David D. Skatrud

Dynamics and tunability of a small optically pumped cw far‐infrared laser

We report the development of an ultrasmall, optically pumped cw far‐infrared (FIR) laser that provides substantial tunability. This laser operates at pressures significantly higher than the maximum allowed by currently accepted theory. We also report the development of a new theoretical model for diffusion limited optically pumped FIR lasers which accounts for this behavior. It is shown that the consideration of additional higher energy vibrational states, along with appropriate energy transfer mechanisms, fundamentally alters the behavior of the system in the high pressure, high pump intensity regime. Although 13CH3F is used for both the experimental demonstration and the theoretical model, the concept is general and should apply to all diffusion relaxed FIR lasers.

The millimeter and submillimeter spectrum of CN in its first four vibrational states

The rotational absorption frequencies of 65 new lines in the millimeter and submillimeter region of the spectrum have been measured for the CN radical in its ground electronic state. These measurements were made in a low pressure glow discharge of methane and nitrogen and include 25 lines from the v = 2 and v = 3 vibrational states, in addition to 40 lines from v = 0 and v = 1. The Dunham constants, as well as the spin-rotation and hyperfine constants of these four vibrational states, were calculated by means of a global nonlinear least squares fit of these data.

A time-resolved study of rotational and vibrational excitation and relaxation in the 13 CH 3 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.

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.

Study of the ν3 and 2ν3 ← ν3 bands of 12CH3F by infrared laser sideband and submillimeter-wave spectroscopy

Abstract An infrared laser sideband spectrometer operating in the CO 2 laser region with 8- to 18-GHz sidebands has been used to record 266 transitions in the ν 3 band and 84 transitions in the 2 ν 3 ← ν 3 band of 12 CH 3 F. The accuracy of the measured frequencies is estimated to be 1–3 MHz. A millimeter/submillimeter-wave spectrometer has been used to record the spectra of 48 pure rotational transitions in the ground vibrational state and 55 transitions in the v 3 = 1 vibrational state with an accuracy of 20–90 kHz. The new measurements have been combined with previous radio frequency and infrared laser results to derive sets of constants for the ground, v 3 = 1, and v 3 = 2 states for this molecule. Tables of the vibrational dependence of the parameters and of the near coincidences of the ν 3 and 2 ν 3 ← ν 3 band transitions with CO 2 laser frequencies are given.

Dynamics of the HCN discharge laser

Millimeter/submillimeter rotational absorption techniques are used to monitor the post‐discharge decay of vibrational state populations in a gated cw HCN far‐infrared discharge laser. The data provide a detailed map of the important energy flow pathways in the laser and make possible the construction and verification of a simple model for the laser excitation and inversion mechanisms. In addition, these data provide the requisite information for the quantification of this model. The resulting predictions are in excellent agreement with our extensive new data set and a substantial body of work reported in the literature.

Time‐resolved double resonance study of J‐ and K‐changing rotational collisional processes in CH3Cl

Time‐resolved double resonance spectroscopy using infrared pump radiation and millimeter‐wave and submillimeter‐wave probe radiation (IRMMDR) has been used to study rotational energy transfer (RET) in CH3Cl. A collisional energy transfer model using only five parameters for RET plus those needed for vibrational processes is shown to accurately model 350 IRMMDR time responses for two different pump states and 43 probe transitions covering a wide range of rotational states. Previous studies in this laboratory have revealed that J‐ and K‐changing RET have vastly different characters in CH3F [J. Chem. Phys. 92, 6480 (1990)]. Both J‐ and K‐changing RET were accurately modeled with four parameters—one for dipole–dipole collisions, two for the ΔJ scaling law, and one for the cumulative rate of K‐changing collisions. As was found for CH3F, J‐changing rotational collision rates in CH3Cl are modeled accurately by both the statistical power gap (SPG) law and the infinite order sudden approximation using a power law ...

Theory of millimeter wave nonlinearities in semiconductor superlattices

A study of the impact of semiconductor superlattices on millimeter wave nonlinearities is reported. χ(3) as large as 10−2 esu can be achieved by using 250 GHz radiation. Self‐induced transparency is also predicted, and its competition with the third‐harmonic generation process is quantified.

Excitation, Inversion, and Relaxation Mechanisms of the HCN FIR Discharge Laser*

Millimeter/submillimeter rotational absorption spectroscopy has been used as a diagnostic probe of a cw HCN discharge laser. This sensitive technique allowed in situ absolute population measurements of a number of vibrational states of HCN (including the upper lasing state) and other pertinent molecules. This unique set of data, which was obtained under a variety of discharge and laser conditions, is directly related to excitation, inversion, and relaxation processes. Along with elucidating several fundamental aspects of inversion and relaxation these results also strongly indicate that the primary laser excitation mechanism is near-resonant energy transfer from vibrationally excited nitrogen, N2 (v=1), to the fundamental symmetric stretching mode of hydrogen cyanide, HCN(100), and subsequent thermal population exchange between HCN (100) and the upper laser state, HCN (110).

Frequency stability and reproducibility of optically pumped far-infrared lasers

Direct measurements of the gain profile of optically pumped far‐infrared lasers show that large shifts in the laser frequency can be caused by the absorption from thermal molecules on the laser transition. The absorption shifting greatly exacerbates pump frequency deviations, resulting in an extreme sensitivity to pump offsets and drifts. This pressure‐dependent shifting mechanism is not present in transversely pumped lasers, which explains their superior frequency reproducibility compared to longitudinally pumped lasers, and reconciles two apparently conflicting results regarding laser stability.