Attenuated total reflectance spectroscopy with chirped-pulse upconversion Hideto Shirai,1 Constance Duchesne,1,2 Yuji Furutani,3 and Takao Fuji1∗ 1 Laser
2 Current
Research Center for Molecular Science, Institute for Molecular Science, 38 Nishigonaka, Myodaiji, Okazaki, 444-8585, Japan
´ Address: Ecole Nationale Sup´erieure de Chimie de Paris (Chimie ParisTech), 11 rue Pierre et Marie Curie, 75005 Paris, France
3 Department of Life and Coordination-Complex Molecular Science, Institute for Molecular Science, 38 Nishigonaka, Myodaiji, Okazaki, 444-8585, Japan ∗ [email protected]
Abstract: Chirped-pulse upconversion technique has been applied to attenuated total reflectance (ATR) infrared spectroscopy. An extremely broadband infrared pulse was sent to an ATR diamond prism and the reflected pulse was converted to the visible by using four-wave mixing in krypton gas. Absorption spectra of liquids in the range from 200 to 5500 cm−1 were measured with a visible spectrometer on a single-shot basis. The system was applied to observe the dynamics of exchanging process of two solvents, water and acetone, which give clear vibrational spectral contrast. We observed that the exchange was finished within ∼10 ms. © 2014 Optical Society of America OCIS codes: (300.6340) Spectroscopy, infrared; (190.7220) Upconversion; (320.7110) Ultrafast nonlinear optics.
References and links 1. N. J. Harrick, Internal Reflection Spectroscopy (Wiley, 1967). 2. R. M. Nyquist, K. Ataka, and J. Heberle, “The molecular mechanism of membrane proteins probed by evanescent infrared waves,” ChemBioChem 5, 431–436 (2004). 3. P. R. Rich and M. Iwaki, “Methods to probe protein transitions with ATR infrared spectroscopy,” Mol. BioSyst. 3, 398–407 (2007). 4. Y. Furutani, T. Kimura, and K. Okamoto, “Development of a rapid buffer-exchange system for time-resolved ATR-FTIR spectroscopy with the step-scan mode,” BIOPHYSICS 9, 123–129 (2013). 5. R. A. Kaindl, M. Wurm, K. Reimann, P. Hamm, A. M. Weiner, and M. Woerner, “Generation, shaping, and characterization of intense femtosecond pulses tunable from 3 to 20 µm,” J. Opt. Soc. Am. B 17, 2086–2094 (2000). 6. V. Petrov, F. Rotermund, and F. Noack, “Generation of high-power femtosecond light pulses at 1 kHz in the mid-infrared spectral range between 3 and 12 µm by second-order nonlinear processes in optical crystals,” J. Opt. A-Pure and Appiled Optics 3, R1–R19 (2001). 7. G. Cerullo and S. D. Silvestri, “Ultrafast optical parametric amplifiers,” Rev. Sci. Instrum. 74, 1–18 (2003). 8. K. J. Kubarych, M. Joffre, A. Moore, N. Belabas, and D. M. Jonas, “Mid-infrared electric field characterization using a visible charge-coupled-device-based spectrometer,” Opt. Lett. 30, 1228–1230 (2005). 9. C. R. Baiz and K. J. Kubarych, “Ultrabroadband detection of a mid-IR continuum by chirped-pulse upconversion,” Opt. Lett. 36, 187–189 (2011). 10. J. Zhu, T. Mathes, A. D. Stahl, J. T. M. Kennis, and M. L. Groot, “Ultrafast mid-infrared spectroscopy by chirped pulse upconversion in 1800–1000cm−1 region,” Opt. Express 20, 10562–10571 (2012).
#225963 - $15.00 USD Received 31 Oct 2014; accepted 11 Nov 2014; published 19 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.029611 | OPTICS EXPRESS 29611
11. J. Knorr, P. Rudolf, and P. Nuernberger, “A comparative study on chirped-pulse upconversion and direct multichannel MCT detection,” Opt. Express 21, 30693–30706 (2013). 12. Y. Nomura, Y. T. Wang, T. Kozai, H. Shirai, A. Yabushita, C. W. Luo, S. Nakanishi, and T. Fuji, “Single-shot detection of mid-infrared spectra by chirped-pulse upconversion with four-wave difference frequency generation in gases,” Opt. Express 21, 18249–18254 (2013). 13. Y. Nomura, H. Shirai, K. Ishii, N. Tsurumachi, A. A. Voronin, A. M. Zheltikov, and T. Fuji, “Phase-stable subcycle mid-infrared conical emission from filamentation in gases,” Opt. Express 20, 24741–24747 (2012). 14. T. Fuji and Y. Nomura, “Generation of phase-stable sub-cycle mid-infrared pulses from filamentation in nitrogen,” Appl. Sci. 3, 122–138 (2013). 15. K. F. Lee, P. Nuernberger, A. Bonvalet, and M. Joffre, “Removing cross-phase modulation from midinfrared chirped-pulse upconversion spectra,” Opt. Express 17, 18738–18744 (2009). 16. A. A. Lanin, A. B. Fedotov, and A. M. Zheltikov, “Ultrabroadband XFROG of few-cycle mid-infrared pulses by four-wave mixing in a gas,” J. Opt. Soc. Am. B 31, 1901–1905 (2014). 17. J.-J. Max and C. Chapados, “Infrared spectroscopy of acetone–water liquid mixtures. I. factor analysis,” J. Chem. Phys. 119, 5632–5643 (2003). 18. J.-J. Max and C. Chapados, “Infrared spectroscopy of acetone–water liquid mixtures. II. molecular model,” J. Chem. Phys. 120, 6625–6641 (2004).
Infrared (IR) spectroscopy is used in various scientific and industrial fields. In particular, attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) is a powerful tool to study liquid or solid samples [1]. In a typical ATR-FTIR, an IR beam passes through a total reflectance prism in such a way that it reflects at least once off the internal surface in contact with the sample. This reflection forms the evanescent wave which penetrates into the sample. The penetration depth into the sample is typically between 0.5 and 2 µm. The beam is then introduced into a Fourier-transform infrared spectrometer. In this method, little or no sample preparation is required and samples can be set under various conditions, for example, high pressure, low temperature, etc. ATR-FTIR has been used to obtain fruitful information for understanding biological functions, such as energy conversion and signal transduction [2, 3]. Recently, a new time-resolved method for ATR-FTIR spectroscopy has been developed, which can be utilized for many biological systems just by exchanging a solution w/o a substrate perfused on the sample to initiate the reaction [4]. However, the time resolution of the Fourier-transform infrared spectroscopy (FTIR) with the rapid scan mode is limited by the speed of the FTIR device which needs time (at least several milliseconds) for scanning the delay of the interferometer. The step scan method could be possible to improve the time resolution to the response time of the detector (∼10 ns for a typical photovoltaic MCT detector), but it is only applicable for a system which can be triggered repeatedly (more than several hundreds). Chirped-pulse upconversion is a noteworthy method to improve the time resolution of IR spectroscopy. In this method, an infrared beam is upconverted into a visible beam and the spectrum is measured with a high performance visible dispersive spectrometer. Nowadays, optical parametric amplifiers which produce IR pulses with a duration of femtosecond regime [5–7] are commercially available. Such intense IR pulses are easily converted into visible pulses. So far, several varieties of IR spectroscopy with chirped-pulse upconversion were reported [8–11]. Recently, it becomes possible to obtain IR spectra which spread from 200 to 5500 cm−1 within 1 ms [12]. In this paper, we report attenuated total reflectance (ATR) spectroscopy connected with chirped-pulse upconversion. Single-shot IR absorption spectrum measurement of liquids from 200 to 5500 cm−1 has been realized. To demonstrate the benefit of the high-speed IR spectrum acquisition, we have observed the dynamics of acetone-water liquid exchange process with a rapid solution exchange system by measuring the change of the IR spectrum of the mixture. We observed the dynamic change of the infrared absorption bands in the millisecond time scale and estimated that the exchange was complete within ∼10 ms.
#225963 - $15.00 USD Received 31 Oct 2014; accepted 11 Nov 2014; published 19 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.029611 | OPTICS EXPRESS 29612
Ti:sapphire amplifer 0.85 mJ, 800 nm, 1 kHz
(a)
Chirped pulse
ZnSe Nitrogen
CM3
BBO DP DWP
CM4
ATR ITO
MH BF
ITO FWDFG
CM1
ω1+ω1-ω2→ωMIR
(b)
OP
CM2
Spectrometer
Kr FWDFG
ω1+ω1-ωMIR→ω2
aceton sample
(c)
ATR prism ATR prism water Fig. 1. (a) Schematic of the ATR system with chirped-pulse upconversion. BBO: β BaB2 O4 crystal (Type 1, θ = 29◦ , t = 100 µm), DP: delay plate (calcite crystal, t = 1.7 mm), DWP: dual wave plate (λ at 400 nm, λ /2 at 800 nm), CM1: r = 1 m concave dielectric mirror, CM2: r = 0.5 m concave mirror with a hole (φ = 7 mm), CM3, CM4: r = 1 m concave silver mirror, MH: aluminium mirror with a hole (φ = 7 mm), ITO: indium tin oxide coated plate, ATR: ATR top-plate (Golden Gate ATR, Specac), OP: off-axis parabola ( f = 50 mm), BF: blue filter. (b) Side and (c) top views of the ATR system with the solution-exchange system.
The schematic of the experimental setup is shown in Fig. 1(a). The light source is a Ti:sapphire multi-pass amplifier (790 nm, 30 fs, 0.85 mJ at 946 Hz, Femtopower compactPro, FEMTOLASERS). The beam was split into two with a beam splitter (15% reflection). The transmitted beam was used to generate an ultrabroadband IR pulse and the reflected beam was used as a chirped-pulse. The reflected pulse was stretched by passing through two zinc selenide (ZnSe) plates with the thickness of 10 mm with a Brewster angle, which corresponds to the group delay dispersion (GDD) of ∼22,000 fs2 . To avoid nonlinear effects in the crystal, we expanded the beam to the diameter of 15 mm. The pulse duration of the chirped-pulse was estimated as 4.9 ps by measuring the spectrally resolved cross-correlation between the chirped-pulse and the IR pulse [12]. The value of the instantaneous frequency was used for retrieving IR spectra from the upconverted spectra. The frequency resolution of the system is estimated as 4.2 cm−1 from the duration of the chirped-pulse. The IR pulse was generated by using four-wave mixing through two-color filamentation. The detail of the generation scheme is shown in the references [13, 14]. The generated pulse was reflected by two indium tin oxide coated substrates, which reflect the IR pulse and transmit the other visible beams. The IR pulse was gently focused into a diamond prism attached in the ‘ATR top-plate’ (Golden Gate ATR, Specac) by using a concave mirror (r=1.0 m). The prism is a right angle prism with the reflection surface size of 2 mm×2 mm. The spot size on the back surface of the prism is estimated as ∼ 0.5 mm. After the total reflection, the IR pulse was
#225963 - $15.00 USD Received 31 Oct 2014; accepted 11 Nov 2014; published 19 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.029611 | OPTICS EXPRESS 29613
0.6
6
0.4
4
0.2
2
0 −100
−50
0 Time (fs)
50
0 100
4
(b)
1
3
0.8
2
0.6
1
0.4
0
0.2
−1
0
0
2000 4000 Wavenumber (cm−1 )
−2 6000
Power (arb.units)
Fig. 2. The intensity (shaded curve) and phase (open squares) of the IR pulse after its reflection by the ATR diamond prism in (a) time and (b) frequency domain. 105 104 103 102 101 100
FTIR CPU 0
1000
Phase (rad/π)
8
Power (arb. units)
0.8
1.2
10
(a)
Phase (rad/π)
Intensity (arb. units)
1
2000 3000 Wavenumber (cm−1 )
4000
5000
Fig. 3. Measured IR spectrum with chirped-pulse upconversion (red) and conventional FTIR spectrometer (blue). The light source of the FTIR spectrometer is a halogen lamp.
combined with the chirped-pulse through a mirror with a hole. The combined beam was tightly focused into krypton gas with a parabolic mirror ( f =50 mm). The generated four-wave mixing signal was introduced into a spectrometer which consisted of an imaging spectrograph and an EMCCD camera (SP-2358 and ProEM+1600, Princeton Instruments). The whole path for the IR pulse was purged with nitrogen gas. One of the largest advantages of ATR spectroscopy over transmission IR spectroscopy in terms of the combination with chirped-pulse upconversion is that neither delay shift nor spatial displacement between the IR and chirped pulses exists by exchanging the samples. With transmission IR spectroscopy, it is not possible to avoid the delay shift between the IR and chirped pulses when the sample is exchanged. It causes systematic error of the frequency values. In addition, some chirp might be added to the IR pulse due to the dispersion of the sample at transmission IR spectroscopy. Here we discuss the effect of the absorption and dispersion of the diamond prism. In principle, as long as the dispersion of the crystal is small enough that the duration of the IR pulse is kept much shorter than the chirped-pulse, a simple algorithm can be used to retrieve IR spectra from the upconverted spectra [15]. We measured the duration of the IR pulse after the total reflection by using four-wave mixing XFROG [13, 14, 16]. The retrieved time and frequency domain pictures are shown in Fig. 2. The duration of the pulse was 25.7 fs, which was more than three times longer than that of the incident pulse, 7 fs. However, it is much shorter than that of the chirped-pulse (4.9 ps). The GDD of the diamond crystal was estimated as +51 fs2 at 3000 cm−1 for 1.4 mm of the propagation length. At first we measured an upconverted spectrum without any samples. The measured spectrum is shown in Fig. 3. Absorption of the diamond prism at ∼2000 cm−1 is clearly visible. In spite of the absorption, diamond prisms are commonly used for ATR-FTIR to take advantage of the transparency in the low frequency region,
2 Current
Research Center for Molecular Science, Institute for Molecular Science, 38 Nishigonaka, Myodaiji, Okazaki, 444-8585, Japan
´ Address: Ecole Nationale Sup´erieure de Chimie de Paris (Chimie ParisTech), 11 rue Pierre et Marie Curie, 75005 Paris, France
3 Department of Life and Coordination-Complex Molecular Science, Institute for Molecular Science, 38 Nishigonaka, Myodaiji, Okazaki, 444-8585, Japan ∗ [email protected]
Abstract: Chirped-pulse upconversion technique has been applied to attenuated total reflectance (ATR) infrared spectroscopy. An extremely broadband infrared pulse was sent to an ATR diamond prism and the reflected pulse was converted to the visible by using four-wave mixing in krypton gas. Absorption spectra of liquids in the range from 200 to 5500 cm−1 were measured with a visible spectrometer on a single-shot basis. The system was applied to observe the dynamics of exchanging process of two solvents, water and acetone, which give clear vibrational spectral contrast. We observed that the exchange was finished within ∼10 ms. © 2014 Optical Society of America OCIS codes: (300.6340) Spectroscopy, infrared; (190.7220) Upconversion; (320.7110) Ultrafast nonlinear optics.
References and links 1. N. J. Harrick, Internal Reflection Spectroscopy (Wiley, 1967). 2. R. M. Nyquist, K. Ataka, and J. Heberle, “The molecular mechanism of membrane proteins probed by evanescent infrared waves,” ChemBioChem 5, 431–436 (2004). 3. P. R. Rich and M. Iwaki, “Methods to probe protein transitions with ATR infrared spectroscopy,” Mol. BioSyst. 3, 398–407 (2007). 4. Y. Furutani, T. Kimura, and K. Okamoto, “Development of a rapid buffer-exchange system for time-resolved ATR-FTIR spectroscopy with the step-scan mode,” BIOPHYSICS 9, 123–129 (2013). 5. R. A. Kaindl, M. Wurm, K. Reimann, P. Hamm, A. M. Weiner, and M. Woerner, “Generation, shaping, and characterization of intense femtosecond pulses tunable from 3 to 20 µm,” J. Opt. Soc. Am. B 17, 2086–2094 (2000). 6. V. Petrov, F. Rotermund, and F. Noack, “Generation of high-power femtosecond light pulses at 1 kHz in the mid-infrared spectral range between 3 and 12 µm by second-order nonlinear processes in optical crystals,” J. Opt. A-Pure and Appiled Optics 3, R1–R19 (2001). 7. G. Cerullo and S. D. Silvestri, “Ultrafast optical parametric amplifiers,” Rev. Sci. Instrum. 74, 1–18 (2003). 8. K. J. Kubarych, M. Joffre, A. Moore, N. Belabas, and D. M. Jonas, “Mid-infrared electric field characterization using a visible charge-coupled-device-based spectrometer,” Opt. Lett. 30, 1228–1230 (2005). 9. C. R. Baiz and K. J. Kubarych, “Ultrabroadband detection of a mid-IR continuum by chirped-pulse upconversion,” Opt. Lett. 36, 187–189 (2011). 10. J. Zhu, T. Mathes, A. D. Stahl, J. T. M. Kennis, and M. L. Groot, “Ultrafast mid-infrared spectroscopy by chirped pulse upconversion in 1800–1000cm−1 region,” Opt. Express 20, 10562–10571 (2012).
#225963 - $15.00 USD Received 31 Oct 2014; accepted 11 Nov 2014; published 19 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.029611 | OPTICS EXPRESS 29611
11. J. Knorr, P. Rudolf, and P. Nuernberger, “A comparative study on chirped-pulse upconversion and direct multichannel MCT detection,” Opt. Express 21, 30693–30706 (2013). 12. Y. Nomura, Y. T. Wang, T. Kozai, H. Shirai, A. Yabushita, C. W. Luo, S. Nakanishi, and T. Fuji, “Single-shot detection of mid-infrared spectra by chirped-pulse upconversion with four-wave difference frequency generation in gases,” Opt. Express 21, 18249–18254 (2013). 13. Y. Nomura, H. Shirai, K. Ishii, N. Tsurumachi, A. A. Voronin, A. M. Zheltikov, and T. Fuji, “Phase-stable subcycle mid-infrared conical emission from filamentation in gases,” Opt. Express 20, 24741–24747 (2012). 14. T. Fuji and Y. Nomura, “Generation of phase-stable sub-cycle mid-infrared pulses from filamentation in nitrogen,” Appl. Sci. 3, 122–138 (2013). 15. K. F. Lee, P. Nuernberger, A. Bonvalet, and M. Joffre, “Removing cross-phase modulation from midinfrared chirped-pulse upconversion spectra,” Opt. Express 17, 18738–18744 (2009). 16. A. A. Lanin, A. B. Fedotov, and A. M. Zheltikov, “Ultrabroadband XFROG of few-cycle mid-infrared pulses by four-wave mixing in a gas,” J. Opt. Soc. Am. B 31, 1901–1905 (2014). 17. J.-J. Max and C. Chapados, “Infrared spectroscopy of acetone–water liquid mixtures. I. factor analysis,” J. Chem. Phys. 119, 5632–5643 (2003). 18. J.-J. Max and C. Chapados, “Infrared spectroscopy of acetone–water liquid mixtures. II. molecular model,” J. Chem. Phys. 120, 6625–6641 (2004).
Infrared (IR) spectroscopy is used in various scientific and industrial fields. In particular, attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) is a powerful tool to study liquid or solid samples [1]. In a typical ATR-FTIR, an IR beam passes through a total reflectance prism in such a way that it reflects at least once off the internal surface in contact with the sample. This reflection forms the evanescent wave which penetrates into the sample. The penetration depth into the sample is typically between 0.5 and 2 µm. The beam is then introduced into a Fourier-transform infrared spectrometer. In this method, little or no sample preparation is required and samples can be set under various conditions, for example, high pressure, low temperature, etc. ATR-FTIR has been used to obtain fruitful information for understanding biological functions, such as energy conversion and signal transduction [2, 3]. Recently, a new time-resolved method for ATR-FTIR spectroscopy has been developed, which can be utilized for many biological systems just by exchanging a solution w/o a substrate perfused on the sample to initiate the reaction [4]. However, the time resolution of the Fourier-transform infrared spectroscopy (FTIR) with the rapid scan mode is limited by the speed of the FTIR device which needs time (at least several milliseconds) for scanning the delay of the interferometer. The step scan method could be possible to improve the time resolution to the response time of the detector (∼10 ns for a typical photovoltaic MCT detector), but it is only applicable for a system which can be triggered repeatedly (more than several hundreds). Chirped-pulse upconversion is a noteworthy method to improve the time resolution of IR spectroscopy. In this method, an infrared beam is upconverted into a visible beam and the spectrum is measured with a high performance visible dispersive spectrometer. Nowadays, optical parametric amplifiers which produce IR pulses with a duration of femtosecond regime [5–7] are commercially available. Such intense IR pulses are easily converted into visible pulses. So far, several varieties of IR spectroscopy with chirped-pulse upconversion were reported [8–11]. Recently, it becomes possible to obtain IR spectra which spread from 200 to 5500 cm−1 within 1 ms [12]. In this paper, we report attenuated total reflectance (ATR) spectroscopy connected with chirped-pulse upconversion. Single-shot IR absorption spectrum measurement of liquids from 200 to 5500 cm−1 has been realized. To demonstrate the benefit of the high-speed IR spectrum acquisition, we have observed the dynamics of acetone-water liquid exchange process with a rapid solution exchange system by measuring the change of the IR spectrum of the mixture. We observed the dynamic change of the infrared absorption bands in the millisecond time scale and estimated that the exchange was complete within ∼10 ms.
#225963 - $15.00 USD Received 31 Oct 2014; accepted 11 Nov 2014; published 19 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.029611 | OPTICS EXPRESS 29612
Ti:sapphire amplifer 0.85 mJ, 800 nm, 1 kHz
(a)
Chirped pulse
ZnSe Nitrogen
CM3
BBO DP DWP
CM4
ATR ITO
MH BF
ITO FWDFG
CM1
ω1+ω1-ω2→ωMIR
(b)
OP
CM2
Spectrometer
Kr FWDFG
ω1+ω1-ωMIR→ω2
aceton sample
(c)
ATR prism ATR prism water Fig. 1. (a) Schematic of the ATR system with chirped-pulse upconversion. BBO: β BaB2 O4 crystal (Type 1, θ = 29◦ , t = 100 µm), DP: delay plate (calcite crystal, t = 1.7 mm), DWP: dual wave plate (λ at 400 nm, λ /2 at 800 nm), CM1: r = 1 m concave dielectric mirror, CM2: r = 0.5 m concave mirror with a hole (φ = 7 mm), CM3, CM4: r = 1 m concave silver mirror, MH: aluminium mirror with a hole (φ = 7 mm), ITO: indium tin oxide coated plate, ATR: ATR top-plate (Golden Gate ATR, Specac), OP: off-axis parabola ( f = 50 mm), BF: blue filter. (b) Side and (c) top views of the ATR system with the solution-exchange system.
The schematic of the experimental setup is shown in Fig. 1(a). The light source is a Ti:sapphire multi-pass amplifier (790 nm, 30 fs, 0.85 mJ at 946 Hz, Femtopower compactPro, FEMTOLASERS). The beam was split into two with a beam splitter (15% reflection). The transmitted beam was used to generate an ultrabroadband IR pulse and the reflected beam was used as a chirped-pulse. The reflected pulse was stretched by passing through two zinc selenide (ZnSe) plates with the thickness of 10 mm with a Brewster angle, which corresponds to the group delay dispersion (GDD) of ∼22,000 fs2 . To avoid nonlinear effects in the crystal, we expanded the beam to the diameter of 15 mm. The pulse duration of the chirped-pulse was estimated as 4.9 ps by measuring the spectrally resolved cross-correlation between the chirped-pulse and the IR pulse [12]. The value of the instantaneous frequency was used for retrieving IR spectra from the upconverted spectra. The frequency resolution of the system is estimated as 4.2 cm−1 from the duration of the chirped-pulse. The IR pulse was generated by using four-wave mixing through two-color filamentation. The detail of the generation scheme is shown in the references [13, 14]. The generated pulse was reflected by two indium tin oxide coated substrates, which reflect the IR pulse and transmit the other visible beams. The IR pulse was gently focused into a diamond prism attached in the ‘ATR top-plate’ (Golden Gate ATR, Specac) by using a concave mirror (r=1.0 m). The prism is a right angle prism with the reflection surface size of 2 mm×2 mm. The spot size on the back surface of the prism is estimated as ∼ 0.5 mm. After the total reflection, the IR pulse was
#225963 - $15.00 USD Received 31 Oct 2014; accepted 11 Nov 2014; published 19 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.029611 | OPTICS EXPRESS 29613
0.6
6
0.4
4
0.2
2
0 −100
−50
0 Time (fs)
50
0 100
4
(b)
1
3
0.8
2
0.6
1
0.4
0
0.2
−1
0
0
2000 4000 Wavenumber (cm−1 )
−2 6000
Power (arb.units)
Fig. 2. The intensity (shaded curve) and phase (open squares) of the IR pulse after its reflection by the ATR diamond prism in (a) time and (b) frequency domain. 105 104 103 102 101 100
FTIR CPU 0
1000
Phase (rad/π)
8
Power (arb. units)
0.8
1.2
10
(a)
Phase (rad/π)
Intensity (arb. units)
1
2000 3000 Wavenumber (cm−1 )
4000
5000
Fig. 3. Measured IR spectrum with chirped-pulse upconversion (red) and conventional FTIR spectrometer (blue). The light source of the FTIR spectrometer is a halogen lamp.
combined with the chirped-pulse through a mirror with a hole. The combined beam was tightly focused into krypton gas with a parabolic mirror ( f =50 mm). The generated four-wave mixing signal was introduced into a spectrometer which consisted of an imaging spectrograph and an EMCCD camera (SP-2358 and ProEM+1600, Princeton Instruments). The whole path for the IR pulse was purged with nitrogen gas. One of the largest advantages of ATR spectroscopy over transmission IR spectroscopy in terms of the combination with chirped-pulse upconversion is that neither delay shift nor spatial displacement between the IR and chirped pulses exists by exchanging the samples. With transmission IR spectroscopy, it is not possible to avoid the delay shift between the IR and chirped pulses when the sample is exchanged. It causes systematic error of the frequency values. In addition, some chirp might be added to the IR pulse due to the dispersion of the sample at transmission IR spectroscopy. Here we discuss the effect of the absorption and dispersion of the diamond prism. In principle, as long as the dispersion of the crystal is small enough that the duration of the IR pulse is kept much shorter than the chirped-pulse, a simple algorithm can be used to retrieve IR spectra from the upconverted spectra [15]. We measured the duration of the IR pulse after the total reflection by using four-wave mixing XFROG [13, 14, 16]. The retrieved time and frequency domain pictures are shown in Fig. 2. The duration of the pulse was 25.7 fs, which was more than three times longer than that of the incident pulse, 7 fs. However, it is much shorter than that of the chirped-pulse (4.9 ps). The GDD of the diamond crystal was estimated as +51 fs2 at 3000 cm−1 for 1.4 mm of the propagation length. At first we measured an upconverted spectrum without any samples. The measured spectrum is shown in Fig. 3. Absorption of the diamond prism at ∼2000 cm−1 is clearly visible. In spite of the absorption, diamond prisms are commonly used for ATR-FTIR to take advantage of the transparency in the low frequency region,
