Ultrafast Spectroscopies

Ultrafast spectroscopy encompasses many different techniques with the common denominator of using femtosecond (fs) or – less commonly – picosecond (ps) lasers. In general femtosecond pulses are used whenever the highest temporal resolution is required, while picosecond pulses are preferred when the sample excitation requires higher spectral resolution. For example, a 35 fs pulse from a Titanium Sapphire laser provides excellent temporal resolution and has a bandwidth of about 30 nm, while a 1 ps pulse has a bandwidth of just about 1 nm, enabling improved spectral resolution, when required by the experiment and sample.

The simplest form of ultrafast spectroscopy is a time-resolved experiment requiring a single beam; an example is Time Correlated Single Photon Counting (TCSPC). Here the temporal decay of the fluorescence of a sample is accurately determined by illuminating the sample with femtosecond pulses and collecting at the most a single fluorescence photon for each excitation pulse. Many repeated and averaged measurements of the time interval between the excitation pulse and the arrival time of the single detected fluorescence photon enable an accurate reconstruction of the fluorescence decay and its time constants. TCSPC usually requires a mode-locked femtosecond laser operating at many tens of megahertz and at a wavelength that matches the excitation spectrum of the sample. Ideal laser sources for TCSPC are Coherent Chameleon, Mira, Vitara and their OPOs, as they cover the required wavelengths and temporal resolution. TCSPC is also deployed as an important contrast mechansim in microscopy applications.

Pump-and-probe experiments are conducted with at least two pulses separated by a controllable and adjustable time interval. Generally, the two pulses have different wavelengths. The more powerful pulse is referred to as “pump” and is used to excite the sample; the subsequent less powerful pulse is referred as “probe” and reaches the sample to “freeze” and measure the dynamic evolution of the sample initiated by the pump pulse. These experiments typically require energy levels higher than the ones provided by oscillators. For this reason, ultrafast amplifiers like Coherent Astrella, Legend Elite and Monaco with their respective wavelength extensions (Harmonic generators and OPAs) are ideally suitable for these experiments.

Pump and probe experiments can be performed at a variety of wavelengths, depending on the atomic or molecular transitions being studied. Coherent provides OPAs that cover wavelengths between 190 nm and 20 µm. The flexible and configurable Coherent Legend Elite family is available with pulse durations of 25 fs, 35 fs, 110 fs and up to 1 ps, to satisfy every time resolution and sample bandwidth requirements. Depending on the nature of the specific ample and data acquisition chain, different repetition rates and pulse energies are available. Coherent titanium sapphire amplifier products provide energies from 7 mJ up to 20 mJ at repetition rates from 1 kHz to 10 kHz while ytterbium (Yb:) amplifier products deliver up to 2 mJ. Thanks to their ultimate flexibility Monaco Yb amplifiers can be operated from single shot to 50 MHz and provide higher average power than much larger titanium sapphire amplifiers.

Pump and probe experiments can also be performed in the EUV (30 nm to 200 nm) or terahertz (0.5-20 THz) regions, as described elsewhere. A substantial number of Coherent Legend Elite and Astrella femtosecond amplifiers are being used as engines to produces pulses in these spectral regions with commercial or user-built EUV, attosecond and THz generation systems.

More complex experiments include two-dimensional vibrational (2DIR) and electronic (2DES) spectroscopy, femtosecond stimulated Raman spectroscopy. They are discussed in dedicated sections.

Other types of experiments use fs or ps pulses but are not time-resolved in the strict sense of this expression. These include Sum-Frequency Generation (SFG) at surfaces or interfaces and Angularly Resolved Photoelectron Emission Spectroscopy (ARPES) and are also discussed in dedicated sections.

Two-Dimensional Spectroscopy is a sophisticated experimental technique used to uncover the dynamics of vibrational (2DIR) or electronic (2DES) states. Proposed by Isao Noda in 1990 and popularized by Hochstrasser in the late 1990s, two-dimensional spectroscopy gained momentum and popularity to address a variety of specific problems in physical chemistry, biochemistry and applied physics.

Differently from pump and probe experiments, the typical output of a 2D experiment does not necessarily contain temporal information on the evolution of the system under investigation, but rather a 2D chart quantifying the cross-correlation between various vibrational or electronic states. The use of femtosecond lasers is however dictated by the need to have a succession of at least three pulses that determine coherence time, waiting time and detection time. Short pulses are also required to produce the broad bandwidths required to address the various vibrational states under study. Finally, by varying the waiting time, the 2D experiment becomes resolved in time and able to determine the temporal evolution of the correlations between the various levels involved.

Because of its recent development and the diversity of the systems that are studied with 2D spectroscopy – from protein folding to the electron transfer in photosynthesis systems – the actual experimental set-up may vary considerably from laboratory to laboratory. Requirements on pulse arrival times and overlap are very stringent, especially when moving from IR to visible and – more recently - to UV part of the spectrum.

The laser equipment is however relatively well-defined as it typically requires a single beam with very large bandwidth in order to address the broadest possible number of excited levels. The broad bandwidth can be provided by NOPAs, by spectral broadening with hollow-fibers or by a linear OPA pumped with short pulses (25-35 fs) from an amplifier. Depending on the specific set-up, amplifiers like Legend Elite or Astrella are perfect matches for 2D experiments. Generation of UV, visible or IR wavelengths is achieved via Opera Solo, Topas, or home-built OPAs. Monaco HE with a non-collinear OPA offers the advantage of a flexible repetition rate that can be optimized for rapid data collection tailored to the system under study. In situations where the sample is in solid-state form and the experiment requires only modest energies/pulse, directly accessible by titanium sapphire lasers, oscillator like Vitara or Mira can also be conveniently used.

Sum-Frequency Generation (SFG) Spectroscopy has been used since the 1980’s to assess the properties of surfaces and interfaces by studying their vibrational behavior. Differently from IR or Raman spectroscopy that are sensitive to the bulk properties of a sample, SFG spectroscopy is exquisitely sensitive to physical and chemical properties of the few molecular layers at surfaces and interfaces. This is because the process of sum-frequency generation (of which SHG is a degenerate case) takes place at these discontinuities that, by definition, are non-centrosymmetric.​

In SFG spectroscopy, a mid-IR beam is sent onto the surface or interface overlapped with a visible more powerful beam; the specific properties (polarization, intensity, etc.) of the resulting sum frequency beam produced by the two beams provide information on the interface under investigation. ​

Energy and separation of the vibrational levels require the use of mid-IR pulses with relatively narrow linewidth (a few cm^-1). Since SFG is a non-linear process, to maximize its signal it is necessary to use short pulses. The narrow linewidth requirement means that at least one of the pulses should be a time-bandwidth limited picosecond pulse. Because of this, SFG spectroscopy was initially performed with frequency-doubled picosecond YAG lasers pumping a mid-IR OPA, where the green YAG output provides the visible beam and the OPA is tuned to scan the vibrational levels. The SFG beam will be in the visible part of the spectrum and conveniently detected.​

More recently, titanium sapphire amplifiers became more popular because the broad-bandwidth mid-IR beam generated with the femtosecond pulses from the amplifier can address many vibrational levels at the same time; the narrow linewidth requirement is instead satisfied by doubling part of the amplifier output down to 400 nm and filtering it spectrally to match the required vibrational linewidth.​

Important laser parameters for SFG spectroscopy experiments are spectral stability of the pump to insure a stable narrowed output, a broad mid IR spectrum, high pulse-to-pulse stability and very stable pulse duration from the amplifier to insure a stable output of the mid-IR OPA.​

Coherent amplifiers like Legend Elite USP, Astrella-USP with their OPAs or a home-built OPA address perfectly SFG spectroscopy by providing a broad mid IR spectrum and a powerful beam that can be frequency doubled and spectrally filtered.​

Angularly Resolved Photoelectron Emission Spectroscopy (ARPES) is used to analyze the electronic levels in advanced solid-state materials like superconductors, 2D materials and topological insulators. In ARPES a deep UV laser beam is sent onto the surface being studied and the electrons emitted by photoelectric effects are collected by a detector that is scanned around the sample. The photon energy (i.e. wavelength) minus the work function and the measured kinetic energy indicate the energy level of the electron in the sample while the momentum parallel to the surface is a faithful representation of the momentum in the sample. Analyzing the three-axis component of the emitted electron speed provides a representation of the properties in the materials, specifically the so-called Brillouin zones.​

The ideal laser source should produce deep UV photons at high repetition rate (hundreds of kHz or higher). This is to expedite data collection as ideally each laser pulse should excite a single detected electron. Energy per pulse and pulse duration are not critical as much as the photon energy. Typically, the employed wavelengths are below 250 nm to get reasonable electron kinetic energies, after overcoming the material work function. While wavelengths between 190 and 250 nm can be achieved with harmonic generation in non-linear crystals, access to 100-200 nm wavelength requires resonant or higher harmonic generation in gas like Xe.​

Coherent lasers that match ARPES requirements are Monaco, Mira and Paladin 355. Monaco can be quadrupled to 259 nm and quintupled to 207 nm. Mira and Paladin have been used to produce harmonics below 180 nm with the exotic crystal KBBF. Monaco has been used to produce 115 nm light by THG in BBO and successive THG in Xe gas.​

In time-resolved ARPES, a much longer wavelength photon (UV to mid-IR) is used to bring electrons above the Fermi level while the UV pulse is used to trigger the photoelectric effect. TR-ARPES can study the properties of the energy levels between Fermi and Vacuum levels specific of advanced solid-state materials. Differently from ARPES, TR-ARPES poses stringent and somewhat contradictory requirements on pulse duration: spectral resolution requires narrow bandwidth, but temporal resolution to study the lifetime of the involved energy level requires short pulses.​

Coherent Monaco/Monaco HE with their OPAs Opera-F and Opera-HP provide compact and convenient solutions for both TR-ARPES and ARPES.​

​Terahertz picosecond pulses can be used to study the dynamics of collective motions in solid or liquid state materials (THz pump and probe spectroscopy). Other types experiments are used to determine the THz response of materials (time-domain spectroscopy or TDS) or to image samples (THz imaging) in this less explored part of the spectrum. Femtosecond lasers are ideal tools to produce THz pulses with the desired bandwidth and pulse duration at substantial energy levels.

Terahertz picosecond pulses can be used to study the dynamics of collective motions in solid or liquid state materials (THz pump and probe spectroscopy). Other types experiments are used to determine the THz response of materials (time-domain spectroscopy or TDS) or to image samples (THz imaging) in this less explored part of the spectrum. Femtosecond lasers are ideal tools to produce THz pulses with the desired bandwidth and pulse duration at substantial energy levels.​

​Coherent titanium sapphire amplifiers are used to produce THz pulses up to Megavolt/cm and frequencies from 0.5 THz to 20 THz and beyond.

THz TDS is a technique used to determine the THz response (absorption and refractive index) of materials to either characterize or identify them. Here a THz pulse is sent onto a sample and the transmitted pulse is Fourier-transformed to retrieve its frequency spectrum. This is compared with the frequency spectrum to extract the THz absorption spectrum of the sample. THz imaging uncovers in a non-destructive way structures and faults that are hidden inside a complex material specimen as the sample is scanned in front of the THz beam and the transmitted THz pulse is measured. Both these techniques require lower fields than pump and probe and are usually performed using a so-called THz photoconductive switch illuminated by femtosecond pulses from a laser oscillator like Coherent Chameleon, Vitara or Axon.​

​An important feature of THz pulses is that both their phase and amplitude can be controlled and measured. This means that THz TDS determines both the imaginary (absorption) and real (refraction) part of the refractive index of the sample under investigation.

A complementary approach to THz pulse measurement is to interrogate samples and detect their Raman spectrum at a wavelength extremely close to the excitation wavelength. This “low-wavenumber” Raman technique is called THz-Raman spectroscopy. Although there are no actual THz fields involved, the response of the material to THz-Raman correlates to the absorption at THz wavelengths the same way that a Raman signal address the same vibrational levels measured with FTIR.​

Coherent uniquely supplies spectrally narrowed diodes (Coherent Surelock family) and volume Bragg gratings (Coherent Sureblock) that are used to extend commercial or home-built Raman spectrometers to the THz-Raman region. Inspection of samples in this region is profitably used for many applications like to distinguish different states of crystallization of active pharmaceutical ingredients. In addition to diodes and filters, Coherent produces also subsystems (THz Raman Probe) and full systems inclusive of well-plate THz Raman analyzer.​

One of the most sophisticated applications of high-energy, ultrashort pulse Ti:Sapphire amplifiers is related to the process of high harmonics generation (HHG).​

In HHG, intense laser pulses are focused into a target gas to fluences above 10₁₄ W/ cm² to generate much higher order harmonics of the optical driving frequency than what is possible with conventional solid-state non-linear materials. The output of the high harmonic process extends well into the extreme ultraviolet spectral regime (EUV). At the same time, the resulting pulse width can be on the attosecond scale, thanks to the large bandwidth and short wavelength of the HHG pulse​

Due to the underlying quantum dynamic effects, the HHG pulses are produced collinearly with the driving laser pulses, in the form of coherent, highly collimated, and laser-like beams. Therefore, HHG and attosecond technology extend conventional ultrafast spectroscopy and strong field studies into EUV domain and with pulse durations otherwise unachievable. This capability advances time resolution and control techniques from the molecular to the atomic-bound electron scale. In fact, with photon energies reaching the tens and hundreds of eVs, core and multi-electron dynamics or even nuclear dynamics can be time-resolved. This capability makes table-top HHG beam lines a cost-effective alternative to synchrotron radiation for some experiments.​

The shortest wavelength and therefore highest photon energy possible with HHG is defined by the cutoff energy and is given by the formula E_cutoff = I_p + 3.17 U_p, where I_p is the ionization potential of the target gas atom. Up is the ponderomotive energy in eV given by U_p = 9.3 x 10^-14 I x λ² with I and λ are the laser intensity in W/cm² and the wavelength in μm, respectively. While energy scaling based on intensity is only linear and strongly limited by saturation effects in the ionization of the gas itself, it is possible to significantly decrease the cutoff wavelength by using longer driving wavelengths, although this may seem counterintuitive. This extension however comes at the price of a substantial decrease in the efficiency of the EUV process as it scales with 1/ λ5.​

The shortest wavelength and therefore highest photon energy possible with HHG is defined by the cutoff energy and is given by the formula E_cutoff = I_p + 3.17 U_p, where I_p is the ionization potential of the target gas atom. U_p is the ponderomotive energy in eV given by U_p = 9.3 x 10^-14 I x λ² with I and λ are the laser intensity in W/cm² and the wavelength in μm, respectively. While energy scaling based on intensity is only linear and strongly limited by saturation effects in the ionization of the gas itself, it is possible to significantly decrease the cutoff wavelength by using longer driving wavelengths, although this may seem counterintuitive. This extension however comes at the price of a substantial decrease in the efficiency of the EUV process as this scales with 1/ λ5.​

Simultaneous with the generation of EUV wavelength, HHG produces very broadband, ultrashort pulses. When driven with 20-100 fs pulses, the duration of the HHG pulses follows the envelope of the drive pulse. Using few-cycle drive pulses from a Ti:Sapphire amplifier that is also carrier envelope phase (CEP) stabilized, precise control of the underlying quantum dynamic effects is possible and results in isolated attosecond pulses. Furthermore, varying of the exact shape and phase within the drive pulse gives access to the shape and phase of the attosecond pulse produced.​

The operational simplicity and stability of one-box Ti:Sapphire amplifiers such as the Coherent Astrella are ideal for tabletop EUV generation of 35-100 fs pulses. The sophisticated CEP options available with Coherent Legend Elite Ti:Sapphire amplifiers unlock true attosecond-scale experiments. Since the photon flux in the EUV is order of magnitude lower than at optical frequencies, many of these EUV experiments lasts for hours. It is therefore imperative that the driving amplifiers be very stable in each beam parameter, inclusive of CEP stability. Both Astrella and Legend Elite amply satisfy these requirements.​

Free Electron Lasers (FELs) and Synchrotron facilities house a variety of ultrafast lasers that fulfil at least three different purposes: photocathode injection, FEL linewidth-narrowing, and UV to THz pump and/or probe to be performed in the experimental hutches. These lasers must support two key requirements: the highest possible level of reliability and femtosecond-level synchronization of the laser pulses with the particle beam or FEL.​

These large and expensive multi-user facilities host teams of visiting scientists that conduct in parallel or sequentially diverse experiments on a 24/7 cycle, therefore down-time is both very expensive and highly disruptive. For this reason, all the instrumentation – including lasers – must provide maximum reliability and uptime, sometimes insured by building redundancy and in-house availability of spare parts. As an example, it is common that the photoinjector room houses two identical frequency-tripled TiS laser amplifiers placed side by side. The lasers have a common optical path to the photocathode and are periodically swapped to verify identical performance on the photocathode. This way, if one laser fails, the other can be started and operated in minutes to re-activate the entire FEL.

The second laser application - used only in FELs designed to produce a narrow-line output beam - is to generate a wavelength at some overtone of the desired FEL wavelength and “seed” the FEL. For example, Fermi Elettra in Trieste (Italy) uses a Coherent Opera Solo OPA pumped by a Legend Elite amplifier to produce a Deep UV output for this specific purpose.​

Finally, the experimental hutches contain other laser amplifiers used for pump and probe experiments in conjunction with the FEL or particle line beam. Differently from conventional pump and probe experiments where both pulse trains originate from the same laser and are therefore synchronized, here the laser amplifiers must be tightly synchronized with the particle or FEL beam. FELs pulses can be as short as a few fs, while the amplifier output can be as short as 25-50 fs, so a tight synchronization is mandatory, often combined with binning the data based on the statistical jitter distribution.​

Many FELs and accelerator installation use Coherent Legend Elite amplifiers with harmonic modules and OperaSolo or Topas OPAs, a testimonial of the reliability and precise, low-jitter synchronization of these lasers.​

Electrons, positrons and ions particle beams can be used for many applications in industrial diagnostic, imaging and medicine. However, the production of these beams requires large accelerators facility, whose substantial cost limit dramatically most practical applications. In 1979 Toshiki Tajima and John Dawson laid the foundation for laser plasma acceleration as a disruptive technique able to accelerate charged particles over distances of a meter or less, compared with the hundreds or meters or longer for conventional accelerators.​

This so-called wakefield acceleration takes place when a very energetic laser beam is injected into a plasma column collinear with the beam. The laser beam separates electrons and ions, with the lighter electrons moving away from the center of the plasma column. Qualitatively, these electrons move back very rapidly towards the center of the plasma trailing the laser pulse. The resulting distribution of charge then creates a longitudinal field that accelerates the charged particles purposely injected in the plasma.​

​Laser beams with extremely high peak powers like the Texas Petawatt laser facility or BELLA at Lawrence Berkeley National Laboratory can accelerate electrons to the GeV level over only a few centimeters. On a smaller scale, a kHz TiS amplifier with 10-20 mJ pulse energy and 20-25 fs pulses may be able to accelerate electrons to a few MeV, once its pulses are compressed to optical cycle duration (~3 fs). Such development would lead to a true tabletop accelerator with applications to industrial diagnostic.

TiS amplifiers like Coherent Legend Elite or Astrella are used to study and determine the possibility of compact, table-top laser plasma acceleration.​