Stark effects in optically pumped CH30H far infrared laser
K. P. Koo and P. C. Claspy
The Stark tuning properties of the 118.8-/m CH30H FIR laser line pumped by the P(36) 9.4-,um CO2 laser
line are investigated. A laser model using a rate equation approach is derived. Observed Stark tuning ef-
fects include (1) frequency shifting of CH3 0H absorption line, (2) frequency shifting of the FIR laser line,
and (3) amplitude modulation of the FIR laser output. Frequency shifting of the CH30H absorption line
closer to the CO2 pump laser line center results in a Stark enhancement of the 118.8-Am FIR laser line by a
factor of 3 with a Stark field of 180 V/cm. Frequency shifting of about 7 MHz of the 118.8-Am FIR laser line
is observed with a Stark field of 362 V/cm. Amplitude modulation characteristics of the FIR laser output
are observed to be similar to those for Stark absorption modulation.
1. Introduction
Stark effects in optically pumped far infrared (FIR)lasers are of interest because of the frequency tuningproperties which can give additional attractive featuresto such systems. Basically, two types of Stark fre-quency tunings are associated with an optically pumpedFIR laser system. The first one is the Stark shifting ofabsorption components with respect to the pump laserline. In this way, the absorption line can be tuned fromnoncoincidence into coincidence with the pump laserlinewidth, thus allowing the generation of new FIR laserlines.' For absorption lines which already have suffi-cient coincidence with the pump laser linewidth, en-hancement of the FIR laser output can be achievedthrough more efficient pumping by tuning the absorp-tion line closer to the pump laser line center.2 4 Thesecond Stark tuning effect is associated with the fre-quency tuning of the FIR laser transition.5 6 In addi-tion, when an ac Stark field is used, AM and FM effectson the FIR laser are possible.2-5 This paper presentsresults of an experimental investigation on the Starkeffects on the CH30H FIR laser system pumped by theP(36) 9.4-,gm CO2 laser line. The Stark-modified FIRlaser gain is also derived using a rate equation approach.The observed interactions among the pump laser po-larization, the Stark field orientation, and the molecularorientation are visualized using a classical model.
II. Background
The CH30H FIR laser lines that are of interest to usare the 118.8-Am and the 170.6-Am lines which are thestrong FIR laser lines pumped by the P(36) 9.4-gm CO2laser line. The energy states and transitions associatedwith these two optically pumped FIR laser lines haverecently been assigned 7 as shown in Fig. 1. CH 3 0H isan asymmetric hindered rotor,- 11 and its rotationalstates are described by four quantum numbers (J, K, A,r). Here, J denotes the total rotational angular mo-mentum, K denotes -the projection of J along the mo-lecular axis, n denotes the tortional rotation of the OHgroup with respect to the CH3 group, and r denotes thequantum number associated with the tunneling of theOH group through the threefold potential of the CH3group.
The Stark effect on an optically pumped FIR laser istwofold, namely, that on the absorption process andthat on the FIR emission transitions. For the absorp-tion transition, since it involves AV = 1 and since allother quantum numbers are unchanged, the Stark effectwill come only from a difference between the amountof Stark perturbation of energy levels in the ground andexcited vibrational states. Stark absorption studiesreported in this paper and in Ref. 12 show that there isa sizable Stark effect in this absorption transition. Asa first-order or linear Stark effect, the frequency shiftP. can be represented by
PS = CE,
The authors are with Case Western Reserve University, Electrical
where C8 is the first-order Stark coefficient which de-pends on the magnetic quantum number M, and E isthe applied electric field.
For the FIR transitions, changes of J, K, and r occur.Therefore, the Stark effects will depend on the partic-ular transition and the Stark field orientation with re-spect to the pump polarization. Stark effects in rota-
1314 APPLIED OPTICS / Vol. 18, No. 9 / 1 May 1979
P(36) 9.4u C02CH3 0H, v5 C-0 STRETCH
Differentiating this equation with respect to the electricfield E, we obtain the Stark coefficient for the Starklinewidth broadening case, namely,
J",(n", r", K")
- 16,(0,2,7)- 15,(0,2,7)418y, 14, (0,2, 7)
C dv- Avc(v- vm)d (dE (AV, + Apv,) 2dE (Avm),
13,(0,2,7)
Fig. 1. Energy states for the 118.8-Am and the 170.6-gm CH 3 0H FIRlaser system.
Ill. Laser ModelA laser model using the rate equation approach is
used to analyze our optically pumped Stark-tuned FIRlaser system. Our analysis follows closely that of De-Temple1 6 except that we are concerned with the CH 3 0HFIR laser system at FIR wavelengths of 118.8 gm and170.6 gm and also its Stark tuning properties. TheStark effects can be easily incorporated into the ir ab-sorption coefficient and the FIR emission cross section.In Fig. 2 are shown the energy levels appropriate for the118.8-gm or 170.6-gum CH30H FIR laser system along
Table 1. Stark Shifting Frequencies for the 118.8-Arm and the 170.6-Am Lines
G', GI, F, F', F", C1, and C2 = constantsE = electric fieldJ = rotational quantum number
a Theoretical.b Experimental.
tion-rotation transitions in CH30H have been studiedin microwave spectroscopy,1 3 and a collection of usefulinformation is tabulated in Table I.
The Stark frequency shifts described by the formulasin Table I are for Stark fields sufficiently large that thedifferent M components of the absorption line are re-solved. At low Stark fields, the absorption linewidthis Stark broadened only because adjacent M compo-nents are not frequency spaced by more than the tran-sition linewidth. When this Stark-broadening phe-nomenon interacts with the narrow resonance of thelaser cavity, a frequency pulling effect5' 15 occurs, andit is mathematically represented by5
AV,
where v = laser frequency,Pm = molecular transition frequency,V = cavity resonant frequency,
Avm = transition frequency linewidth, andAv = cavity resonator frequency linewidth.
rk' Yb
b -
n2
n. r k Y
- , r -
(2)
Fig. 2. A laser model showing the energy states and various impor-tant transitions for the 118.8-Am or the 170.6-Am CH 3 0H laser
system.
1 May 1979 / Vol. 18, No. 9 / APPLIED OPTICS 1315
J', (n', r', K')
16,(O, 1, 8)
15,(0, 1, 8)
14,(0, 1, 8)
13,(0,1, 8)
0K -a.0U,
J,(n,r,K)
17,(0,1, 8)
16 ,(0,, 8)
15,(0,1 ,8)
(3)
r '
STARK FIELD
314 V/cm
251
220189
157
126
94
63
31
16
0
Fig. 3. First derivative Stark spectroscopy of CH 3 0H vapor at 100mTorr at the P(36) 9.4-gim CO2 laser line with AM = 1 and the CO2
laser linewidth scanned.
with some collision processes which are thought im-portant. In the rate equation analysis, the excitationor absorption process is considered to be decoupled fromthe emission or lasing process so that they can be ana-lyzed separately. The effect of pump radiation ab-sorption appears as a source term in the FIR laser gainconsideration, while the effect of FIR emission on theabsorption process is included in the absorption satu-ration intensity.
With the addition of Stark frequency shifts, theStark-modified absorption coefficient is given by
av)=ao exp[-(A - vsi)2/Av2I(4
(1 + IIR/ISat)1/2
where ao is the unsaturated absorption coefficient, Ais the detuning of the pump laser from the absorptionline center, AVD is the infrared Doppler halfwidth at thee-1 point, Isat is the saturation parameter which is de-rived in Appendix A, P, is the Stark-shifted frequency,and IR is the cavity infrared intensity given by
O.5PIR(5
(y + aL)abhvIR (5)
where L is the cavity length, ab is the cross-sectionalarea, PIR is the CO2 pump laser power, -y is the totalinfrared cavity loss coefficient per pass, a is given by Eq.(4), and hvIR is the infrared photon energy.
Using a two-level rate equation model, the FIR lasergain is given by1619
where a(It ) is given by Eq. (4), a- are the Stark-mod-ified FIR emission cross sections in the + and - direc-tion, Vz is the axial velocity of CH 3 0H molecules, r, is
the rotational relaxation rate, and fi is the fractionaloccupation of a rotational level within the ni rotationalmanifold in thermal equilibrium. The Stark-modifiedstimulated emission cross sections are given by
where XFIR is the FIR wavelength, A21 is the EinsteinA coefficient, AVH is the homogeneous linewidth, vo isthe molecular transition frequency, and vs2 is theStark-shifted frequency.
IV. Experiment
Our experiment consists of two parts: (1) the Starkabsorption spectroscopy to study the Stark effects in theabsorption transition of our optically pumped FIR laserand (2) the Stark-tuned optically pumped CH30H FIRlaser to investigate the Stark properties of such an op-tically pumped FIR laser system. Comparing resultsof these two parts can reveal how much of the Starkbehavior in the FIR laser output is due to the Stark ef-fects in the absorption process and how much is due toStark effects in the emission process.
A. Stark Absorption Spectroscopy
The experimental techniques for derivative Starkabsorption studies are similar to those used before.1718
The probe source is a conventional CO2 laser operatingat the P(36) 9.4-,gm line. Laser output power is -0.5W in the TEMOO mode. The Stark cell is 68.6 cm long,2.7 cm wide, and has a 6.35-mm plate separation.
A direct absorption measurement indicates that theCH30H absorption component is located at the high-frequency side of the P(36) 9.4-,gm CO2 laser line width.Application of Stark fields up to 1 kV/cm induces nodirectly measurable change in absorption. However,the first derivative Stark absorption spectroscopy asshown in Fig. 3, where the probe laser is linewidthscanned, reveals that the absorption line shows a generalStark broadening phenomenon. Stark broadeningcauses a decrease in the detected signal because in firstderivative Stark spectroscopy the detected signal isproportional to the slope of the absorption line. Itshould also be noted from the figure that there is a slightincrease in the detected signal level as the dc Stark fieldincreases from 16 to 63 V/cm. This increase of signallevel corresponds to a Stark-induced increase of ab-sorption which has a remarkable influence on the op-tically pumped FIR laster system, as discussed later.
B. Stark-Tuned FIR Laser System
1. Experimental ArrangementsThe setup of our experiment as described previously4
is shown in Fig. 4. The CO2 laser has a peak power of61/2 W at the P(36) 9.4-,gm line. The Stark plates arethe same as that used in our Stark spectroscopy study.The mechanical chopper is inserted only when phasesensitive detection is used. For direct observations ofthe FIR laser output with the oscilloscope, the CO2pump laser is cavity scanned. FIR laser pressure is
1316 APPLIED OPTICS / Vol. 18, No. 9 / 1 May 1979
Fig. 4. A schematic diagramshowing the experimental setup forour optically pumped Stark-tuned
CH3 0H FIR laser system.
Ml M2 --- FLAT MIRRORS D2 --- PYROELECTRIC DETECTORM3M4-- Lmm, 3mm HOLE-COUPLED Al MIRRORS HDM --- HOLLOW DIFFERENTIAL MICROMETER
DI __ G:Au DETECTOR FPMC--- MECHANICAL LIGHT CHOPPER
Table I. Operation Conditions for Studying the Stark Properties of ourOptically Pumped CH3 0H FIR Laser System
Peak pump power at the P(36) 9.4-jim CO2 - 6.5 Wline
CH30H FIR laser pressure - 170 mTorrFIR laser output power at the 118.8-jim line 5 (arbitrary unit)FIR laser output power at the 170.6-jm line _ 1
measured by a Pirani gauge calibrated at least to 100mTorr of CH30H vapor pressure using a mechanicalgauge. Identification of the FIR wavelength is by ametal mesh scanning Fabry-Perot interferometer, andFIR radiation polarization is measured by a grid po-larizer.
2. Results and DiscussionsThe general operation conditions that we used for
studying the Stark properties of our FIR laser systemare listed in Table II. The usual dependence of the FIRlaser output power on the pump laser power and theFIR laser pressure have also been observed, and a dis-cussion of them can be found in Refs. 4 and 19. TheStark effects that we have observed include Stark en-hancement/suppression, Stark frequency shiftings, andStark modulation of the FIR laser output.
For the 118.8-gm FIR laser line and Stark field per-pendicular to the pump laser polarization, we have ob-served an enhancement of the FIR laser output by dcStark field similar to that in Refs. 2 and 3. Figure 5shows the FIR laser signal increases with increase ofStark field up to around 180 V/cm beyond which theFIR laser signal decreases with further increase of Starkfield. The initial enhancement of the FIR laser signal
I --- METAL MESH FABRY-PEROTINTERFEROMETER
10 CH 3 0H, 118 mP 170 m Torr
.~9
7
o -
< -
0 80 160 240 320 400
DC STARK F ELD ( V/cm )
Fig. 5. FIR laser output vs Stark field.
with low Stark field is believed to be due to a slight in-crease in pump power absorption as a result of Starkshifting the absorption component closer to the linecenter of the pump laser linewidth. This Stark en-hancement of the FIR laser output agrees qualitativelywith the results obtained in our Stark spectroscopystudy. The rolling off of the FIR laser signal withhigher Stark fields is believed to be caused by theshifting of the FIR gain curve frequency away from thecavity resonant frequency. The FIR laser frequencyshifting effect will be discussed later.
1 May 1979 / Vol. 18, No. 9 / APPLIED OPTICS 1317
YFIR C' FIR
UIIR
1-Me
-M
E STARK
I FIELDII 173 V/cm
126
31
PUMPLASERLINEWIDTH
Y'FIR
C'FIR C~ 'FIRC'" FIR
A M = 0
t M
ES = V/cm ES = 126 V/cm
Fig. 7. A schematic diagram showing the Stark splitting of the FIRlaser transition states as observed with a fixed laser cavity length.
VERTICAL Fig. 6. FIR laser output vs Stark
SCALE field showing the splitting of the118.8-jum CH 3 0H FIR laser line.
I mV/cm
2
Figure 6 shows the FIR laser output for three differ-ent Stark fields when the pump laser is cavity scanned.Here, the FIR laser cavity length is adjusted so that itscavity resonant frequency is slightly off from the118.8-gm line in the low-frequency side. As the Starkfield is increased from 0 to 126 V/cm, the FIR laseroutput waveform changes from a single peak to a doublepeak with a simultaneous increase in signal level.Further increase of Stark field to 173 V/cm induces anover-all decrease in FIR laser signal strength and achange in FIR laser output waveform. This entirephenomenon can be explained by the frequency shift-ings or splittings of energy levels involved in the FIRemission transition.
With the illustration shown in Fig. 7, one can see thatas the Stark field is introduced, both the FIR laser upperand lower states are split into different M components.As a result, a double-peak FIR laser output waveformis expected, provided that the two M components of the
LASER LINEWIDTH
DC
STARKFIELD VFIR V118.811
Co2 PUMP LASER LINEWIDTH
DC
STARK VFR 118.8p 7 MHz
FIELD
Fig. 8. FIR laser output vs Stark field for two different FIR laser cavity resonant frequencies with AM = 0.
1318 APPLIED OPTICS / Vol. 18, No. 9 / 1 May 1979
VERTICAL
SCALE
1 V/cm
2
1
Co2 PUMP
205 V/cm
181
95
0
362 V/cm
0
I
;
upper FIR laser level are within the pump laser line-width. Since the FIR laser cavity length or the cavityresonant frequency v"Y is fixed, these two FIR peaksshould have the same frequency. This is possible if vP
V2 1 tvY* The frequency detuning of the FIR lasercavity resonant frequency assists the acquiring of thiscondition. Also, it can be seen that the double-peakFIR laser output waveform is more pronounced at 126V/cm than at 173 V/cm when v vffP'ty and 2 >,cavity.VFIR
More convincing evidence of the Stark shifting of theFIR laser output frequency is shown in Fig. 8. In thiscase, the CO2 pump laser is operated with a cavity irisso as to reduce its gain and thus its tuning linewidth.Also, the FIR laser cavity length is adjusted such thatno FIR lasting is observed in the absence of Stark field.This cavity adjustment corresponds to a cavity fre-quency detuning of about -7 MHz from that for max-imum FIR laser output with no Stark field. Then byintroducing a Stark field of 362 V/cm, a FIR laser out-put is observed, indicating that FIR lasing frequencyis tuned into coincidence with the FIR laser cavity res-onant frequency.
The identified 170.6-,gm FIR laser line has been ob-served to be suppressed by the Stark field for the casewhen the Stark field is perpendicular to the CO2 pumplaser polarization (i.e., AM = 1 for ER 6
A). Thechanges of the 170.6-gm FIR laser output as a functionof different Stark fields are observed as shown in Fig.9. It can be seen that the FIR laser output decreaseswith the increase of Stark field and goes through a totalsuppression of the laser action at the Stark field of 157.5V/cm. However, further increase of Stark field inducesa FIR laser output with a double-peak waveform. Thecause of the double-peak waveform is similar to thatdiscussed in the previous paragraph. Although the lowStark field suppression of this FIR laser line is not fullyunderstood at this time, a classical model consideration,as discussed in Appendix B, reveals that such a sup-pression of the FIR laser output is possible.
INCREASING STARK FIELD
When the Stark plates are rotated so that the Starkfield is parallel to the pump laser polarization, only the118.8-um FIR laser line is found to be lasing. The FIRlaser output is again suppressed by the Stark field asshown in Fig. 10. Cavity tuning effects similar to thosediscussed in previous paragraphs are not measurablewithin the detection capability of our system. Since wehave been unable to detect any significant Stark effectsin the absorption spectrum, a quantitative descriptionof the source of the suppression effect has not been
STARK
FIELD
362 V/cm
158
---- ~~95
0
Fig. 9. FIR laser output vs Stark field for the 170.6-jim line.
STARK
MMM ~~~F I ELD
0 V/cm
55
87
Fig. 10. FIR laser output vs Stark field for the 118.8-,um line withAM = 1.
Fig. 11. Modulation character-istics of the FIR laser output as afunction of dc Stark field. Uppertrace, modulation drive (100 Vpeak to peak at 80 Hz). Lowertrace, modulated FIR laser out-
put.
1 May 1979 / Vol. 18, No. 9 / APPLIED OPTICS 1319
possible. Nonetheless, our classical model consider-ation does indicate the possibility of suppressing the118.8-,um FIR laser.
3. Modulation of FIR Laser OutputWhen an ac Stark field is superimposed on the dc
Stark field and the pump laser frequency is fixed, acmodulation of the FIR laser output is achieved. Fre-quency shifting of the absorption component in and outof coincidence with the pump laser frequency is inher-ently an AM effect on the FIR laser output. Frequencyshifting of the FIR emission transition is a FM effectwhich will appear as an AM effect when interacting witha fixed FIR laser cavity resonant frequency.
Figure 11 shows the characteristics of the ac modu-lation of the FIR laser output as a function of dc Starkfields. Observed features include frequency doublingand 1800 phase shift of the FIR laser modulated output.These phenomena, which are also observed in Starkabsorption modulation of CO2 laser radiations,2 0 can bereadily understood by considering the frequencyshifting of the absorption component with respect to thepump laser frequency, or frequency shifting of the FIRlasing frequency relative to the cavity resonant fre-quency.
V. Conclusion
Optically pumped FIR laser system can be Stark-tuned to provide for intensity enhancement or fre-quency shiftings or modulation of the FIR laser output.The Stark plate waveguide structure also allows the FIRlaser to operate at higher pressures than the conven-tional version.
Appendix A
From the model shown in Fig. 2, a set of rate equa-tions can be set up as follows:
where no = population of single rotational state inthe ground vibrational state,
nln2 = population of the upper and the lowerFIR laser levels in the excited vibra-tional state,
nfnbngnd = rotational manifolds for (v = 1, K = 8),(v = 1, K = 7), and (v = 0, K = 8), re-spectively,
nc = the remaining excited state popula-tion,
ng = the remaining ground state popula-tion,
W = (IIR)/(hPIR) = (aIIR)/(NhvIR) = rate ofabsorption transition,
a = absorption coefficient,IIR = circulating pump intensity,
a = absorption cross section,h = fractional occupation of a rotational
level within the ni rotational manifoldin thermal equilibrium,
3af
3b = fractional occupation of na,nb rotationalmanifolds within the total excited statesin thermal equilibrium,
r = relaxation rate due to V-V, V-T, andwall collisions,
r, = rotational relaxation rate,r' or rk = collision rate involving AK transition (F
> when FIR lasing occurs), andN = total number of molecules.
Equations (A1)-(A8) can be solved for (n 1 - n2) interms of the other parameters for the steady state con-dition, with some algebraic manipulation, (no - n1 ) canbe expressed as
An An
WS1+-
if we letan = no - ni,
tO~iarkN(ta B-1)An = ao.'N~B1
0 h + .0', + 1aI'h)(OB 1)W= rFr(Fk + NFr, + a Ok)(B - 1)
Or - W/r30 kA + (B - 2)(rk + Or, + aak)
where An = saturated population difference,Ano = unsaturated population difference,
W = pump rateW = saturation parameter,
(A9)
(A10)
A ,, Qtarha + r) + rkr r >r3rrk(20,a + 1) , ,
4 I' (, r( + ') + 3k rk ( l',rFk(2ka +)F- Iha]
B F lk12¢arr(¢ark + F) + 2FkF] X r >rj rrf2fk(2rk(2Fh a + ) -rFa] = r
rh[4uar(uak + r) + 3rkFrI r= k
Notice that terms A and B represent the couplingeffect of FIR emission on the saturation parameter inthe absorption process.
The saturation intensity is related to the saturationparameter by I = (Whv)/u.
Appendix B: Stark Effects Considerations UsingClassical Model
The absorption component of CH30H at the P(36)9.6-Am CO2 laser line has been assigned 7 21 as a Qbranch (AJ = 0) transition from the ground state to theC-O stretch vibrational state. Therefore, the molec-ular orientation most favorable for absorbing linearlypolarized radiation is that having its total angular mo-mentum parallel to, or its oscillating dipole perpen-dicular to, the pump laser polarization as shown in Fig.12. Here, the drawing depicts a CH30H molecule
1320 APPLIED OPTICS / Vol. 18, No. 9 / 1 May 1979
PUMPPOLARIZATION
MOLECULAR ORIENTATIONFOR MAXIMUMABSORPTION
Fig. 12. Correlation between the CH 3 0H molecular orientation andthe pump laser polarization for maximum absorption resulting in aQ-branch (AJ = 0) transition to the C-O stretch vibrational mode.
PUMPPOLARIZATION
MOLECULAR ORIENTATIONWITH A J , ANDAK-I TRANSITION Qs
I A\\\K-I\\- - \ \
J-I J
Fig. 13. A vector model showing the CH 3 0H molecular orientationrelative to the pump laser polarization and its rotational transition
for the 118.8-gm FIR laser line.
PUMPPOLARIZATION - -
MOLECULAR ORIENTATIONWITH A J= ANDA K I TRANSITION
K , K-I
J
Fig. 14. A vector model showing the CH 3 0H molecular orientationrelative to the pump laser polarization and its rotational transition
for the 170.6-jim FIR laser line.
spinning about its molecular axis and at the same timeprecessing about the total angular momentum vectorJ. Notice that the CH3 0H molecule has two dipolemoments (ll and ) which can interact with a Starkfield. The molecular precision about the J vector hasno effect on gu but will create an oscillating dipole fromA-
For the 118.8-,m FIR laser transition with AJ = 1and AK = 1, the vector model is given in Fig. 13.Applying a Stark field perpendicular to the pump laserpolarization on the randomly oriented CH3 0H mole-cules will have two effects. First, molecules which havetheir ,41 nearly in line with the Stark field will have theirJ vector aligned with the Stark field and therefore awayfrom the favorable orientation for absorbing the pola-rized pump radiation. Second, molecules which havetheir J vector nearly perpendicular to the Stark fieldwill carry a strong induced dipole perpendicular to theJ vector due to the interaction of the Stark field withg . This induced dipole will, in turn, interact with theStark field to align more molecules in the favorable or-ientation for absorption. Notice that since AJ and AK
change in the same direction, it is possible that the Jvector orientation remains unchanged in the FIRtransition as already shown in Fig. 12. Thus, the Starkfield does not hinder the FIR transition. The net effectwill bring about an increase in the number of CH3 0Hmolecules in the favorable orientation if the interactionof the Stark field with the perpendicular dipole isstronger than that with the dipole parallel to the mo-lecular axis. Therefore, the 118.8-gm FIR laser outputcan be enhanced by a Stark field perpendicular to thepump polarization. Following the same type of rea-soning, an applied Stark field parallel to the pump ra-diation polarization will suppress the FIR laser out-put.
For the 170.6-,4m FIR laser line, it involves a AJ = 0and AK = 1 transition. Such a transition requires achange of the J vector orientation as shown in Fig. 14.Applying a Stark field perpendicular to the pump ra-diation polarization will have the same type of effect onthe absorption process as that for the 118.8-Am FIRlaser line. However, this Stark field orientation willrestrict the change of the J vector orientation and hencethe FIR laser transition. The over-all effect will be asuppression of the 170.6-m FIR laser output if thesuppression on the FIR transition is stronger than theenhancement on the absorption process. Applying aStark field parallel to the pump laser polarization willsuppress the absorption by so much that the FIR lasingthreshold has not been reached.
References1. H. R. Fetterman, H. R. Schlossberg, and C. D. Parker, Appl. Phys. Lett. 23,
684 (1973).2. M. S. Tobin and R. E. Jensen, Appl. Opt. 15, 2023 (1976):3. M. S. Tobin and R. E. Jensen, IEEE J. Quantum Electron. 13, 481
(1977).4. K. P. Koo and P. C. Claspy, "Stark Effects in Optically Pumped FIR Laser,"
presented at the 2nd International Conference and Winter School onSubmillimeter Waves and Their Applications, 6-11 December 1976, SanJuan, Puerto Rico.
5. S. R. Stein and A. S. Riseley, Appl. Opt. 16, 1893 (1977).6. H. R. Fetterman, C. D. Parker, and P. E. Tannenwald, Opt. Commun. 18,
10 (1976).7. E. J. Danielewicz, Jr., and P. D. Coleman, IEEE J. Quantum Electron.
QE-13, 485 (1977).8. D. G. Burkhard and D. M. Dennison, Phys. Rev. 84, 408 (1951).9. J. S. Koehler and D. M. Dennison, Phys. Rev. 57, 1006 (1940).
10. K. T. Hecht and D. M.-Denrson, J. Cheim.Phys. 26, 48 (1957).11. E. V. Ivash and D. M. Dennison, J. Chem. Phys. 21,1804 (1953).12. J. R. R. Leite, A. Sanchez, M. Ducloy, and M. S. Feld, "Saturated Absorp-
tion Spectroscopy of CH3 0H," presented at the 2nd International Con-ference and Winter School on Submillimeter Waves and Their Applications,6-11 December 1976,San Juan, Puerto Rico.
13. R. G. Hughes, W. E. Good, and D. K. Coles, Phys. Rev. 84, 418 (1951).14. S. Golden and E. B. Wilson, Jr., J. Chem. Phys. 16, 669 (1948).15. A. Yariv, Quantum Electronics (Wiley, New York, 1975), Chap. 13.16. T. A. DeTemple and E. J. Danielewicz, Jr., IEEE J. Quantum Electron.
QE-12, 40 (1976).17. K. P. Koo, "Characterization of a Waveguide CO2 Laser and its Applications
to Stark Spectroscopy," M.S. Thesis, Case Western Reserve University,Cleveland, Ohio.
18. P. C. Claspy and K. P. Koo, IEEE J. Quantum Electron. QE-12, 647(1976).
19. K. P. Koo, "Stark Effects in Optically Pumped CH3 0H FIR Laser," Ph.D.Thesis, Case Western Reserve University, Cleveland, Ohio (1977).
20. Laser Technology for Communications, Case Western Reserve UniversitySemiannual Technical Progress Report 1, NASA grant 36-027-014, Feb-ruary-July 1970.
