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Observation of infrared free-induction decay and optical nutation signals from nitrous oxide using a current modulated quantum cascade laser
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10.1063/1.4710540
/content/aip/journal/jcp/136/17/10.1063/1.4710540
http://aip.metastore.ingenta.com/content/aip/journal/jcp/136/17/10.1063/1.4710540

Figures

Image of FIG. 1.
FIG. 1.

(a) A schematic diagram of the optical layout for the pulse modulated experiments. (b) A schematic diagram of the quantum cascade laser modulation arrangement and the detection frequency separation (fast and slow channels) system for the pulse modulated experiments. The MCT detector for the 10 cm reference cell is not shown.

Image of FIG. 2.
FIG. 2.

A schematic diagram showing the rectangular current pulse modulation applied to the “current tuning,” and the resultant “frequency tuning” of the fast, current 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. Its tuning rate is related to the heat loss as the laser approaches its equilibrium temperature.

Image of FIG. 3.
FIG. 3.

The response of the quantum cascade laser to the rectangular electrical modulation pulse. The average scan rate of the chirped pulses is 3.08 MHz ns−1. (a) A comparison of the shape of a rectangular electrical modulation pulse with a duration of 200 ns, and the shapes of the fast channel detector signals of the QC laser output, when driven by modulation pulses of 50 ns (red) and 200 ns duration (black). (b) A comparison of the shapes of the fast channel detector signals recorded in an evacuated Herriott cell using electrical modulation pulse durations of 50 (red), 100 (blue), 150 (green), and 200 (black) ns.

Image of FIG. 4.
FIG. 4.

The tuning range of the slowly swept QC laser through a representative part of the absorption spectrum of N2O used for these experiments, (1) P(34), 0001-0000, 14N14N16O, 1254.47269 cm−1; (2) P(41) f, 0111-0110, 14N14N16O, 1254.51180 cm−1; (3) P(29), 0001-0000, 14N15N16O, 1254.56517 cm−1; (4) R(9), 0001-0000, 14N14N18O,1254.63426 cm−1. The scan rate of the slowly swept laser, derived from these data, is 1.895 MHz μs−1. (a) The black line is the spectrum recorded through the 10 cm reference cell and a gas pressure of 0.4 Torr, and the red line is the spectrum recorded using the Herriott cell, with 100 m path length, and a nitrous oxide pressure of 203 mTorr. (b) An expanded part of the spectrum including lines (1) and (2). Herriott cell; (i) blue 0.8 mTorr, (ii) red, 20 m Torr; (iii) black, reference cell, 0.4 Torr.

Image of FIG. 5.
FIG. 5.

Variation with probe pulse detuning of the detector signal recorded with a pressure of 0.8 mTorr of nitrous oxide in the Herriott cell. The duration of the current pulse is 50 ns (175 to 225 ns). (a) The slow channel signal showing the start of the fast scan relative to the center of the absorption line: (i) black, −93 MHz; (ii) blue, −9 MHz, (iii) green, 31 MHz (iv) red, 62 MHz (b) The fast channel spectra, and the turn on and turn off of the 50 ns QC output pulse. (i) Black, red detuned, −93 MHz, the FID structure occurs on the turn-on and turn-off of the current pulse; (ii) blue, just below line center, −9 MHz, only the FID structure at turn on is seen clearly, blue detuned, (iii) green, 31 MHz and (iv) red, 62 MHz, very little structure is seen on either the rising or falling parts of the emitted pulses.

Image of FIG. 6.
FIG. 6.

Variation with probe pulse detuning of the detector signal recorded with a pressure of 0.8 mTorr of nitrous oxide in the Herriott cell. The duration of the current pulse is 100 ns (175 to 275 ns). (a) The slow channel signal showing the start of the fast chirp relative to the center of the absorption line. The starting positions of the fast chirps relative to the line center are red shifted by (i) black, −300 MHz, (ii) blue, −271 MHz, (iii) green, −118 MHz and (iv) red, −87 MHz. (b) The fast channel spectra and the turn on and turn off of the 100 ns QC laser output pulse. The start of the fast chirp is varied from far-red wavelength detuning, (i) black and (ii) blue, to red wavelength detuning, (iii) green and (iv) red. As the scan range of the frequency up-chirped probe pulse is now greater than the span of the Doppler-broadened line, FID signals are observed either at the turn-on or turn-off of the current pulse, and are superimposed on the chirped passage through the Doppler-broadened line during the current pulse. (c) An extended view of the fast channel spectrum to show the effects of optical nutation occurring on the chirped pulses which begin far red detuned, (i) black and (ii) blue, but not on chirps (iii) green and (iv) red, where the chirp terminates close to the absorption baseline.

Image of FIG. 7.
FIG. 7.

Variation with time of the slow scan (a) and fast chirp (b) through the R(9) line of the 14N14N18O isotopologue using a chirped pulse of 50 ns duration and a nitrous oxide pressure of 20 mTorr. (a) (i) Detuned by −112 MHz, (ii) detuned −78 MHz. (b) Both spectra show both leading edge and trailing edge FID oscillations, and nutation signals on a longer timescale.

Image of FIG. 8.
FIG. 8.

Variation with time of the slow scan (a) and fast chirp (b) through the R(9) line of the 14N14N18O isotopologue using a chirped pulse of 100 ns duration and a nitrous oxide pressure of 20 mTorr. (a) (i) Black, red detuned by −274 MHz, (ii) blue, red detuned by −19 MHz. (b) (i) Black, trailing edge FID oscillations only, large nutation at longer time. (ii) Blue, FID signal on the leading edge only, a small optical nutation signal is seen at longer time.

Image of FIG. 9.
FIG. 9.

Variation with time of the slow scan (a) and fast chirp (b) through the R(9) line of the 14N14N18O isotopologue using a chirped pulse of 100 ns duration and a nitrous oxide pressure of 200 mTorr. (a) (i) Black, red detuned, −290 MHz; (ii) blue, red detuned −260 MHz; (iii) red detuned −151 MHz. (b) Fast scans showing (i) black, and (ii) blue, trailing edge FID signals and (iii) red, leading edge FID signal and an interrupted trailing edge FID signal. (c) Fast scans showing (i) black, and (ii) blue, trailing edge FID signals and large (outer) nutation oscillations, and (iii) red, leading edge FID signal and a large nutation signal (inner), corresponding to the interrupted trailing edge FID signal.

Image of FIG. 10.
FIG. 10.

Variation with time of the slow scan (a) and fast chirp (b) through the P(34) line of 14N14N16O using a chirp time of 100 ns and a nitrous oxide pressure of 20 mTorr. (a) Red detuned (i) black, −253 MHz, (ii) blue, −242 MHz, (iii) green −211 MHz, (iv) red, −170 MHz. (b) Fast chirp spectra, range 150 to 400 ns. (c) Extended time span post chirp, 150-700 ns, to allow large optical nutation signals to be seen.

Image of FIG. 11.
FIG. 11.

Power dependence of the FID structure of the P(34) line of 14N14N16O using a chirp time of 100 ns and a nitrous oxide pressure of 20 mTorr. (a) (i) to (iv), the variation of the amplitude of the signal as the laser power is reduced using a linear polarizer. (b) A comparison of the highest power (i) and lowest power (iv) spectra, at maximum power reduction of a factor of 30 the oscillatory FID structure is almost completely damped.

Image of FIG. 12.
FIG. 12.

High nitrous oxide pressure, 200 mTorr. (a) The locations of the origin of the chirped scans relative to the line center for a series of experimental measurements using the 100 m path length astigmatic Herriott cell, (i) black, −500 MHz, (ii) blue, −453 MHz, (iii) green, −323 MHz; (iv) red, −215 MHz and (v) black, −193 MHz. (b) Reference cell spectrum, 0.4 Torr, 10 cm cell path length.

Image of FIG. 13.
FIG. 13.

The fast channel spectra corresponding to the chirp ranges and conditions shown in Figure 12. (a) Large red detuning (i) black, −500 MHz, (ii) blue, −453 MHz and (iii) green, −323 MHz. (b) Scan originating within the strongly absorbing region, (iv) red, −215 MHz and (v) black, 193 MHz.

Image of FIG. 14.
FIG. 14.

Chirp length dependence of the nitrous oxide spectra recorded using a gas pressure of 200 mTorr. Chirp lengths (i) 50 ns, (ii) 100 ns (iii) 150 ns, and (iv) 200 ns.

Image of FIG. 15.
FIG. 15.

Power dependence of the FID structure of the P(34) line of 14N14N16O using a chirp time of 150 ns and a nitrous oxide pressure of 200 mTorr. (a) (i) to (iv), the variation of the amplitude of the signal as the laser power is reduced using a linear polarizer. (b) A comparison of the highest power (i) and lowest power (iv) spectra. At maximum power reduction of a factor of 60 the FID structure is unchanged, whereas the nutation structure in the optically thick region is heavily damped.

Image of FIG. 16.
FIG. 16.

The calculated cumulative polarization of nitrous oxide using a path length of 100 m, and a detuning of three Doppler widths below line center. The pressure of N2O is 1 mTorr and of the damping gas, N2, 0.5 Torr. The fast scan range is 200 ns. F, FID, and R, rapid passage through an absorption line. (a) A comparison of FID only (i) Sf = Sr = 0, with rapid passage induced oscillations in the forward and backward directions (ii) Sf = 0.2, Sr = 0.1. (b) A comparison of FID only (i) Sf = Sr = 0, with rapid passage induced oscillations in the forward direction only (iii) Sf = 0.2, Sr = 0.0.

Image of FIG. 17.
FIG. 17.

The calculated cumulative population of nitrous oxide using a path length of 100 m, and a detuning of three Doppler widths below line center. F, FID and R, rapid passage. (a) A comparison of FID only (i) Sf = Sr = 0, with rapid passage induced oscillations in the forward and backward directions (ii) Sf = 0.2, Sr = 0.1. (b) A comparison of FID only (i) Sf = Sr = 0, with rapid passage induced oscillations in the forward direction only (iii) Sf = 0.2, Sr = 0.0.

Tables

Generic image for table
Table I.

The intensities and calculated integrated absorbance of the main nitrous oxide absorption lines observed in the wavenumber range from 1254.45 to 1254.65 cm−1, and with an absorption path length of 100 m.

Generic image for table
Table II.

Approximate times between collisions of nitrous oxide molecules based upon a self broadening parameter for the P(34) line of 8.64 MHz Torr−1 (see Ref. 26).

Generic image for table
Table III.

Parameters used in the rapid passage calculations.

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/content/aip/journal/jcp/136/17/10.1063/1.4710540
2012-05-07
2014-04-17
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Observation of infrared free-induction decay and optical nutation signals from nitrous oxide using a current modulated quantum cascade laser
http://aip.metastore.ingenta.com/content/aip/journal/jcp/136/17/10.1063/1.4710540
10.1063/1.4710540
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