measuring Techniques (D)

N.N. Winogradoff

Instrumentation Division, Washington, D.C. 20234 Institute for Applied Technology

The objective of this work was to investigate the general phenomena of the interaction of intense light

with matter from the point of view of using these interactions to provide alternative means for measuring

pulse shape, power and energy in laser beams.

Approaches Used:

CI) Elemental silicon as an attenuator.

(2) Optical rectification in non-linear optical materials.

Results and Conclusions : (Reported in the above sequence)

(1) In principle the pulse shape, power and energy in a ruby laser flash can be observed and measured by conventional photomultipliers and other photo-emissive detectors if the incident beam is attenuated by a suitable filter or diffusing screen. Most of the filters and screens commonly used

consist of glass or other chemical compounds, and, as such, are susceptible to bleaching. Since radiative recombination of excess carriers in an indirect band gap material is a very inefficient process, it seemed feasible to use silicon as a "wavelength converting" attenuator. The ruby radiation was absorbed

on the front surface of a polished Si wafer 0.10 cm in thickness, mounted directly on the window of an

SI pnotomultiplier . The 1 . 1 p,m recombination radiation emitted from the rear surface was then measured

by the photo-current in the detector.

Although the Si used had a lifetime of 400 u, sec, the photomultiplier current clearly reproduced laser pulses down to 2 x 10"° seconds (the limit of the electronic equipment used).

This technique enabled us to display the details of the spikes of a ruby laser on an oscilloscope.

The chemical inertness, high melting point and elemental nature of the Si eliminate the bleaching

problems found in compounds. Attempts to obtain quantitative data relating the dependence of the attenuation factor on the beam intensity and semi-conductor characteristics showed that tight control

over the flash lamp voltage and rod temperature was essential.

A Korad ruby laser system with a 25 joule capability and voltage and temperature control of ± 1 volt in

5000 and ± 1/4 F respectively was ordered and is being installed.

*1Winogradoff, N.N., and Kessler, H.K., - Radiative Recombination Lifetimes in Laser Excited Silicon,

Appl. Phys. Letters 8, 99 (1966).
(2) Conversations with Dr. J. Ward at the Clarendon Laboratories, Oxford, confirmed our belief

that the dc field produced in a suitable non-linear crystal during the generation of second harmonic

radiation could be measured and used to measure the E field in the laser beam. Theoretical relationships between this dc field and the other electro-optical coefficients of the material would provide means for discriminating between the voltage due to the optical rectification and other spurious effects.

It is hoped to undertake this investigation when the above Korad laser has been installed.

2.8. Far and Near Field Studies (D)

M.M. Birky

Atomic Physics Division, Washington, D.C. 20234 Institute for Basic Standards

Preliminary studies (under ARPA support) of near and far field patterns of ruby are being made as a means of evaluation of optical rod quality. These patterns give information on homogeneity of active

material and on beam divergence.

The far field pattern was obtained by placing a photographic film at the focal plane of a lens as shown

in Fig. 1. If the lens has a focal length f, the beam divergence of the laser is given by a = D/f,

where D is the diffracted spot size as recorded. In order to get a number of exposures of different

intensities for each laser shot, a plane-parallel glass prism was inserted between the film and lens,

and inclined at a small angle to the optic axis of the laser and lens system (Fig. 1). The prism was

used in a fashion similar to a Lummer-Gehrcke plate except in that it served only as an attenuator, and

the thickness of the prism was such that there was no overlap of the beams. A number of beams of successively reduced intensity emerged from the two faces of the prism, the intensity ratio of successive

beams being determined by the angle of incidence $ and the index of refraction of the prism. For the far field pattern the photographic film was placed behind the prism at the focal plane of the lens, i.e. at

If from the lens.

With this arrangement, there was the possibility of using light from either or both faces of the prism.

If the lens of focal length f is placed a distance 2f from the output end of the laser, the far field

pattern will be recorded at a distance of 3f from the laser, or as stated above, at the focal plane of

the lens. However, at a distance 4f from the laser, or 2f from the lens, the laser head will be in focus, and—the pattern recorded here on the film will be the near field pattern- So, with the prism in place, the light emerging from the two sides of the prism could be used to obtain near and far field

patterns simultaneously. The near field patterns, also recorded at various intensities, give information on the homogeneity of the active rod, and the complexity of the mode structure.

FILM AT If FROM LENS

FILM AT 2f FROM LENS

Figure 1. Arrangement for obtaining near and far field patterns generated by a laser
2.9. Mode Structure, Near and Far Field Patterns of C0„ and Other

Long Wavelength Lasers (D)

N.N. Winogradoff

Instrumentation Division, Washington, D.C. 20234 Institute for Applied Technology

Objective : According to Capt. Axelrod of the Kirkland Air Force Base, there is an urgent need for a simple large aperture method of determining the above characteristics of C09 and other long wavelength

lasers without cryogenic cooling of detectors.

2 Approach : At power levels in excess of a few milliwatts/cm , thermal quenching of a u.v. pumped phosphor

provides a method for such observations.

Results and Conclusions : In collaboration with Dr. A. Ronn, and using a commercial phosphor, the feasibility and reasonable resolution of the method was demonstrated with a C0„ laser.

A wide range of brilliant phosphors based on Ge0» and covering a wide spectral range have been found

to be unimpaired after exposures to temperatures up to some 1200 C.

We plan to evaluate these phosphors for use in determining the above laser characteristics at higher

power levels, and apply the same principles to ruby and Nd lasers.

Marinace, J. and Winogradoff, N.N., Ge0„ Phosphors, Patent Applied for and assigned to I.B.M. (1960).

3. Materials

3.1. Bulk Optical Properties of Laser Materials (M)

G.W. Cleek

Inorganic Materials Division, Washington, D.C. 20234 Institute for Materials Research

This project was authorized in November 1967, with the objective of evaluating the bulk optical properties of laser crystals and glasses, particularly synthetic single crystal ruby and neodymium-doped glass. An analysis of the thermal distortion introduced into ruby and glass laser rods by flash lamps indicate

that experimental data are needed on the change of refractive index with temperature, dn/dT, the change

of refractive index with density, dn/dp, and the thermal expansion, a. It is also necessary to know the elastic and photoelastic constants. The elastic constants of the glass will be determined by a pulseecho technique, and the elastic constants of ruby are already known. It is planned to measure the remaining properties using a combination of interferometric and polarimetric techniques. Equipment is available to make appropriate measurements over the temperature range from -190 to 1000 C and at pressures up to 1000 bars. In addition, the specimens will be examined for homogeneity and for inclusions.

A piece of Nd-doped glass in the form of a cylinder 8 cm in diameter and 2.9 cm thick has been supplied

by the American Optical Company, as material for measurement of dn/dT, dn/dp, a, elastic constants,

photoelastic constants, density and refractive index. This specimen was marked for identification as 0835-AT. The two faces were polished and photographs were made of the fringe pattern in the Twyman- Green interferometer and of the shadowgraph pattern. These give an indication of the refractive index

difference within the piece and of the optical homogeneity. The photographs are shown as Figures la and

b, respectively.

The specimen was then examined microscopically for inclusions with side illumination against a dark

background. Three inclusions that appeared to be metallic were found. Two seeds were also found in

the specimen. A photomicrograph of the largest metallic inclusion is shown in Fig. 2.

When inspected between crossed polarizers, the specimen showed an optical path difference of about

15 m /cm at the edge. The birefringence was high only at the edge and was practically zero over a large area near the center. It was felt that the birefringence could be reduced by annealing, so the specimen was annealed by holding for 24 hours at its annealing temperature of 464 C. It was then cooled at the rate of l°C/hr. to 350° and at 2.5°C/hr. to 220°C at which point the furnace was cut off and allowed to cool to room temperature. The annealed specimen exhibited a maximum optical path difference of about

2 nvi/cm at the very edge, and was practically zero elsewhere.

The cylinder is now being cut to provide samples for the measurement of dn/dT, dn/dp, a, photoelastic

constants, elastic constants, density, and refractive index. The largest of the inclusions will be cut

out and an attempt will be made to verify its composition by analysis.
Earlier, a sample of American Optical Company Nd-doped glass identified as C-835-E was obtained. Measurements of refractive index on a prism sample of this glass were made by Irving Malitson of NBS. The results are given in Table 1 and are plotted in Fig. 3. Over the temperature range from 10° to 22°C, he found that dn/dT was about +3 x 10" 6 /°C.

Figure la - Fringe pattern of specimen of glass 0835-AT photographed in Twyman-Green

interferometer

Figure lb - Shadowgraph photo of specimen of glass 0835-AT

7
Figure 2 - Microphotograph of inclusion found in specimen of glass 0835-AT. A millimeter

scale at the same magnification is shown at top for comparison.
CD LO sr ro CM

m m m LO LO in

O

m

X3QNI 3AllOVdd3d

Figure 3 - Dispersion curve for glass C835-E
Table I

Refractive Index, Glass C-835-E

American Optical Co.

X, nm Refractive Index

313.0 1.55372

325.1 1.54938

340.4 1.54539

346.6 1.54375 365.1 1.53968

404.7 1.53295 435.8 1.52891 467.8 1.52566 480.0 1.52460

508.6 1.52242 546.1 1.52006 643.8 1.51574 667.8 1.51491

• 706.5 1.51377

794.7 • 1.51194

1014.0 1.50810

1083.0 1.50726

1128.7 1.50667

1362.8 1.50405

1529.5 1.50228

1709.1 1.50038

Uncertainty = ±1 x 10 dn/dT for 10°C temperature range about +3 x 10 / C

3.2. Characterization Standards for Laser Materials (M)

J.L. Torgesen

Inorganic Materials Division, Washington, D.C. 20234 Institute for Materials Research

This project was initiated in mid-November 1967, to develop precise methods for the chemical analysis of

solid state laser materials and to provide characterization of laser materials as to physical defects,

dislocations, etc.

Preliminary experiments in spark-source mass-spectrometry have been conducted by Mr. Paul Paulsen,

Analytical Chemistry Division, NBS, with a sample of ruby of unknown source and a sample of neodymium

glass. Mass spectrograms of good quality were obtained in both cases by employing (1) a high-purity

gold counter-electrode directly and (2) repeating after sputtering a gold film on the sample surface in situ. No attempt has been made to interpret the mass spectrograms quantitatively in these trial experi- ments designed to explore methods of sparking with non-conducting materials.

Small disc specimens have been cut from ruby laser rods supplied by Mr. Mirarchi, Fort Monmouth. Both

Verneuil-grown and Czochralski-pulled specimens are represented. Attempts are in progress (1) to spark

specimen pairs directly; (2) to provide comparative analyses on specimens grown by different techniques;

and (3) to establish the feasibility of using such pairs as reference specimens in the analysis of unknown samples in the spark-source mass spectrometer.

New ruby boules of high quality will shortly become available. These boules will contain chromium oxide

dopant in nominal concentrations of 0.03, 0.05, and 0.07 weight percent. Specimens are to be taken from

10
these boules for research on the chemical analysis of chromium with respect to its absolute concentration level in various oxidation states and its use as an internal standard for the determination of trace quantities of impurities. In addition, specimens are to be cut from the boules for the precise measurement of refractive index as dependent on chromium concentration and for the determination of stress-optical coefficients. These latter measurements are conducted under a related project.

3.3. Semiconductor Laser Materials and Laser Device Studies (M)

N.N. Winogradoff

Instrumentation Division, Washington, D.C. 20234 Institute for Applied Technology

Scope - The work carried out was a continuation of the program described in NBS Report No. 9591 covering

the period January 1, 1967 to July 1, 1967, and included:

1. A comparison of the characteristics of:

(a) diffused

(b) solution regrown, and

(c) compensated, vapor deposited, epitaxial GaAs injection lasers.

2. The evaluation of GaAs lasers made by incorporating a wide range of dopings (Sn, Te and Zn)

and degree of compensation of the p-type side of the junction with donors, by vapor phase epitaxy, to determine the optimum laser material specifications.

3. A study of the mechanisms responsible for the decrease in light power output when GaAs laser diodes are operated at other than cryogenic temperatures.

Results and Conclusions (Reported in the above sequence)

1. There has been considerable disagreement in the measurement of power levels of GaAs laser

diodes when identical units were evaluated in different establishments.

Although this has been attributed to uncertainties in the calibration of the spectral response

of the detector (usually a biplanar photo emissive cell) we have shown that in the pulsed mode, the differences observed were primarily due to the pulse shape of the current pulse generator, and that

standardization of the rise and fall times of the current pulse, its duration and repetition rate is of primary importance in such comparisons. Slow rise times increased the temperature of the diode before

an inverted population could be produced and resulted in low power output and high threshold currents.

Similarly, when diffused laser diodes exhibiting a time lag between the onset of lasing action and the leading edge of the current pulse ' were evaluated with short duration pulses, the production of an inverted population required very high peak currents, again giving the impression of high threshold

currents.

Pulse generators producing 40 ns pulses with rise and fall times of ~ 4 ns at current levels

of up to 140 A and having repetition rates of up to 400 pps have been constructed by H. Kessler and appear to be well suited to injection laser pumping.

2. Owing to difficulties in obtaining useable material, the evaluation of compensated, vapor

grown epitaxial GaAs p-n junctions has been slower than expected. Since only 5 out of the 25 samples

planned for, were obtained, no definite conclusions could be drawn so far.

3. The investigation of the temperature dependence of radiative recombination in GaAs by means

of the Radiative Band Pinch Effect showed that under ruby laser excitation, GaAs emitted radiation

centered at 0.900 y,m and that this radiation had a width at half height of 0.007 eV at room temperature.

Discussion of the experiment with Dr. Broom, of the University of Berne (Switzerland), lea the latter to suggest that this emission could be due to bound excitons.

1Winogradoff, N.N., and Kessler, H.K., "Light Emission and Electrical Characteristics of Epitaxial

GaAs Lasers and Tunnel Diodes," Solid State Commun. 2, 119, 1964.

*2Winogradoff, N.N., and Kessler, H.K., "Compensation and Band Tailing Effects in High Power Room

Temperature GaAs Lasers" Solid State Commun. 5, 155, 1967.

*3Winogradoff, N.N., Owen, K., and Curnutt, R.M., "The Radiative Band Pinch Effect and Temperature

Dependence of Radiative Recombination in GaAs" Int. J. Electronics 22, 229, 1967.

11
However, we could find no trace of such a line at low level injection but did observe a systematic shrinkage of the spectral width (observed by means of a thermocouple) as the ambient

temperature was increased from 35 C to 230 C.

We have interpreted the new phenomenon of line narrowing with increasing temperature in terms

of a temperature dependent shift of the Fermi level in heavily doped material exhibiting exponential

density of states tails in the conduction and valence band edges. The experimental results again

supported our contention that the decrease in power output of GaAs laser diodes at higher temperature was due to a decrease in the quantum efficiency for radiative recombination rather than to internal

absorption.

*4Winogradoff, N.N., and Neill, A.H., Jr., "Band Tailing Effects and the Temperature Dependence of Radiative Recombination in Compensated Epitaxial GaAs Laser Materials," IEEE J. Quant. Electronics

1968 (in the press).

Pankove, J. I., "Absorption Edge of Impure Gallium Arsenide," Phys. Rev. A140 , 2059, 1965. Winogradoff, N.N., "Field Control of the Quantum Efficiency of Radiative Recombination in Semiconductors," Phys. Rev. A138 , 1562, 1965. *7Winogradoff, N.N., Radiative Recombination within a Space-Charge Region in a Semiconductor," J. Appl.

Phys. 37, 3916, 1966. *8Winogradoff, N.N., and Kessler, H.K., Fermi Controlled Recombination as a Junction Design Factor in GaAs Laser Diodes," Int. J. Electronics 2_1, 329, 1966. *9 Kessler, H.K., and Winogradoff, N.N., "Surface Aspects of the Thermal Degradation of GaAs p-n Junction Lasers and Tunnel Diodes," IEEE Trans E.D. ED13 , 688, 1966.

3.4. Optical Evaluation of Laser Rods (D)

F. Rosberry

Metrology Division, Washington, D.C. 20234 Institute for Basic Standards

Until recently, the program for the measurement of homogeneity in optical materials was primarily con- cerned with variations of the refractive index (measured to the 5th or 6th decimal place) in cylindrical

samples measuring about 50 mm in diameter and 10 mm in thickness.

The results were reported in the form of contour maps with refractive index gradient lines enclosing

areas of similar index as shown in Fig. 1.

The small frontal area and long length of the laser rods presented several problems in that small variations in refractive index tended to neutralize one another as the depth of the rod increased,

while striae, inclusions, and voids increased with depth. Laser rods with beveled ends and end coatings

made the optical evaluation of the rod material even more difficult.

In view of this, a procedure for evaluating the rod material by observing its transmission fringes and

shadowgraph was developed. The interferograms were made on a Twyman-Green interferometer using a HeNe gas laser source at 632.8 nm. The shadowgraphs were obtained by using a tungsten source in the apparatus shown schematically in Fig. 2.

The overall quality found was somewhat below that of high quality optical glass due in part to the increased depth. Neodymium doped glasses and ruby crystal rods totaling eleven different samples have been examined to date. Figure 3 is a shadowgraph showing the striae present in a disk of neodymium

glass. Windows were polished on the edge of this disk for re frac tome trie measurements. Figure 4a shows

a transmission shadowgraph through the length of a seven inch ruby crystal rod with beveled ends. Figure 4b is a reflection shadowgraph of a poorly finished end surface on a ruby rod. Figure 5a shows

a transmission shadowgraph through about ten inches of a neodymium rod. The lower picture shows the interferogram of the same sample and presentation.

Four ruby crystal rods are now in our optical shop being refinished with ends perpendicular to rod axis

in hopes of improving ease of transmission mea=iiT-f>ments.

12
VALUE FOR VARIATION OF INDEX OF REFRACTION

AnXIO"6

PHYSICAL WEDGE

THICK

10.0 THIN MAX GRADIENT PER CM. = 10.9 X I0"6 TOTAL GRADIENT 16.4 X I0"6

SILICON SINGLE CRYSTAL I • I* I "P" TYPE AnXIO"5

10.0,

10.0

MAX. GRADIENT PER CM=I2.4XI0"5 TOTAL GRADIENT 17.6 X 10* 5

Figure 1 - Diagrams illustrating the comparison of the refractive index contour maps

(showing areas of similar refractive index) in (a) a sample of high quality

optical glass and (b) a single crystal of p-type silicon

13
LU 1

^*

CO <-> => cz ^

h- -> V LU | < o 1 htr CO 1 ^v,^ co < I 1 >

Q. 1 CO

Q_

1 1 1 < III w °

1 -1 2

CJ? 9 CO Z ! x z

X i Z LU Q_ 51 9 < z

tr 1 o

e?

°

£ 1 o

Q

<

2

X u. 1 CO \- 1 O

CJ

co<

iz

o

LU

_J

Li_ <

GO Q

o

< o L r

_ q:

LU

X

o

^ ' f

1 crLU X

CJ

CO

X . CO v\ < \. -1

Figure 2 - Diagram of the shadowgraphing technique used in evaluating a laser rod

14
Figure 3 - Shadowgraph showing striae present in a disk of neodymium doped glass

15
Figure 4a - Transmission shadowgraph through the length of a seven inch ruby crystal

rod with beveled ends

Figure 4b - Reflection shadowgraph of a poorly finished end surface on a ruby rod

16
Figure 5a - Transmission shadowgraph through about ten inches of a neodymium doped

glass rod

Figure 5b - Interferogram of the same sample

17
3.5. Magnetic Resonance Studies of Laser Materials (M)

Te-Tse Chang

Inorganic Materials Division, Washington, D.C. 20234 Institute for Materials Research

Since iron is present as an impurity in Nd glass laser rods, and the absorptive losses are governed by

the relative amounts of iron in the Fe^+ and Fe J+ valency states, it was of interest to develop a technique for determining this ratio.

The approach used was to attempt to identify these ions by the fine structure of their paramagnetic

resonance spectra. Preliminary results showed a somewhat asymmetrical line of about 2000 gauss in width

at 4°K. It is hoped that heat treatment, partial crystallization or irradiation might produce a more distinctive spectrum which could be used for quantitative studies.

3.6. Laser Rod Holography (D)

K.D. Mielenz and A. Funkhouser

Metrology Division, Washington, D.C. 20234

Institute for Basic Standards

The various characteristics such as power and coherence of the light produced by a solid state laser

are dependent on the laser rod makeup and the geometry of the rod and its cavity. It is of interest

then to study the distortions introduced in a rod during pumping and lasing. In the past this has been

difficult but currently the technique of holographic inter ferome try is enabling us to make such a study.

In this method, a hologram is made of the laser rod by shining light from a continuous laser through it and recording the interference between this light and light which has taken a path outside. When the hologram is returned to the position in which it was taken, interference occurs between the laser rod

and its reconstructed image. If the arrangement has not changed in any way, no fringes will be formed. However, any changes made in the laser rod subsequently will show up as interference fringes. By the shape, spacing, and orientation of the fringes, the exact nature of the change can be determined.

To date, relaxation of distortions caused by heating a laser rod with a heat lamp and with pumping it have been observed. These measurements have demonstrated the basic sensitivity and accuracy of the method. Future plans currently call for increased time resolution so that observations can be made

during the actual pumping and lasing conditions.

3.7. Crystal Characterization Studies by Optical Means (M)

R.F. Blunt and M.I. Cohen

Inorganic Materials Division, Washington, D.C. 20234 Institute for Materials Research

During this period the study of optical properties of SrTiO„ in the region of the absorption edge has been completed. SrTiO- is an excellent prototype of the perovskite system. It lends itself well to many types of measurement (including transport properties, superconductivity, as well as the optical

properties discussed here). The calculated band structure of Kahn and Leyendecker1 serves as a most

important "cornerstone" upon which meaningful investigations can be based. In this work absorption

coefficient, reflectivity, and electro-reflectance measurements were made. The absorption coefficient

shows an exponential rise (Urbach rule) over 5 orders of magnitude with a slope of 1/kT. The absorption data indicate a band gap of about 3.4 eV from the Urbach rule. This rule, simply stated as a (hv - E ) a = a

o

e kT-^ ' gives a cross-over of the exponential edges for all temperatures at hv = E . Here a is the absorption

coefficient, hv is the incident photon energy, and a is a constant. Figure 1 shows the absorption edge

at four temperatures ranging from 300°K on the left to near liquid helium temperature on the right.

The extrapolated edges cross at a = 1.3 x 10"^ cm" 1 and 3.37 eV.

o Both the D-C reflectivity and the electroreflectance spectra showed broad bumps near 3.4 eV in support

of a 3.4 eV band gap.

*1 Kahn, A.H., and A.J. Leyendecker, Phys. Rev. 135 , A1321 (1964).

18
AAAA «--l,-.no' 2 Additional results on a heavily reduced sample show that Redfield's model for an exponential edge is not applicable to SrTiO„ . This model (which would be appropriate for covalent semiconductors) requires

freeze-out of carriers in order to yield the observed temperature dependent slope of an exponential

edge. The reduced sample we measured, however, did not exhibit carrier freeze-out^ and yielded the same temperature dependence as the pure material.

The electroref lectance spectra revealed several interesting features. The signal was quite large, ex- tremely broad in energy, and of the wrong "sign," hence makes an identification with the Franz-Keldysh

effect difficult. More typical F.K. data is presented by Shaklee, Pollak, and Cardona,^ while Frova

and Boddy-3 give electroreflectance spectra on several other "perovskite" crystals that resemble, in many

respects, our SrTiO- data. The electroref lectance signal was appreciable down to at least 2.5 eV, well

into the "window" of SrTiOo- We thus feel that the electroreflectance effect observed could be due to a shift or a splitting in the major oscillator (4.4 eV), responsible for the refractive index, with

induced lattice polarization.

The optical properties of several other materials are being studied also. The solids are primarily

high- symmetry crystals with high melting points and high dielectric constants. Included are TiO„ ; the tetragonal scheelites, such as PbMoO, ; and "perovskites, " such as CsPbCl_ and LiNbO,.; the latter class

being of considerable current interest since many exhibit "non-linear" optical behavior, and some have inordinately large electro-optical coefficients.

The room temperature absorption edges of several crystals are given in Fig. 2, showing the exponential

behavior of the edge and the large anisotropy of PbMoO, . This figure is a plot of log a (cm~l) vs photon energy (eV).

To the above list we have recently added several crystals of the monoclinic NiWO, type. Some of these are known to be magnetic with very low Neel temperatures. Examples currently include ZnWO, and MnWO.

.

The former appears to have a fundamental edge near 4 eV, while the latter (at least for crystals

currently available) has a strongly anisotropic absorption band at about 2.2 eV.

In the past several months we have nearly completed the setting up of infrared equipment appropriate to

a detailed comparative study of the "blue-band" that appears in reduced (or suitably doped) TiO„, SrTiOTM,

and Ba TiO_. Measurements will involve optical absorption at various temperatures, and ultimately,

photoconductivity. These will be correlated with transport measurements.

A paper, "Optical Properties of SrTiO, in the Region of the Absorption Edge," by M.I. Cohen and R.F. Blunt, is to be published in an April 1968 issue of The Physical Review.

2D. Redfield, Phys. Rev. 130, 916 (1963).

*3H.P.R. Frederikse, W.R. Thurber, and W.R. Hosier, Phys. Rev. 134, A442 (1964).

4K.L. Shaklee, F.H. Pollak, and M. Cardona, Phys. Rev. Letters 15, 883 (1965).

5A. Frova and P.J. Boddy, Phys. Rev. 153, 606 (1967).

M. Cardona, Phys. Rev. 140, A651 (1965).

19
£

o

I-

•ZL UJo

Li. U_

UJo

o

z

o

H

Q.

Ql

O

(nCD<

3.2 3.4

PHOTON ENERGY (eV)

Figure 1 - Diagram illustrating the temperature dependence of the absorption edge of SrTiO„

20
I0 :

I0 2

£

10

10-I 2.8

PbMoQ

SrTi03

3.0

hv (eV)

3.2

Figure 2 - Diagram showing the exponential nature of the absorption edges of TiO„,

SrTiCL and PbMoO, at room temperature

21
3.8. Liquid Lasers-Light Scattering in Liquids (D)

R.D. Mountain

Heat Division, Washington, D.C. 20234

Institute for Basic Standards

The frequency distribution of initially monochromatic light which is scattered by density or composition

fluctuations in a dense fluid is directly proportional to the frequency spectrum of those fluctuations.

The frequency spectrum of the fluctuations can be calculated in terms of the material properties (specific

heat, viscosity, etc.) of the fluid using the linearized hydrodynamic equations of irreversible thermodynamics. Such calculations have been carried out for fluids with internal degrees of freedom which are weakly coupled to the translational degrees of freedom. These calculations provide criteria for making

a distinction between frequency dependent transport coefficients and additional thermodynamic variables

which are not in local thermodynamic equilibrium.

Various aspects of critical opalescence in binary mixtures have been studied. The late Peter Debye and

coworkers studied the shift in the critical mixing temperature when an electric field is applied and were able to observe the approach to the new equilibrium state of the fluid. We have developed a con- sistent means of relating these measurements to measurements of the frequency spectrum of light scattered

by concentration fluctuations in the fluid.

The light scattering measurements presently conducted by Dr. Wims in collaboration with Dr. Sengers are aimed at studying the behavior near the critical point of binary mixtures. Simultaneously the critical

behavior of the coexistence curve and the surface tension are being studied.

Relaxation processes which are not due to the exchange of energy between vibrational and translational

degrees of freedom have been observed in many liquids. These are called "structural relaxation"

processes as it is assumed that the relaxation mechanism involves changes in the local order of the fluid in response to fluctuations in the density "Structural relaxation" has been observed in C0„ near

the critical point. An effort is being made to characterize this effect in terms of correlation functions.

*1Mountain, R.D. and T.A. Litovitz, "Negative Dispersion and Brillouin Scattering," J.O.S.A. 42, 516

(1967).

*2Mountain, R.D., "Density Fluctuations in Fluids Having an Internal Degree of Freedom," J. Res. NBS

72A , 95 (1968).

*3Mountain, R.D. and Cooper, M.J., "Interpretation of Relaxation Times in the Electric Field Effect

on Critical Opalescence'," J. Chem. Phys. (1968) (to be published).

*4Larsen, S.Y., Mountain, R.D. and Zwanzig, R., "On the Validity of the Lorentz-Lorenz Equation Near

the Critical Point," J. Chem. Phys. 42, 2187 (1965).

*5 "Spectral Structure of Critical Opalescence: Binary Mixture," J. Res. NBS 69A, 523 (1965).

*6Mountain, R.D., "Spectral Distribution of Scattered Light in a Simple Fluid," Rev. Mod. Phys. 38, 205 (1966).

*7Mountain, R.D., "Interpretation of Brillouin Spectra," J. Chem. Phys. 44, 832 (1966). *8Mountain, R.D., "Thermal Expansion and Brillouin Scattering in Liquids," J. Res. NBS 70A , 207 (1966).

3.9. Laser Transitions in HCN and DCN (D)

D.R. Lide, A.G. Maki and L.J. Schoen

Atomic Physics Division, Washington, D.C. 20234 Institute for Basic Standards

The so-called CN laser has been known for several years. One commercial firm has been selling such

lasers for some time. Yet, it was not until studies were initiated in the Infrared and Microwave

Spectroscopy Section of the Bureau of Standards that it was shown that the HCN molecule is responsible

for the laser action.

Studies of the HCN and DCN laser systems are continuing in the hope that all known laser transitions can

be identified. The work on the laser assignments for HCN and DCN has resulted in several successful

Gebbie, H.A., Stone, N.W.B. and Findlay, F.D., Nature 202 , 685 (1964). *2

Lide, D.R. and Maki, A.G., Appl. Phys. Letters 1_1, 62 (1967).

22
attempts to find new laser transitions on the basis of theoretical predictions. The Bureau of Standards

group has been in contact with Professor Javan at M.I.T. and his group has been able to verify the assignments by finding new laser lines at frequencies predicted by the Bureau's assignments. '^'^

It seems that laser lines for both HCN and DCN are due to an unusually high population of molecules in certain vibrational levels. Where those levels come close to other unpopulated levels, laser action

is possible as the energy in the populated levels is so to speak dumped into the unpopulated levels. This dumping can take place only if the transition is rather strongly allowed. In most cases the transitions are too weak for laser action. In a few cases the levels are connected through a Coriolis interaction. That is to say, the vibrational and rotational motion of the molecule gets mixed-up in such a way that the two vibrations resemble each other and, if they are close together, the molecule can hardly

tell in which vibrational state it belongs. Under those circumstances the molecule can easily pass from

one vibrational state into the other. That means that the transition between the two states is strongly

allowed and laser action can result. This explanation holds for both HCN and DCN although the energy

levels involved are different for the two isotopes.

The Infrared and Microwave Spectroscopy Section is continuing its study of the rotational and vibrational

energy levels of HCN in an effort to provide more positive assignments for the remaining laser lines. In

the past three months new infrared spectra have been obtained which verify some of the laser assignments

and which make it possible to predict some more new and as yet unobserved laser lines.

A secondary result of the explanation of the HCN and DCN lasers was the fact that it demonstrated a new mechanism for explaining laser action. As a result, other workers were then able to explain the laser

transitions observed in H„0 and D„0 lasers. That explanation had also eluded workers for several years although many laboratories throughout the world were studying such lasers.

Hocker, L.O. and Javan, A., Phys. Letters 25A, 489 (1967).

4Hocker, L.O. and Javan, A., Appl . Phys. Letters 1_2, 124 (1968).

*5Maki, A.G., Appl. Phys. Letters 1_2, 122 (1968).

4. General

4.1. HCN Laser Frequency Measurement (D)

D.A. Jennings

Radio Standards Laboratory, Boulder, Colorado 80302 Institute for Basic Standards

The objective of this task was to measure the frequency of the HCN laser. This has been done by beating a 74 GHz klystron with the laser output in a silicon diode. The frequency of the two prominent

laser lines were found to be 890,759 ±0.1 MHz and 964,312.1 ± 0.2 MHz.

4.2. Giant Pulse Mode Control (D)

D.A. Jennings

Radio Standards Laboratory, Boulder, Colorado 80302 Institute for Basic Standards

Experiments for the control of modes in giant pulsed lasers are being performed.

It is expected that axial mode control will be accomplished by using an etalon as an output mirror, non- axial modes will be controlled by one of two methods: (a) an aperture in the cavity which will restrict

operation to the fundamental mode or (b) the use of optical components to force the entire rod to operate in the fundamental mode. When the laser is operative applications will be tried.

This project was started in November 1967; however, a new water cooled laser head has been constructed

with emphasis on temperature control and uniform pumping of the laser rod. Preliminary investigations

have been started of several types of etalons for mode control. A power monitor is also being constructed.

23
4.3. The Pressure Dependence of the (3s_-2p, ) 6328A Transition in Neon (D)

M.M. Birky

Atomic Physics Division, Washington, D.C. 20234 Institute for Basic Standards

The use of a stabilized laser for various types of length measurements requires a knowledge of its wavelength reproducibility whether it be used as a primary or secondary standard. One convenient means of stabilization that is commercially available is to lock the laser to the bottom of the "Lamb dip." How

close this point is to the true atomic resonance frequency is dependent on the gas pressure and other

effects. This investigation is limited to the Doppler broadened (3s„-2p.) 6328A 2 %e transition.

Two lasers attached to a vacuum system with helium and neon reservoirs are being used to study the effects

of gas pressure on the laser output frequency. Each laser consists of a 31 cm hemispherical cavity, 3 mm

I.D. tube and is rf excited. The flat mirror is mounted on a piezoelectric transducer that is used for

the length scan and the stabilization. They are independently stabilized on the bottom of the Lamb dips. The construction of the system allows the lasers to be filled to different pressures.

The frequency change as the pressure is increased in one laser is detected by looking at the beat between

the two lasers, with a phototube, a spectrum analyzer, a counter and a digital recorder. The direction

of the change is also recorded by continuously monitoring the error signal from the servo-system used to control the laser in which the pressure is changed. The correlation between the frequency shift and error signal voltage was verified initially with the aid of a Fabry-Perot interferometer.

Data taken between 1.2 to 3.2 torr show that the bottom of the Lamb dip increases in frequency with in- creasing -'He pressure. The magnitude of the shift is about 17 MHz/torr in this pressure range.

The experimentally obtained Doppler broadened line with its Lamb dip superimposed has some asymmetry.

The immediate problem is to separate any instrumental asymmetry (for example, mirror tilt during scan)

from asymmetry in the atomic line, and to extend the measurements to lower pressures. Also, since all measurements by others have employed dc excitation of the laser, plans are to heterodyne a dc laser with

an rf excited laser to determine if there is a Doppler shift as a result of a net drift of the radiating

a toms

.

4.4. Fluorescence and Double Resonance Phenomena in C0„ (D)

A.M. Ronn

Atomic Physics Division, Washington, D.C. 20234

Institute for Basic Standards

New laser induced infrared fluorescence experiments have been performed. These experiments consisted of studying the infrared emission spectrum of gases which were irradiated by a C0„ laser. The laser raised

the molecules from the ground vibrational state to some higher vibrational state. Fluorescence resulted

as the molecules then emitted radiation in falling back to the ground vibrational state. These experi- ments should provide more information on both rotational and vibrational relaxation times and on the selection rules which govern collisional relaxation mechanisms. So far all experimental attempts to make quantitative measurements have been unsuccessful due to equipment difficulties. Some of this work

is being done in collaboration with Dr. George Flynn's group at Columbia University. Qualitative work

has been done on about ten compounds. Ammonia was the lightest molecule studied. It is potentially a very good molecule to use for these studies, because its rotational spectrum is fairly well understood.

However, one really should have somewhat better resolution than is currently available to us in the 10 micron region.

Very little further work has been done on infrared and microwave double resonance experiments. In the last semi-annual report experiments were desc

certain molecules while observing the effect

some more double resonance work in the future

last semi-annual report experiments were described where a C09 laser is used to vibrationally excite

certain molecules while observing the effect on the microwave pure rotational spectrum. We hope to do

*1Ronn, A.M. and Lide, D.R., Infrared and Microwave Double Resonance Using a C09 Laser, J. Chem. Phys.

47, 3669 (1967).

24
4.5. Crystal Defect Studies Using Magnetic Resonance (M)

Te-Tse Chang

Inorganic Materials Division, Washington, D.C. 20234 Institute for Materials Research

When ions of certain impurities are introduced into suitable crystals, the impurity ions interact with

the surrounding host ions much in the same manner as the host ions interact among themselves.

However, the impurity ions will retain some of their own characteristics and may distort the local environment.

If the crystal is an insulator (or, in some cases, a conductor or semiconductor) and the impurity ions are paramagnetic (i.e. have some unpaired electronic spins), the method of electron paramagnetic

resonance could be used to study the properties of both the host lattice and the impurities. In addition,

if the energy states of the spin system between which the transitions were observed could be equally

populated (i.e. the system is saturated) by pumping with relatively high intensity radiation, the relaxation processes toward thermal equilibrium may shed some light on the properties of either the host lattice,

impurity ions, or both.

According to Van Vleck the relaxation processes are temperature dependent. At very low temperatures,

the spin system will relax by the direct emission of a phonon, i.e. by contributing to a certain mode of lattice vibration. The relaxation rate for this process is linearly dependent on temperature.

At higher temperatures where thermal lattice vibration is appreciable (i.e. the phonon spectrum extends

into higher frequencies), the spin system can give up its energy to a phonon by collision, resulting in

a phonon of higher energy. The relaxation rate for this two phonon or Raman process is proportional to the ninth power of the temperature.

2 At still higher temperatures, Orbach's mechanism where the spin system is first raised to a higher

(real) excited state by phonon absorption and then relaxes to the ground state with an exponential time dependence, dominates.

In general, if we denote the instantaneous population difference between two levels by n, and the popu- lation difference at thermal equilibrium by n , then

n = n

Q [1 - exp (- t/T^] (1)

where t is time, and T, is a characteristic time called the "spin lattice relaxation time" which depends

on both the ions, the host lattice and on the temperature at which the relaxation takes place.

The above temperature dependencies of the relaxation times 1/T. for the various temperature ranges have been verified experimentally for many ions and particularly for ions of the rare earth elements.

Experiments with M in TiO„ showed that at 4 K saturation could easily be obtained with only a few milliwatts of microwave power, so that the study of the relaxation mechanism in this material required very low sampling power.

A high sensitivity, superheterodyne detection system was therefore constructed, and preliminary measurements with this system are shown in Figs. 1 and 2. The temperature dependence of the relaxation rates were found to be T

9 between 3°K and 210°K, linear below 1.5°K, and T

5 between 1.5°K and 3°K. The latter

is a new result and although this temperature dependence was reproduced under different experimental

conditions and with different samples further experimental work to establish a true T dependence is under way.

Van Vleck, J.H., Phys. Rev. 57, 426 (1940).

2Finn, C.B.P., Orbach, R. and Wolf, W.P., Proc . Phys. Soc . (London) 77, 261 (1961),

3 Chang, T., Phys. Rev. 136, A1413 (1964).

4Yang, G.C. and Chang, T., Bull. Amer. Phys. Soc. 11, 220 (1966).

25
1000

100

CO

CO<

<£>

I I

I-o

1

' f

i

1 1 1—i—I—i—I—i—I—i

r~r

110)

J L_L

• H II C AXIS

o H ± C AXIS

I I I I 1 80 100 120 140 160 180 200 220 240 260

TEMPERATURE °K

Figure 1 - The temperature dependence of the linewidth of Mo :TiC> 2

. The circles and dots

are experimental points. The line marked T is a theoretical curve

26
Io

UJ

<r>

I UJ

<r2

g

x

<

_jUJ

cr

.0001

.001

2.0 3.0

TEMPERATURE °K

5.0

Figure 2 - The temperature dependence of the relaxation rate of Mo

are experimental points. The lines marked T, T , and T

c urve s

5+

TiO„. The dots

are theoretical

27
oUJ

UJ

<

X

<

_l

UJ .00

.0001

3.5 4.0 5.0 6.0

MAGNETIC FIELD - KILO-OERSTED

Figure 3 The magnetic field dependence of the relaxation rate of Mo :Ti(>

2

. The dots and circles are experimental points. The line marked

H* is a theoretical curve

28
1.3 1.5 2.0 2.5 3.0 TEMPERATURE, °K

Figure 4 - The temperature dependence of the relaxation rate of Mo :TiO . The dots are experimental points. The lines marked T, T

5

, and

T* are theoretical curves

29
4.6. The Measurement of Optical Frequencies and a Redetermination of the Velocity of Light by Microwave Modulation of Laser Beams (D)

Z. Bay and G.G. Luther

Atomic Physics Division, Washington, D.C. 20234 Institute for Basic Standards

The method used is based on the principle, that if the difference and the ratio of two optical fre- quencies is measured, the frequencies themselves can be determined. A simultaneous measurement of the wavelengths then yields the velocity of light. The project consists of three phases.

I. Stabilization of the optical frequencies to be measured. The possibility of locking a He Ne 6328A laser to the center frequency of the atomic transition by the use of loss modulation was experimentally demonstrated. The precision in the frequency lock was found to be better than 1 part in 10 . In order to see and utilize the full possibilities of this new locking method, the building of two systems was started with the cooperation of the Electronic Apparatus Design Section. This developmental

work is now nearing completion.

II. Modulation of the stabilized laser beam to produce sidebands whose difference frequency is known. For accuracy, the highest possible modulation frequency is required. The technique first

applied by R. Targ, G.A. Massey and S.E. Harris-*- at 3 GHz was extended to 25 GHz.

An intracavity electro-optical loss modulation was applied to a 6328A He Ne laser by the use

of a KDP crystal excited at microwave frequencies of 10 GHz and 25 GHz.

The crystal was oriented with the c axis nearly parallel to the direction of the optical beam

propagation, the a and b axes parallel and perpendicular to the beam polarization, respectively. The crystal was placed in a tuned microwave cavity with the electric vector along the c axis. In this

position a part of the energy of the optical beam was converted to the opposite polarization and was

coupled out by reflection at the Brewster window.

The sidebands, v + uu and v - (i), (where v is the optical and ou the microwave frequency) so generated were observed visually in the ring system of a Fabry-Perot interferometer. For the measurement of the intensities in the sidebands relative to that of the main band, a scanning interferometer was used with a low frequency (~ 50 Hz) sweep over the transparency curve. The intensity ratios were determined by using a photomultiplier the output of which is phase -de tec ted against the sweep.

Preliminary experiments showed that the energy converted to the sidebands was a few tenths of

a percent of the output of the laser for a one watt microwave excitation. It was shown that one can easily obtain 10 ^ photons per sec (10^0 photoelectrons per sec with commercial photomultipliers) for

two tenths of a watt microwave excitation with a laser of about 50 cm in length. These figures are not

thought to be an upper limit; there are several ways of increasing the energy output in the sidebands

(for example, by the use of a Glan-Thompson prism). There was no indication in the preliminary experi- ments that the electro-optical response of the KDP was less at 25 GHz than at 10 GHz.

Quantitative experiments are underway to relate the observed effects directly to the Pockelscoef f icients.

III. Our theoretical studies have shown that provided the order numbers are known, the ratio of

the two optical frequencies can be determined by tuning a passive cavity (Fabry-Perot interferometer)

to both sidebands. Preparations for experimental utilizing this technique is now under way.

It should be noted that the accuracy in any determination of the velocity of light is limited

by the accuracy of the present wavelength standards to about 1 part in 10°. Our optical frequency

determinations are based on frequency standards of much higher accuracy (about a part in lO^). Thus

the possibility exists that future wavelength standards can be based on optical frequency measurements

and on a value of the velocity of light, specified so as to be compatible with present length standards.

Targ, R., Massey, G.A. and Harris, S.E. (Proc. IEEE 52, 1247 (1964).

30
NBS TECHNICAL PUBLICATIONS

PERIODICALS NONPERIODICALS

JOURNAL OF RESEARCH reports National

ireau of Standards research and development in lysics, mathematics, chemistry, and engineering.

>mprehensive scientific papers give complete details the work, including laboratory data, experimental

scedures, and theoretical and mathematical analyIllustrated with photographs, drawings, and

larts.

iblished in three sections, available separately:

Physics and Chemistry

kpers of interest primarily to scientists working in

;se fields. This section covers a broad range of

physical and chemical research, with major emphasis on standards of physical measurement, fundamental

constants, and properties of matter. Issued six times

a year. Annual subscription: Domestic, $5.00; for- eign, $6.00*.

• Mathematical Sciences

Studies and compilations designed mainly for the

mathematician and theoretical physicist. Topics in mathematical statistics, theory of experiment design,

numerical analysis, theoretical physics and chemistry, logical design and programming of computers

and computer systems. Short numerical tables. Issued quarterly. Annual subscription: Domestic,

$2.25; foreign, $2.75*.

• Engineering and Instrumentation

Reporting results of interest chiefly to the engineer

and the applied scientist. This section includes many

of the new developments in instrumentation resulting

from the Bureau's work in physical measurement,

data processing, and development of test methods.

It will also cover some of the work in acoustics,

applied mechanics, building research, and cryogenic

engineering. Issued quarterly. Annual subscription:

Domestic, $2.75; foreign, $3.50*.

TECHNICAL NEWS BULLETIN

The best single source of information concerning

the Bureau's research, developmental, cooperative

and publication activities, this monthly publication

is designed for the industry-oriented individual whose

daily work involves intimate contact with science

and technology

for engineers, chemists, physicists, research managers, product-development managers, and

company executives. Annual subscription: Domestic,

$1.50; foreign, $2.25*.

; 'Difference in price is due to extra cost of foreign mailing.

Applied Mathematics Series.

tables, manuals, and studies. Mathematical

Building Science Series. Research results, test methods, and performance criteria of building materials, components, systems, and structures. Handbooks. Recommended codes of engineering

and industrial practice (including safety codes) developed in cooperation with interested industries,

professional organizations, and regulatory bodies.

Special Publications. Proceedings of NBS conferences, bibliographies, annual reports, wall charts,

pamphlets, etc. Monographs. Major contributions to the technical literature on various subjects related to the Bureau's scientific and technical activities.

National Standard Reference Data Series.

NSRDS provides quantitative data on the physical

and chemical properties of materials, compiled from

the world's literature and critically evaluated.

Product Standards. Provide requirements for

sizes, types, quality and methods for testing various

industrial products. These standards are developed

cooperatively with interested Government and in- dustry groups and provide the basis for common

understanding of product characteristics for both

buyers and sellers. Their use is voluntary.

Technical Notes. This series consists of communications and reports (covering both other agency

and NBS-sponsored work) of limited or transitory

interest. CLEARINGHOUSE

The Clearinghouse for Federal Scientific and

Technical Information, operated by NBS, supplies

unclassified information related to Governmentgenerated science and technology in defense, space, atomic energy, and other national programs. For

further information on Clearinghouse services, write:

Clearinghouse

U.S. Department of Commerce

Springfield, Virginia 22151

Order NBS publications from:

Superintendent of Documents

Government Printing Office Washington, D.C. 20402 
U.S. DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20230

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE

OFFICIAL BUSINESS