Optically Pumped Far-infrared Research Articles

A high-efficiency regime for gas-phase terahertz lasers

We present both an innovative theoretical model and an experimental validation of a molecular gas optically pumped far-infrared (OPFIR) laser at 0.25 THz that exhibits 10× greater efficiency (39% of the Manley-Rowe limit) and 1,000× smaller volume than comparable commercial lasers. Unlike previous OPFIR-laser models involving only a few energy levels that failed even qualitatively to match experiments at high pressures, our ab initio theory matches experiments quantitatively, within experimental uncertainties with no free parameters, by accurately capturing the interplay of millions of degrees of freedom in the laser. We show that previous OPFIR lasers were inefficient simply by being too large and that high powers favor high pressures and small cavities. We believe that these results will revive interest in OPFIR laser as a powerful and compact source of terahertz radiation.

Discovery of Optically Pumped Far-infrared Laser Emissions from Formic Acid and its Isotopologues

An optically pumped molecular laser system with a transverse excitation scheme has been used to generate far-infrared radiation. Over 100 laser emissions with wavelengths up to 1.03 mm have been detected with this system using formic acid and several of its isotopologues. This includes ten new laser lines which are reported with their operating pressure, power, polarization with respect to the CO2 pump laser, and wavelength measured to an uncertainty of ± 0.5 μm.

Electric field effect on the optically-pumped far-infrared laser

This study investigates the effects of the electric field on the optically-pumped far-infrared (OPFIR) laser. The main mechanisms responsible for the change of the laser output power due to the electric field are considered to be the non-linear Hanle effect (NLHE) and Stark splitting. However, both the rate-equation calculation and the experiments with the circular hollow Stark waveguide OPFIR laser show that the conventional NLHE provides a relatively limited power increase via enhanced absorption. Significant power enhancement is observed in the rectangular waveguide only when both the absorption and emission processes are ΔM=±1 transitions, where the classical Hanle effect (CHE) is considered to be applicable. By including constructive interference, the power increase due to the CHE is calculated for the parallel and orthogonal lines. Experimental results from the rectangular waveguide OPFIR laser show good agreement with the new calculation taking into account the classical Hanle and Stark splitting effects.

Energy Exchanged Process of Superradiant Optically Pumped Far-Infrared Laser with Buffer Gas

By means of solving the density matrix equations, base on the mechanism model of buffer gas, the output intensity of FIR and the FIR signal gain coefficient Gs and IR pumping signal absorption coefficient Gp were calculated, energy exchanged process of superradiant OPFIRL with buffer gas was analyzed. It was found that after adding buffer gas into the laser medium, the FIR output could be increased, and the active region of a FIR laser was lengthened.

A New Model of Calculating Miniature Optically Pumped Far-Infrared Laser

Considering the broadening of infrared pumping laser, the spectrum of miniature optically pumped superradiant NH3 far-infrared laser pumped by CO2-10R(8) has been calculated by solving density matrix equations and using iteration method. It is first time that the theoretical spectrum has been in good agreement with experimental results.

Theoretical Study on NH3 Superradiant Miniature Optically Pumped Far-Infrared Laser with Buffer Gas

A miniature superradiant NH3 optically pumped far-infrared laser (OPFIRL) with buffer gas N2 was theoretically studied. Base on the mechanism model of buffer gas, synthetical relaxation time was calculated. By solving the semi-classic density matrix equations, spectra of NH3 superradiant OPFIRL with buffer gas N2 was calculated, also the operating gas pressure. It was found that by adding buffer gas, FIR power could be raised, and there existed an optimum ratio of gases mixture and an optimum operating gas pressure which was higher than that of pure NH3 operating.

New Optically Pumped FIR Laser Lines and Frequency Measurements from CH3OD

This work reports the discovery of 45 new optically pumped far-infrared (FIR) laser lines from CH3OD in the range 42.6 μm to 207.2 μm. A highly efficient CO2 laser was used as the pump laser for an Optically-pumped Fabry-Perot FIR laser. The frequencies of most of the new lines were measured in the range 1.7 to 6.4 THz. Twenty-eight CO2 lines were used as pump lines; nineteen of these had frequencies lower than those previously used to pump CH3OD in a FIR laser. Sixteen of the new FIR lines had absorption transitions which belong to the OD bending vibrational mode.

Application of high-resolution Fourier transform spectroscopy to Fermi resonance and the assignment of far-infrared laser lines in CD 3OH

High-resolution Fourier transform spectra of the infrared (IR) CO stretch band and the farinfrared (FIR) torsion-rotation band of CD 3OH have been analyzed to find new assignments of reported optically-pumped FIR laser lines and to confirm previous assignments proposed by other workers. The identified CD 3OH FIR laser emission includes a substantial group of transitions involving CO stretch levels which are significantly shifted by Fermi resonance with high-lying torsional levels of the vibrational ground state. The results are supported by frequency combination relations for closed IR FIR transition loops, which also yield improved accuracies for a number of observed or predicted FIR laser frequencies. The previous assignments have generally been confirmed, with modifications for two of the pump systems. Offsets of the assigned IR pump transitions from the corresponding CO 2 laser lines are also presented, as determined from the spectroscopic data.

Collisional energy transfer in optically pumped far-infrared lasers

A numerical simulation of collisional energy transfer in optically pumped far-infrared (OPFIR) lasers is presented along with a discussion of each collisional process and how it affects laser operation. A simulation is required to adequately describe these lasers because of the relatively large number of nonequilibrium states and thermal pools that are significant in the dynamics of their excitation and relaxation. The results of diagnostic studies and theoretical considerations are used to restrict the numbers of degrees of freedom so that a numerically tractable and physically satisfying model results. The resulting simulation accurately characterizes collisional energy transfer in these lasers over a much wider range of physical conditions than is possible with analytic models. These conditions include the very low pressure regimes most suited to the recovery of fundamental molecular collisional energy transfer parameters. Significantly for OPFIR laers, it predicts a new operating regime, which has been experimentally verified, that leads to small, tunable CW laser systems.

A time-resolved study of rotational and vibrational excitation and relaxation in the<sup>13</sup>CH<inf>3</inf>F optically pumped far-infrared laser

The rotational and vibrational energy transfer processes of the13CH 3 F optically pumped far-infrared (OPFIR) laser have been studied in a time-resolved experiment. The experiment uses a tunable millimeter and submillimeter spectroscopic technique as a diagnostic probe. Included are observations of the fast \Delta J processes that closely connect other J states within K = 3 to the directly pumped J = 5 level, a vibrational swapping mechanism that transfers excitation from K = 3 to other K states, vibrational relaxation due to both wall collisions and molecule-molecule collisions, the nonunity probability of vibrational deactivation in a wall collision, and pump saturation and hole burning effects due to the CO 2 pump laser. All of these observations are accounted for in the context of a numerical simulation. This results in a complete map of all of the collision-induced rotation-vibration transitions of importance to this basic OPFIR system including quantitative cross sections for the relevant processes.

Radio-frequency Stark tuning of optically pumped far-infrared lasers

We have directly monitored the rf Stark-field-induced splitting of the gain profile of the 1222-microm (13)CH(3)F optically pumped far-infrared (OPFIR) laser transition. The resulting data are found to be in good agreement with ac Stark theory. Comparison with dc Stark tuning techniques shows that the rf Stark effect possesses a number of attractive attributes for frequency tuning cw OPFIR lasers, including significant increase in the turing range and straightforward implementation.

Photon-assisted tunneling and AC Josephson effect at 246 and 604 GHz in small-area superconducting tunnel junctions

We report the first observations of photon-assisted quasiparticle tunneling and AC Josephson effect in superconducting tunnel junctions irradiated with far-infrared (FIR) radiation. Radiation at 246 GHz (λ=1.22 mm) and 604 GHz (496 μm) from an optically-pumped FIR laser source was used. Tin-tin oxide-lead junctions of ∼1 (μm) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area were fabricated on crystal quartz substrates with integral planar dipole antennas of resonant length at the frequency of the incident radiation. The observed photon-assisted tunneling features are in excellent agreement with the Tien-Gordon theory, and the inferred responsivity approaches the quantum limit at low temperatures for photon energies less than the gap. At 604 GHz, with a 176 ohm junction, we have seen 7 Josephson steps, comparable to point contact performance. The variation of the step widths with laser power is found to agree quite well with both the RSJ model and the Werthamer theory. For low resistance junctions (e.g. 16 ohms), we find the Josephson steps to be flat and to agree well with the shape predicted by the RSJ model without noise rounding, while noise rounding is very evident with the higher resistance junctions. In all cases, the step shape is in reasonable agreement with the theory of P. A. Lee, using a noise temperature of 10-20K with an appropriate small junction capacitance.