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' 1

NBS TECHNICAL NOTE 574

ARPA-NBS Program

of Research on High

Temperature Materials

and Laser Materials

Reporting Period

July 1 to December 31, 1970
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and

technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety.

The Bureau consists of the Institute for Basic Standards, the Institute for Materials

Research, the Institute for Applied Technology, the Center for Computer Sciences and

Technology, and the Office for Information Programs.

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 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 a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics—Electricity—Heat—Mechanics—Optical Physics—Linac

Radiation2—Nuclear Radiation2—Applied Radiation2—Quantum Electronics3—

Electromagnetics3—Time and Frequency3 —Laboratory Astrophysics3—Cryogenics3

. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research lead- ing to improved methods of measurement, standards, and data on the properties of

well-characterized materials needed by industry, commerce, educational institutions, and

Government; provides advisory and research services to other Government agencies;

and develops, produces, and distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials and the following divisions: Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Reactor

Radiation—Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to pro- mote the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of technological standards (including mandatory safety standards), codes

and methods of test; and provides technical advice and services to Government agencies

upon request. The Institute also monitors NBS engineering standards activities and

provides liaison between NBS and national and international engineering standards

bodies. The Institute consists of the following technical divisions and offices: Engineering Standards Services—Weights and Measures—Flammable Fabrics

Invention and Innovation—Vehicle Systems Research—Product Evaluation

Technology—Building Research—Electronic Technology—Technical Analysis

Measurement Engineering.

THE CENTER FOR COMPUTER SCONCES AND TECHNOLOGY conducts re- search and provides technical services designed to aid Government agencies in improving cost effectiveness in the conduct of their programs through the selection, acquisition,

and effective utilization of automatic data processing equipment; and serves as the principal focus within the executive branch for the development of Federal standards for automatic data processing equipment, techniques, and computer languages. The Center

consists of the following offices and divisions: Information Processing Standards—Computer Information—Computer Services —Systems Development—Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination

and accessibility of scientific information generated within NBS and other agencies of

the Federal Government; promotes the development of the National Standard Reference

Data System and a system of information analysis centers dealing with the broader

aspects of the National Measurement System; provides appropriate services to ensure

that the NBS staff has optimum accessibility to the scientific information of the world,

and directs the public information activities of the Bureau. The Office consists of the following organizational units: Office of Standard Reference Data—Office of Technical Information and

Publications—Library—Office of Public Information—Office of International

Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Part of the Center for Radiation Research.

8 Located at Boulder, Colorado 80302.
iAl BUREAU OF SMNOUtCS

UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director tJS2

1 *" <*MtfC*> TECHNICAL NOTE 574

ISSUED MAY 1971

Nat. Bur. Stand. (U.S.), Tech. Note 574, 44 pages (May 1971)

CODEN: NBTNA

ARPA-NBS Program of Research

on High Temperature Materials

and Laser Materials

Reporting Period

July 1 to December 31, 1970

Edited by A. D. Franklin and H. S. Bennett

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

Supported by the

Advanced Research Projects Agency

of the Department of Defense

%

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.

(Order by SD Catalog No. C 1 3.46:574). Price 50 cents.

Stock No. 0303 0854.

TABLE OF CONTENTS

Page

J_ o J.X1 C jTOClU.C C J_ Orl oooooooo«oooooooeo««*ooo J2. High Temperature Materials Program . . . . . . <» . . . . . . 2

2 1. Objectives and Summary of Program in High

Temperature Materials . . . . . . . . . . 2

z. • j. « jl o uojeccives .....»oo»«oo.o«... ^ 2.1.2. Summary of Progress to Date . 00 . 00 oo. 4

2.2. Project Summaries in High Temperature Materials . „ . 5

2.2.1. Diffusion in Oxides ... ....... B s . 5

2,,2olol. Crystal Growth from Vapor ..... 5

H. So Parker and C. A„ Harding

2o2 lo2 Diffusion in Refractory Materials . 6

A. L„ Dragoo

3. Laser Materials Program .....Co...., 11

3.1. Introduction to Laser Materials .... ...... . 11

3.2. Objectives and Summary of Program in Laser Materials . 11

J Q £ O JL O UDJ 6C C XVcS •OOOOOOOQ000090000 J- JL 3.2.2. Summary of Progress to Date ......... 13

3.2.2.1. Characterization of Laser Materials 13

3.2.2.2. Damage to Laser Materials ..... 13

3.3. Project Summaries in Laser Materials ••• 14

3.3.1. Damage to Laser Materials .......... 14

3.3.1.1. Laser Induced Damage Studies .... 14

Albert Feldman, Roy Waxier, and

Deane Horowitz

3.3.1.2. Effect of Microdomains Formed by

Liquid /Liquid Immiscibility Upon

the Damage Threshold of Glass ... 22

Wolfgang Haller and Hank Dardy

3.3.2. Appendix: Trip Reports ........... 26

3.3.2.1. Foreign Trip Report, H. S. Bennett . 26

3.3.2.2. Domestic Trip Report, H. S. Bennett 35

III
ARPA-NBS PROGRAM OF RESEARCH ON HIGH TEMPERATURE

MATERIALS AND LASER MATERIALS

Work Performed at the National Bureau of Standards

Supported by the Advanced Research Projects Agency,

Department of Defense

(ARPA Order 373-62)

Reporting Period July 1 to December 31, 1970

Edited by

A. D Franklin and H. S„ Bennett

Progress reports are given for projects on the growth

of ultra -pure A1_0« crystals, the development of a mass spectrometer -based sectioning technique for measuring oxygen

diffusion in oxides, the development of a precision facility

for measuring the threshold energy in a laser beam producing

damage in a transparent substance, and the study of the

factors influencing this damage threshold in glass

„

Key words: A1_0~; crystal growth; damage threshold; glass;

high temperature materials; laser; oxides; oxygen diffusion;

pure materials; sapphire.

IV
Io INTRODUCTION

The National Bureau of Standards, with the support of the Advanced

Research Projects Agency (Office of Materials Sciences) of the Department of Defense, is carrying out a program of research on High Temperature and Laser Materials. The overall objective of this program is to

provide some of the key elements needed by DoD in its purusit of new technology through research and development. These elements include:

i. The preparation and characterization of materials needed for

critical studies where exacting criteria of chemical purity,

crystalline perfection, etc. must be met if progress is to

be made,

iio The development of new measurement techniques where these are required, and the acquisition of critically-needed data

on certain material properties, and : iii. The elucidation of the basic mechanisms contributing to or limiting the use of materials under extreme conditions

(e.g. high temperatures, high intensity laser light, etc.).

A summary of the results achieved in the period July 2 to December 31,

1970, is given here.

Among the many areas of Defense Technology where advances of the

kind described above are needed, this Program has focussed upon two.

Ceramics are used in many ways, and many problems in their use relate

ultimately to mass transport effects. Highly important improvements

could be made by developing a firmer understanding of and control over mass transport processes in oxides. Since NBS has had considerable

experience in this field, it is a natural one upon which to concentrate

a portion of this Program.
The other area of concentration was introduced into this Program

several years ago in response to an urgent need seen by the Sponsor,, As

laser power has moved upward, laser beams, especially pulsed beams of

very short pulselength, have been found to damage the materials (amplifier laser rods, windows, lenses, prisms, and other optical elements)

through which the beams must pass,, Not only must a better understanding

be developed of the processes leading to the several types of damage so far observed, but also there is a need for more reproducible techniques

for studying these damage processes and for characterizing the response

of given materials with respect to them,,

Corresponding to these two areas of concentration, this Report is

divided into two parts, High Temperature Materials and Laser Materials.

Each part will open with a brief summary of objectives and accomplishments, followed by detailed Project progress reports.

2. HIGH TEMPERATURE MATERIALS PROGRAM

2.1. Objectives and Summary of Program in High Temperature Materials

2.1.1. Objectives

The operation of rocket and jet engines involves exposing metals

at high temperatures to oxidizing conditions, and raises the problem

of oxidation protection of high temperature refractory metal structural

components. This same problem, currently for superalloy turbine blades,

is posed again by jet engines. The oxidation protection of metals

requires some form of barrier layer to prevent diffusion. This layer,

either applied or developed by the early stages of oxidation itself,

will most likely be an oxide, and will function to keep the metal and

oxygen apart. It can work only if the diffusion rates of oxygen and

metal atoms through the layer are small enough. Thus, the strategy for

the development of oxidation protection of otherwise susceptible metals

at high temperatures will depend in part on being able to control the

rates of diffusion, and therefore, in having data on the diffusion

rates themselves

.
Diffusion in oxides is important to other classes of DoD problems.

Thus improvements in the reliability and efficiency of military

communication, computer, and radar circuitry could be made if ceramic

magnetic and ferroelectric components could be made more reproducible

and reliable. Because such important properties as initial susceptibility and coercive force of ferrites, and dielectric and piezoelectric properties of ceramic ferroelectrics, are very sensitive to the

micros true tural features (porosity, grain size) controlled by sintering,

the strategy for improvement here requires control over sintering in

these ceramics. The same is true of ballistic response of ceramic armor. The development of transparent armor requires new techniques for the

suppression of grain growth during otherwise complete sintering,

mainly of Al .

Since sintering in ceramics is mainly brought about by diffusion

processes, the development of successful, long-range strategies to meet

these DoD needs rests in part on knowledge of the rates of diffusion

of metal and oxygen atoms in oxides.

Reliable techniques for measuring oxygen diffusion rates are not

yet available, and therefore reliable data for oxygen diffusion does not

exist for any save a few oxides. Thus one objective of this program is

to develop a reliable technique for measuring the rate of diffusion of

oxygen in oxides

„

Further, diffusion rates are notoriously sensitive to impurities,

and strategies for control require knowledge of the basic rate in the

pure material and of the way various classes of impurites influence it.

This in turn requires very pure crystals for study. To date, almost no

oxide crystals have been prepared pure enough to reveal the intrinsic

diffusion rates. The second objective in this area is to prepare

intrinsically pure crystals of those materials for which mass transport

data is sorely needed. The first candidate has been Al , an important

constituent in many ceramics, in particular for ceramic armor.

3
2 1.2o Summary of Progress to Date

In previous work on the preparation of very pure crystals of Al , the growing technique had been perfected and extremely pure crystals

prepared. The probably sources of residual impurities had been identified. During the past period, modifications to the growing apparatus

have been made designed to eliminate or strongly lower the remaining

major impurities, particularly silicon. Ultra -pure aluminum has been

obtained as source material „ During the next reporting period growth

will be attempted of crystals sufficiently pure to make possible the

study of the intrinsic mass transport processes in A1~0 .

In the project on the development of a technique for the measurement of Diffusion Coefficients in Oxides, the diffusion anneal

apparatus has been used to perform diffusion anneals of ZnO crystals

for Prof. A. Heuer of Case-Western Reserve University, working under

another ARPA contract „ Work has continued on developing a method for

18 moving tracer from ground-off sections of the sample into the massspectrometer for analysis. An investigation was made of one possible

method whereby graphite is reacted with the specimen to form CO, which

is then converted to C0„ by a hot iron filament before introduction into

the mass spectrometer,, The conversion step appears to be too slow to

allow use of the method. Instead, reaction with halogen fluorides to

release oxygen for direct analysis will be triedo The necessary equipment has been designed and is being assembled.
2.2. Project Summaries in High Temperature Materials

2.2.1. Diffusion in Oxides

2.2.1.1. CRYSTAL GROWTH FROM VAPOR

H. So Parker and C. A. Harding

Inorganic Materials Division

Institute for Materials Research

The objectives of the program are: first, the growth of aluminum

mono- and bicrystals of sufficient physical perfection and chemical

purity for use as research materials; second, the reduction of both

cation and anion impurities to sufficiently low levels to permit

meaningful property measurements at the intrinsic level, and third, the

extension of the technique to other materials of interest.

During the present reporting period, growth of single and bicrystals

has continued and specimens have been supplied to Dr. Heuer at Case

Western Reserve University. A further characterization of chemical

purity under consideration at the present time is that of precise

measurement of thermal conductivity at low temperatures. This will be

pursued further if samples of sufficient size can be grown.

Modifications have been made in the seed holding technique for

bicrystal growth which should result in better maintenance of alignment

during the critical first growth cycle. A very small amount of high

purity alumina cement is placed over one end of the seeds and prefired

to 1400°C in a separate furnace with the seeds held in the proper

orientation by alumina blocks. This has the further advantage of

leaving more seed surface available for growth because less cement is

needed to provide the required strength.

Poor quality growth in one bicrystal run, the crystal having the

appearance of being grown under a very high supersaturation, was

traced to a minute leak in the alumina growth chamber. In the pressure

range over which the system operates, 0.3 to 5 Torr, the leak was too

5
small to be detected under dynamic pumping conditions. Rate of pressure

rise in a blanked -off system is now used to establish the vacuum tight

integrity of the system before growth.

New heaters have been designed and wound for the two vapor growth

furnaces . The present temperature control system operates by sensing

current level and maintaining a constant current through the furnace.

The new heaters incorporated a second concentric winding of small

diameter tungsten wire, electrically isolated from the heater winding.

This auxiliary winding will form the temperature sensitive arm of a bridge circuit, thus the control system will respond directly to

temperature changes rather than current changes, providing better

temperature control.

Preliminary testing of components of the new vapor growth system

is in progress and it is anticipated that the new system will be fully

operative in about one month's time. A supply of zone refined

aluminum metal of nominal 99.9999% purity has been obtained from a commercial source . Upon completion of a growth run with this source material, a series of comparative analyses will be made.

A paper describing the growth and characterization of ultra-high

purity aluminum oxide crystals by the vapor growth technique is in

preparation.

2.2.1.2. DIFFUSION IN REFRACTORY MATERIALS

A. L. Dragoo

Inorganic Materials Division

Institute for Materials Research

The objectives of this program are to measure and to interpret

diffusion of oxygen in single crystal oxides. Experimentally these two

objectives consist of (1) extending the sensitivity of present techniques

6
18 so that the distribution in a single crystal sample can be measured,

(2) comparing the results of these concentration profile measurements

18 with measurements of the rate of exchange of between the gas phase

and the oxide crystal, (3) determining the dependence of the diffusion

rate on temperature, oxygen partial pressure, impurities in the solid,

etc. and (4) obtaining reliable diffusion and exchange coefficients

„

Oxygen self -diffusion in oxides provides a tool for uncovering the type

of point defects involved in ionic migration in oxides and the energies

required for the formation and motion of these defects . Since oxygen

diffusion can be the controlling step in the solid state reaction and

sintering of oxides, the diffusion coefficients are needed for interpretation and estimation of these rates.

Our initial studies have been carried out on single crystals of

rutile because of its relatively high oxygen self -diffusion rate along

the C axis in the neighborhood of 1200°C'. Our results at 1275 and

1350°C agree well with those of Haul and Dumbgen(l) who studied oxygen

exchange with single crystal fragments and with those of Lees, et al(2)

18 who use a proton activation technique on O-diffused single crystals.

The problem of platinum deposition during diffusion anneals in the

platinum furnace tube was eliminated by hanging a series of heat shields

above the specimen. I estimate that the thermal isotopic fractionation

which may occur should be no more than a fraction of a per cento

Having established the gaseous exchange technique, the next step

is the development of an analytical procedure for determining the

16 18

distributions of and in the oxide crystals. A possible technique for liberating 0~ from the small amounts of powder removed from

the crystal during sectioning is the reaction of the oxide powder with

hot carbon in molten platinum Precise measurement of the isotopic

ratios necessitates the conversion of the CO to C0

? prior to the mass

spectrometer analysis

„
Oishi and Kingery(3) reported the catalytic decomposition of CO to

CO- using a red-hot, electrically heated, iron filament previously

reduced by hydrogen. The C0~ produced by the reaction

2C0(g) = C0

2 (g) + C(s)

was collected on the liquid nitrogen cooled walls of the tube during

the course of the reaction which required 2 to 4 hours to complete

depending upon the surface conditions of the iron wire.

I attempted to increase the conversion rate by using a dense

bundle of iron wire which had a much larger surface area than the coiled

iron filamento The wire was contained in a quartz tube and was heated

externally, as indicated in Fig. 1. After reducing the wire with

-3

hydrogen, a small quantity of CO (10 to 20 u moles at 6 x 10 to

-3 2

12 x 10 N/m pressure) was admitted to the tube e The gas and iron

wire were heated; the temperature of the heater was controlled

between 780° and 1180°C for periods of 2 to 4 hours „ The lower end of

the tube, filled with glass balls to provide a large surface area, was

cooled with liquid nitrogen., The pressure was observed with a thermocouple gauge. I did not observe any pressure drop which would

have occurred if C0„ had been formed and had been frozen out.

Since this reaction, which is the critical step in the graphitereduction procedure, did not promise a quick success, the procedure has

been abandoned for the much more thermodynamically favorable bromine

pentafluoride reaction in order to liberate

? from the oxide samples.

Two diffusion anneals were carried out on ZnO specimens for

Dr Heuer of Case -Western Reserve University,,
METAL VALVE

O-RING CONNECTOR

MANIFOLD

TC GAUGE

TEFLON VALVE

QUARTZ TUBE

GLASS BALLS

IRON WIRE

FURNACE

_LIQUIDnItrogen"

Fig. 1 Apparatus for the Conversion of CO to C0„.

9
References

1. R. Haul and G. Dumbgen, J» Phys. Chera. Solids 26^, 1 (1965)

2. D Lees, J M. Calvert and A, J. Derry, "A Technique using

Resolance Capture of Protons to Study Oxygen Diffusion in Titanium

Dioxide," paper presented to the Thomas Graham Memorial Symposium

on Diffusion Processes, University of Strathclyde, Glasgow,

Scotland, 1969=

3o Yo Oishi and W Do Kingery, J» Chem. Phys 33, 480 (1960)

10
3. IASER MATERIALS PROGRAM

3.1o Introduction to Laser Materials

The National Bureau of Standards, with support from the Advanced

Research Projects Agency (Office of Materials Sciences) of the

Department of Defense, is carrying out a program of research on laser

materials o This program includes studies to characterize laser

materials, to define and evaluate damage measurements in laser materials,

and to investigate the degradation of laser performance . A summary of

the results achieved during the reporting period is given here.

3.2. Objectives and Summary of Program in Laser Materials.

3o2.1o Objectives

One of the important factors which is limiting the advance of

ultra high power laser technology is the failure or degradation of

laser materials due to optically induced damage. In the past, the

empirical approach to laser materials has been adequate. That is, the

damage or degradation to laser materials has been circumvented by

equipment design or by a cut-and-try improvement in the laser materials.

This problem of optically induced damage now poses a serious threat to

the successful application of laser devices to an important class of

needs of the Department of Defense , This fact has been recognized and

the Advanced Research Projects Agency supports research to improve the

performance of laser materials. The convening of three cxsnmittees to

examine the optical damage to laser materials illustrates the present

concern and importance of solving this class of problems. These

committees are the Subcommittee II of ASTM Committee F-l, the

Institute for Defense Analysis Studies, and the National Materials

Advisory Board Committee on the Fundamentals of Damage in Laser Glass.

The success of these efforts depends upon having the knowledge

and proper skills available when they are needed. In the case of

laser technology, they include the science and techniques of laser

11
materials preparation and characterization and of the measurement and

evaluation of those properties which are relevant to the damage and

degradation of lasers. It is the overall objective of the present NBS

program on laser materials to identify some of the more important

problems concerning damage and degradation of lasers and to provide the

necessary advances in those areas for which NBS possesses the competence

to do so. These efforts will be directed mainly toward providing the

techniques for the preparation and characterization of laser materials

to be used in tests for the measurement and evaluation of material

properties pertinent to the damage and degradation of lasers, for the

tabulation of key damage data, and for the improvement in the knowledge

and skills necessary to advance the performance of lasers

In pursuit of this program, a continuing attempt is being made to

analyze the technological problems associated with high performance

lasers and the materials from which they are constructed, and to distill

from this analysis major scientific problems whose solutions would

advance laser technology. In addition, the problems chosen should fall

within areas in which NBS has available competence,, Also, the number of

problems included in the program should be kept small enough so that

each one can be approached in a reasonably compreshensive manner.

The level of effort on laser materials has been reduced at the

suggestion of the sponsor. This has been accomplished by terminating

the projects on the Characterization of Laser Materials and on the

Degradation of Laser Materials. In accordance with these changes and

the above mentioned criteria, the present program consists of the

project on Damage to Laser Materials. Damage and Degradation as used

here are distinguished in that Damage is localized and occurs during the

course of a very few pulses whereas Degradation implies the much more

gradual and general deterioration of performance over many pulses „ There

may, of course, be some overlap.

12
3.2.2. Summary of Progress to Date

3.2 2olo Characterization of Laser Materials

The projects on the Characterization of Laser Materials have been

terminated. The results of the past work have been or will be published

in the open literature. In particular, four papers have been written:

i) R. M. Waxier and E„ N. Farabaugh, Photoelastic Constants of

Ruby, NBS Journal of Research, Sec. A (Physics and Chemistry) 74A.

No. 2, 215 (1970).

ii) R. M. Waxier, Laser Glass Composition and the Possibility of

Eliminating Electrostrictive Effects, to be published in the April 1971

issue of the IEEE J. of Quantum Electronics.

iii) R. M. Waxier, et al., Optical and Mechanical Properties of some Nd-doped Laser Glasses, accepted for publication in the NBS J. of

Research, Sec. A (Physics and Chemistry).

iv) R. F. Blunt, G. A. Candela, R. A. Forman, A. H. Kahn, and B. W,

3+

Mangum, Magnetic Susceptibility and Optical Studies of Cr in Al 0.,

(Ruby); submitted to the Journal of Applied Physics, Jan. 1971.

3.2.2.2. Damage to Laser Materials

Inclusion damage (bulk) has been a serious source of difficulty for

the laser systems engineer concerned with ultra -high power pulses.

Although surface damage and intrinsic damage (self -focusing or beam

trapping) contribute additional problems, these are at present apparently not as great an obstacle to the advance of laser technology as the

inclusion damage is. A laser glass damage test facility is being

constructed at NBS to study the damage problem and to attempt to bring

some unity to the great proliferation of damage data. This test

facility will center around a one gigawatt neodymium glass laser system

and will be used to develop standards for performing meaningful

(reproducible) measurements on the damage mechanisms which are produced

by high energy laser pulses. The construction of this facility includes

detailed characterization of the laser beam. Near field intensity

profiles of this laser have been made and have been compared with other

neodymium lasers . These profiles will be used as a guide to make

13
certain that the NBS laser beam has a satisfactorily homogeneous beam.

The necessity for having a homogeneous beam in order to obtain meaningful damage measurements has been stressed at several conferences

„

During this report period, with the benefit of valuable consultations with Dr J. Emmet t and Mr., Jo McMahon (Naval Research Laboratory)

and with Dr. J. Swain (Lawrence Radiation Laboratory, Livermore), the

present NBS Nd-glass laser is being improved in order to ensure sufficient beam uniformity and reproducibility to allow meaningful damage

threshold measurements

„

A theoretical model for studying the thermal stresses produced by

inclusions in laser materials has been developed. Three papers on this

problem will be published; namely,

i) H. S. Bennett, Absorbing Centers in Laser Materials, to be

published in the February 1971 issue of the J. of Applied Physics.

ii) H, S. Bennett, Inclusions in Laser Materials, to be

published in the NBS Journal of Research, Sec. A (Physics and Chemistry).

iii) Ho So Bennett, Heat Diffusion near Absorbing Centers in Laser

Materials, to be published in the NBS Journal of Research, Sec. A

(Physics and Chemistry)

3o3. Project Summaries in Laser Materials

3.3ol. Damage to Laser Materials

3.3.1olo LASER INDUCED DAMAGE STUDIES

Albert Feldman, Roy Waxier, and Deane Horowitz

Inorganic Materials Division

Institute for Materials Research

Objectives

The objectives of this work is the development of a laser test

facility to determine intrinsic bulk damage threshold parameters of

laser glasses used in pulsed high energy laser systems. To achieve this

objective, it is necessary to produce a laser beam in the TEM mode.

14
This work complements the work at the Naval Research Laboratory

which is concerned with developing high power laser systems and testing

for inclusion damage.

Technical Approach

The present report period has been spent testing and evaluating the

performance of the laser system, described in our previous report (NBS

Technical Note 565, 1971), which had been delivered by a commercial

supplier.

Burn pattern tests made on the oscillator rod showed the beam to be

extremely inhomogeneous and non-reproducible from shot to shot. It was

thus necessary to examine every element in the laser system, and to

improve a number of them.

Figure 1 shows a shadowgraph of the oscillator rod. The structure

at the center of the pattern indicates damage which occurred in the

ant iref lection coating on the rod during preliminary testing of the

system. The rod was returned to the manufacturer for repolishing and

has been returned to us.

Figure 2 shows Twyman-Green interferograms of the two tent polarizers in our system. Both indicate they are acting as weakly positive

elliptical lenses in the system. Figure 3 shows a Twyman-Green interferogram of the first amplifier rod in our system. The jog in some of

the fringes indicates that there is an abrupt change in optical path in

the rod which might adversely affect any attempt to obtain diffraction

limited operation of the system. The second amplifier rod appears to

have reasonable optical path uniformity as shown by the relatively

evenly spaced straight interference fringes in Fig. 4.

The Pockels cell, which was returned to the manufacturer for

improvement of the shuttering characteristics, was also found to have

damaged windows

.

15

;
System Evaluation

After consultations with Dr. J. Emmet t and Mr. J. McMahon of the

Naval Research Laboratory we have come to the conclusion that this

commercial system requires extensive modification.

It is necessary for every component of the oscillator system to be

of extremely high optical quality because any optical distortion is

magnified by the many passes which the laser light must make in the

oscillator cavity ,> Thus a cavity with the fewest number of elements is

desirable. We have therefore decided to Q-switch by means of a rotating

mirror and hence eliminate two polarizer stacks and the Pockels cell.

In addition, a specially selected oscillator rod has been ordered which

will produce less than 1/10 wave of wavefront distortion with the

632.8 nm HeNe laser line.

Present Activities

We are presently assembling components for construction of a Twyman-Green or Mach-Zender interferometer „ These will be used for

examining all optical components to be used in our laser system, and

also to measure the optical distortions introduced into the laser rods

by pumping. It is planned that these tests will be performed under

dynamic conditions.

Acknowledgments

We would like to thank Dr. J. Eramett and Mr. Jo McMahon of the

Naval Research Laboratory for their valuable assistance in this work.

The Twyman-Green interferograms and the shadowgraphs were done by

Fred Rosberry of NBS.

16
Fig. I. Shadowgraph of oscillator rod„ The structure at the

center is caused by damage in the antireflection coating.

17
Fig„ 2(a) Twyman -Green interferogram of ten plate tent polarizer

in oscillator system.. Three elliptical fringes are shown

which indicate that the polarizer is acting as a weak

positive lens.

18
Fig. 2(b). Twyman-Green interferogram of five plate polarizer

stack in oscillator system. Two fringes are seen. This polarizer is acting as a weak cylindrical lens

19
Fig. 3. Twyman-Green interferogram of first amplifier rod. The

abrupt shift in the fringes indicates a sharp gradient in

the optical path across the rod.

20
Fig 8 4o Twyman-Green interferogram of second amplifier rod. The

relatively evenly spaced fringes indicate good optical

quality , The small spots are caused by dust.

21
3.3. 1. 2 . EFFECT of MICRODOMAINS FORMED by LIQUID /LIQUID

IMMISCIBILITY UPON the DAMAGE THRESHOLD of GLASS*

Wolfgang Ha Her

Inorganic Materials Division

Institute for Materials Research

and

H. Dardy

Catholic University of America

Washington, D.Co 20017

Objective

The objective of this associated project was to determine whether

a correlation exists between the possible microdomain structure of

glass laser rods and the intrinsic bulk damage threshold,,

Approach

In a previous report(l) the question was raised whether metastable

immiscibility, which is now being recognized as a widely occurring

phenomenon in many glass -forming systems, plays a significant role in

the mechanism of optically induced damage to laser glass rods. This

question assumed even greater importance, because it was found (1) that

a glass composition, close to one used in commercial laser-glasses, had

a measurable liquid/liquid immiscibility gap with a critical temperature in its transformation region. It was therefore quite likely that

the forming and annealing of such a glass would lead to the presence of

a microdomain structure in the laser rod.

There is no doubt that microdomains cause some scattering and beam

attenuation of a magnitude depending on the domain size and the

* An associated project

22
difference of the index of refraction at the laser wave length between

the coexistent glass phases in the laser rod. Such scattering, however,

would mainly affect the lasing efficiency. Because of its diffuse, nonlocalized nature, the energy absorption associated with the microdomains

would most likely not contribute to damage to the laser rod. The

correctness of this assumption has been tested to the following

investigation.

The glass used in this investigation is a sodium borosilicate of

the composition (weight per cent): Na~0, 6.0; BO, 25.6; Si0„, 68.4.

This glass composition is at the top of the metastable imraiscibility

dome of the Na~0. Bo0o» Si0~ system(2) and its domain growth kinetics

have been studied extensively (3). Domains of controlled size are

introduced in the glass by heat treatments of pre-selected duration and

temperature. The method of heat treatment is described elsewhere (3).

After the preparation of a surface by fracture, selective etching,

shading and replicating, the size of the domains can be measured by

replica electromicroscopy. Another way of assessing domain size is by

selectively removing one of the coexistent phases in a series of leaching treatments and subsequently measuring the pore size distribution

of the remaining skelton by mercury intrusion porosimetry(4) . The pore

size diameter found by mercury porosimetry measures the narrow parts of

the interconnected pore system,, These values are about one-third of

the grain diameters measured by electromicrography. The glasses used

have domain diameters (from mercury intrusion-porosimetry) of 130A,

200A, 250A, 380A, and 565A. Glasses up to 250A domain size appear

clear to the eye. Glasses of 380A and 565A domain size are blue-yellow

opalescent. The glasses are cut and polished to give 1 x 3 x 3 cm prismatic samples „ The damage threshold of the glass samples is tested

by exposing them passively to a focused laser pulse.

A nominal 10 megawatt ruby laser pulse of 15 nanoseconds duration

was employed in the study. The laser operated in a single mode by means

of a transmission sapphire etalon and had a cryptocyanine-methanol

23
Q-switch<, The expected damage site was detected by aligning a He-Ne

laser coaxially through the ruby laser beam,. This enabled one to

inspect visually the focal region in the glass both before and after

each actual laser pulse. The samples could be moved with micrometer

drives in all directions „ Only such regions were selected for a ruby

shot which did not show any foreign particle inclusions when inspected

in the coaxial He-Ne laser beam. The relatively good reproducibility

is believed to be due to this pre -select ion. A ballistic thermopilecalorimeter monitored the integrated energy of each pulse. Power

control was achieved by rotating a Glan-Thompson prism external to the

laser cavity. This permitted the laser to be uniformly pumped so as to

reproduce the same Gaussian beam profile and frequency spectrum from

shot-to-shot o A 9 centimeter focal length lens was used to focus the

laser beam into the sample.

All measurements were reproducible to within better than 10 per

cento The experimental configuration was not changed during these

runs. A relative comparison between glass systems was thus assured

even if absolute energies were not exactly readable. Absolute energy

measurements require that a time profile of each pulse be obtained

through use of a fast photodiode and wideband oscilloscope. Such

equipment was not available at the time of these measurements. Previous

calibration of the laser system has shown that reproducibility from

shot-to-shot can be maintained provided temperature equilibrium of the

laser cavity is established,, It is on this uniformity that we base our relative comparison of measured energies even though we are not able to

. 2

convert these into absolute power densities in terms of MW/cm . Multiple

firing of the laser below the critical energy levels needed for damage

produced no damage to the glass samples provided sufficient time for

temperature equalization was allowed.

o The glass sample with the smallest domain size (130A) began to

fracture at 0.55 joules. The sample with the largest domain size

o (565A) required 0.73 joules to fracture. The other samples ranged in

24
between. This difference in energy levels was probably due to beam

attenuation in the more turbid samples.

In order to establish a comparison with present materials and other

glasses, damage thresholds of the following glasses was determined

Fused silica and low thermal expansion borosilicate glass

required both 0.55 joules. A medium thermal expansion mixed alkali

borosilicate glass was slightly below 0.55 joules. Two commercial,

Ne -doped laser glasses showed very low damage thresholds, approximately one-fifth of that exhibited by fused silica. Energy measurements

to determine the exact cut-off were below the range of operation of the

ballistic thermopile employed in the experimental system. The damage

in both samples, however, was identical.

Conclusion

Under the conditions of the particular damage threshold test, an

immiscible sodaborosilicate glass with submicroscopic domains displayed

a threshold value similar to that for fused silica and two alkali

borosilicate glasses. The damage threshold was substantially independent of domain size and was approximately five times greater than the

thresholds of two commercial laser glass compositions <>

References

1. J. H. Simmons and W. Haller, "Liquid -Liquid Phase Separation in a Laser Glass", NBS Tech. Note No. 531, 31 (1970).

2. W. Haller, D. H. Blackburn, F. E. Wagstaff and R. J. Charles,

"Metastable Immiscibility Surface in the System Na_ 0-B

2

-SiO "

J. Amer. Ceram. Soc, 53, [1] 34-39 (1970).

3. W. Haller, "Rearrangement Kinetics of the Liquid-Liquid

Immiscible Microphases in Alkali Borosilicate Melts",

J. of Chem. Phys. 42, 686 (1965).

4. W. Haller, "Chromatography on Glass of Controlled Pore Size",

Nature, 206, 693-696 (1965).

25
3.3.2. Appendix: Trip Reports

3.3. 2d. FOREIGN TRIP REPORT

Report by: Dr. Herbert S. Bennett

Phys ic is t National Bureau of Standards

Institute for Materials Research

Inorganic Materials Division

Physical Properties Section

Locations Visited:

University of Grenoble, France 9/13/70 - 9/17/70,

International Conference on Magnetism and Laboratoire de Spectrometric

Phys ique

University of Rome, Italy 9/18/70, 9/21/70 - 9/23/70, Physics

Department

University of Amsterdam, the Netherlands 9/25/70, Physics

Department

Atomic Energy Research Establishment, Harwell, England 9/28/70 - 9/30/70 and 9/6/70, Theoretical Physics Division

Royal Radar Establishment, Malvern, England 9/30/70 - 10/2/70,

Nonlinear and Electro-Optics Section

Imperial College, London, England 10/5/70, Department of Metallurgy

Purpose of Trip

a) To attend the International Conference on Magnetism, 1970, and

to visit the high magnetic field laboratory at the University of

Amsterdam.

b) To visit the researchers at the University of Rome, at the

26
Atomic Energy Research Establishment, Harwell, and at Imperial College,

who have performed experimental and theoretical studies on color centers

in ionic crystals.

c) To visit the Royal Radar Establishment, Malvern, and the

University of Grenoble . Researchers at these laboratories have experienced damage or degradation in systems containing lasers and laser

materials.

d) To investigate possible areas of laser damage research in which

the National Bureau of Standards might make a unique and definite

contribution..

e) To present a paper at the Magnetism Conference and to present

formal talks at each of the above laboratories, except at Imperial

College o These talks, which I gave, treated three different research

areas to which I have made contributions . The areas are magnetism,

damage to laser materials, and electronic states of color centers.

Summary:

This trip report summary is written in chronological order and not

according to the subject areas.

1. International Conference on Magnetism

The 1970 International Magnetism Conference was extremely large

and suffered from the usual problems common to large meetings with

parallel sessions. The single most important point from this conference

for me was to learn that the researchers in Prof. Prigogine's group at

the University of Brussels are working on an expansion technique from

which the transport properties (e.g., spin diffusion and thermal

conductivity) for magnetic systems near their critical points may be

computed. Prof. Prigogine discussed an unpublished calculation on such

transport properties. His group found that the contributions of the

various modes of the system are similar to each other to all orders of

the expansion for a class of mean-field models. Thus, the usual perturbation series does not converge. In fact, a renormalization procedure

is required to describe the behavior near the critical point.

27
The papers presented in the session on the "Critical Phenomena"

indicated that progress has been made since the last International

Conference on Magnetism (Boston 1967) in applying scaling concepts to

critical phenomena such as the second order ferromagnetic and antiferromagnetic phase transitions. Prof. Gaunt (King's College, London)

reported on the considerable amount of evidence which supports scaling

concepts for the equation of state in the critical region. He gave

evidence that the scaling function which occurs in the equation of

state depends on the lattice dimension and not on lattice structure.

Also, by examining the higher order terms in the equation of state, one can assess the region of validity of the scaling law form.

2. Laboratoire de Spectrometrie Physique, University of Grenoble

I discussed with Dr. J. Lajzerowicz (Grenoble) the second harmonic

light scattering by domains in ferroelectric materials such as triglycine sulfate and KDP. The latter material is used as the Q switch

in the NBS Nd -glass laser. If domains are induced in the KDP Q switch,

then inhomogeneities in the laser beam due to the generation of second

harmonic light, which will reduce the spatial intensity at the

fundamental frequency in the vicinity of a domain wall, might arise.

Dr. Lajzerowicz believes that if the electric field which induces the

dipole moments in the KDP Q switch is perpendicular to the optic axis,

then the generation of second harmonic light should not occur. Hence,

it is most important for the NBS laser effort to determine that these

conditions obtain. At present, the Nd -glass laser has inhomogeneities,

and we should make certain that the electric field is perpendicular to

the optic axis of our KDP Q switch. Dr. Lajzerowicz also pointed out

the importance of having a single crystal of KDP for the Q switch. He

feels that polycrystalline KDP would certainly introduce inhomogeneities in the intensity of the laser beam.

3. University of Rome, Physics Department

Dr. P. Perfelti is developing he teroj unctions of GaAs and PbS.

Such junctions may be used as pressure sensitive detectors for 7 H to

28
14 H radiation. The CO^ laser radiates at 10.6 M.„ The atmosphere also

exhibits relatively little absorption for this same range of wavelengths .

Prof. G. Chiarotti and Prof. G. Margaritondo recently have

measured several properties of the relaxed and unrelaxed F -center states

in KC1, KBr, RbCl, and NaCl. They studied the F center in absorption

and also in emission. In particular, the Stark effect in the F center

absorption has been observed in the above mentioned alkali halides.

Changes of the absorption coefficient due to the electric field have

been measured as a function of photon energy. From these data and by

using the method of moments given by Slichter (University of Illinois)

the energy difference between the 2s and 2p levels and the dipole

matrix element <2s|z|2p> have been evaluated. These are unpublished

results of which I was not aware when I was writing my most recent

paper on the F center. In that paper, I considered only the Stark

effect for emission. I plan now, as the result of my conversations

with Prof. Chiarotti and Prof. Magaritondo, to compute the Stark effect

for absorption.

I learned also that in addition to reporting my F center lifetimes

in terms of the ratio of dipole matrix elements, I should have reported

the values of the dipole matrix elements for absorption and emission.

These values are available and the researchers at the University of

Rome have suggested that I publish them in a separate paper. I underestimated their worth to experimentalists and did not include them in

my most recent paper in an effort to reduce the length of that paper.

4. University of Amsterdam, Physics Department

The University of Amsterdam has a new high magnetic field laboratory. The installation is equipped with a computer determining the

field strength as a function of time. This permits a very sensitive

test of the proportionality between the magnetic moment and the field

strength in paramagnetic substances.. Dr. R« Gersdorf and others in his

29
group described the magnetic field installation. It has been constructed

to produce a field which increases from to ,32? 6T in about 250 ms,

remains constant at 32. 6T for 60 ms and then decreases to zero in seven equal steps of about 80 ms each. Experiments can be performed at

4°Ko They have detected deviations from linearity of Pd metal at 4°K.

2 They find that the magnetization M = x H (l + $H), where x is the low

field limit to the susceptibility, H the magnetic field, and

p = (6.1 ± 1.0) x 10 T . Hence, they argue that a sensitivity of

1 part in 10 is possible with their new installation.

I consulted also with Prof, de Chatel at the University of

Amsterdam on his problem of understanding the highly damped but propagating modes in magnetic spin systems.

Profo Po de Chatel is attempting to construct a theory for the very

weak itinerant ferromagnetism in the binary alloys Ni,,Ga and Ni-Al.

He would like this theory to be valid in the vicinity of the transition

temperature T and to include the highly damped propagating modes. Such

modes frequently are called paramagnonso He has computed the response

of these systems to a weak external magnetic field at temperatures less

than the critical temperature. We discussed the possible analogy

between the poorly propagating modes around the critical temperature

which I found in the Heisenberg systems and the spin wave spectrum of

itinerant electrons which Prof, de Chatel believes exists in these

binary alloys, assuming that the alloys NiJSa and Ni-Al are free from

the complications caused by composition clusters (local deviations from

stoichiometric composition).

5. Atomic Energy Research Establishment - Theoretical Physics

Division

Dr. Marshall Stoneham and I are working on a theory to treat the

long lifetime of the relaxed excited state of the F center in KCl and

NaClo Such studies are related to the speed with which photochromic

materials may be switched. The problem is to determine how the local

30
electric field varies from one F center to another. I have recently

developed a theory which gives the lifetime as a function of the at - present unknown local electric field. Dr. Stoneham has computed

recently the fluctuating electric fields for the dynamic and static

Jahn-Teller effects . We feel that the elements of his work can be

applied to my F center theory. While at the Theoretical Physics

Division we thought about how to accomplish this We have ideas on this

now but the mechanics for developing the necessary theoretical techniques are not straightforward and will require considerable more

thought. A gross simplification of our present understanding is as follows. Due to the local vibrational modes of the lattice a distribution of E center types exists . Some F centers experience large

electric fields and other experience only small electric fields. So the

lifetime measured in the laboratory is a nonlinear average over the

distribution of F center types. The problem is to compute the distribution of F center types. We plan to collaborate on this problem in

the future.

I discussed with Dr. Alan Lidiard the changes which have occurred

at the Atomic Energy Research Establishment and in particular the

changes which his Theoretical Physics Division have undergone since

1966. Many researchers now apply their knowledge in theoretical physics

to very practical problems of private industry and government. One

example is the control and reduction of fuel cell swelling in fast

breeder reactors.

60 Royal Radar Establishment, Malvern

Dr. Ken F. Hulme was my host and arranged a most informative

visit for me.

Dr. Hulme is in charge of the Nonlinear and Electro-Optics Section

at Royal Radar Establishment. Their original aim was to develop new

materials for the variable part of the spectrum. But they soon became

involved with the optical mixing crystals. First they grew KDP, ADP

31
and triglycine sulfate and then Ag AsS , LiNbO_, Ba NaNb 0. , and LilCL,

They are now interested in the coumarin derivatives. Such derivatives have been used previously as Q switches for ruby lasers. But now

single crystals of the coumarin derivatives may be used for frequency

doubling the beam from neodymium glass and neodymium YAG lasers.,

Articles appearing during the past year in Applied Physics Letters claim

that single crystals of coumarin derivatives are highly resistant to

optically induced damage.,

Other recent nonlinear optical materials which they have studied are

GdHg (NS) , lithium forma te, Te0

2

, Yt alumina te, Ho fluoride, and FAP.

Dr. Hulme's group now is phasing out research on new materials and

concentrating more on how best to use presently available materials in

electro-optical devices (Q switches, up-conversion and infrared imaging

to the visible.)

Researchers at NBS are growing crystals of pyrargyrite (Ag~SbS_).

This crystal is related to proustite (Ag AsS-) and has nonlinear optical

applications from 1 M- to 10 M-o Pyrargyrite appears more promising than

proustite because pyrargyrite has a damage threshold which is much

further from the nonlinear optical threshold than that for proustite.

It also should have better optical qualtities c Because the techniques

for growing pyrargyrite are similar to those for growing proustite

crystals, I was particularly interested in their results on proustite.

Researchers at RRE have had considerable experience in growing single

proustite crystals for up-conversion, which requires large crystals

„

Using a modified Bridgeman technique they have grown crystals 2.6 cm

in diameter by 1 cm thick. They have used the Stockbarger method to

study crystal growing for the following crystals, AgGaS

2

, CdGa_S,,

ZnGa„S,, ZnIn„S,, and AgInS„. The Stockbarger method has been used also

to grow proustite boules 15 mm long by 12 mm in diameter at the growth

rate of 4 mm per day- Controlling the stoichiometry of proustite is

difficult and still uncertain.. They use excess sulfur in some cases to

attempt to control the stoichiometry of proustite.

32
The Royal Radar Establishment has a materials information center.

The center, called Electronic Materials Unit, exists to make available

research quantities of those electronic materials developed by the

Materials Research Division of which Dr. Beardsley is the Chief, which

are not otherwise available because of their novelty or of the

difficulty of their preparation.

7o Imperial College, Department of Metallurgy

I took this opportunity to visit Nancy Tighe, who is on a leave of

absence from the National Bureau of Standards to work with Prof. Peter

Swann on the 1 MEV electron microscope at Imperial College. This 1 MEV

electron microscope is in the final stages of being installed in a laboratory which was previously used for hydraulic studies. The microscope is an impressive 2 1/2 stories high. Prof. Swann and Nancy Tighe

plan to study the reduction of hematite (Fe-CL) to magnetite (Fe_0.) and

then to FeO. At present the Fe o 0„ and Fe„0. samples do not thin as

2 3 3 4

easily as the previous Al-0 samples with which Nancy Tighe has worked

at NBS. Relatively thin samples less than 50 u- are required even for

the transmission experiments planned with the 1 MEV electron microscope.

In addition, they are concerned about obtaining good magnetite crystals

and may ask the National Bureau of Standards to provide them with such

crystals.

Prof. Boswarva and Dr. Tharmalingam, who are also at Imperial

College, have calculated theoretically the formation energies of

Schottky and Frenkel defects in calcium fluoride, uranium oxide, and

cerium oxide. Their model for calcium fluoride is similar to the model

which Dr. Alan Franklin at NBS has used. Using Dr. Franklin's calcium

fluoride results to back their model, they then proceed to do the

f luor ite -structured oxides U0 ? and Ce0». They find that to order,

1st order and 2nd order, anion Frenkel pairs are the intrinsic defects

in calcium fluoride. However, the order calculations predict that

anion Frenkel pairs are the intrinsic defects in U0„ and higher order

calculations predict that the Schottky defects are the intrinsic defects.

33
I also spoke with Prof o D. Holt about laser beam uniformity

problems with GaAs lasers. Such lasers are noted for their hot spots

(f luctations) in the spatial intensity of the laser beam. A GaAs laser

is a p-n junction for which the p layer is GaAs doped with Zn and the n layer is GaAs doped with Se, Te, or Si. The two different dopants

change the crystal parameters somewhat and lead to incomplete interstitial lattice planes near the junction. Prof. Holt has found a correlation between laser performance and the number of defects near

the p-n junction.

At all of the laboratories which I visited, I asked whether they

had any staff members who were familiar with damage or degradation in

any of the laser materials. In particular after visiting the Royal

Radar Establishment, University of Grenoble, and Imperial College, I

have formed the subjective opinion that there is little, if any, effort

in Western Europe to improve the characterization of laser beams for

damage measurements. The science of measuring damage thresholds at

present needs considerable improvement,. Even at the Royal Radar

Establishment there is not a group which plans detailed experiments

to determine the spatial uniformity, the temporal properties, and the

frequency spectrum of the laser beams used in studying damage measurements. Once these parameters are known it is also important to improve

them» This is an area of laser technology which has been neglected and

is the area in which the National Bureau of Standards can have the

greatest impact if its program to develop a damage test facility is

successful.

34
3.3.2.2. DOMESTIC TRIP REPORT

Report by: Dr. Herbert S. Bennett

Physicist

National Bureau of Standards

Institute for Materials Research

Inorganic Materials Division

Physical Properties Section

Locations Visited:

Lawrence Radiation Laboratory, Livermore, 11/9/70

Microwave Laboratory, Stanford University, 11/10/70

U. S. Navy Postgraduate School, Monterey, 11/12/70 - 11/13/70

Hughes Research Laboratories, Malibu, 11/16/70 - 11/17/70

Purpose of Trip:

lo To visit and consult with the researchers at the above

laboratories, who have experienced damage or degradation in systems

containing lasers and laser materials.

2. To investigate possible areas of laser damage research in

which the National Bureau of Standards might make a unique and definite

contribution.

3. To present invited colloquia at the Lawrence Radiation

Laboratory, the U. S. Naval Postgraduate School, and the Hughes

Research Laboratories.

Some of the information which I acquired during visits to laboratories concerned with damage to laser materials is reported here. These

laboratories and my host at each laboratory were: The Lawrence Radiation Laboratory, Livermore, (Dr. J. Swain, 9 Nov.), the Microwave

Laboratory, Stanford University (Prof. Robert Byer, 10 Nov.), and Hughes

Research Laboratories, Malibu, (Dr. Conrad Giuliano, 17 Nov.).,

This trip is written in chronological order

„

35
lo Lawrence Radiation Laboratory, Livermore, 11/9/70, Dr. Jo Swain.

Dr c J. Swain was my host. Researchers in Dr. Swain's group at LRL,

Livermore, have constructed a 15 element-5 inch disc laser and are planning to build a 12 inch disc laser. These lasers contain neodymiumdoped laser glass and radiate at the 1.06 u neodymium wavelength. The

15 element-5 inch disc laser produces a laser beam with an energy of 200

2

J over 50 cm in a Ins pulse „ If the laser damage thresholds of the

optical components can be increased, the present configuration has

sufficient energy storage to produce a laser beam with 1000 J in a 1 ns pulse. I also visited this laboratory in June 1969. Since that time

researchers have encountered additional damage problems. Operating at

lower power densities than they now do, the researchers experienced in

early 1969 only surface damage on the non-las ing components in the laser

2

beam. Their attempts to exceed the present limit of 400 J over 50 cm

in a 1 ns pulse usually lead to surface damage of the die lee trie -coated

mirrors and to self -focussing damage in the laser rods. Using

commercially-available high quality optical glass, the find

that the damage due to inclusions becomes negligible. In order to solve

the surface damage of the dielectric -coated mirrors, they are replacing

the mirrors with prisms constructed from borosilicate crown glass BK 7.

They hope that the bulk extrinsic and intrinsic damage thresholds for

prisms will be greater than the surface damage threshold of the prisms

and of the presently used mirrors by at least a factor of two. Because

meaningful data on the damage thresholds of optical materials at 1.06 u do not exist, the researchers at Livermore will have to experiment with

several materials and techniques in their attempts to achieve the

1000 J pulses for which their present configuration has been designed

Dr. Swain has devoted considerable effort and time to characterizing the temporal and spatial properties of the laser beam. The

Lawrence Radiation Laboratory 5 inch disc laser contains a very complex

oscillator section to achieve a reproducible beam which exhibits

Gaussian behavior in time and in space. He has determined the uniformity of the nine KDP Pockei cells (Q-switches). These Q-switches

36
together with apertures are used to control the laser beam. In addition,

because every time the laser beam is apertured or pulse shaped the

energy content of the beam decreases, the researchers have a total of 9

additional amplifier rods in the complex oscillator section with feedback to control spatial and temporal properties of the laser beam.

2. Microwave Laboratory, Stanford University, 11/10/70, Prof.

Robert Byer.

Professor Robert Byer directs the research on the non-linear laser

materials effort at the Microwave Laboratory. His group produces laser

materials and measures the non-linear optical properties of potentially

important crystals for laser applications. During the past year, the

Stanford researchers have grown from solutions crystals of LiI0« which

are about 3 cm in diameter and 5 cm long. These crystals of LiI0« are uniaxial, polish easily, and have non-linear coefficients which are

about nine times larger than the non-linear coefficients for KDP. They

2

also have surface damage thresholds of about 100 MW/cm . LiI0„ is a promising new laser material. Other laser materials which have been

examined are proustite and dimethyl-sevenethyl-coumarin. Because the

surface damage threshold for proustite is very close to its parametric

threshold, Prof. Byer does not consider proustite to be a suitable laser

material. The various damage and non-linear optical thresholds for the

related pyargyrites are not known. Measuring such thresholds for

pyargyrite (Ag SbS„) is a worthwhile research project. The coumarin

crystals, which were grown at Stanford from coumarin derivatives dyes

supplied by a commerical source, have a high surface damage threshold.

However, the optical quality of present crystals is very poor. The

crystals are yellow. If coumarin dyes can be purified substantially,

then dimethyl-sevenethyl-coumarin would be a very promising laser

material.

Prof. Byer and one of his students have completed a quantitative

study on the advantages and disadvantages of Raman, resonance, and

absorption scattering for air pollution measurements. I have called

Dr. McNesby's attention to this work.

37
3. Hughes Research Laboratories, Malibu, 11/16/70 - 11/17/70

Dr. Giuliano.

Dr. Giuliano has completed the first in a series of intrinsic bulk

damage threshold measurements on ruby. His test apparatus is an improved

ruby laser which is Q -switched by a solution of crypt ocyanine and which

operates very reliably (95%) in a single TEM mode. The improvements

were required in order to achieve negligible change in the laser beam

parameters from one pulse to another pulse and to monitor the space,

time and spectral properties of the laser beam for each damage threshold

measurement o The spatial properties of the beam are controlled by

apertures made from machine die for wire pulling, a resonance reflector

(two uncoated Fabrey-Perot plates) and the saturable dye. The resonance

reflector is heated to change the separation between its plates and

thereby to aid in spatial mode control. A MgO diffuser in a nineelement camera is employed to determine the spatial uniformity of the

laser beam at several locations along the laser beam path. Coated

Fabrey-Perot plates are used to monitor the frequency spectrum and the

number of longitudinal modes. This laser produces a beam having an

energy of about 15 mJ, a pulse length of 20 ns, and a beam radius of

1 mm (1/e for the electric field).

Dr. Giuliano has studied the thresholds for the appearance of the

filamentary damage tracks (intrinsic bulk damage) as a function of focal

length of the lens which focuses the ruby laser beam inside the test

samples. The samples are ruby samples having different Cr concentration

and sapphire. The focussing lens is made from fused silica, is

diffraction limited, and is designed to minimize the longitudinal

spherical aberration. In addition, he has determined the time behavior

of the damage tracks by means of a streak camera. The filamentary

tracks begin inside the sample and grow toward the entrance face of the

sample. The growth velocity has also been determined as well as the

spectral content of the light scattered by the damage tracks.

38
I also spoke to Dr c Gentile (Hughes Research Laboratories, Malibu,

California) about growing crystals of Ag AsS., (proustite) and

Ag., SbS (pyrargyrite)o He has grown these two crystals by a modified

Bridgman technique and by a Czochralski technique . The latter method

is not recommendedo Even though an Ag-Sb alloy sinks in the melt so that the proper composition can be pulled, excess sulfur occurs and

presents an exceedingly difficult disposal problem. The Bridgman

techniques are more promising and have a greater potential for improve

mento He has also obtained by a modified Bridgman technique single

crystals of Ag AsS (proustite). He has not been able to grow single

crystals of Ag_SbS„ (pyrargyrite). The starting materials for the

pyrargyrite studies were supplied by a commercial source and

may not be of the best purity. Also, pyrargyrite is more sensitive to

oxygen contamination than proustite. Proustite and pyrargyrite are nonlinear optical materials which have applications from .6 u to 10 u.

In conclusion, during the past eighteen months, the researchers at

the above three laboratories have demonstrated successfully ways to

control and to monitor the laser beam parameters. These efforts should

form the basis for additional techniques by which reproducible and

meaningful damage threshold measurements can be made in the future.

4. Uo S. Navy Postgraduate School, Monterey, 11/12/70 - 11/13/70

Profo Neighbours.

Prof. Neighbours was my host at the Naval Postgraduate School., He

has had considerable experience in ultrasonic problems in solids. His

group is thinking about setting up a small laser facility with which to

study the interactions among acoustic phenomena and laser beams. After

presenting my colluquium, I consulted with them as to what were some of

the more challenging and interesting physics problems associated with

sound and light experiments.

39

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ARPA-NBS PROGRAM OF RESEARCH ON HIGH TEMPERATURE MATERIALS

AND LASER MATERIALS , Reporting Period July 1 to December 31,

1970

5. Publication Date April 1971

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A. D. Franklin and H. S. Bennett (Edited by)

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A1-0-; crystal growth; damage threshold; glass; high temperature materials; laser;

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