Mr. 'Ian S. NAME OF AUTHOR/NOI# DE L'AUTEUR, GORDON h

A --Pumped Dye . - T %tE OF THESIS/TfTRE DE LA TH&

Phission is haQy (panted to ths NATIONAL LlEWW OF L'autwisetiim rst, par !a pr~*(nte:KCOI~@ 4 ,le ~UOTH~- -. 2 -, )-->- >CANADA m rnicrdilrn hi-4 mi s adto lend or ael l wisr WENATIONALE LJU-CANADA dr &rd&er cett* th&m at af the film. de pr4ter w ds VS~Rdes exonpl~jresdu film.

The euB## reserves other publication raw,and neithgr the L'sutew se @serve fes eutrtw &&Is do puliHc&tion; ni la -

thesis M* extbisiw extpzts from it mybe printed or olheii thdsenl de longs eqreits aGP cslt~ine doivent &re impritnds A NITROGEN- LASER-PUMPED

- - * OF T~ REQUIREMENTS FOR THE DE6mE OF MASTER OF SCIENCE* in the Dapartnent

A> -A .. is my not be reproduced in whole or in part, by photocopy or other means, without @rmission of the author. %

. APPROVAL . \ . . I '\

Name : Ian Sydney 'Gordon

Degree : ast tar of Science t -~itl$of Themis : A Nf trogen Laser Pumped Dye Lmer

- Examining Cowttee : Chairman: Be ?. Clayman

.,- J. C. Irwin Senior Supervisor 0 8 I hereby grant to Uproa F -ty th~ r- , ' my thesis or d'ieeertation (the ti of "whgch =--below) to usen

of the Simon Freser university Libr,ary, and to make partial or einglc

@ copies only for such users or in response to a request from the library :

of any other university, or other educational iitstitutim, on i'te 'own s

behalf or for one of fts usere. I further agree that permisatcan for A

- m7riprripre-cFpTing of this thesis for scholarly purposes &y be granted

by me or the Dean of Graduate Studies. It is understood that copying

without my written permission.

Title of ihedis/~issertstion: *A Nitrogen-Laser-Pumped system ? ' a

(signature )

Pfr. Ian S. Gordon I +. > 1

(date) I

, , P ...... ,.

STRACT . . . ~ 11. . . . .

%

, A fiitrogen-'l~ser~pu~~peddye laser system has been ,.- .\ designed. constructed, and evaluated. me nitrogen laser ? # -ou'tput was in the form of pubes whose width between the half-pawer points wds about. 9 nsec. The nitrogen laser Y gave a peak power of 500 kW in each pulse and at the

average parer of 38 xWwa.8 obtained, The nitrogen-laser

, , was in turn used to pump a dye laser; The dye laaer, ------

with Rhodamine 60 as the active medium, gave a maximum " i 4 . v average pqJer output of approximately .064 mW. The d= laser system has been used t'og perform preliminary Rman

scattering experiments'on ZnSe and ZnTe. The fizst-order ,I

. Raqan spectra of both ccrmpbunds have been obtained and I ' \ in addition a very brief investigation of the Resonant I -. ZnTe has been Raman effect in carried out. The- fettsibitity of usingsoch &laseraystera for Raman scatteringexpeziments ' - is discussed and poeshle improvements suggested. e pleasure working with , him.A Thankrr are due to Frank Wick . ' e and-others in the machina shop twit weill aa Wally Hall kn the P /'ronicshop. USO, thank you' T.B. B for stixnulaeing -Y/ c. many usef 4% discussions. I also thank my colleagues,

---- many valuable Acknowl '?dgment is made to'the National Research Council

Assistantships, aild to J.Q. lmii who provided f inanciai P"" assistance through his .grant. i d Table of Contents

I

Abstract - iv 1'- Acknowledgements * a - ~istof Tables vii k , , \

. List of Figures + vifi d Chapter 1 Introduction 1. \ Chapter 2 The.Pulsed Molecular Nitrogen -Laser 6 0 -

er Tr*si#$ons in "the Nitrogen ' 6

.2 The Rate Equations for the 8 U

i

-- - 'Inversion Criterion 11

' 2.4 Power Output 14 2.5 The Nitrogen-, ~aser

I 2.6 Descript~onZofthe Nitrogen Laser q.

I_

2,6a The Laser Box , 20 a . .. - 2.6b The ~iectrodea 22 2.6~'The Gas Supplr system 24

A t

I I 2.6d The Electrical Circuit L 0 . - 2-?+ De.tmztora 27 I * ~07 + 2; 8 ~ischar~eCharacteristics of the 27 Nitrogen Lasee 2.9 Output Energy of the'gitrogen Yes29 $ b.

r Chapter 3 Tuneable pulsed Dye 37 - -. < -". 3.1 t he Dye ~olecule 37 - Pr= 3.2 Light Ab$Mpti% by Dyes ' 38 i , 3.3 Dye Laser.Stirnulation Wfth a -43 Pulsed Molecular Nitrogen Laser i .3.4 The Description of the Dye.~saer , 44 '

3.4a The Dye Cell F 46

3.5 The Output of the Dyepaset 51

* r 3.6 Dye8 52 I - Etfrapted---Sc------4.1 ~ntxoductiona 56 ' 4.2 The Apparatp , 56 4 4.3 Results 60 ?\

f e h 4.4 Resonant Rman Effect 63 4 1 .Chapter 5 Cone lusion's - 69

List of References - 72 9

'. 4"

\

7 4i- d

/

------A------

A- - L % v

------

w Table * ,. Page I The Lasing Limits and Laser output for Dyes Used LIST; OF FIGURES

- - d Figure Page

An Energy Level Diagram of the Nitrogen 7 P SP' The Pump cycle of Nitrogen 9 Calculated Laser Power Density Using the 12 Saturation~~pproximation~- .

The,.Laser Box Cross-Section, 23 '

------7 T--- u------L-~ - - A- -ALL w- The Electrical Circuit- f ?he Bias Circuit for the Phototube 28

------

T& Dependence of Pulse-Energy P-~=sNEF- - -31- The Dependence of Pulse Energy on Repetition Rate with a ~ctrogenFlow 0of 3 Litets pet Minut * *.

The Dependence of Pulse Energy on Repetition ' 33 t Rate with a ~itrogenFlow of 5 Liters per Minute

The Dependence of Average Power Output on 34' ~epetitionRate 36 Pulse Shape Measurement fi s

37 7 The Molecule Butadiene ,. Eigenstates of a Typical Dye Molecule wdth 42 Radiative and Non-Radiative Transitions . . A Simple Tuneable Dye Laser. 44

The Pump Cycle of Dye Molecules '45 The Layout of the Dye -Laser 47

------The Frame of t* Dye Cel& t ,-46 - -- Cr -- Relative Laaer Intensityas a L'unction or 53 * davelength for Rhodamine 66 and 7-Diethylamino-4-Methylcoumkfin Page

Sek-up for Scattering Experiments '. 57

A Ranfan Spectr of ZnSe at Roqn Temperatare ,, '61 dkSpectra '7 o ZnTe at Roam Temperature+ -,62 Scattering Amplitude as a Function of the Rdduced Resonance Enesg)', Ai

The Absorption yge in undbped\ ZnTe at.300 K - ? . 'he ideal light'source for luminescence and light ,scatterlYng studies would provide an arbitrarily istense, m&ochromatic beamwhose frequency could be varied continuously throughout the electro-magnetic spectruxu.'

dssc~veryin 1966 (Schaf , 1973, p. 1) of the dye laser , I - i provides an approximate equivalence in the visible ------2 ------

region of thqfspectrum. Witha suitable selection of A *i = \

dyes and excitation source, the laser can be tuned e

throughout the visible spectrum. At present, there are , eseentially three types of dye lasers. classijied .acc&ding * & to the type of excitation, These three sources of * excitation are the flashlamp. cw argon ion lasers , and % the nitrogen laser.

One podsibility of pumping a dye laser is to use a f lashlamp. Thbe, however, are not very efficient for several reasons. First oT all, they pr0duce.a lot of light which 4s not of a suitable wavelength to pump the dye; only a fraction the light produced can be

of P------used for this puzpose, It is also-necessary to filter A~ ---*.--- out phot&herai~ally active wavelength; which will decom-

'pose the- me, Secondly, itcis an extended, diffuse - *. g 5'A 7 risetime of the pulse is slow, In the dye rnolecules,~' 1 1 i I a the maximum population of the excited state responsible 2 ii i f + for thelaser tranjiition can be reachid before the & I. intensity of the f lashlamp is a mqximum: Furthermore, 5, f - 9 [ there are many problems associated with flashlamps in general, These include a lack of pulse to pulse repro- - >" ------>

cw lasers, Until very recently, 'cw lasers operated only

a., a., down to the green, so that they were unable to pump t the ; dyes to lower wavelepgths. Recently, however, an

.argon laser operating in the ultra-violet has became a

comerckally available. At present this is the most , I efficient and noise fre&itation source for use with h dye lasers but it is- very expensive and has a rather I short lifetime. \ FI t The third excitatiog aource for dyes is the nitrogen L e

11 laser. Until recentlyr this was the only laser which b I operated in the near ultra-violet with a reasonqble ------power output; ~hestrong lirX6f thiCliisF i* at-%~2 - .

p-pppp . ma iind is thus well suited for pumping dyes. ~T~trogen.. d T- laser output is in the form of zt pulse lasthg for about c' -8 I 10 . se&nda, - d"so - that with a typical peak Gwer of 100 kilowatts and pulse repetition rate of 100 pulses per ~ second, a cw output of .lwatts is obtained. This laaer can be built of readily available materials, and the gases required, .nitrogen and helium, are readily available. Furthemre, the durability of the laser. is very good.

L. There are two main disadvantages of this particular '1

A A- -- excitation- -- -- source.- - - -Firstly, the-- laser------produces--- a---A large - - P amount of electrical noise interferes with any

- 'sursounding equipment, difficult to separate

low. ?.An output of .1 watts, wheh combi&ed with a dye laser efficiency of somewhat less than 101, yields an output of less than 10 milliwatts. Thus, to obtain

results from a scattering experiment, qui- a sophisticated detection procedure is required. These two main dis- -advantages, however, are.also shared with the flashlamp

r* -4 excitation source. The nitrogen laser has many advantages over the flashlamp; its output beam is monochromatic and collimated

9 to allow easy focussing, its risetime is short, and its ... - *. reproducibiXity is good. It was therefore decided to

------7-

build a n~t~gen-fas&-Pumpesddye laser ~~st&. This @ laser syst- has been designed and constructed, and then used to prform preliminary scattering experintents on

- - single crystals of ZnSe and ZnTe. The first order spectra of both these crystals have been observed and the spectrum of ZnTe has been investigated as a function of the excita- tion energy. Although both compounds are familiar and have been extensively studied, the resonant Raman effect in these compounds has not been investigated for a lack of a suitable source. The organization of the thesis is as follows. Chapter 2 contains the theory of the nitrogen laser and explains why this laser when operated in the ultra-violet cannot be made to work continuously. It describes the nitrogen laser, giving details on design considerations and construction of the laser cavity, the electrodes, the gas supply system, and the electrical circuitry. The discharge characteristics, pulse shape, and power output as a function of various parameters such as gas mixture and pulse repetition rate are also given. Chapter 3 contains a brief review of the structure of dye molecules, and the important transitions concerned in laser operation. The dye laser is then described; its various components are described and the output characteristics for 4 different dyes are given. The next chapter describes Raman scattering experiments performed on ZnSe and ZnTe using the nitrogen-laser- pumped dye laser system. These were difficult because electrical noise from the nitrogen laser interfered with p.

4 the signals, and the signal was weak. To help rectify the situation, the detection system was physically removed from the room of the nitrogen laser, and the experiments carried out in this manner. The final chapter suggests 8 possible improvements to the system that would further simplify the of such experiments and discusses CHAPTER

THE PULSED MOLECULAR NITROGEN LAS

2.1 Laser Transitions ih the Nitrogen Molechle

The nitrogen molecule .is diatomic and has many , s

bands of electronic transitions. An energy level diagram for the nitrcqeh molecule is shawn in figure 1, and the firsz and secon-dt-posZC1ve bbahldZlllare fa?.ed . ~~ex-Xt%'&%~~~~- in both these first and second positive bands has been

------The, -- observed. laser lines corresponding- - --to - - the-- - - electronic ------G- - -- - transition B'E +. A" or, the first' positive group; was g &' F reported by Mathias and Parker (1963) in March 1963. Associated with the electronic transition are maw vibrational lines and the resulting bands fall in the near infra-red. The laser lines corresponding to the electronic transition c3nU-+ B'I~~,or the second positive group,, occur in the near ultra-violet. Lasing action in . thiq band was reported by Heard (1963) in November 1963. I There are at least seventeen lines corresponding to the 0-0 rotational band of this electronic transition (Kasuya

# f and Lide, 1967). These lie bhtween 337.139 nm and'

337.24 nm in vacuum, and range in- --intensity -- -from -- very------

strong (337.18 nm) to weak (7 different-- lines). This discovery caused a great deal of -excitement since it was

the highest frequency las-er availabk _at that ti=. - - F'ig. 1. An Energy Level Diagram @f the ~itrogen Molecule (Herzberg, 1967. p.449).( a

collision laser, Electrons of sufficiently mgh energy & (produced by a field of geveral thousand volts per inch) collide with nitrogen molecules which are excited into higher energy levels, The populat' on density of each l d- B level can be determined from a suitable- set of rate equa- tions and these equations can in turn be used to provide a t guide for the desiQand constrkction of the nitrogen Laser.

" therefore be described.

*. 2.2 The ate bquations for the Nitroqen Laser 7 d The ultra-violet nitroge laser is basically a /r' laser, .thea first C three-level since n#trogen molecule is

raised from its ground state to the C electronic state,

ahd then lasing action between the C and B electronic.

states*occurs. These three states are shown on the energy le2 el diagram of the nitrogen molecule (figure 1) and in figure 2 with abbreviated notation. Udng this notation, the equations governing a three-level laser in general, and the nitrogen laser in particular, can be

------written as (Ali, KoLb , 46% Anderson, 1967) :

------337.1 nnt

Fig. 2. The Pump Cycle of Nitrouen Molecules.

------dN1 and -= -(X12+Xtt)N~+(r;:+Y~I)N~+(r;:+Y,1)~~e. (3) dt

,. c* Here Nl rN2ir and N are the population densities ~f the

is the rate of collisional ekcitation from level i 'ij . to level j where i

, and the electknic levels B" and C 'll respectively. ' ' Q U Also r;; << T;: : Y 1 and Y21 a& very small compared to other tentis in question; and gz=gs (the B and C states

are both triplet states and have the same' istatistical I - - and (2) may now be written as - 11 - ,

dNp b a z and - .= XI~NI+~~;;+Y~~)N)-(T;;+X~3)~2+~32 (NI-NI) (5)

* J In the case of the nitrogen laser, equations (4) and (5) above are not exact, since there are additional

levels above the C level. Furthermore, there is the 1 effect of electrons colliding with the excited moladules

i to these equations are those which inc'lude the ionization

sign3ficantly altered; peak power is reduced by 3% and t the half-width is reduced by 10%. These equation- 8- -+ thus' in a suitable form for obtai'ning estimates of how long it is possible for the inversion criterion td hold.

2.3 he Inversion 'criterion In this sectidn it is shown that the condition

N+N* holds only for times smaller.,. than ~32(40 nsec,. 2 Ali, Kolb, and Anderson, 1967), so that the u&tra-vialet

i nitrogen laser can never be comtinuous~it must be a 6 - ..pulsed laser. Fig. j. Calculated Laser Power Density Using the Saturation Approximation (Ali, Anderson, .and Kolb, 1967). which can be integrated, assuming Nl and the pumping . rate constant, to yeild

- - -A -- A ------" 7- -- I- --

& N2+N3 X1 3Nlt0 (7) .

3 , thjs term the inversion criterion will only hold for " Y even shorter times), and substituting the &-ove equation , - into (4) , it is found that

Solving for N / 4" C Thus, for N s>N2 to hold,

This means inversion can pnly take place in times small

this .time. Y 32 , "the collisional deexcitatioh. term. is 1 not always small canpared to rY'2, tm radiativ %r decay term. 'In fact, with a high density of low energy electrons present, electron - excitedan~ftrogen morecuke collis.ions may drop more nitrogen molecules from the- electronic C state (Ali, Kolb, and Anderson, 1967) than spontaneous . 'a radiative decay. We will, how&&, alway* have the

-maximum condition t<~~~;that $s, inversion can only

last for times less than.T 31 (40 nsec) . K It & thus seen +at the output of $he molecular nitrogen laser will be in the form of pulses of duration

In order to predict the power output of the nitrogen laser, the energy density wi-thiri the laser is required.

I This can be determined from 'the rate eqiiatio4 s, sinde "s, the change in population density bf the lasing states will give the dens.ity , and E~T,the energy of the laser p8oton is knwn, The rate equations, however, * - are closely coupled to the-ionization rate .equation and. j - de electron energy equation ,* since the energy of the electrons are responsible for exciting the nitrogen d

The change in energy density p in the laser at'the laser frequency can be obtained 'from

El dt 9' i C . --a The rate 'of ionization , assuming binary coklisions n

t where Ne is. the electron density, ai is the i iation 7

t 0, cross-section, and v is the ele9 rori welocity. d- B nally, the rate of chnge of electron energy is

------\- - - given by " . . P

- 16 - , C - where k is Boltztaann's constant, Te is the electmn .. -

i, temperature, 3 is'the current density, E are the ji energies of the transition j to i, E, is the ionization energy of the nitrogen molecule, and -XVE, is the r~>

of energy loss by, the ,electrons to the ground state - -L-j 1 3 ' vibrational levels. * .-

i To obtain the current density from the above equation, the remainder of the electrical circuit must be considered. .- The circuit, shown in detail in figure 5, essentially ,

, r

voltage Vo being discharged through the laser oavity . % This cipuit inevitably has a small but finite 2-

where 1 is the discharge length, -wd A is the &ischarge area. -1 The ekcitation rate coe'rf icient (=X1 rNe f ," -

the ioniaaltion rate coefficient (,

-- (involving terms such as ' _A, A , - ij ijNe. yd ji) or the nitrogen &aser have been measured by various authors 3 9. - 'C m (~erj,1965; Ali, Kolb, and Addereon, 1967);' Once these L * J t es are known, it is possible to solve the equations .

previously developed numerically (equations (4) ,(5) I' and (13) to (16) ) . The laser power density n& then be plotted as a fqnctfon of time for diff&! ent circuit parameters (including different , initial volt-

press,ures&of nitromn. It is thus possible to predict conditions for maximizing laser output as a function of

equations further rather than obtaining the exact' solution.

If the laser transition is sqturated (saturation has been observed, Gerry, 1965) , then NI-N2 <

and . .

-- - a ------Wi

-- ---P = -(X13-X12) - -- + (-1 - N(Y32-X23). (18)A- -3 2- 7 ' -T32 2T2I '. 4

This simplifies equations (4) and (5) so that one may - - - obtain' numerical solutions of power,putput vs. @be

9 L as a function of time is shown in figure 3.

The maximum gain of the laser may be calculated -t from the equation (Fowles, 1968, p.2671,

Here, m is the mass of the nitrogen molecule, T is the

absolute~ temperature of the nitrogen gas, ,c,is the speed

r;*--~ -- --- ~ - -- -~ ~ -

of light, f is the frequency of' the laser light, and A2 I

is Einstein's coefficient (l/lOnsec; Ali, Kolb, andp-- --

Anderson, 1967) . All kits are MKS. Typical values of -- --- the inversion density, N3-N2, are of the order of 10"

. + (Ali, Kolb, and Anderson, 1967). This will yield a gain

of about 20dB/m in the nitrogen laser. =e

2.5 The Nitrogen-Helium

When the nitrogen laser h pulsed at more th& twb 4 or three pulses per second, the power output per pulse

drops noticeably. This is most likely due to a decrease in tde population density of the ground state nitrogen.

-- r- - Thus, in order to puIse- thelaser %any tmes per second,~- - 1 llr is necessrq to reduce the relaxationtixneof the *. molecules to their ground state. In nitrogen lasers - operating in theinfra-red, it has been observed that the recovery time. of the laser (the &nimumrequired waiting time between pulses) has a linear dependence upon gas pressure in the range of 0.5 to 5 torr (Kasuya 9 7. and Lide, 1957). This suggests that the process of molecyles couiding with each other-is the dominant * factor in the relaxation time, which is the critical factor in the recovery time. For nitrogen lasers Qpr-

pqocess is also likely very important since in both cases

state A to the ground state. If helium is ~addedto the -- system, this process is hastened, and the relaxation

time to the ground state is reduced. -n Another way to increase the pulse repetitipa rate is to rapidly flow the nitrogen gas through the laser. This process removes long lived ionized particles and . r neutral mlecules not in the ground state and renews the supply of ground state, nitrogen.

2.6 Description of the Nitrogen Laser. In order to produce electrons of sufficiently high *

energy to collide with and excite.the nitrogen molecules ,

------to the e&ctronic C state, it -is rrecessary-twpme--- --

a~reiet-..anw kthin the laser cavity. Many problems were encountered , in trying to get a good designso that the discharge would indeed be fairly uniform along the length of the laser. These problems will be dealt with under,the appro- priate sections describing the laser box, the electrodes, i ly system, and the electrical circuitry.

2.6a The Laser Box

a' a' As earlier mentioned, the gain i nitrogen

-* .- - A -2L-- -- "------laser could be 20 dB/m. In order to n as much

power output as possible, it is desirable to make the

--- - - f aser-long tcrtakeadtvarrtage--of -this gain; The -ferigth ------od the laser, however, is limited by the short du3 ation of\thegain. After about 15 nsec, the pulse is absorbed and extinguished because the lower laqer level is meta- stable, and the 7'per laser statd is no longer populated. 1215 nsec, lishq travel3 a distance (d=ct) of hut4.5 metres. If a single pass laser is built with a reflecting mirror at one end and ,an output window at the other, som2 of the light will travel twice the length of Me laser. Since this light aannot travel greater thh , 1 about 4.5 metres before being absorbed, it is senseless to_build a laser longer than roughly 2 metres.

within the cavity, it*was necessarya to '.% - use a trarlsverse discharge geometry. Since It2is -diffi- cult to produce an even discharge in a large volume, the

width of the laser cavity is limited to about 1 inch. To meet the length and width requirements, the laser

cavity built was 1 inch wide and 6 feet long.

Care must be taken when constructing the laser box that arcing cannot take place along or near a cavity wall. This is complicated by the fact that at the high voltages being used, the electrical path along a non- conducting surface offers a lower resistance to a discharge than in a gas not close to any surface. Therefore, in

order tp obtain a discharge within the gas and not near a cavity wall, it is necessary to make the distance between the electrodes which are contained in the cavity less than one-half the distance along any other possible path

the discharge can take when it travels near a cavity wall. In this way, the discharge will not travel from the cathode, to a cavity wall, along the cavity wall, and to the anode.

Also, it may be noted that the laser cavity does not have to maintain a high vacuum. This is due to the fact that air itself is 80% nitrogen (the active medium of the laser) and that the laser operates best at about 100 torr or higher. The laser box itself was constructed of ordinary materials, namely, plexiglass (lucite) , aluminum, and brass. The laser box had inside dimensions of 1-1/Zmx. 2-3/4"x6'-On, The individual pieces for the laser were.

- screwed tdgether. O-rings were compressed between the 'B sv A 9pieces of material as to make a seal. cross-section LJI of the laser cqvity is shown in figure 4. The end pieces were of lucite and were bolted to the ends of the

laser box. A gasket between the end pieces qdd the laser -- - --A -- ---Lu>L ------D box was used to make a seal, A length of 2-112" diameter d,lucite tubing protruded from each end piece, upon which

------7 ------was attached a 4". diameter window (obtained from ESCO Products and was optical grade) at brewster's angle. The laser box was bolted onto an inverted U-beam, which was attached to a heavy table.

As mentioned earlier, only one mirror was used in .,

thig laser. A front silvered mirror was mounted in a

.holder which was in turn bolted securely onto the inverted

U-beam that the laser rested on.

i

2.6b The Electrodes @ The cathode consisted of a bandsaw blade 14 teeth per inch, and was 5'10," long so as to fall 1" 9

- - short of the endsof the caviky. Itprotruded-approx0 -- - - - , imately 1f2" into the inside of the lager box. The purpose of the bandBaw blade was to produce an

- even discharge -originating-from -the teeth of the hl'ade; Aluminum - -

1-1 1-1 Plexigloas Fig. 4. The Laser Box Cross-Section. t would be easy for the discharge to start -A pointed tooth of the bandsaw blade.' If the a simply a @inted edge, alignment between the would be much more critical in order to obtain an even I discharge along the six foot length of the cavity. -

The anode consisted of a 6'-On long piece of aluminum, which protruded 1" into the laser box: It~~sideswere

tapered down from 1/4" to a narrow rounded edge. Its ' e ends were also rounded.

2.6~ The Gas Supply System ,- The nitrogen gas was obtained from a wall supply C pressurized, at between 15 and 20 -p.s.i. The helium f source was a pressurized tank with a' standard regulator to reduce pressur! to 20 p.,s.i. Precision nebdle valves controlled the flow of the gases, The gases were mixed' in' a *ing chamber and passed from there into the laser i chamber. The Gases wer'a pumped throzh the laser chamber and out the opposite end. A manometer at the pump end - was used to measure the pressure of the gases in toxr.

Using this gas supply system, it was possible to ~omp~letely 4

accomplished two thing;. Firstly, . a fresh-- supplk of nitrogen in its ground state would replace nitrogen molecules that would no longer be in their ground - state. Thus a better power output was achieved. Secondly, . 2.6d The Electrical Circuit 9 The electrical circuit used ie sh in figure 5.

A high voltage supply charges up the .(e5 microfarad capacitor. When the thyratron is trighred, the capacitor

laser are twenty 500 picofarad ceramic ca4 acitors. The' leads from the capacitor to the laser

------,- ---L------

cables, . each about 4 ' ' long, The small and the cable& are evenly spaced along the 6 foot length of the laser. The purpose of this arrangement of the 'cables and isto distribute the charge evenly r9 along the length of the laser, so that tffe discharge within the cavity will be fairly uniform along its lgngth, and-the discharge everywhere within .khe cavity will occur simultaneously. The trigger unit consisted of a timex and pulse generator, which triggF d an SCR> The SCR was reqdred - to produce a pulse whohi rate of rise of $ltage was at

least 1800 volts per microsecond and whose peak, voltage ,

------was a minumurtl of 550 volts. This pulse was fed into a 1:l pulse transformer which was required to drive the thryratron. The thyratron would thqn be in a conducting

------c

s vVolts PC

2d-500pf

' ceramic capacitors

Fig. 5. The Electrical Circuit state for ap~roximately2 microseconds during which time r - the capacitor would dump its charge across the laser.

2.7 Detectors Two different types of detectors were used in this

experiment? one was a phototube, and the other was .a

'""pyroelectric detector;

and was used to measure'the pulse shape of the nitrogen b laser. The bias circuii for the'phototuba (figure 9 reduced

p.160). It was thus,possible to observe pulses $hose width was the-order of 10 ngec,

A pyroeledtric detector, Molectron model PI-71, was

i 1 used to measure the energy output of the nitrogen laser. The peak voltage output of the pyroelectric detector is di'rectly proportional to the energy of the laser pulse

2.8 Discharge Characteristics of the Nitrogen Laser

uniform along the mat is the length of electrodes is. 0

obtained here was not hrfectly uniform but consistedL& a aeries of narrow channels originating at each tooth of the bhndsaw blade cathode and spreading out towards ------GR circuit short Tektronix -- - - GR coaxial type 847-WN cgpacieor typq 847-K

Laser light pulse

4 50Q coaxial /

~i-a1 BNC

Battery BNC .

L ------

Fig. 6. The ,Bias Circuit for the Phototube. I

- - - - the anode. The discharge characteristics depended upon the voltage, gas composition, and pulse repetition rate. There was a distinct range of values of these parameters beyond which the discharge became irregular, with most of the discharge passing through one or several points. In this case, most of the energy would be in one or two bright arc discharges. Most of the gas would remain unexcited, and c~ns&~uentl~the laser output would be reduced and become highly irregular. With an optimum choice of the parameters mentioned above a fairly uniform discharge was'obtained, although some discharge channels always appeared somewhat brighter than others.

2.9 Output Energy of the Nitrogen Laser An attempt was made to find the best operating condi- tions for the laser. The power output of the laser in- creased with increasing voltage. Therefore, in all measure- ments, the voltage used was 26 kilovolts, the highest volt- aqe available from the power supply. It was found that a mixture of nitrogen and helium was best, with helium being 2.7 times more concentrated than nitrogen. A variation of the relative concentrations of these gases from this ratio would result in a drop in the output. When 3 liters per minute of nitrogen, and 8 liters per minute of helium were flowed through the cavity, the h - and M optimum pressure was about 120 torr resulteh in v

output of 3.8 millijoulee per pulse at a repetition rate' , 1 \of 6 ~ulsasper second; while when 5 liters per &.nuts 'b I of nitroq*n, and '13.5 liters of helium were flawed through I ' the cavity, the optimum pressure was about l40 tom.. The output pex pulse here increaged to 5 millijoules per 1 - I pulse at 6 pulses .per second. Also, it was noted that ------L---L--L A--A- & : ps. the optimm pressure seems to fall slightly as the repetition cC rate is ihcreased. These above results are shown in

above, the partial pressub of ktrogen is 32 torr and . 38 torr respectively. These p~ssur-es'ckpare with a

theoretical opt%mum of about 28 torr (~li,1969). .

As the pulse rate is increased, the energy output / . - ,per pulse ia reduced, as shown in 'figures 8 9.

, However, &n though the energy output falls C, as -b + the regetitiorrrate ,is increased, the total average power output (the energy per pulse times the puloe repetition rhte) is increased. Thus, the a*erage power +output inqxeases from about 19 mill%~attsat 2 pulses

n for a fill pres~ureof 100 ton, and a flow raterod

% 5 liters per ainute 6f nitrogen and 13.5 liters per a

- A ------minute of helium, as shown in- figure 10. At repetition - J *

Fig. 7- .The Dependence of PuSsa Energy on Pressut&. ..& -&,6 pulses per second, 5 liters per minute of N - - A 0 purmr -parse-; 5- literrpar minute-of N$ - -- + 6 pulses per 'second, 3 liters per 'dnute of N2 010 pulses per second, 3 liters per minute of NZ. 2 *. 4 6 8 10 % Putse Repetition Rate (secaV

t -- Fig. 8. The Dependence of Pulse Energy on Repe'tition WZth a Nitrogen -3' biters per*-. A 80 torr pressure X 100 torr pressure

0'120 torr pressure - . L- - fJ 140 tori pressure + 160 torr pressure. Pulse Repetition Rate kec-'1 '

- - - - P ------Pig. 9- ~ha~ependmceof 6ul.e Energy on Repetition RateikmgenFZmudNitrocranPlowofrs -@r Minuke~. + 100 torr pressure X 140 torr preeske 0 160 torr pressure. Fig. 10. The Depehdence of Average Power Output,on Repetition Rate. + 80 torr pressure, 3 liters per minute of nitrogen flow- X 120-torr pressure, 3 liters per minute of nitrogen flow 140 torr greesure, 5 liters per minute of nitrogen flow. rates greater than 10 pulses per second, however, the discharge becomes highxy irregular and arcing takes place. Pulse to puhe consistency is very poor, and power out- , put falls. * . The shape of the output pulse of the nitrogen laser " was measured as a function of time. It is seen from figure 11 that the pulse width at haXf maximum is 9 nsec ---L--u-L -- -" a2u------u-----~---- - and the total duration is 15 nssc. A 5 millijoule pulse,

pulse as a function of pressure, while figures 8 and 9 give the energy output perpulse as a*function of pulse repetition rate. From these figures, it is seen that the maximum output energy per pulse at 6 pulses per second is 5 millijoules, with a pulsewidth of 9 naec at half maximum (figure 1311, corresponding to a peak power of =SO0 kilowatts, and an average power of 30 nriLliwatts (figure 7. Pe gure 10 also shows that a power output of 38 hlliwatts is obtained at the maximum obtainable repetition a rate okl0 pulses per sepnd. .srr

3

TUNEABLE PULSED DYE LASERS

/' 3.1 The Dye Molecule The chemical definition of a bye (Schaf fer , p.6) is any substance which contains conjugated double

bonds are separated by a single bond, as in the molecule L .

Fig. 12. The Molecule Butadiene.

All dyes contain at least one conjugated set of double . bonds. To understand how these conjugated double bonds give rise to the nature of dye@, which is all as & active medium of the dye laser, it'is secesaary to review the

------types -of -- bonding- - - electrons, u electrons- - and a electrons two - I - found in the conjugated set of double bonda formed by the I A-

\ carbon or lieteroatom in the conjugated chain. There are three-q electrons, which are characterized by rota-tional symmetry of their wave functions with respect ti the

I bond diregtion. These u electbns form three 6 bonds. Using the example of butadiene above, the two end carbon atoms form two cf bonds with the two adjoining hydrogen

L atoms and one a bond with- a second carbon atom, and the

i

carbon atoms in the middle of the chain fop one a bond a

- with a hydrogen atom and two a bonds wikh the tm other

, \ left over, which is a,n electron. This R electron is

nucleus and has rotational symmetry about an axis normal I - 1 to the $lane subtended by the orbitals of the three

electrons. A T bond is formed by the latergl overlap of these n electrons and is a maximum when the &nunetry - axes of the orbitads are parallel. In this position,

then, the bond energy is minimized. As a result, a conjugated .chain, which consists of atom linked bypboth .- o bonds and these n bonds is plans; and of high rigidity.

3.2 Light Absorption by Dyes

The light absorption by aes can be ,understood qualitatively (Schaffer, 1973, p.9) using a highly sim- /

of the conjugated chain lying in a common plane and linked

- - by o bonds. he n electrons have a node in the plane of P

' f the molecule and form a charge clovd above dnd below this '

*i I 1 J / plane. The centres of this cloud are about one-half a bond length distant (above and below) from the nodal plane. The electrostatic potential, to a first approx- > imation, is constant along the length of the conjugated

a 'and large outside the lengthyof the chain. In

othecha$ words,. . the T electron appears as a parti-cle in a

------me dimensional 4ox.-c& hngkh-L ,-Inthiscase tha._Len~_th _, eL is the distance between the end atoms of the conjugated chain, plus two bond lengths (one bond lengeh on each end of the chain). The quantum mechanical result of a . I .--- particle in a box is well known (Schiff, 1968, p.37). The energy En of the nth eiganstate of the electron is

'where h is Planckls constant, and m is the mass of khe 9 electron. Each state can be filled by exactly two electrons - 7 of opposite spin. Hence, fdr N electrons, we have N/2 , states, for in dyes hl is always even. The transition,

then, of the highest oecrtpied%e the-lowerst-uj'toc-bd------1. The corresponwng wavelength of a photon abeorbed in such a transition is given by

where c is the speed of light.

-- - - Thus,-tMe-atode&-indieate~lthat toa-first approx------intation, the position of the absorption band is determined only by L (the length of the conjugated chain) and N

(the number of n elegtrons). The minimum value thatgmax

can have is about 200 nm. Different dyes have different values of Amax; these vary from the near ultra-violet to

+% 8 the near inf ra-red,

i' An absorption hand of. a we usually cowre tens of

nanometers {Schaffeq, 1973, p.18). This large width is I . " I due to $ fact,that since the dye molecules contain a . - large number qf at- (typically 50 or more), it has a ' large number of normal vibrations (typically 150 or

cm-', and this energy interval is superimposed on the

- electrorric transitions. Ultra-violet absorpti6nt wIll generally cover either thb first or the second absorption qa

% i electrostatfc. perturbations. caused by the surrounding

solvent molecuSes. These perturbations further broaden I the individual lines of a vibrational series. Further- \ more, we hhve superimposed on these lines a ladder of rotationaily excited levels. These are very broa6, since ------the coll~ft%s-~x~ht%e soknt molecules are pve'lp"fre- n,-r

quent, reaching the order of loL2collisions per second. I+

These_-lines-are broaden~d~s~much_ that-we hue -aaa?ntially_ _- - a continuum in.the absorption band. Consequently, 4 absorption is nearly continuous over this sand. The

same is true, of course, for the florescent emission . which corresponds to a reverse transition from an elec-

- i tronically excited state .to the ground electronic, state. The dye molecule has an even number of n electrons.

This means that the excited states of the dye molecule ean be singlet states or triplet. states, and that for each singlet state there is a triplet state af lower * -BT P / energy (S~haffer,1973, p. 26) . Finally we have a picture of the eigenstates of the I i A dye- (see figure 13 1 , The -transi$ian,Sp ~o- SJ IS- radiation- ----

The transitions from a singlet state to a triplet state t 4 are' spin forbidden and are induced through pertu~bations. Florescence / I ~zlnsec / 1. The transitton TI to G is also spin forbidden. Usually Tt non-gadiative processes whose lifetime is generally of B

4 the order of % microsecond dominate this transition. However, if this transition is radiative, it is termed

% - phosphorescent. This fphosphorescent li f etine usually ranges between milliseconds and seconds. .* * . . *

1: Nitrogen 'Laoer

C * - pumps~urcefor - - -- h B 't nanoseconds. In this time, it &s impossibh to transfer many of the J. dye malecules~roman excited singlet state to a triplet state, since this transfer time is of% he order of 100 nqec. Hence, to a first appxoxihration we may neglect all triplet' states-since they are not populateds The dje nblecules absorb pump radiation and are . " lifted from the around state into a higher vfionic I oi the first or second excited singl=t state. Radiation- A less deactivation of the dye molecules to the larest r- vibrational level the first electronic singlet state i

.owurs_in picoaeeonds ,and- sti$rmlate&emissfc)n- then~~~~rs ------4 # - from the first excited singlet skate to highervibronic-

levels of the ground state. A further radiationleas' , * w 7 .- - ;I 44 - - 9

.? state within picoseconds. This php cycle isshown inr ?' figure 15, A simple tuneable dye laser- consists of a dye solution' contained within a cell, a diffraction grating, e

md an output window as shown in.the diagr4 3 below, 2 i e t dif fractidn axis A- A- laser >------.--- dye -- - -- =-- - cer .

3-L---

a Fig. 14. A simple Tpneable Dye Laser , s

When & photon of the laser frequency is emitted along the axis of the laser and strikes the grating, it

II ibretufned back along the laser axis, but a phot~of a different' frequency is returned along a different axis. s' When the grating is rotatedr the laser frequency a&cordingly so that the photon of the new laset is returned'back along the laser axis. It is this sblectn I I

L ive feedback process provided by the grating that make the dye laser tuneabls.

--- The essential components of tIiiZ@i laser werb a

I c '8 dye cell to contain the active medium, a diffractf~n

..c- _ -- - - grating to the laser cavity to the desired wavelength, 9 Stimulated / Emission

e- Fig- 15 -me Pulrlp Cycle of Dye MQle&ules_._------

- - bF

7 in,, +& i . scope expand* the beam so as to cover more lines of the

$lidfraction grating, thereby increasingA* the resolution , , ' ' of the grating and reducing the- linewidkh of - the- laser \ beam. A schematic outline of the component ]configurat&n is ehown in figure 16. Note that the dye laseru axis i>

- lliilL1---lll- UI-Li--i--^----- fC p=+i?ndiculer to the ~isof the nitrogen laser. A brief description of the components of the dye laser is given )

3.4a The a3ye.eell The dye cell consisted of a stainless steel frame (shown in figure 17) , a quartz front wiodw , and two &ti-reflective coated side window. The front part of the frame had.a section 2.6 cm x 0.1 9 the width if

I the frame (1.0 cm) milxed out. The quartz window, through which the nitrogen laser beam was passed, was glued onto the front of the frame, and the anti-reflective coated glass was glued onto the sides of the frame. Thus a cavity 1.0 dm long and 0.1 cm deep was famed to contain the dye. The nonnal of the anti-reflective coated glass

------~4sat 5'. & the axis of the laser .so that ,any reflections from this -+lass would. not be amplf ied when returned back 5 towkds the aye cell. The dye had to be circulated within the dye cell and

--- Fig.17. The E'rP&te of the Dye Cell. ' dye was pumpcd through the cell at a speed of &out a metre par second. ~oleswere drilled through the hack i +of the cell cavity, and appropriate attachments made ta ! a dye pump and reservoir- dye was pumped fro? the , me 1 reservoir, through the pump to the dye cell. and through dye was regulated by keeping the pump in a water bath.

For _the dye ?hdaminebG,-_the temperatzurk of_-the-_b&h-- _- - -- - was room temperature or slightly higher, whereas qr other dyes used (7-Diethylamino-4-Methylcoumarin, POPOP, and Dimethyl POPOP), the bath was ice -water. 1

Using the above-arrangement of .the dye pump .and .

reservoir, it was also a simple mattex to changb from

one dye to another. - i

1 3.4b &tide The diffraction grating wad blyed at 1.0 microns l and had 1200 grooves per ailiimatra. The grating was used in second ordek for all &yes used in this experiml?nt. The holder forpthe grating had fine adjustments so that I - - --- _ _- _ -_ -- - - _ _ ------the gaat&'eoufd be accurately positioned an& the laser a -

-- easily tuned to a particular wavelength. e The beam expanding telescope consisted of two " _ -- _ - - a_- - - - > _ - coated &hr&tic &ensas with focal 122 naa and, * hpq@s . , 20.6 mm,,to give a magnification of about 6. The dhtance -I b between the lenses could be accurately set through the use of a fine adjustment. The axes of the lenses did qot

exactly cokncide with tb%axis of the laser itself, so . \ -that-any- ~Eteetiomr~-th~~;errses-'of-t;h.~------would not be amplified. i-'

1 mi with the telescope installed. The outputbmirror was interchangeable and two types 4 . were used. One was a quartz flat with a 4% reflectiv& , and the other As a coat& mirror whose refxectivity varied fr~m20% at 385 nm to 57% at 475 nm, and 35% at'

The beam from the nitrogen laser was focuesed to a thin line on. the inside of the dye cell caY ity,.by the use of two fucred silica lenses; one plano-convdt wd the other cylindrical. Both these lenses were required to reduce n nitrogen beam to 6 line of suitable ------r- length to match the dye cell; the plano-convex lens to match the length of the'be& with. the dye cell, and the

cylindrical lentil to further focu8 - thebean- to a line.- - This thin line excited the dye molecules and' defihad the \ axis of the dye laser. i 3.5 The 0 of the Dye Laser he output of the dye lasir con&ted af the laser line, and also of a weaker broadband output due to the spontaneous florescence of the dye. SSnce *is-broad f background' florescence tends to obscure signals to be i observed, it was necessary to dass the laser output beam through-a~h;r=o~\&~~-Ite&t~-oAL~e-~~ After eliminating this baqkground, the measured beam

POPOP, and a factor of twenty using the dye Rhodamfne 66. The ~~anochromator,usedwas a 0.25 m grating spectro- meter- (Spex minimate). 0.25 h slits were %;.sad to obtain a bandpass of approximate$ 1 nm and a rejection bf about 10' at 3 nm from the centre of the bandpass. After the beam had passed through the monochromator, measuremnts~ were .mde to determine the tunaability of the be-laser.

b The power output of the dye laser as a function of wavelength was determined. .To determine the wavelength

of the,'laser, the beam' was attenuated and then passed through a Spex model 1460 nbnochramtor, and detected with an ITT FW 130 photomultiplier tube. The autput of the +photomultiplikwas pasaed through a preamplifier (Ortec modal

from the boxcar was achieved. To-detedne the power b output of the dye laser, the output (attenuated if necessary) was focussed dnto the pyroelectric detector. The detectox then'meas'ed the energy-output of the dyi F'-

reflective mirror. The intensity of We laser line as. a function of wavelength was plotted as shown in figure - ..

18. The maximum obtainable cw power was ,022 milliwatts at 455 nm. * With a 5x10~~molar concentration of Rhodkne 66 as the dye, the laser worked between the limits of 572 nm

arid 606 nm, with a 38% reflective mirror. The maximum C average power output was ,964 milliwatts at 590 nm, as 2 shown ih figure 18.

.Replacing the reflective mirror with a quartz plate,

-- -of 4 or 5 in each of the above C-L

Two oeher dyes were 180 testad fqr their upper and r lower laser limi,ts, as-well 48 their ma- cw-power -- - - - WAVELENGTH lnml

Fig. 18. Relative Laser In sity 9s a Function of Wavelength for d'7-~iethyldno-4- + Methylcoumsrin. output. A 7.5x1f4 molar solution of POPOP in tolupe 4 lased between the limits of 413 nm and 427. nm with a 44% reflective mirror. Its'rnaximum cw power output I was .011milliwatts. AXSO, a 7.5~10~molar solution - of Dimethyl POPOP in toluene lased between the limits

- - -A -A L --- of - 4-zE-x~~-&- 44-1--m. --Ee--e ~~h:mr-w~-4-9%~------* reflective, and the maximum cw power was .050 milliwatts.

Mof the mirror for each of the four 'dyes used is stqmarizea in Table -I.It is noted that using the combination of

k three dyes, -namely, POPOP, Dimethyl POPOP, and 7-Diethyl* amino-4Methlycoumarin, it was possible to continuously I tune the dye laser between the Limits of 413 nm'and 477 nm, a range a 64 nm. Additional dyes (Bell and Tyte, - 1974) can be obtained to cover the remaindeq of the visible

'4 spectral kegion.

$ With all dyes, it was observed that the tuning range

dropped considerably if the quartz flat replaced the .

reflective mirror. In Dimethyl POFOP, for example, the The Lasing Limits and Laser Output for Dyes Us . . I w, L ?a&$, Limits Maximum Reflectivity, (m) ,Power Ou :put (mw of Mirror E ~ - I r-

POPOP

I k* I 1 Dimet OPOP 422 - 441 ' .OSO C

14 4 8

+ - 56 - b' I cy- a, . -

t

SCATTERING EXPERIKENTS

-- 4.1 Introduction 4 - b 1 To test the applicability and perfomancs\ of the

I+ nitrogen-laser-pynped dye laser systep preliminary 'Rmh

-Ap-- -Ap-- LL- -- -- A------.--A- scattering experiments were carried out on two wide band 5 semiconductors, ZnSe and ZnTe. The first order Raman

ly (Irwin and e, 1972) an'd the spectra are we11 3 knbwn. The resgant Raman effect (RRE) has not been studied in detail in either ZnTe of XnSe, however,

because of the lack of a suitable source. .BeOause of , their relatively simple crystal structure (zincblende)-

they are ideal crystals in which to study the RRE and ir the-preliminary results of -such an investigation are

,. . presented in this chapter. P'

4.2 The ~~p~ra&s meexperimental apparetus is as shown in figuke

T

- - . monochrontat@, is focuss$ onto a cyystal , employing-a 90 degree seateerzng geometry: The scattered light from

double monochromator (Spex model 1400) . The output of t- J

the monochromator is detected by an ITT E'W 130 photo- multiplier contained in a thermoelectrically cooled housing. Pulses from the photomultiplier are amplified by an Ortec model 113 preamplifier and model 454 amplifier and are then passed into a boxcar detector (PAR model 160). The boxcar is operated as a linear gate and is triggered by the electrical noise,from the nitrogen laser which is picked up by an antenna. The boxcar gate is open for approximately 1 microsecond and thus dark pulses occuring outside this time interval are prevented from reaching the counting electronics. The output of the boxcar is applied to a discriminator (Ortec model 486) whose threshold is set to reject electrical nodse, but will accept single photon pulses. The output pulses from the discrim- inator are then counted on a Hewlett-Packard model 3734A electronic counter. Although the system will accept single photon pulses, if there is more than one photon per pulse, the electronics will not resolve the individual ; the system cannot resolve separate counts within one pulse that is only a few nanoseconds long. This leads to the problem that it is possible to have at most one count per pulse, and at high counting rates, some counts represent two or more photons reaching the detector. Let us therefore consider closely what is being counted (Bell and Tyte, 1974). probability that a signal is produced.by the laser pulse,

. . I the counting rate is '

-- -~ ~ ,- --

" In scattering experiments, however, the value r is

very small (0; the order of 10-'), so that we may approx- imate in(~-R/N)~ by -R: Thus we find the telationship, &

- Therefore,,yhen a counting rate is ob.served, we must e use equation 425) in order to find out the true counting \ rate: If the observed counting rate is small, then the B +J t i true counting rate is virtually the same, but for high

i'

- - - cawrting r aZss , -the_03semd and - tine coumlng -rqtesare -

( J iie dliffekmt. -- While a spectrum is being taken, Mat is, while * scatteredphotons are being countd, it is desirable to monitor the intensity of the dye laser beam. For this Q .I r 11 fraagf t& A divertad, nuated if nkessary, and focuss& w pyroelcrctrid detector. The output of the defector is, b V i then passed into a boxcar integrator. ~husthe intensity of the dye laser beam can be monitored while a spact - f" is taken.

4.3 Results Preliminary measurements indicated that Raman spectra

can be obtained with the present experimental set- ' we . ,

I. " The slit width of the monoahromater was about 4 cnt , - and the maximum count rates obtained wede about 25 counts I Per minute.' Three spectra are shown in figures 20 and 21. In figure 20, the crystal is polycrystaline ZnSe.

There 'are four lines in evidence. These a*' at 143 clam',

204 cmv I, 251 cm-l, and 501 cmol, and have an experimental -1 error of about *6 cm . These values compare favourably

' with the previously observed values of 139 6' (2TA) ,

and Lacombe, 1972). ? % r )?j In figure 21, the eta1,is a single crystal of ZnTe,

------n- - - '3 cut with one 100 face and two 110 faces. Incident focussed , light struck the 100-face and war~atteredout the 110

face. Two lines corresponding to the LO and M phonons

- - - - - + ------1-

are in evidence, These lines occur at 207a Po and 177 . +- . To- A Ramun Spectrum of XnSc at' ?oom Tebratk I P-"L? I +4 . Fig. 21. Raman Spectra of ZnTe at Room Temperature. .P C

d

* 2 I 1 I -1 1 .,an and compare favourably to the values of 208.3

and 1!7.5 cm-f (L~tCombe,~J971).,

4: 4 Resonant Raman ~ffect a , - * e The ratio of t e intensity of the m+lineto the . . < 3, .intensity of the scattered lager line was measured as

a function of wavelength in ZnTe. ' According to Martin (1974) ,. it is predicted, that 'the amplitude of the LO line should increase relativet

of -photon is increased towards the band gap energy. '

- Figpa 22 shows this thaoretical curvef where A 0 - h~p/E lg

-. which for ZnTe is 10-13 meV, hua is the energy of the +- - . - phonon which is 25.6 meV (207 CIC?), and hui is the bhergy difference between the laser Zine and the band gap energy. The results of the measured inteneity of the-scattereiX laser line, and the ffjpeak. are shown in Table XI: The .%.. a Value of" ths inten& of- the LO line divided by .the ,

I intensity of the scattered laser lide js shown under the * - r column IdIlilser. These values were then acorrected for

not well defined at room temperature, and may bk reufted . 0 ------as well 'because-oT i&*i

8 T 3

aThe-&sorprion coerricient 'was takein rrm rigure 23 0

distance Qas taken to be 1 millirmetre. The column * under I~d'laser is corrected for absorption and is -\ * relative to the lowest value of ILdIlaser. he values ' 1 predicted by Martin (1974) for the values given above

and an energy gap of 2.25 eV are, also given in able 11 a -L ------under the~columtheoretical ILO/Ilaset. 'J' There are many possible sources of error +.in *he*

band gap be shifted, but impurities may drastically

4' 4 change absorption. . . The laher line in this geometry may scatter off the

face of the crystal.without actually penetrating the , crystal. Furthermore, this scattering may also be a ,' function of wavelength. e many In thi experiment, because of uncertaf.c nties the agreement between the experimental and theoretical. values of Im/Ilaser must be considered satisfactory',

# The value is the' righfiorder of magnitude and fs also increasing as the band edge is appraached, as expected.

- t has thus beerrdemonstrat& that-the-nktmgen--7 -L -

Raman scatteltfng experimen,ts and also study the resonant - Raman effect . Although tgia Bystem sky b= ~uccessfuXLy Fig. 23. The Ab8orption Edge in Undopsd lnTe at 300~~.A

- - - - , - -- - '* - used, it has the disadvantage that the cw power output is low, and it is therefore necessary to count for long J i periods of time and to oqpate with very poor resolution. \ As a result the experiments are very tedious and lengthy and the results lack precieion. . . d- CHAPTER 5

. A nitr~~eh-laser-~~ed dye laser system has bean constructed 'ad used to perform preliminary Raman scatter- , .R ing and..resonant Raman experiments on ZnSe and ZnTe. 8 . form ~h?enitrogen laser output was in the of -.pulses with a width at half maximum of 9 nsec, and a total duration

kilowatts peak power from the laser operating at 6 pulses. I'

30 milliwatts. If it were pwsible to operate at con- siderably higher pulse repetition rates, more cw power

, output could be obtained from the 1yer. For repetition

li rates greater than 10 puJses per nd with the pres nt s" ' + - became very along 'the length of the laser, the energy per pulse dropped con- siderably, and.the pvlse to pulse consi$tency bgcw

extremely wr. 1t would ~SObe possule lo incrf ase , e - the power :L tpbt $F the laser by reducing the inductance in the elqctrical circuitry, since a ma~imumdischarge ,f,, " 6 * within a nanosecLhds is desirable. A meaningful .. reducticm ,in. would_he -ach;haued iCth_a ty ------

- were shortened and capperbar instead of cables were b8ed to catry the discharge current*. In designing the .

nitrogen laser, a gas- flow &stem vas&viaad- to re&ve - - 4- excited and ionized molecules ad.replace them with F' 1 molecules in their ground state. A more **id egubange c of'gas, for example by means of a transverse gee-try, ? would also enable higher repetition rates to be employed. L-- With-the dye laser constructeq and 4.differTt

5 1 dyes used, the laser output could be tuned from 413+ to

varied from .Oil milliwatts for POPOP to ,064 milliwatP for ~hbdamine66. Using other dyes (Bell and Tyte, 1970, I

- . .;between 437 and 602 nm, The linewidth of the dye laser 1, b was about 0.3 nm. I~rovementsin both power outpuf anb , a reduction of the linewidth may be possible by increasing , the power of the telescope 'and tnsertfng a Fabry-Pexot -. R' R' - ., etalon betwe 7 the telescope and diffraction grating. I The laser system was use4 to perfarm preliminary ,, a scattering experiments. A first order Raman spectrum "*., of both ZnSe and ZnTe was obtained, and.the results were ' I in gqod agreement with previous work (Irwin d IaCombe, 4' 4' de 1972). Thei resonant Ranran effect wa$ also % ZnTe, and satisfactory agre-nt with the theory of

- - . - --- - . ------IWrtin (1974) was nated. In performing kheae ex@rhts, the problem encountwed were the electrica1,noise . . I. t-n t-n at a OE .th& -,

nitrogen laser. Further improvements may be made by either shieldingLthe &trogen lalrer, or shielding the \ detecting equipme&. To telieva the tedium caused by long counting times due to the &ow power o the laser. 9 3 automatic data acquisition equipment lady be used. Also. ------the power output of the la~ermy high& repetition rates. .The incorporation of some or

------aLIef -these i~rovementa-to_ the@xe_ee&-lasgr~~stemp- would provide an extremely useful'excitation souroe for . . + luminescence and scattering studies. 5

*

, - -. , - t A i6

I

r \

14

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------" --- -A ? - -- -+ \ t 1 t jL4 'd I-ri I 1 Ali, A.W., Kolb, A.C., and Anderaon, A.D. 1967.- Appl. Opt.' -6, 2115. a- A1iI A.W. 1969. Appl. Opt.'. -81-993.

I Brtmea, PoAo 1970. wudies of Laser Lnduded BreakBawn Pheria~nain Liquid Water, Ph.D. Thesis, Sinon Fraae~University, Burnaby, B.C. Bell, M.I., and Tyte, R.N.' 19?4> ~p~l.Opt.33, 1610.. ------

Heard, HOG. 1963. ~atase,%N,- 667. Xeraberg, G. 1967. . HoLe'culeir' Spec'tta ah&8"~o:le'c~l'ar Stmcture I. ' ' Spim'tYaa'o'f DiatoWC I@'Xd'cp'Xea, van Nostrand-Co., Inc. e J', Irwin, Jd!., and I;aCombe,*Je 1972. Can. J. Phy&"fiO,- 2596.

- Laconbe, J? 1971. R& Studiea of ~hononDisper~ion ' in Zincblende Semiconductore, e Praser University, Burnahy, LC simn k PFm,*. 1 tbrtin-,R.M. , 1974. Privpe .Conmunication. P. Mathiae , 5. , and Parker, *l963. A&. php. Lett , 2, f . 16.. I

.L Schaffez, F.P. 1973. Dye' L'aserr, Springer-Verlag, Betlin. ,' , 5 Ahif f , L. I. 1968. " puah'tmi Tag'c'ffahics, ~~Graw- ill. - ?+------Ad -- 1- b > - F /"- 2. v . .: % - -

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