Femtosecond Stimulated Raman Spectroscopy

FSRS is an ultrafast nonlinear optical technique that enables acquisition of the ground and time resolved excited state Raman spectra of a sample, giving vibrational --- and consequently structural --- information.  The technique is performed in the near collinear "Pump-Probe" geometry by time-ordering three pulses.  A narrowband picosecond duration Raman pump pulse causes Raman scattering in the sample of interest at the same time as a coincident broadband femtosecond white light supercontinuum pulse that effectively amplifies the Raman shifted light to increase the signal.  Finally an optional electronically resonant pulse, the "actinic" pulse, initiates a photophysical or photochemical process in the sample of interest at a variable time before the Raman pulses.  

FSRS is experimentally very similar to transient absorption spectroscopy (TAS) with a  supercontinuum probe, but with the advantage of high time- and spectral-  resolution vibrational spectroscopy.  This allows for additional tools in interpreting and assigning molecular dynamics and detection of "dark" molecular states.  Even without time resolution, FSRS has the advantage that molecules which are very difficult to study using spontaneous Raman spectroscopy become easier to study the gain of the stimulated Raman process and relative insensitivity to fluorescence. 

The actinic pump is short in duration and is resonant with an electronic transition in the sample of interest.  It instigates a photochemical change in the sample.  At a later time, the sample is probed via a femtosecond white light supercontinuum probe pulse, which is coincident with some part of the Raman pump pulse, which conveys the high spectral resolution to the measurement.  The Raman pump wavelength can, in some sense, be picked arbitrarily, however in practice, often resonance enhancement is required to increase the signal of the sample over the solvent, for example.  Another concern is the spectral coverage of the probe and the relative probe intensity or sample absorption at the wavelengths of interest. 

Flexible and Upgradable System

FSRS is designed with flexibility in mind.  The basic system comes with the essential components required for ultrafast spectroscopy; however, almost every system that is shipped is specially built and tailored to meet the customers’ needs.  The system can perform femtosecond stimulated Raman and transient absorption spectroscopies, and can be upgraded to do two-dimensional visible or NIR spectroscopy as well.  Some things that are frequently customized or upgraded are:

  1. Addition of ultraviolet probe (CaF2 crystal) and motion system
  2. Addition of near infrared probe (YAG or sapphire crystal) and InGaAs camera
  3. Addition of internal second harmonic generation for actinic pump
  4. Gratings to support additional spectral ranges or resolutions
  5. Pump reference channel
  6. Sample – moving stage or thin flow cell with pump
  7. Additional polarization optics for pump and probe
  8. Automated pump power control – shutter or motorized filter wheel
  9. Automated system alignment/realignment using patented GuideStar™ technology
  10. Dual optical chopper for actinic and Raman pumps

Newport scientists and engineers are always working to support new additions and types of pump-probe spectroscopy, so, if you have an idea that you don’t see listed, feel free to inquire.

Most features are offered as upgrades and can be added in the future as they are required, or as budget allows.

A grating and slit based filter is used to create a tunable narrow-band Raman pump pulse with adjustable bandwidth from the laser fundamental or OPA, allowing <10 wavenumber resolution.  This method of generating the Raman pump is very flexible and can support many spectra ranges, however a relatively powerful input pulse is required.  Consequently, there are also several other options for generating the Raman pump.  Please contact Newport for more details.

High Quality Components and Worldwide Support

Newport’s Femtosecond Stimulated Raman Spectrometer is assembled and thoroughly tested and calibrated by a dedicated team of scientists and engineers.  FSRS is constructed from Newport’s highest quality components, for stable performance as well as increased durability and lifetime.  Some examples are: 

The use of high quality components improves the resistance to vibration induced by the moving components, such as chopper, motion stage, and sample stirrer or recirculator, as well as reduced drift due to thermal changes. 

More importantly, support and training is provided either locally or remotely, before, during and after installation.  FSRS features free software upgrades for life, with support for new, customer inspired, features as they are added.  Source code is available upon request. 

Each system includes installation and training by a Newport scientist or engineer and includes high quality beam routing for steering the ultrafast amplified laser beams to the FSRS system.

State of the Art Probe Generation and Referenced Detection

The probe can be generated between 320 to 1600 nm using a combination of calcium fluoride (CaF2), sapphire, and yttrium aluminum garnet (YAG) crystals.  It is collected and refocused on the sample using off-axis parabolic mirrors in order to obtain a small and uniform spot size free from chromatic aberration.  The beam is split before the sample for the purpose of shot-to-shot probe referencing.  After the spectrograph, the light is detected by a dual-sensor camera, which features two CMOS arrays, which have the ideal combination of sensitivity and dynamic range.   

The MS260i spectrograph is a hallmark of the flexibility of the FSRS system.  Due to the likelihood of utilizing different Raman pump wavelengths for different samples, wavelength and resolution agility is required.  When integrated with the FSRS software, the MS260i supports automatic or computer controlled adjustment of spectral range, resolution, and camera or other light sensor.  This is due to the MS260i’s support for up to two output ports and three gratings.  Since the MS260i is an imaging spectrograph, the probe and a reference can take virtually the same optical path and be imaged on two separate image sensors.  This improves the stability and repeatability of the measurements, especially in the case of short integration times with photosensitive samples, or in the case of an unstable laser.  Additionally, the MS260i is an excellent spectrograph and is an essential laboratory tool that can be used for many experiments besides FSRS.  With the full set of gratings offered, the instrument can function from 200 nm to 20 um, with spectral resolution ranging from 0.13 to 3.95 nm.  Even more Richardson gratings can be ordered directly from www.gratinglab.com.

Optimized Delay Stage for Spectroscopy

Example layout for standard FSRS with tunable 800 nm Raman pump

The FSRS benefits from the newly released Delay Line Stages, which are ideal for ultrafast spectroscopy due to the fast speed, acceleration, and repeatable positioning.  The standard maximum optical delay between pump and probe in FSRS is 4.3 ns.  This is limited by the length of the stage (325 mm) and the number of optical passes that the probe beam makes to and from the high quality retroreflector used on the stage.  In standard configurations, there are four passes (to and from the retroreflector twice).  

The resolution changes depending on the mode of operation.  In the best case, the resolution is set by the minimal incremental motion (MIM) of the delay stage.  This is applicable when performing a single measurement where the stage is incremented linearly in only one direction.  In the standard FSRS configuration, this is ±1.0 fs.  If many scans are to be averaged, the repeatability of positioning must be considered.  If performing the same measurement, the uni-directional repeatability must be accounted for, which is 1 fs in the standard FSRS configuration.  Lastly, if the set of time points is acquired randomly, as is often the case to minimize the effect of laser and sample changes to the measured dynamics, the bi-directional repeatability is the most important factor.  For the standard FSRS, this has a value of ±2 fs.  In a fast experiment, delay stage settling time is also a consideration, and the positioning may suffer if the settling time is low.

Since the time resolution is set by the convolution of the pump and probe duration -- with some small consideration for crossing angle -- the motion stage is more than sufficient for a pump-probe experiment when pulse widths are greater than 20 fs.

Multiple Pump Options

Many samples require a pump with wavelength different from that of the fundamental wavelength of the ultrafast laser.  For this reason, FSRS is often sold with one or more external optical parametric amplifiers (OPA) or harmonics generators.  FSRS is also offered with an addition of an internal second harmonic generation (SHG) option for the actinic pump beam.

The energy of the pump beam can be controlled internally by using a variable reflective neutral density filter imprinted on a thin glass substrate.  This reduces the GVD introduced to the beam while allowing power control over a range of 0-4 OD.

In order to reduce the effect of laser fluctuations, a FSRS measurement is most typically performed at half the repetition rate of the laser using a New Focus phase locked optical chopper to block every other Raman pump pulse.  The spectrum of the transmitted white light supercontinuum probe is compared to the spectrum when the pump is present at half the repetition rate of the laser, the fastest rate possible, in order to minimize the effect of any laser noise and fluctuations on the end measurement.  When switching to transient absorption measurements, or to see the effect of the actinic pump on the measurement, the chopper can be instead positioned in the actinic pump beam.  This can be achieved by a simple rotation of the chopper about its axis.  Alternatively, an additional chopper can be added and synchronized to ¼ the laser rate for acquisition of all signals at high repetition rate. 

The Raman pump pulse is generated from the fundamental in the default arrangement, but can be generated from an external source and directly coupled to the spectrometer.  External sources might include filters, etalons, narrow bandwidth OPAs, NOPAs, or second harmonic crystals. 

Alternatively, a tunable Raman pump can be filtered from an OPA input if the energy is high enough.  As a frame of reference, we estimate that to obtain 10 cm-1 resolution with 100 fs input pulses greater than 100 μJ input energy is required.  This may vary depending upon the sample and wavelength.  At shorter pulse durations, more energy is required.

Easy-to-use Software and Data Analysis

Data collection in the FSRS software is very easy due to automation of many crucial components, including control of multiple motion stages, cameras, optical chopper and spectrograph.  This allows for time-overlap of the pulses, automated calibration of the FSRS spectrum in wavenumbers, and systematic noise removal.  During experiment setup, the Raman gain signal and total camera signal can be monitored live for optimization of the pump and probe overlap and signal levels as well as coupling to the spectrograph.  During acquisition, the integration time can be increased in order to find small signals.  The actinic pump delay with respect to the timing of the Raman pump and probe can be increased in an incremental manner or randomly based on a predefined set of desired delay locations.  During the scan, you can see live display of the three dimensional data set as well as cross section views of the kinetics and spectrum.  After data acquisition, the data can be reloaded for viewing and data processing in the FSRS software.  The data is easily saved in custom format for loading into any other software package, such as Glotaran™, Matlab™, Python, or Origin™. 

Signals in femtosecond stimulated Raman spectroscopy can be quite weak, and can be very easily obscured by the features of the solvent or the ground state spectrum of the species of interest.  Due to these reasons, data processing is very important.  In FSRS, we've simplified the process of data processing and allow for the user to remove the solvent spectrum, transient absorption baseline, and ground state spectrum, as well as correction for chirp of the probe pulse.