Contains reports on two research projects.Joint Services Electronics Programs (U.S. Army, U. S. Navy, and U. S. Air Force) under Contract DAAB07-71-C-0300National Aeronautics and Space Administration (Grant NGR 22-009-526)National Science Foundation (Grant GP-24254

Muehlner, D. J.

Weiss, Rainer

Contains research objectives, summary of research and reports on two research projects.Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E)National Aeronautics and Space Administration (Grant NGR 22-009-526)M.I.T. Cabot Solar Energy Fun

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VIII. GRAVITATION RESEARCHAcademic and Research StaffProf. R. WeissDr. D. J. MuehlnerR. L. BenfordGraduate StudentsM. HartwellD. K. OwensN. A. PierreRESEARCH OBJECTIVES AND SUMMARY OF RESEARCHThe research in this group is in the field broadly labeled experimental relativity,in particular, experiments related to the gravitational interaction and measurableconsequences of cosmological models.Specific Projects1. Astronomy in the Region 1-0. 1 mmIn this region of the electromagnetic spectrum there is some important astrophys-ics, such as the spectral peak of the alleged 2.7" K isotropic cosmic black-body radi-ation, thermal radiation by interstellar and possibly intergalactic dust, as well asintense radiation from galactic centers, and doubtless other information concerning theearly history of an evolving universe.An experiment carried out with a balloon-borne far infrared radiometer in the fallof 1969 indicated that the isotropic background radiation may not have a thermal spec-trum and may be dominated by a strong line between 10 cm and 12 cm Theseresults are tentative, since we were not able to vary several important parameters inthe experiment. An improved radiometer is under construction which is scheduled tofly in the spring of 1971. Future flights will measure the spectrum of the isotropicbackground with a liquid helium-cooled lamellar grating interferometer that has-lI cm-1 resolution. Measurements of the small- and large-scale spatial isotropy of thebackground radiation are planned.2. Far Infrared Detection(a) Development of low-noise InSb photoconductive detectors.(b) Far infrared detection by parametric up-conversion in CdS pumped by the5145 A line of the Argon laser.3. Test of the Strong Principle of EquivalenceAn experiment to test the strong principle of equivalence and, in particular, toThis work was supported in part by the Joint Services Electronics Programs (U. S.Army, U. S. Navy, and U.S. Air Force) under Contract DA 28-043-AMC-02536(E), andin part by the National Aeronautics and Space Administration (Grant NGR 22-009-526),and the M. I. T. Cabot Solar Energy Fund.DOCUMENT OFFIC' 26-327RESEABCH LAKRPATOR'7 IT .l,'••'HONICSQPR No. 100 51 MASSACHUSETTS INSTITUTE OiF TECHNOLOGY(VIII. GRAVITATION RESEARCH)search for evidence of the Brans -Dicke scalar gravitational interaction which violatesthe strong principle has been studied by this group in conjunction with the MeasurementsSystems Laboratory of M. I. T. In this proposed experiment an atomic oscillator anda gravitational oscillator are both placed in a space probe; one measures the ratio ofaxis crossings of one oscillator relative to the other as the probe passes through varyinggravitational potentials. Several gravitational oscillator configurations have beenstudied, one of which may be tested in Earth orbit in Skylab B.4. Pulsar Gravitational Wave Antenna5. Sensitive Intracavity Laser Polarization InterferometerThis may permit measurement of intrinsic laser phase noise.R. WeissA. MONOLITHIC CAPACITIVE GRID INTERFERENCE FILTERSFOR THE FAR INFRAREDFor a balloon-borne liquid helium-cooled far infrared radiometer l , 2 we need low-pass filters with well-defined cutoff frequencies. Moreover, the filters have to bemechanically rugged and indifferent to repeated cycling between room and liquid-helium temperatures. The interference filters described by Ulrich 3 appeared to haveoptical properties that would meet our requirements, but they were mechanicallyunsuitable. His filters consist of multiple layers of "capacitive grids" or arrays ofsmall metallic squares. The individual grids are formed on thin mylar films that arestretched over rings and separated by spacers. This method of construction insuresflatness and parallelism of the reflecting surfaces, but the resulting filters are delicateand not suitable for cooling to low temperatures.We have developed a method of construction which produces very rugged filters atthe expense of some loss of control over the flatness and parallelism of the grids.Capacitive grids of silver are deposited on polyethylene substrates by evaporation, withwire screening placed flush against the polyethylene as masks. A 4-grid interferencefilter, for example, is assembled by fusing 5 layers of polyethylene (including oneblank) together in a vacuum between heated pressure plates. A temperature ofapproximately 110 0 C and a pressure of roughly 3 ib/in. 2 applied for 15-30 min areadequate to fuse the individual components of a filter into a single piece. In orderto avoid excessive flowing of the polyethylene it is important that the temperaturebe kept as low as possible in this step; hence, some experimention is required.The spacing between the grids in a finished interference filter is determinedby the thickness of the polyethylene substrate, which must be chosen appropriately.Ulrich 3 gives formulas by which the filter parameters, such as grid constants andthe spacing between grids, may be selected. These formulas are for grids in vacuum;when the grids are embedded in a dielectric the filter parameters must be scaledQPR No. 100(VIII. GRAVITATION RESEARCH)by the index of refraction (~1.5 for polyethylene).-iSeveral 4-grid filters, with nominal cutoff wave numbers of 6, 8, and 12 cm- , havebeen constructed. Figure VIII-l shows the transmission spectra of two filters with 6-iand 12 cm-1 cutoffs, as measured with a Michelson interferometer. The grids for thesefilters were deposited on standard 0.012 in. and 0. 006 in. polyethylene sheet. Thescreens used for masks had openings of 0. 354 mm and 0. 117 mm, wire diametersof 0. 131 mm and 0. 247 mm, and were cut from sieves No. 45 and No. 80 of theU.S. Standard Series. Grids in the same filter were oriented at random angles10010zZOzv-N\//I10 20 30WAVE NUMBER ( cm- 1Fig. VIII-1. Transmission spectra of two lowpassinterference filters.of 20-30' with respect to each other. The filters after trimming were 4.5 cmin diameter.In summary, we have found a method of making very rugged far infrared interfer-ence filters, suitable for cooling to liquid-helium temperatures. The ruggedness-lof the filters is illustrated by the fact that the three 12 cm-1 filters that were used inthe first balloon flights of our radiometer l , 2 survived a crash that otherwise com-pletely destroyed the apparatus. The same filters will be used in future flights of anew apparatus.D. J. Muehlner, R. WeissQPR No. 100(VIII. GRAVITATION RESEARCH)References1. D. Muehlner and R. Weiss, "Measurement of the Isotropic Background Radiationin the Far Infrared," Phys. Rev. Letters 24, 742 (1970).2. D. J. Muehlner and R. Weiss, Quarterly Progress Report No. 96, Research Labo-ratory of Electronics, M. I. T., July 15, 1970, p. 61.3. R. Ulrich, "Effective Low-Pass Filters for Far Infrared Frequencies," InfraredPhys. 7, 65 (1967).B. FAR INFRARED DETECTION BY PARAMETRIC UP-CONVERSION1,2In a classic paper on nonlinear optics Bloembergen proposed that a nonlineardielectric medium could function as a parametric up-converter for infrared radiation.The scheme is shown in Fig. VIII-2, in which wl is the visible laser pump radiation,and w2 the far infrared signal to be detected. The pump and signal wave generate non-linear polarization waves in the material at many different frequencies; in particular,there is a time -dependent polarization at frequency w3 = I + co2. If the polarizationwave at w3 has the same phase velocity as that of the traveling light wave at w3 in themedium, the amplitude of the traveling wave may increase linearly with distancetraveled in the medium. Depending on the amount of nonlinear polarization, as wellas the precision of the velocity match, it is possible, in principle, to convert all pho-tons at c2 into photons at w3 if there is sufficient pump power at cl. The converted wave,W3, is separated from the pump by high-resolution optical spectroscopy and detected bya photomultiplier.1 LASER PUMP UP-CONVERTED SIGNAL" NONLINEAR OPTICAL PHOTO-S DIELECTRIC SPECTROMETER MULTIPLIER02 IR SIGNALFig. VIII-2. Parametric up -conversion scheme.In principle, there is no intrinsic noise introduced into the signal by the up-conversion, since there should be no output at w3 unless there is input at w 2 - thereis no spontaneous emission noise. Thermal noise at w 2 is converted with the sig-nal and can only be reduced by cooling the nonlinear dielectric, although it is notclear that the noise temperature of the dielectric is the lattice temperature. Inthe idealized case, it might be expected that the limiting noise is photomultiplierbackground that might limit the noise to a few photons/sec or a noise equiva-lent power at X2 = 1 mm of ~10 W/HzQPR No. 100(VIII. GRAVITATION RESEARCH)1. Some Equations Relating to Parametric Up-conversionW1 = visible pump.W2 = far infrared signal.W3 = up-converted sum signal.The conversion efficiency is given byphotons out at wC3/s 2 _z1 ==sinphotons in at C 2 /s sin ntwhereS16 Tr XNLI1 2 3int c 3 n2 n 3withXNL = magnitude of the nonlinear polarizability in esuI, = visible pump intensity erg/s/cm 2n 2 = index of refraction for w2n 3 = index of refraction for c 3c = velocity of light cm/sin t = twice the length in the medium over which there is completemtconversion of w2 - W3z = the length over which there is a phase match.The perfect phase -match condition for collinear waves isk3 = k 1 + k 2 or n3 W3 = nlc 1 + n2W2.For imperfect phase matching there is a maximum length of medium, Zmax, suchthat the sum wave does not begin to feed energy into the polarization wave. This beginsto occur when the two waves are Tr phase apart.Trmaxmax k 3 - (kl+k2)QPR No. 100(VIII. GRAVITATION RESEARCH)To give a typical example: Assume a cw Argon ion laser pump and a CdS crystalas nonlinear dielectric:115l = 3. 6 X 10I 1  102 W/cm2-7XNL ~ 4 X 10 cgsn2  n 3  2W02 2 X 1012z ~Icmmax-5l = 2 X 10NEP at = 1 mm ~ 107 W/Hz 1 /2Several investigators have demonstrated that up-conversion from the near infraredis possible. 3 - 5 None have approached detection efficiency that even rivals conventionaldetectors, however. The principal difficulty is that since the nonlinear polarizabilityof most materials is small, a long phase-matched path is required in the material; how-ever, the useful path lengths are limited by optical inhomogeneities. Several investi-gators 3 , 4,6 have used high-power pulsed lasers to achieve a conversion efficiency of-6as much as 1% in a small phase-matched path length. The duty cycle is only 10 - 6 , how-ever.2. Specific Problems of Up-conversion in the Far Infrared(a) Separation of the pump from the up-converted signalThe relative frequency shift of up-converted light from the pump is small, for 1 mm-4radiation Aw// ~ 5 X 10- 4  To separate the pump from the signal requires high-resolution spectroscopy with little scattering. This is not a small problem when weconsider that the pump beam may typically be 1018 photon/s, while the up-convertedbeam may include a few photons/s.(b) Scattering processes in the medium that contribute at w3Rayleigh scattering by inhomogeneities in the medium will be elastic, and shouldnot convert o 1 - 3; however, Rayleigh scattering can scatter pump light from oneQPR No. 100(VIII. GRAVITATION RESEARCH)polarization state into another. This may be troublesome if pump vs up-converted sig-nal rejection is accomplished by giving these two waves orthogonal polarizations. Thismethod can be used in a crystal with off-diagonal terms in the nonlinear polarizabilitytensor. Brillouin scattering is not a problem for up-conversion of radiation at 1 mmor shorter wavelengths. The relative frequency shift of an incident photon at Wl thatrecoils at an angle 0 after collision with a phonon is given for small 0 byS= v 10 - 5 6,w cwhere v is the velocity of sound in the medium.Raman scattering may be a problem; however, this depends on the nonlinear medium.A~o -2Raman spectra in most materials are shifted by -- 3 X 10 from the pump.3. Proposed Experiments(a) Up-conversion in CdS using the 5145 A line of the Argon ion laser as pump anda resonance absorption cell of molecular Iodine as a narrow-band absorptionfilter to eliminate the pump after the crystalFor collinear propagation, phase matching can be achieved by bi-refringence in CdSif the pump is propagated as the extraordinary ray and the far infrared signal, and theup-converted signal as ordinary rays. The experimental arrangement is shown inFig. VIII-3.FAR 5145INFRARED + 50 ASOURCE y FILTERSINGLE-MODE ARGON IONLASER 5145 A X-POLARIZEDFig. VIII-3. Experimental arrangement.Pump rejection is achieved by (i) giving the pump wave orthogonal polarization tothe up-converted wave, (ii) a narrow-band (Aw/• ~ 10- 5 ) absorption filter 8 of gaseous12, (iii) a bandpass transmission filter, 50 A wide, at 5145 A to eliminate the fluores -cence of the 12, and (iv) a piezoelectrically tunable Fabry-Perot spectrometer.The nonlinear polarizability of CdS is not known at 5145 A; at 6943 A, (XNL)4 X 10 esu. The band gap of CdS at 300 0 K is ~5120 A and, since the nonlinear polar-izability becomes large near resonance, it may be an order of magnitude larger atQPR No. 100(VIII. GRAVITATION RESEARCH)5145 A than at 6943 A. In the initial experiments the far infrared source will be anHCN laser operating at 337 lm.(b) Phase matching and nonlinear polarizability in the region near anomalousdispersion in the mixing mediumIt seems interesting to investigate the possibility that both phase matching andenhancement of the nonlinear polarizability can be achieved if the nonlinear medium ispumped in the region of anomalous dispersion. The frequency and phase conditions areC3 = + 2 k3 - (kl+k2 ) = 0 = n 303 - (n 2022+nl 1)(n3 -n l) c 1 = (n 2 -n 3)W 2.Since n2 and n3 are very closeAnn2 - n3  Ai Wo IAnso that in a region of anomalous dispersion An/Aco is sufficiently large to compensatefor the difference in index for far infrared and optical radiation. The experiment canbe tried in two ways, either by using different pump frequencies near 02 or by varyingthe location of the anomalous dispersion by changing the temperature of the medium. Forexample, in CdS the band-gap wavelength changes from 5120 A at 300 0K to 4900 A at 4°K.R. WeissReferences1. J. A. Armstrong, N. Bloembergen, J. Ducuing, and P. S. Pershan, "Interactionsbetween Light Waves in a Nonlinear Dielectric," Phys. Rev. 127, 1918 (1962).2. N. Bloembergen, Nonlinear Optics (W. J. Benjamin, New York, 1965).3. M. Takatsuji, "Theory of Far Infrared Detection Using Nonlinear Optical Mixing,"Jap. J. Appl. Phys. 5, 389 (1966).4. J. E. Midwinter and J. Warner, "Up-conversion of Near Infrared to Visible Radiationin Lithium-Meta-Niobate," J. Appl. Phys. 38, 519 (1967).5. R. C. Miller and W. A. Nordland, "Conversion of Near Infrared to Visible Light byOptical Mixing," IEEE J. Quant. Elec. , Vol. QE-3, p. 642, 1967.6. J. Warner, "Photomultiplier Detection of 10.6 [1 Radiation Using Optical Up-conversion in Proustite," Appl. Phys. Letters 12, 222 (1968).7. G. D. Boyd, T. J. Bridges, and E. G. Burkhardt, "Up-conversion of 10.6 L Radiationto the Visible and Second Harmonic Generation in HgS," IEEE J. Quant. Elec.,Vol. QE-4, p. 515, 1968.8. S. Ezekiel and R. Weiss, "Laser-Induced Fluorescence in a Molecular Beam ofIodine," Phys. Rev. Letters 20, 91 (1968).QPR No. 100

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