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Sub-Doppler spectra of infrared hyperfine transitions of nitric oxide using a pulse modulated quantum cascade laser: Rapid passage, free induction decay, and the ac Stark effect
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10.1063/1.4710542
/content/aip/journal/jcp/136/17/10.1063/1.4710542
http://aip.metastore.ingenta.com/content/aip/journal/jcp/136/17/10.1063/1.4710542

Figures

Image of FIG. 1.
FIG. 1.

A schematic diagram of the optical layout for the NO experiments.

Image of FIG. 2.
FIG. 2.

The calculated spot patterns are plotted on the entrance mirror (Figs. 2(a) and 2(b)), and on the second mirror (Figs. 2(c) and 2(d)). The reflection numbers of some of the most closely spaced spots, labelled on the figures, correspond to very different reflection numbers so that interference effects are minimised.

Image of FIG. 3.
FIG. 3.

An alternative way of showing the large separations of spots of successive bounces as a function of reflection number. The spacings between successive spots are sufficiently large that the interference effects are minimised.

Image of FIG. 4.
FIG. 4.

A schematic diagram showing the rectangular current pulse modulation and the resultant frequency tuning of the rapid, pulse-induced, laser tuning. As the frequency tuning depends upon the Joule heating provided by the current pulse, the tuning reverses at the termination of the pulse. The reversed tuning rate, or rebound, is related to the heat loss as the laser approaches its equilibrium temperature. As a result the rebound tuning is more non-linear.

Image of FIG. 5.
FIG. 5.

Schematic diagrams of the Doppler broadened envelopes and hyperfine structures of the transitions studied in these experiments: (a) the 1-0 R(11.5)3/2 unresolved doublet and (b) the 1-0 R(18.5)1/2 resolved doublet, of 14NO. The positions and integrated intensities of transitions between the hyperfine levels are given in Tables I and II, and are taken from the compilation of Maki and Wells29 and HITRAN.30

Image of FIG. 6.
FIG. 6.

Slow experimental scans through the 1-0 R(11.5)3/2 unresolved doublet showing the typical locations of the perturbation pulses. In Fig. 6(a) the origin is red detuned (to lower wavenumber) by approximately one Doppler width, and in Fig. 6(b) blue detuned (to higher wavenumber) by approximately the same amount. The calculated shape of the envelope of the unresolved Doppler structure is shown as a dashed line.

Image of FIG. 7.
FIG. 7.

Slow experimental scans through the 1-0 R(18.5)1/2 resolved doublet showing locations of the perturbation pulse. These are (a) red detuned by about 3 Doppler widths, (b) between the components near the centre, and (c) blue detuned by approximately one Doppler width. The calculated shape of the envelope of the unresolved Doppler structure is shown as a dashed line. The shape of the experimental spectra shows that some power saturation is occurring.

Image of FIG. 8.
FIG. 8.

Rapid, current pulse-induced experimental QC laser scans through the 1-0 R(11.5)3/2 unresolved doublet: (a) red detuned, initial scan blue (to high frequency), (b) blue detuned, initial scan red (to lower frequency). In both (a) and (b), the spectra were recorded using laser radiation passing through the reference cell (i) and the 100 m Herriott cell (ii). There is evidence for partial resolution of the hyperfine structure. In (a) the doublet e-e component is seen first, whereas in (b) the un-split f-f component is recorded first. In the spectra recorded using the Herriott cell there is some evidence for triplet structure in the f-f component. After the current pulse terminates, the probe reverses its scan direction, leading to the observation of a rebound spectrum, in which the order of the e-e and the f-f components is reversed.

Image of FIG. 9.
FIG. 9.

A schematic diagram of the hyperfine structure and the e-f Λ-doubling of the 1-0 R(11.5)3/2 transitions in 14NO. Possible coupled pairs of V, (f side), and inverted V, (e side), are shown. The dashed lines show the dipole-allowed, laser field-induced coupling between the e and f Λ-doublet states.

Image of FIG. 10.
FIG. 10.

Calculated behaviour of the real part of the polarization, P r, when rapid passage occurs, by numerical solution of the Maxwell-Bloch equations (see Refs. 7 and 9). Our approach differs slightly from the earlier work7 in that we include the various possible hyperfine transitions, but neglect the M-dependent alignment. The field induced connection between the e and f components is made by including the coherent interactions between equivalent F-components of the e-e and f-f transitions. The chirp rate, which was as chosen 2 MHz/ns, is similar to that used in the experimental fast chirp. The calculated patterns are (a) red detuned with a blue chirp, and (b) blue detuned with a red chirp. The effective splitting of the hyperfine structure, given by the zero-crossings, is much larger than that in the zero field limit shown in Figure 3, and given in Table I. It is, however, similar to that observed in the experimental spectra, where the e-e splitting is ∼10 MHz. Near the zero crossings the calculated e-e pattern splits into two, with a separation of ∼10 MHz, whereas the f-f components exhibit very small splittings both experimentally, and in the calculated spectrum.

Image of FIG. 11.
FIG. 11.

Expanded difference plots of the observed field-dependent hyperfine structure of the 1-0 R(11.5)3/2 unresolved doublet in the (i) reference cell and (ii) Herriott cell: (a) red detuned, blue chirp, (b) blue detuned, red chirp. In (c) the scale in (b) has been expanded to show free induction decay-like behaviour on the e-e lambda doublet. The field-induced splittings of the f-f lambda doublet are more marked in the rebound phase.

Image of FIG. 12.
FIG. 12.

Rapid experimental scans through the 1-0 R(18.5)1/2 resolved doublet, starting red detuned by at least one Doppler width. The initial scan direction is blue (to higher frequency) in (i) the short reference cell and (ii) the 100 m Herriott cell. In (a), a full scan showing the resolved doublet in the spectrum (i) from the short reference cell, whereas in the Herriott cell (ii) a much more complex pattern may be seen. In (b), the region around the doublet is expanded and the complex pattern in (ii) appears to have six components. The relationship between the e-f splitting seen using the short reference cell and the six line pattern seen using the Herriott cell is indicated via the two parallel lines. In order to demonstrate the power dependence of this complex spectrum in (c), spectra are shown at fixed transmission values as the power through the cells is reduced by rotating a linear polarizer (see Table IV). At the lowest power used, the doublet structure is almost recovered. In (d) the power dependence of the structure seen in the spectrum from the Herriott cell is compared with that of the doublet seen via the short reference cell. To facilitate the comparison, the intensity of the reference cell spectrum has been reversed. The positions of the e and f components seen in the reference cell correspond to the centres of the two strong e and f doublets. The remaining structure overlaps the rapid passage induced dip which follows the short reference cell doublet.

Image of FIG. 13.
FIG. 13.

Rapid experimental scans through the 1-0 R(18.5)1/2 resolved doublet, starting red detuned by at least one Doppler width. The initial scan direction is red (to lower frequency) in the (i) short reference cell and (ii) Herriott cell. In the full scan, no spectra are observed in the direct scan. In the rebound phase the doublet is observed, followed at longer time by the doublet seen via nutation generated by the large laser field. In (b) the region around the doublet is expanded. The pattern in (ii) is complex, but with poorer resolution than that shown in Figure 12. In Fig. 13(c) the region around the nutation generated doublet is shown, and for (ii) the Herriott cell spectrum, a large Autler-Townes splitting is seen.

Image of FIG. 14.
FIG. 14.

Rapid experimental scans through the 1-0 R(18.5)1/2 resolved doublet, starting blue detuned by at least one Doppler width. The initial scan direction is red, to lower frequency, in the (i) reference cell, and (ii) Herriott cell. In (a) the full scan shows the resolved doublet in spectrum (i), whereas in the Herriott cell spectrum (ii) a more complex pattern may be seen. In (b) the region around the doublet is expanded and the complex pattern in (ii) is not as well resolved as the equivalent pattern in Figure 12. In (c) an expanded view of the spectra is shown as the power is reduced using a linear polariser. At low power the doublet structure is almost recovered as it was in Figure 12(d). The doublet spectrum (i), from the short reference cell, is inverted and enlarged vertically to facilitate the comparison with the Herriott cell spectra.

Image of FIG. 15.
FIG. 15.

Rapid experimental scans through the 1-0 R(18.5)1/2 resolved doublet, starting the probe pulse between the two components. The initial direction of the fast scan is to lower frequency (a red chirp). On the rebound it scans through both components leading to the observation of a doublet. However the signal, which occurs in the nutation region, has only one component. The full scan is shown in (a), in which the power dependence of the Autler-Townes splitting of the single component is seen. In (b) the scan is expanded to show the region of the direct scan and rebound, where the doublet structure in the rebound phase resembles that shown in Figure 13. In (c), the power dependence of the doublet structure is shown, collapsing at low power to the doublet structure of the spectrum obtained using the short reference cell.

Image of FIG. 16.
FIG. 16.

Scans through the R(24.5) transition of the 2Π1/2 band of 15NO, see Table III. The origin of the perturbation pulse is approximately one full width at half maximum from the centre of the doublet. For traces labelled (i) the initial direction of the perturbation pulse is a red chirp, for (ii) the initial pulse direction corresponds to a blue chirp. (a) Full range, note the Autler-Townes splitting in each component of the doublet. (b) An expanded view of the origin, showing that the power broadened doublet is seen using the blue chirp only. Since the nuclear spin of 15N is 0.5, the number of hyperfine components is reduced, and little hyperfine structure may be seen in either the direct or rebound signals. As with 14NO the large nutation signals are anti-correlated with the observation of the power broadened doublet in the direct scan.

Tables

Generic image for table
Table I.

Positions and integrated intensities of the hyperfine components of the R(11.5) transition of the 2Π3/2(v = 1) − 2Π3/2(v = 0) band of 14N16O.

Generic image for table
Table II.

Positions and integrated intensities of the hyperfine components of the R(18.5) transition of the 2Π1/2(v = 1) − 2Π1/2(v = 0) band of 14N16O.

Generic image for table
Table III.

Positions of the Lambda doublet components of the R(24.5) transition of the 2Π3/2(v = 1) − 2Π3/2(v = 0) band of 15N16O.

Generic image for table
Table IV.

Changes in the intensity of the signal of the linearly polarised beam of the quantum cascade laser transmitted through the Herriott cell due to rotation of the linear polariser.

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/content/aip/journal/jcp/136/17/10.1063/1.4710542
2012-05-07
2014-04-24
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Sub-Doppler spectra of infrared hyperfine transitions of nitric oxide using a pulse modulated quantum cascade laser: Rapid passage, free induction decay, and the ac Stark effect
http://aip.metastore.ingenta.com/content/aip/journal/jcp/136/17/10.1063/1.4710542
10.1063/1.4710542
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