In this work we developed a setup consisting of an Optical Tweezers equipped with linear and non-linear micro-spectroscopy system to add the capabilities of manipulation and analysing captured objects. Our setup includes a homemade confocal spectrometer using a monochromator equipped with a liquid nitrogen cooled CCD. The spectroscopic laser system included a cw and a femtosecond Ti:sapphire lasers that allowed us to perform Raman, hyper-Raman, hyper-Rayleigh and two photon Excited (TPE) luminescence in particles trapped with an Nd:YAG cw laser. We obtained Raman spectra of a single trapped polystyrene microsphere and a single trapped red blood cell to evaluate the performance of our system. We also observed hyper-Rayleigh and hyper-Raman peaks for SrTiO3 with 60s integration time only. This was possible because the repetition rate of the femtosecond Ti:sapphire lasers, on the order of 80 MHz, are much higher than the few kHz typical picosecond laser repetition rate used before in hyper- Raman experiment, which required acquisition times of order of few hours. We used this system to perform scanning microscopy and to acquire TPE luminescence spectra of captured single stained microsphere and cells conjugated with quantum dots of CdS and CdTe and hyper-Rayleigh spectra of a noncaptured ZnSe microparticle. The results obtained show the potential presented by this system and fluorescent labels to perform spectroscopy in a living trapped microorganism in any neighbourhood and dynamically observe the chemical reactions changes in real time.
Up to now optical spectroscopies have analyzed the scattered light or the heat generated by absorption as a function of the wavelength to get information about the samples. Among the light matter interaction phenomena one that has almost never been used for spectroscopy is the direct photon momenta transfer. Probably because the forces involved are very small, varying from hundreds of femto to tens of pico Newtons. However, the nowadays very popular Optical Tweezers can easily accomplish the task to measure the photon momenta transfer and may be the basis for the Optical Force Spectroscopy. We demonstrate its potential as such a tool by observing more than eight Mie resonance peaks of a single polystyrene microsphere, and showed the capability to selective couple the light to either the TE, TM or both microsphere modes depending of the beam size, the light polarization and the beam positioning. The Mie resonances can change the optical force values by 30-50%. Our results also clearly show how the beam polarization breaks the usually assumed azimuthal symmetry by Optical Tweezers theories. We also obtained the spectrum from the two photon excited luminescence using the Optical Tweezers to hold a single bead suspended and a femtosecond Ti:sapphire laser for the non-linear excitation. This spectrum shows the pair of peaks due to both TE and TM spherical cavity modes. We have been able to observe more than 14 Mie resonance peaks in the TPE luminescence. Our results are in good agreement with optical force calculations using Maxwell stress tensor and partial wave decomposition of the incident beam approximated to a 3th order gaussian beam.
We developed a set up consisting of an Optical Tweezers plus linear and non-linear micro-spectroscopy system to add the capabilities of manipulation and analysing the captured object. For the confocal micro-spectrometer we used a 30 cm monochromator equipped with a cooled back illuminated CCD. The spectroscopic laser system included a cw and a femtosecond Ti:sapphire lasers that allowed us to perfom raman, hyper-raman, hyper-rayleigh and two-photon excited (TPE) luminescence in trapped particles with an Nd:YAG cw laser. With the cw Ti:sapphire laser we obtained raman spectra of a single trapped polystyrene microsphere and red blood cells and silicon to evaluate the performance of our system. The femtosecond Ti:sapphire laser was used to observed hyper-rayleigh and hyper-raman peaks of SrTiO3 with 60s integration time only. In the past, hyper-raman measurements required integration times of few hours, but the huge intensity together with the 80 MHz repetition rate of the femtosecond laser decreased this time for the seconds range. The sensitiveness of our system also permitted to observe more than 14 Mie resonance peaks in the TPE luminescence of a single stained trapped microsphere, which agrees well with the calculations. This system opens up the possibility to perform spectroscopy in a living trapped micro-organism in any desired neighbourhood and dynamically observe the chemical reactions and/or mechanical properties change in real time.