1. Could you explain the significance of your article to the non-specialist? 

Measurement of molecular details of the interfaces, films and membranes is crucial for understandings and technology developments. For example, direct measurement on how surface water molecule sits and moves is one of the most challenging problems, and such knowledge is key for understandings on how life started and works, and how the climate and the environment on Earth evolve.

In this work, recent developments on one of the most unique and powerful experimental tools are reviewed on a unified ground. This ensures many further applications in interface and film studies, including solutions for the surface water problem. 

2. What has motivated you to conduct this work? 

When I entered the field of interface studies with nonlinear optical techniques, namely, the Second Harmonic Generation (SHG) and the Sum-Frequency Generation Vibrational Spectroscopy (SFG-VS) techniques, in the early 1990s, their unique surface specificity and sensitivity have been well demonstrated by the pioneers, such as Ron Shen and Ken Eisenthal (my Ph.D. advisor) etc. in the field, and the promises of employing SHG and SFG-VS for molecular interface and film studies was very high. However, most of the promises have not been delivered as expected, because some of the key issues in quantitative analysis of the SHG and SFG-VS data have not been solved.

People started questioning the value of these techniques at the turn of the century. (For example, Phys. Rev. B, 61, 13283, 2000.) When I started building my independent research program in 1999, my students and I were trying to measure the details of phase transitions in the Langmuir films using SHG. The idea to study Langmuir as well as LB films with SHG was not new, but very detailed study had not been achieved. From well designed experimental apparatus, we observed strong polarization dependence of the SHG signal from the Langmuir monolayer, while these data were not easy to be interpreted with the existing formulations.

In order to quantitatively interpret such polarization dependence, we developed some novel ideas and formulated the SHG and SFG-VS problem with the concept of orientational functional.

The key in this development is to unify the polarization analysis of the SHG data, the tensorial and symmetry analysis of the molecular polarizability tensor elements, and the orientational analysis of the molecular at the interfaces of in the films into the concept of the orientational functional.

These developments led us to re-evaluate and to solve the key problems which had limited the development of this field. In these developments we relied on the fundamental treatments of SHG and SFG-VS by Professor Ron Shen, Tony Heinz, Dr. Xing Wei et al, and some recent development on orientational analysis by Professor G. J. Simpson et al. 

3. Where do you see this work developing in the future? 

A lot of possibilities have been opened up with the ideas and tools for quantitative analysis with SHG and SFG-VS for molecular interface and film studies. Basically we believe that the practice, including designing of experiments and performing data interpretation, in this field shall be put on a relatively more solid theoretical foundation, and application of these techniques shall bring a lot of new information on the details of structure, dynamics and interaction in the molecular interfaces and films. The following directions shall be among the most significantly influenced. 

a. Characterization of molecular interface and films. A lot of measurements in the past decade were either incomplete, or the interpretations were flawed. Now new measurements and interpretations in a relatively unified manner shall change the practices and warrant a lot more new discoveries. Better use of these techniques shall have significant impact to the material and membrane studies. 

b. Molecular properties and interactions can be studied in detail by studying molecules in the two-dimension. Understanding the detail of interactions between oriented molecules is crucial for both fundamental and practical purposes. Now such studies are possible and one can consciously perform such studies for simple and complex interfaces and films. Quantitative measurement of model molecular systems under different conditions can help construction of interaction potentials and force fields for prediction of structures and dynamic behaviors of molecular films and biological membranes, as well as protein folding. 

c. Electronic properties of molecules can be systematically studied by studying the aligned molecules at the interfaces. Molecule-substrate interactions can also be systematically studied in detail. 

d. Quantitative measurement of molecules at the electrochemically and catalytically important semiconductor and metal surfaces can be similarly developed. Through symmetry analysis and polarization measurement, well separation of the contribution from the substrate and the molecular adsorbates can be achieved. This shall enable detailed measurement of these surfaces, and shall bring new discoveries at the molecular level. 

e. SHG Imaging and micro region surface analysis can become powerful competing technique for fluorescence and Raman methods. The intrinsically coherent nature of the SHG technique ensures many advantages over the incoherent fluorescence and Raman techniques, such as image contrast, orientational analysis etc. A recent example is by Professor Eisenthal and collaborators for measurement of electric potential response of neuron cells (Imaging membrane potential in dendritic spines, PNAS-USA, 103, 786, 2005.). 

f. Many fields of photonics studies with films and nonlinear optic , either material or biological, shall see interesting developments and applications with these ideas and tools. 

4. Are there any particular challenges facing future research in this area? 

The major challenge in application of SHG and SFG-VS for interface and film studies has always been to understand the usefulness and limitation of these techniques. This requires both methodologies with explicit concepts and ideas, and well-trained people.

Our unified treatment of the problem provides new tools and concepts, and it has bridged the major gaps in this field of studies. It is much easier now for the chemists and other researchers to understand and to apply SHG and SFG-VS for the particular problems they are interested in.

Since this field of studies requires knowledge and skills from many disciplines, such as optics, laser spectroscopy, quantum mechanics, as well as knowledge on particular subjects and problems, to integrate effectively the trainings of students and researchers in these interdisciplinary subjects remains to be the primary challenge.