Molecular Holography
In stimulated or coherent Raman spectroscopy, two optical fields coherently drive a vibrational mode while one of these fields (SRS), or a third one (CARS), probes this coherent molecular vibration. The third-order non-linear susceptibility, which enables the process, has resonances at the vibrational frequencies. Therefore, by tuning the frequency differences between the lasers, on can probe the vibrational spectrum of a molecule. When the frequency difference matches a molecular vibration, the intensity of the pump field experiences a loss (Stimulated Raman Loss – SRL) while the probe field experiences a gain (Stimulated Raman Gain – SRG) because for each probe (Stokes) photon created by stimulated emission, a corresponding pump photon is annihilated. The gain and loss are usually many orders of magnitude less than the overall intensity (1 in 10^6-10^7) so lock-in techniques are required to extract the small signal. For co-linear pump and probe beams propagating through an isotropic Raman active sample the SRG/SRL is proportional to the third-order susceptibility, path length and incident beam intensity.
Note that in incoherent probing, spectroscopic signals linearly depend on the number of molecules, while coherent Raman signals show a quadratic dependence. SE-SRS benefits from eight order of magnitude increase in the enhancement factor.
To probe such non-linear effects one normally employs picosecond or femtosecond pulses to obtain the high peak powers (~ 10 MW/cm2) needed to excite third order processes. Typically, however, the intensity fluctuations in SERS would preclude its use in such an experiment because these fluctuations would swamp the small changes in SRS intensity at resonance. However, we are able to overcome this difficulty because of the unique stability of scattering from the Oxonica nanoparticles
To establish the field of molecular holography, we must achieve two main objectives: (1) demonstrate pulsed and continuous wave (CW) Stimulated Raman spectroscopy using highly scattering nanostructured surfaces (e.g. metal nanoparticles) and (2) demonstrate that the coherent SE-SRS emission from molecules can be used to create a molecular hologram. We have already demonstrated two subsets of each objective – (1a) SE-SRS using pulsed lasers and (2a) conventional holography of metal nanoparticles. In the long term (>5 yrs.) we hope to establish a label free approach to molecular holography.
We have successfully completed two proof-of-principle experiments that demonstrate: (i) coherent light emission can be generated from specialized nanoparticles by stimulated Raman scattering emission at chosen molecular vibrational frequencies using low laser power pulsed and CW and (ii) a hologram of gold nanoparticles can be created. We were the first group in the world to demonstrate CW Stimulated SERS as reported in August at the SERS Faraday Discussions conference (C. L. D. Lee and K. C. Hewitt, SERS Faraday Discussion – in press (2017)). Thus the realization of Molecular holography using CW sources appears very likely, since the most recent experiments demonstrate one can simply switch between parallel crossed polarization states of the pump and probe beams to turn on and off stimulated emission (see attached).
A molecular holography approach to imaging would compete primarily with fluorescence imaging techniques. Molecular holography would offer two major advantages. Raman linewidths are an order of magnitude, or more, smaller than fluorescence linewidths. Within the same spectral bandpass, therefore, it becomes possible to identify hundreds of spectrally unique particles instead of the few now possible with fluorescence imaging techniques. In addition, holography becomes possible because of the coherent nature of the SRS signal, allowing researchers to track these particles in 3D, within mm^3 volumes, at video capture rates.