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Physical Review A

Rydberg-atom masers. II. Triggering by external radiation and application to millimeter-wave detectors

P. Goy, L. Moi, M. Groß, J. M. Raimond, Claude Fabre, S. Haroche

Published Apr 1, 1983• 42 citations
Publisher

Abstract

The "triggering" of transient maser systems using excited Rydberg atoms as active medium is investigated theoretically and experimentally. We show how the maser pulse characteristics are modified by the interaction of the active medium with small amounts of resonant mm-wave radiation. The phase, polarization, and emission delay of the radiated field are modified by this triggering: this can be used to detect very small powers of mm-or sub-mm-wave radiation. A complete discussion of the quantum and thermal noise of these detectors is presented. A demonstration of the feasibility of this technique is reported. Triggering signals corresponding to a detectivity of ${3\ifmmode\times\else\texttimes\fi{}10}^{\ensuremath{-}17}$ ${\mathrm{W}/\mathrm{H}\mathrm{z}}^{1/2}$ at 108 GHz ($\ensuremath{\lambda}=2.8$ mm) have been measured at room temperature. The quantum-noise limit---corresponding to ${6\ifmmode\times\else\texttimes\fi{}10}^{\ensuremath{-}19}$ ${\mathrm{W}/\mathrm{H}\mathrm{z}}^{1/2}$---should be reached with liquid-helium-temperature-cooled detectors.

DOI
10.1103/physreva.27.2065

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The Journal of Chemical Physics•Jan 2, 2013•26 citations
The "triggering" of transient maser systems using excited Rydberg atoms as active medium is investigated theoretically and experimentally. We show how the maser pulse characteristics are modified by the interaction of the active medium with small amounts of resonant mm-wave radiation. The phase, polarization, and emission delay of the radiated field are modified by this triggering: this can be used to detect very small powers of mm-or sub-mm-wave radiation. A complete discussion of the quantum and thermal noise of these detectors is presented. A demonstration of the feasibility of this technique is reported. Triggering signals corresponding to a detectivity of 3\ifmmode×\else\texttimes\fi10\ensuremath−17{3\ifmmode\times\else\texttimes\fi{}10}^{\ensuremath{-}17}3\ifmmode×\else\texttimes\fi10\ensuremath−17 W/Hz1/2{\mathrm{W}/\mathrm{H}\mathrm{z}}^{1/2}W/Hz1/2 at 108 GHz (\ensuremathλ=2.8\ensuremath{\lambda}=2.8\ensuremathλ=2.8 mm) have been measured at room temperature. The quantum-noise limit---corresponding to 6\ifmmode×\else\texttimes\fi10\ensuremath−19{6\ifmmode\times\else\texttimes\fi{}10}^{\ensuremath{-}19}6\ifmmode×\else\texttimes\fi10\ensuremath−19 W/Hz1/2{\mathrm{W}/\mathrm{H}\mathrm{z}}^{1/2}W/Hz1/2---should be reached with liquid-helium-temperature-cooled detectors.
The operation of maser devices using highly excited Rydberg atoms as active medium is investigated. These systems, working in a transient regime reminiscent of super-radiance generate few-hundred-ns-long bursts of mm-wave radiation. They are characterized by extremely low inversion density thresholds (${N}_{0}\ensuremath{\sim}{10}^{4}$ atoms typically) and very small peak power outputs (in the ${10}^{\ensuremath{-}11}$- to ${10}^{\ensuremath{-}13}$-watt range). This study is interesting in the context of new tests of super-radiance and electrodynamics theory. It also opens new possibilities in the important domain of mm-wave amplification and detection. In this paper, we present a simple semiclassical theory of transient Rydberg maser operation and we report the experimental observation of a large number of maser emission lines on Na atoms, spanning a wide frequency interval ($60<\ensuremath{\nu}<950$ GHz). The technique used to monitor the emission is the indirect field ionization procedure which reveals fast radiative transfers occurring between Rydberg levels. Another method involving the direct detection of the maser radiation by a heterodyne receiver is described in a subsequent paper, along with a report on the investigation of various effects related to the triggering of the transient maser emission by external radiation.
Superradiance and transient maser action have been obtained at millimeter wavelengths with alkali Rydberg atoms as the active medium. The emission was detected by monitoring the evolution of Rydberg-state populations using field ionization. Because of the strong Rydberg-atom-radiation-field coupling, the number of radiators at threshold is far smaller than in conventional masers. The estimated radiated energy is extremely low, 10 to 103{10}^{3}103 eV. Applications for spectroscopy and microwave detectors are considered.
A small number N (105≥N≥1) of sodium atoms in an atomic beam are prepared by pulsed laser excitation in a nS Rydberg level (n≂30) while transversing a millimeter wave Fabry–Perot cavity tuned to a transition connecting the nS state to a nearby n′P level. The atoms absorb the radiation in the cavity (nS→nP upward transition), or amplify it in a maser process [nS→(n−1)P, nS→(n−2)P... downward transition]. Due to the huge size of the electric dipole matrix element between nearby Rydberg levels, the system is extremely sensitive to the millimeter wave radiation field (enhanced by the tuned cavity) in both absorption and emission processes. Simple numerical expressions are given. The experiments demonstrate that the system can be used as an absolute power meter, an ultrasensitive millimeter photon counter, an absolute quantum thermometer, and a transient millimeter-wave amplifier.
We present various experiments involving Rydberg atoms interacting with resonant microwave electromagnetic fields inside a cavity, allowing to check different features of the theory presented in the previous paper. We first describe experiments about superradiant emission by Rydberg states in the cavity, also called transient Rydberg atoms maser ; the evolution of the system is detected either on the populations by selective field ionization technique or on the microwave field itself by heterodyne detection. Very low thresholds, down to 100 atoms, for the maser emission have been observed. We then describe various aspects of collective absorption of radiation by very excited atoms including the observation of Rabi nutation in a small applied resonant field, the study of the quantum efficiency of this set-up considered as a microwave photon counter, and finally the unexpected characteristics of the blackbody radiation absorption in the cavity.
Abstract Various experiments involving very excited atoms and millimetre-wave photons are described, including high-resolution spectroscopy of Rydberg transitions, Rabi nutation and transient maser action between Rydberg levels. The large number of extremely narrow Rydberg spectral lines observed throughout the millimetre-wave spectrum provide a very good ‘calibration' of the scale of Rydberg levels of Na and Cs near the ionization limit, which could be used for the realization of precise infrared frequency markers. The observation of Rabi nutation and maser signals involving an unusually small absolute number of atoms and photons reveals the very great strength of the Rydberg atom to radiation coupling and is a clear indication that these atoms could be used as extremely sensitive millimetre-wave detectors. At a more fundamental level, the study of transient Rydberg masers provides a test of superradiance theory. Extrapolation of these maser experiments to the investigation of a situation where a single atom would exchange a single millimetre-wave photon with a resonant cavity is considered.
We apply the chirped-pulse millimeter-wave (CPmmW) technique to transitions between Rydberg states in calcium atoms. The unique feature of Rydberg-Rydberg transitions is that they have enormous electric dipole transition moments (~5 kiloDebye at n* ~ 40, where n* is the effective principal quantum number), so they interact strongly with the mm-wave radiation. After polarization by a mm-wave pulse in the 70-84 GHz frequency region, the excited transitions re-radiate free induction decay (FID) at their resonant frequencies, and the FID is heterodyne-detected by the CPmmW spectrometer. Data collection and averaging are performed in the time domain. The spectral resolution is ~100 kHz. Because of the large transition dipole moments, the available mm-wave power is sufficient to polarize the entire bandwidth of the spectrometer (12 GHz) in each pulse, and high-resolution survey spectra may be collected. Both absorptive and emissive transitions are observed, and they are distinguished by the phase of their FID relative to that of the excitation pulse. With the combination of the large transition dipole moments and direct monitoring of transitions, we observe dynamics, such as transient nutations from the interference of the excitation pulse with the polarization that it induces in the sample. Since the waveform produced by the mm-wave source may be precisely controlled, we can populate states with high angular momentum by a sequence of pulses while recording the results of these manipulations in the time domain. We also probe the superradiant decay of the Rydberg sample using photon echoes. The application of the CPmmW technique to transitions between Rydberg states of molecules is discussed.