heterodyne detection of rydberg atom maser emission

Volume 33, number 1 OPTICS COMMUNICATIONS April 1980

HETERODYNE DETECTION OF RYDBERG ATOM MASER EMISSION

L. MOI *, C. FABRE, P. GOY, M. GROSS, S. HAROCHE, P. ENCRENAZ Laboratoire de Physique de l'Ecole Normale Supdrieure, 75231 Paris Cedex 05, France

G. BEAUDIN Observatoire de Meudon, 92190 Meudon, France

and

B. LAZAREFF Institut d'Astrophysique, 75014 Paris, France

Received 13 February 1980

Using a very sensitive heterodyne receiver developed for millimeter radioastronomy, we have for the first time directly observed the microwave emission of Rydberg atoms in the laboratory. The source is a small system (~10 s atoms ) of Na Rydberg atoms optically pumped in a microwave cavity, which acts as a pulsed maser on the 33 S ~ 32 P transition at 107.8 GHz. Typical signals have a power of several 10- 12 watts and a duration of 0.3 ~s.

Microwave emission of Rydberg atoms in the centi- meter wavelength domain has been detected about fif- teen years ago in the interstellar medium [ 1 ] and since then has been extensively studied in radio astronomy [2]. In the last few years, considerable interest has also been given to the properties of laboratory produced Rydberg atoms in interaction with microwaves (double resonance spectroscopy [3], study of multiphotonic processes [4] ...). Due to severe detection sensitivity problems however, the radioemission properties of these atoms have not been investigated until very re- cently. A few months ago, pulsed maser action in the millimeter range has been observed on transitions between Rydberg states of sodium atoms optically pumped by dye lasers [5]. The active medium is characterized by huge electric dipole moments (~ 1000 Debye), which gives to these masers extremely low thresholds and very small saturating powers. As a result, the total energy radiated per each maser pulse is only in the electron-volt range. Extremely sensitive techniques must be used to detect such small signals. In the first

* On leave from Laboratorio di Fisiea Atomica e Molecolare, C.N.R., Pisa, Italy.

experiment [5], evidence of maser action was indirectly obtained, by monitoring the population of the Rydberg levels involved in the maser transition, using the selective field ionization method [6]. This very sensitive technique has made it possible to observe maser oscil- lation on many nS ~ (n - 1) P transitions (24 < n < 34) with wavelengths X ranging between 1 and 3 mm. In this letter, we report the first direct detection of the extremely weak maser pulses. Using a very sensitive heterodyne receiver developped for radio astronomy [7], we have detected the millimeter radiation emitted around 107.8 GHz (X = 2.78 mm) by the maser operating on the 33 S ~ 32 P transition in sodium.

Fig. 1 shows a general sketch of the experiment. A N a atomic beam crosses a semi-cofocal Fabry-Perot microwave cavity (distance between mirrors --- 72 mm ; quality factor Q = 10000; finesse f = 200; mode waist radius = 12 mm). Two synchroneous dye laser pulses focused at the same spot in the cavity prepare the atoms in the 33 S level (stepwise pumping process : 3 S 3P1/2 -'- 33 S). The laser pulses last 3ns and the pulse repetition rate is 10 pps. After this excitation, the atoms can "mase" on the two fine structure transitions 33 S

32 P3/2 and 33 S ~ 32 P1/2 whose frequencies are

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Fig. 1. Experimental set-up: Rydberg atom maser (top left), sensitive radioastronomy receiver (bottom) and processing of the detected signal (top right).

very precisely known from high resolution measure- ments of the sodium nS [8] and nP [9] levels quan- tum defects (107714.3 -+ 0.7 and 107892 -+ 1.3 MHz respectively). Gross tuning of the cavity to one of these frequencies is achieved by adjusting the mirrors sepa- ration. Fine tuning is then performed by checking the occurrence of maser action on the corresponding transition. A field ionization detector(FID): placed down- stream in the atomic beam allows us to monitor the Rydberg level populations which undergo a resonant change when the cavity is precisely tuned (the FID and the maser adjustment procedure are described in detail in ref. [5].

The radiation field escapes from the cavity through a 1.3mm diameter hole drilled in the spherical mirror and is sent to the detector in a waveguide. A local oscillator (Varian VRB 2113A Klystron) generates a reference signal whose frequency around 112.5 GHz is set exactly 4696 MHz above the expected maser frequency. The reference and the signal from the maser are coupled through an injection cavity and beat together on a state of the art GaAs Pt/Au Schottky diode. The diode chip was made by G.T. Wrixon (Uni- versity of Cork, Ireland) and mounted by P. Landry

(Radioastronomy Department, Meudon Observatory, France) using a microstrip technique on fused silica substrate [7]. The 4696 MHz beat note is amplified (low noise Avantek ASD 9262 M amplifier), down- converted to 630 MHz by beating against a 4066 MHz solid state source, amplified again and once more down. converted to 130 MHz by beating with a 500 MHz oscillator (this last down-conversion step has been added at the output of the radioastronomy receiver, which usually yields output signals in the 5000-1000 MHz range). Finally, the resulting beat note is selectively amplified, filtered by a Av F = 4 MHz bandwidth filter, and sent to an R-7912 Tektronix fast transient digi- tizer scope, which is triggered by the pumping laser pulses. The modulated beat note, proportional to the microwave field amplitude, is either directly observed or rectified through a diode to yield a d.c. signal rough- ly proportional to the microwave power (the diode operated in the quadratic part of its characteristics). The scope is interfaced to a videotape recorder, and we can thus register single microwave bursts at a rate of 10 pps.

Figs. 2 (beat notes) and 3 (rectified signals) show sets of typical signals observed in the same experimental

48


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Fig. 2. A set of typical r.f. maser signals corresponding to the 33S ~ 32P1/2 transition observed on the scope. The sine wave at 130 MHz is not resolved. The noise average amplitude corresponds to 2.8 X 10-taW. At the top, the detected maser pulse corresponds to a maximum power of 1.3 X 10 -11 W.

conditions. A microwave emission is detected about 0.5/as to 1/as after pumping and lasts about 0.3 to 0.5/as. The actual t ime scale of the maser emission is a little bit shorter, since the observed signals are slowed down by the finite response time r F = 0.2/as of the narrow band detect ion filter. The ampli tude, shape and delay of the signals strongly fluctuate from pulse to pulse. This reflects the intensity and frequency fluctuations of the pumping lasers, and also the in- trinsic fluctuations of this transient maser effect which is triggered by the random blackbody radiation noise. Fig. 4 shows (trace a) the rectified maser signal aver-

I I t I

- - w

I ] I I 0 1 2 3 4 5

Time ~s

Fig. 3. Typical rectified maser signals corresponding to the 33 S ~ 32 P1/2 transition.

aged over 200 pulses, and the effect on the signal of cavity detuning (trace b: the signal completely dis- appears when the cavity is tuned 30 MHz off-resonance). The averaging procedure increases the effec- tive detect ion time, which results in an improved signal to noise ratio. The pulse to pulse delay and intensity fluctuations also produce a t ime-broadening of the signal which appears to last longer than the single pulses of figs. 2 and 3.

Absolute amplitude calibration of the signals is achieved by statistical analysis of the single pulse signal to noise ratio. The background signal in fig. 2 corresponds to the 2500 K blackbody noise of the

-F I I L

0

b)

I I I 0.5 1 1.5

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Fig. 4. Rectified maser signals corresponding to the 33 S 32 P3/2 transition, averaged over 200 pulses, a) Cavity on

resonance at 107 714.3 MHz, b) cavity + 30 MHz off-resonance, c) recording of the laser pulse giving the time origin.

49


detector, observed at 107 GHz in an 8 MHz bandwidth ~ • (The noise temperature of the detector has been determined by detecting b lackbody sources at various temperatures). According to Planck's law, this noise corresponds in this bandwidth to a power of 2.8 X 10-13'W. The beat note signals (proport ional to field amplitude) are 2 to 7 times larger than the noise, which corresponds to a peak power between 1.1 × 10 -12 and 1.3 × 10 -11 W, and to a total pulse ener-

gy between 2 and 30 eV (for an average pulse duration of 0.3/~s). In order to estimate the energy actually emit ted by the maser, we have studied the various cavity and transmission losses. The cavity is damped by an oversized coupling hole in each mirror, so that nearly all the energy emit ted inside escapes, with neg- ligible wall absorption. Half of the energy thus goes towards the detector , with a waveguide loss of about 7 dB. The emit ted energy is thus about 10 times the detected one, i.e. 20 to 300 eV, corresponding to a number of millimeter photons between 40 000 and 600 000 (each photon carries 0.5 meV at 107 GHz.). These values have to be compared to the threshold of the maser emission, which we have independent ly evaluated by field ionization detect ion on the same transition and found to be about 20 000 inverted atoms. (Lower thresholds had been evaluated in ref. [5] for a maser operating in a smaller cavity at shorter wavelength).

The Rydberg maser studied in this let ter operates in the superradiant regime (i.e. the cavity damping time rca v = Q/co ~ 15 ns is shorter than the emission time). In this regime, the emission peak should be proport ional to the square of the number of radiators, with a delay inversely proport ional to this number. A

, The noise is detected in the two symmetric 4 MHz wide sidebands around 112.5 • 4.7 GHz. This noise could be greatly reduced by cooling down the mixer at 20 K and following it by a parametric amplifier cooled at 20 K and a FET amplifier cooled at 77 K. In this configuration (see ref. [7], the total noise temperature of the receiver is only 450 K.

qualitative evidence of this phenomenon is given in figs. 2 and 3, where pulses of decreasing intensities (corresponding to decreasing numbers of excited atoms) clearly exhibit increasing delays. Further investigations (with faster detect ion response time) are under way for a more quantitative study of microwave pulse intensity, delays and shapes.

This experiment can be considered as a demonstra- tion of selective amplification of the broadband blackbody radiation noise by Rydberg atoms. We believe it shows that these atoms could be used for preampli- fication of millimeter or far infrared radiation in front of sensitive receivers. Rydberg a tom amplifiers could be used for developing extremely low noise detectors in these wavelength domains with great potential interest in radioastronomy for example.

We would like to thank Mr. J.C. Pernot for techni- cal assistance.

References

[1] B. Hoglund, P.B. Mezger, Science 150 (1965) 339. [2] A. Dupr6e and L. Goldberg, Annual Rev. Astronom.

Astrophysics 8 (1970) 231. [3] K.B. McAdam and W.H. Wing, Phys. Rev. A 15 (1977)

678; T.F. Gallagher, R.M. Hill, S.A. Edelstein, Phys. Rev. A 13 (1976) 1348; C. Fabre, S. Haroche and P. Goy, Phys. Rev. A 18 (1978) 229.

[4] J.E. Bayfield, Phys. Reports 51(1979) 317. [5] M. Gross, P. Goy, C. Fabre, S. Haroche and J.M.

Raimond, Phys. Rev. Letters 43 (1979) 343. [6] T.F. Gallagher, L.M. Humphrey, R.M. Hill and S.A.

Edelstein, Phys. Rev. Letters 37 (1976) 1465; C. Fabre, P. Goy and S. Haroche, J. Phys. B 10 (1977) L 183.

[7] G. Beaudin, B. Lazareff, J.R. Mahieu, 7th European Microwave Exhibitions and Publishers Ltd., Great Britain.

[8] P. Goy, C. Fabre, M. Gross and S. Haroche, J. Phys. B to be published.

[9] C. Fabre, S. Haroche and P. Goy, Phys. Rev. A., to be published.

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heterodyne detection of rydberg atom maser emission

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