The most critical element of an SFG spectrometer
is still the infrared laser that must provide the adequate high-peak power
and be widely tunable over the spectral range of the molecular vibrations.
The infrared OPO provides the SFG spectrometer with
- High SFG sensitivity is demonstrated by the figure of merit one order of magnitude above those achieved using conventional lasers.
- This high sensitivity is obtained for unchallenged resolution of 1.8 cm-1 over all the spectral range from 2.5 to 10 µm.
The performances of the OPO for SFG spectroscopy are illustrated by the following spectrum of a monolayer of NAT and TPN(of w-(4-nitroanilino)-dodecane thiol) self-assembled on silver or latinum, respectively. The accumulation time per point and the scan step were set to 2 second and 0.5 cm-1, respectively.
Thanks to its close collaboration with scientists, LaserSpec is able to manufacture an integrated SFG spectrometer.
As with any non-linear optical process, SFG is enhanced upon focusing the pump laser beams. The maximum non-linear response must be evaluated with respect to a maximum allowed temperature rise of the substrate. In the framework of the thermal response of metals upon sub-nanosecond laser pulse irradiation, we can predict that maximum SFG yield per time unit (SFGmax) obeys the scaling law :
where Pir and tir are the mean infrared power and infrared pulse duration, respectively. Several assumptions have been made in ordre to derive the eqn (1) : (A) the duration of both visible and infrared pulses are equal, (B) the pulse duration and pulse separation must be much smaller and much longer, respectively, than 1 ns, (C) the focusing is not limited by diffraction, and (D) the visible beam power is not limiting since the later beam can be produced at low cost.
Eqn(1) shows that pulse shortness enhances the nonlinear signal. This, however, occurs at the expense of spectral resolution. The figure of merit given by Eqn (1) must therefore be considered along with the infrared laser linewidth. The infrared OPO provides the SFG spectrometer with a figure of merit one order of magnitude above that achieved with conventional single-pulse laser system. This performance is achieved for a spectral resolution of 1.8 cm-1 which is still unchallenged for picosecond lasers. We note, however, that some lasers listed in Table 1 were designed to achieve high temporal resolution for time-resolved spectroscopy at the expense of the spectroscopic performances
|Spectral range||Technology and reference||pulse duration||Resolu-
|infrared power||Merit (Eq.1) (mW/ps)|
|~ 3 µm||OPO/LiNbO3†||11 ps||1.8 cm-1||250 mW||20|
|~3 µm||OPA/KTP 2||1.9 ps||18 cm-1||10 mW||5|
|~ 3 µm||OPO/AgGaS2†||11 ps||1.8 cm-1||40 mW||4|
|~3 µm||OPA/LiNbO3 3||20 ps||8 cm-1||2 mW||0.1|
|~3 µm||OPA/AgGaS2 4||20 ps||3-8 cm-1||1.5 mW||0.08|
|~3 µm||OPA/LiNbO3 5||30 ps||13 cm-1||2 mW||0.06|
|~3 µm||OPA/LiNbO3 6||20 ps||10 cm-1|
|~3 µm||Raman 7||3 ps||5 cm-1||0.2 mW||0.05|
|~3 µm||Raman 8||7 ns||0.2 cm-1||70 mW||0.01|
|~3 µm||Raman 9||6 ns||0.5 cm-1||20 mW||0.003|
|~3 µm||Raman 10||6 ns||0.2 cm-1|
|~ 5 µm||FEL CLIO 11||2 ps, tvis=70 ps||10 cm-1||150 mW||3*|
|~ 5 µm||OPO/AgGaS2†||11 ps||1.8 cm-1||30 mW||3|
|~ 5 µm||OPA/AgGaS2 12||6 ps||6 cm-1||2 mW||0.3|
|~ 5 µm||OPA/AgGaS2 13||20 ps||3-8 cm-1||1 mW||0.05|
|~ 5 µm||Raman 14||60 ps||1.5 cm-1||0.4 mW||0.007|
|~ 5 µm||Raman 8||7 ns||0.2 cm-1||50 mW||0.007|
|~ 9 µm||FEL Felix 15||7 ps||5 cm-1||150 mW/4||5*|
|~ 9 µm||FEL CLIO 11||2 ps, tvis=70 ps||10 cm-1||150 mW||3*|
|~ 9 µm||OPO/AgGaS2†||11 p||1.8 cm-1||4 mW||0.4|
|~ 9 µm||OPA/AgGaS2 12||6 ps||6 cm-1||0.15 mW||0.03|
|~ 9 µm||OPA/AgGaS2 13||20 ps||3-8 cm-1||0.1 mW||0.005|
|~ 20 µm||FEL Felix 15||7 ps||5 cm-1||150 mW/4||5*|
|~ 20 µm||FEL CLIO 11||2 ps, tvis=70 ps||10 cm-1||150 mW||3*|
* see ref. 1 for the generalisation of Eqn (1) when the visible and infrared pulse durations differ.
1. "Vibrational spectroscopy at interfaces by ir-vis sum-frequency generation using CLIO FEL.", A. Peremans et al., Nuclear Instruments and Methods A 375, pp. 657-661, 1996.
2. "High-power broadly tunable picosecond IR laser system for use in nonlinear spectroscopic applications.", D.E. Gragson, B.M. Mc. Carty, G.L. Richmond, and D.S. Alavi, J. Opt. Soc. Am. B, pp. 2075-2083, 1996.
3. "Surface vibrational spectroscopic studies of hydrogen bonding and hydrophobicity.", Q. Du, E. Freysz, Y.R. Shen, Science 254, pp. 826-828, 1994.
4. "Coherent picosecond pulse tunable from 0.41 to 12.9 µm.", H.-J. Kraruse and W. Daum, Appl. Phys. B 56, pp. 8-13, 1992.
5. "Infrared-visible sum frequency generation study of HCOOH on a MgO(001) surface.", K. Domen, N. Akamatsu, H. Yamamoto, A. Wada and C. Hirose, Surf. Sci. 283, pp. 468-472, 1993.
6. "Ethylene Hydrogenation on Pt(111) Monitored in Situ at High Pressures Using Sum Frequency Generation.", P.S. Cremer, X. Su, Y.R. Shen, and G.A. Somorjai, J. Am. Chem. Soc. 118, pp. 2942-2949, 1996.
7. "Vibrational energy relaxation of a polyatomic adsorbate on a metal surface: methyl thiolate (CH3S) on Ag(111).", A.L. Harris, L. Rothberg, L. Dhar, N.J. Levinos, L.H. Dubois, J. Chem. Phys. 94, pp. 2438-2448, 1991.
8. "In situ surface vibrational spectroscopy of the vapor/solid and liquid/solid interfaces of acetonitrile on ZrO2.", S.R. Hatch, R.S. Polizzotti, S. Dougzl, and P. Rabinowitz, J. Vac. Sci. Technol. A 11, pp. 2232-2238, 1993.
9. "Monolayer vibrational spectroscopy by infrared-visible sum generation at metal and semiconductor surfaces.", A.L. Harris, C.E.D. Chidsey, N.J. Levinos, and D.N. Loiacono, Chem. Phys. Lett. 141 (4), pp 350-356, 1987.
10. "Surface Vibrational Spectroscopy of Organic Counterions Bound to a surfactant Mononlayer.", D.C. Duffy, P.B. Davies, and C.D. Bain, J. Phys. Chem. 99, pp. 15241-15246, 1995.
11. "Vibrational Spectroscopy of Electrochemically deposited hydrogen on platinum.", A. Peremans and A. Tadjeddine, Phys. Rev. Lett. 73, pp. 3010-3013, 1994.
12. "Vibrational dynamics of the Si-H stretching modes of the Si(100)/H:2x1 surface.", P. Guyot-Sionnest, P.H. Lin, and E.M. Miller, J. Chem. Phys. 102 (10), pp. 4269-4278, 1995.
13. "Spectroscopic investigations of adsorbates at the metal-electrolyte interface using sum-frequency generation.", P. Guyot-Sionnest and A. Tadjeddine, Chem. Phys. Lett. 172, pp. 341-345, 1990.
14. "Vibrational energy transfer on hydrogen-terminated vicinal Si(111) surfaces: interadsorbate energy flow.", M. Morin, P. Jakob, N.J. Levinos, Y.J. Chabal, and A.L. Harris, J. Chem. Phys. 96, pp. 6203-6212, 1992.
15. "Self dispersive sum-frequency generation at interfaces", E.W.M. van der Ham, Q.H.F. Vrehen, and E.R. Eliel, Optics Letters 21, pp. 1448, 1996.
Vibrational spectroscopy of interfaces by sum-frequency generation.
The vanishing of the second-order non-linear polarizability in centrosymmetric media means that Sum-Frequency Generation (SFG) is forbidden in the bulk of the majority of the materials such as gas, liquid, amorphous solid, and most of the isotropic crystals. As first demonstrated in 1987, this property can be exploited to selectively probe the interfaces where bulk symmetry is broken .
FIG. 1. Principle of surface SFG.
As represented in Fig. 1, SFG requires two laser beams, one tunable in the infrared (wir) and one at a fixed visible frequency (wvis). Both beams are mixed at the interface to be analyzed. The second order susceptibility of the few monolayers thick interfacial region causes a coherent beam to be generated at the sum frequency (wSF= wvis + wir). The SFG process can be regarded as the diffraction of the visible beam on a modulation of the interface polarizability induced by the infrared beam. The SFG yield is enhanced when the infrared beam couples to a Raman active vibration. The interface vibrational signature is readily obtained by scanning the infrared beam frequency.
The following specificities of SFG spetroscopy make it complementary or advantageous to the conventional linear vibrational spectroscopies of Electron energy loss, Helium atom scattering, Infrared absorption and Raman scattering.
The SFG technique is very versatile and can be used in a wide variety of environments (UHV, high pressure gas, liquid ...). It is interface-specific in the majority of cases where the media are centrosymmetric and provide absolute vibrational spectra (no need for a reference spectrum) with high resolution. The performances of the SFG technique have now been demonstrated for various interfaces formed by molecules adsorbed on metals in UHV , in ambient atmosphere or in liquids , chemical layer on semiconductors in UHV  or in ambient atmosphere, molecular layers on insulator in ambient atmosphere ) or in liquid .
SFG has spectacularly opened up new investigation fields in surface science by enlarging the applicability of vibrational spectroscopy to other types of interfaces which cannot be probed by any of the linear techniques: gas/liquid interfaces  (Fig. 2), molecules adsorbed on a metal in a high pressure environment  (Fig. 3), electrochemical interface at overpotential  (Fig. 4).
The specificities of SFG, which probes both the dynamic dipole moment and the Raman polarisabilty tensor associated with the interfacial vibrations, makes this technique particularly complementary to linear vibrational spectroscopies, to study adlayer properties such as bidimensional anisotropy  or Raman activity of vibrations at ordered surface . The temporal resolution intrinsic to using short laser pulses makes SFG particularly suitable for the study of the interface vibrational dynamics [2, 11] or for the investigation of the surface reaction kinetics with millisecond to picosecond time-resolution unachievable with linear techniques.
All the above pioneering SFG works were performed after surmounting over the first challenging step of developing the infrared laser. Owing to the continuing advances in laser technology, infrared lasers that are broadly tunable in the mid-infrared are now becoming available commercially. This is expected to allow further expansion of surface vibrational spectroscopy by non-linear optical techniques.
1."Surface properties probed by second-harmonic and sum-frequency generation.", Y.R. Shen, Nature 337, 519 (1989), and references therein.
2."Vibrational energy relaxation of a polyatomic adsorbate on a metal surface: methyl thiolate (CH3S) on Ag(111).", A.L. Harris, L. Rothberg, L. Dhar, N.J. Levinos, L.H. Dubois, J. Chem. Phys. 94, 2438 (1991).
3."Spectroscopic investigations of adsorbates at the metal-electrolyte interface using sum-frequency generation.", P. Guyot-Sionnest and A. Tadjeddine, Chem. Phys. Lett. 172 (5), 341 (1990).
4."Lifetime of an adsorbate-Substrate Vibration: H on Si(1111).", P. Guyot-Sionnest, P. Dumas, Y.J. Chabal and G.S. Higashi, Phys. Rev. lett. 64, 2156 (1990).
5."In situ surface vibrational spectroscopy of the vapor/solid and liquid/solid interfaces of acetonitrile on ZrO2.", S.R. Hatch, R.S. Polizzotti, S. Dougzl, and P. Rabinowitz, J. Vac. Sci. Technol. A 11 (4), 2232 (1993).
6."Surface vibrational spectroscopic studies of hydrogen bonding and hydrophobicity.", Q. Du, E. Freysz, Y.R. Shen, Science 254, 826 (1994).
7."Pressure dependence (10-10-700 Torr) of the vibrational spectra of adsorbed CO on Pt(111) studied by SFG.", X.C. Su, Y.R. Shen, and A. Somorjai, Phys. Rev. Lett.77, 3878 (1996).
8."Vibrational Spectroscopy of Electrochemically deposited hydrogen on platinum.", A. Peremans and A. Tadjeddine, Phys. Rev. Lett. 73 (22), 3010 (1994)
9."Interfacial Atomic Structure of a Self-Assembled Alkyl Thiol Monolayer/Au(111): A Sum-Frequency Generation Study.", M.S. Yeganeh, S.M. Dougal, R.S. Polizzotti, and P. Rabinowitch, Phys. Rev. Lett. 74 (10), 1811 (1995).
10."Dynamical charge transfer at an Interface: K doping of C60/Ag(111).", A. Peremans, Y. Caudano, P.A. Thiry, P. Dumas, W.Q. Zheng, A. Le Rille, and A. Tadjeddine,, Physical Review Letters 78 (15), 2999 (1997).
11."Photon echo Si-H.", P. Guyot-Sionnest, Phys. Rev. Lett. 66, 1489 (1991).