Paper / Articles

Single-pulse coherently controlled nonlinear Raman spectroscopy and microscopy
N. Dudovich, D. Oron, Y. Silberberg
Nature, Vol. 418, p. 512 (2002).

Interferometric Fourier transform coherent antistokes Raman scattering
M. Cui, M. Joffre, J. Skodack, J. Ogilvie
Optics Express, Vol. 14, No. 18, p. 8448 (2006).

Observing time-dependent vibrational quantum dynamics in deuterium hydride molecular ions
J. McKenna, W.A. Bryan, C.R. Calvert, E.M.L. English, J. Woods, D.S. Murphy, I.C.E. Turcu, J.M. Smith, K.G. Ertel, O. Chekhlov, E.J. Divall, J.F. Mccann, W.R. Newell and I.D. Williams
Journal of Modern Optics Vol. 54, No. 7, p. 1127–1138 (2007).

Ultrafast optical excitations of metallic nanostructures: from light confinement to a novel electron source
C. Ropers, T. Elsaesser, G. Cerullo, M. Zavelani-Rossi, C. Lienau
New Journal of Physics 9, p. 397 (2007).

Standoff detection of trace amounts of solids by nonlinear Raman spectroscopy using shaped femtosecond pulses
O. Katz, A. Natan, S. Rosenwaks, Y. Silberberg
Appl. Phys. Lett, Vol. 92, p. 171116 (2008).

more Ultrafast Spectroscopy Papers

 

Ultrafast Spectroscopy


Optical Spectroscopy

Optical spectroscopy in general means to measure the wavelength dependent properties of light-matter interaction, the result being a “spectrum”. The power of spectroscopy is the fact that this interaction is dependant on the properties, type and state of the material under study. Therefore spectroscopy is often used to determine the material composition of a sample or sometimes even the internal state of that material. There are different types of spectroscopy, dependant on which particular aspect of light-matter interaction is exploited and in which wavelength (or photon energy) region of light the study is done. Spectroscopy can further be done with spatial resolution – it is then called spectral imaging. Another variant of spectroscopy is so-called “time-resolved” spectroscopy. This method allows to look e.g. at the molecular motions of a material and is often used in basic science to study chemical reactions.

As spectroscopy looks at the result depending on the wavelength of the light it either requires a wavelength tunable light source or a broadband light source. In the first case a spectrum is measured sequentially for each different wavelength while tuning the light source. In the second case it is possible to measure the full spectrum with one shot, provided the light source covers the required wavelength range. The advantage when using tunable light sources is that the detector can be rather simple because the wavelength is given by the light source and doesn’t have to be measured by the detector. The advantage when using broad band light sources is that the light source can be simpler and a spectrum can be acquired with higher speed. It is also possible to use simple detectors with broadband light sources by employing the Fourier-transform technique.

Standard optical spectroscopy is often done with Optical Parametric Amplifiers (OPA) as tunable source. These OPAs can generate pulses in the UV to far infrared range in the microjoule range when using the FEMTOPOWER™ compact™ PRO as pump source. At the same time this amplifier delivers < 30 fs pulses in the few hundred microjoule range which can be used as pump pulses for time-resolved studies. Further shortening of these pulses to < 7 fs is possible with the KALEIDOSCOPE™ our hollow fiber compressor which provides access to even faster processes (J. McKenna, 2007).

For solid state spectroscopy often a lower pulse energy in the nanojoule range is sufficient. For example the RAINBOW™ oscillator with < 7 fs pulse duration can be used for ultra broadband near-field optical spectroscopy (C. Ropers, 2007) as well as the SYNERGY™ and FUSION™ series with pulse durations in the < 10 fs to < 50 fs range.

The large bandwidth and short pulse duration of the RAINBOW™ and SYNERGY™ PRO models can be used for Coherent-Anti-Stokes-Raman Spectroscopy (CARS) or microscopy to the determine the molecular composition of microscopic samples or for diagnosis of high-temperature gases (N. Dudovich, 2002). In combination with the Fourier-transform technique (FT-CARS) very fast data acquisition at variable spectral resolution can obtained (M. Cui, 2006). A very recent result is the detection of trace amounts of solids from 10 meters distance using broad-band single-shot CARS (O. Katz, 2008).

To study phase transitions pulse energies > 100 nJ are required which are available with the chirped-pulse oscillator series FEMTOSOURCE™ scientific™ XL at 11 and 5 MHz repetition rates. Models with 100, 200 and 500 nJ at < 50 fs pulse duration are now available. Typical applications include time-resolved studies of melting, magnetization dynamics and metal-insulator transitions. Almost all our models can be synchronized to external references like other lasers or even synchrotrons and free electron lasers (FEL) with our FEMTOLOCK™ synchronization unit. In case of synchrotrons this makes the wavelength regions from the far infrared to the x-ray region accessible to femto- and picosecond time-resolved studies with lasers.
    



THz Spectroscopy

Terahertz spectroscopy with ultrashort pulse lasers

In the last decade THz spectroscopy has turned out to be one of the most powerful methods for studying properties of various materials from dielectrics or semiconductors to explosives or pharmaceutical products or several kinds of illegal narcotic/toxic substances. The particular advantage of THz radiation is that most common materials are transparent but at the same time the radiation does not harm biological tissues (unlike x-rays for instance). Interest in Terahertz technology from the biology, ultra-fast chemistry and health science communities has grown exponentially and new instrumentation and techniques have begun to make their way into many laboratories world-wide. The most striking advantage however is the fact that many complex molecules have unique spectral fingerprints in the Terahertz range. Broadband THz radiation sent through any object under consideration can be used for non-destructive and contact-less material identification, e.g. for security or quality monitoring applications. Therefore broad THz bandwidths are required to identify many chemicals with highest accuracies.

 

THz generation via photoconductive switch

Pulsed THz radiation is generated by ultrashort laser pulses impinging on a semiconductor material like GaAs thus creating very short coherent THz pulses from quickly induced electron-hole pairs. THz frequencies exceeding 5 THz are generated depending on the duration of the optical pulse. Advances in the design of biased electrodes to amplify THz radiation and novel technologies to minimize losses are extending the frequency range towards the 10 THz range. Advantages of these biased photoconductive switches are powerful THz generation and a high signal to noise contrast ratio over several orders of magnitude.

 

THz generation via optical rectification

In contrast to photoconductive switches optical rectification allows the generation of THz frequencies up to several tens of THz. By using ZnTe or GaSe crystals for instance THz radiation arises from a nonlinear process (called difference frequency generation) within the bandwidth of the laser pulse itself. Thus the bandwidth of the THz radiation scales proportionally with the bandwidth of the laser pulse. Very broad THz spectra can be generated with this technique: for example in their 2007 paper Optics Express Vol. 15 No 9, p. 5775, Zentgraf et al. report generation of THz pulses of up to 40 THz with a sub-10 fs FEMTOSOURCE™ synergy™ PRO oscillator and up to 130 THz with a FEMTOPOWER™ compact PRO amplifier followed by the KALEIDOSCOPE™ hollow fiber compressor using difference frequency generation in a LiIO3 crystal.

 

How to choose the right laser for your spectroscopic measurement?

Here are some important criteria you might have to consider before buying a femtosecond laser for your experiment. Please feel free to contact us for specific details.

Medium term stability: stable over periods of tens of seconds to tens of minutes.

Long term stability: will the laser have the same power and same beam direction if you turn it off for a week then turn it back on?

Optical noise: the primary noise source on THz experiments.

Output power: optical rectification needs 100s of mW in contrast to photoconductive switches, which require 10s of mW only.

Pulse width: determines the THz spectral bandwidth as well as the maximum of our THz spectrum. In general: the shorter the pulse duration, the higher your THz range.

Mode quality: this plays a role in focusing the beam tightly.

Center frequency: the THz source is typically GaAs or LT-GaAs which have bandgaps around 800 nm so Ti:Sapphire lasers are prefect driving sources for generation of THz. Tunability of the laser is not required for THz generation. The tuning of the THz frequency is rather performed by changing the phase-matching angle of the generation crystal.



  FEMTOSOURCE™
  scientific™ XL SYNERGY™ TWIN SYNERGY™
  100 200 300 500 650 20
PRO
20 PRO
Ultrafast Spectroscopy Synchrotron and FEL experiments
Solid state
Liquid
Gas phase                  
THz-Spectroscopy



  FEMTOSOURCE™
  INTEGRAL™ PRO INTEGRAL™ 20 INTEGRAL™ 50
  100
200
300
400
100
200
300
400
100
200
300
400
Ultrafast Spectroscopy Synchrotron and FEL experiments      
Solid state      
Liquid      
Gas phase      
THz-Spectroscopy



  FEMTOSOURCE™
  RAINBOW™ FUSION™
    CEP 20
BB
M1
PRO
Ultrafast Spectroscopy Synchrotron and FEL experiments  
Solid state
Liquid      
Gas phase            
THz-Spectroscopy      



  FEMTOPOWER™
  compact™
  V PRO
V PRO CEP
PRO CEP
PRO
PRO CEP HP/HR
PRO HE | 1.6 mJ
PRO HE | 2 mJ
PRO HE CEP | 1.6 mJ
PRO HE CEP | 2 mJ
PRO HP/HR
Ultrafast Spectroscopy Synchrotron and FEL experiments
Solid state
Liquid
Gas phase
THz-Spectroscopy



  Accessories
  FEMTOLOCK™ FEMTOMETER™
    kHz Detector
MHz Detector
Ultrafast Spectroscopy Synchrotron and FEL experiments
Solid state  
Liquid  
Gas phase  
THz-Spectroscopy