How to setup Femtosecond pump-probe transient absorption spectroscopy? Check Simtrum Solution

Setup-Guide: Femtosecond pump-probe transient absorption spectroscopy

+ultrafast spectroscopy  +pump-probe  +transient absorption  +data acquisition  +harmonic generation  +continuum white light module  +delay stage

Author:

Majid Panahandeh (PhD NTU 2014)

At a glance: • Knowledge article • Method •Setup

Ultrafast transient absorption (reflectance) spectroscopy is a technique developed to study dynamics and evolution of excited states on the sub-picosecond timescales in various photo-sensitive chemicals (i.e. inorganic semiconductors, organic compounds, and biological systems). This technique is a non-contact measurement for carriers that can be generated in the absence of an external electric field. Moreover; this technique has widely been used for studying the highly resistive materials since it is difficult to inject carriers from metal contacts into the materials.

This application note will discuss fundamental of this technique, its application, experimental setup and required optical components. It also shed light on measurement in the reflection geometry.

Contents

1.      Application of pump-probe spectroscopy

2.      Methodology and Setup

3.      Editor's Notebook:

3.1   Components for consideration

3.2   Technical tips

4.      Consultation (Active as of: +8GMT 12^TH Nov 2020)      

Application of Pump-Probe Specroscopy

In semiconductors and oxide materials, the timescales for transient processes such as carrier-carrier scattering, intervalley scattering, and carrier-optical phonon scattering are usually in the order of few to hundreds of femtoseconds. In contrast, the slower optical processes such as carrier diffusion and inter-sub-band scattering take place within one picosecond or less. Other transient processes such as carrier-acoustic phonon scattering, hot carrier-phonon interaction and carrier recombination, typically take place at a longer timescale, 10 to 100 ps or even more. You may refer to our tutorial, Jablonski diagram, for more details. This technique is being used to provide information about elementary processes, which take place in semiconductors and oxide materials. Some of the specific information that can be obtained are:

-Carrier photogeneration

-Relaxation paths 

-Hot charge transfer states

-Hybridization of localised and delocalised states

Principle of photoinduced absorption spectroscopy

This technique uses two ultra-short pulses, namely pump and probe, to create and probe the excited states. Since excited states species are typically short-lived, we need short (～100 fs) duration pump and probe pulses to get the temporal evolution of excited-states. In this technique, an ultra-short (～100 fs) pump pulse is used to photoexcite the material and a low-intensity, ultra-short white probe pulse is used to monitor absorption changes in the material, specifically kinetics of excited states after excitations. By delaying probe pulse respected to pump pulse, we can measure the evolution of photogenerated carriers within different delay times from a few femtoseconds (fs) to a few nanoseconds. Figure 1 shows the working principle of the transient pump-probe technique. Typically, photoexcited carriers can go through different relaxation pathways before returning to the initial thermal equilibrium states.

In general, photoinduced absorption is measured by monitoring the change in transmittance of a weak probe beam induced by a stronger pump beam. For small values of the differential transmittance, ΔT/T, and uniform excitation, photoinduced absorption is defined as:

Where T is the transmission of the probe beam when the pump is on while T₀ is the transmission of probe beam when the pump pulse is blocked. In other word, by measuring the difference of the light transmission with and without pumping, one can determine the difference in light absorption (Δα) between the excited sample and that in equilibrium. Through varying the delay time and recording the ΔT at each delay time, the temporal profile of the dynamics can be gained (see Figure 1c). By choosing different wavelengths λ, the spectral profile of the dynamics can also be acquired. The bandwidth of the pump pulse should be narrow in order to enable selective excitation of an optical transition.

Figure 1: Schematics of pump probe technique. (a) Principle of pump probe technique, which uses a narrow band pump and a broadband probe beam. (b) Energy levels of a single molecule (due to the absorption of the pump pulse, electronic transitions occur from the ground state (S0) to upper excited states) (c) a typical transient absorption spectra measured by SIMTRUM R & D team.

Methodology

Mathematical formalisms

When light hits an object, different phenomena may happen, resulting in some ratio of incident light to be reflected, refracted, absorbed, or transmitted. If we assume that scattering is negligible, the total incident intensity is conserved as:

where 10, Ir, It, and Ia are incoming, reflected, transmitted and absorbed light intensity respectively. In general, the transmission coefficient of a thin film with thickness d, refractive index of n, absorption cofficient a, and reflection coefficient R, is given by:

where δ=2πnd/λ is the optical phase shifting per pass.

Similarly, the reflection coefficient R for an incident electromagnetic plane beam through a film with a thickness d and absorption coefficienta, may be described by:

where n and k are the real and imaginary parts of the complex refractive index (ñ=n+ik). k can be directly related to the absorption coefficient through:

In equation 2, we can safely neglect the interference effects since we assume that the thickness of the spin-cast film is much lower than the incident wavelength (d<<λ). In addition, due to the roughness of the spin-cast film, the interference effects are expected to cancel out. Therefore, according to Lambert-Beer law the transmission of the probe light can be rewritten as:

As a first approximation, we assume R<<1 then (1-R)2=1, therefore, the transmitted probe light through the sample can be simplified as:

Upon absorption of light, contributions of absorption bands due to the photoexcitation will alter absorption coefficient. Therefore,

The absorption coefficient is defined as product of the population of photoexcitations and a cross section of those photoexcitation as:

The normalized differential change in transmission can be expressed as density of photoexcitation and their optical cross section:

where s is the optical cross-section of the photoexcited species, and n_e is the density of photoexcitation. Equation 9 implies that in the case of very weak reflectance, modulation transmittance is dominated by the change in the absorption coefficient.

In the above-mentioned derivation, we assumed there is a feeble reflection on the surface of the film that can be neglected. Also, we assumed that the reflection coefficient is the same for the back and front sides of the sample. Now we consider that reflection on the surface of the film cannot be neglected. By neglecting the multiple reflections, the derivation of equation 5 with respect to modulation filed F can be written as:

Dividing by the unperturbed instensity, we obtain: 

For normal incidence yields,

Therefore, the modulated of transmittance of a film can be expressed as:

If we assume there is a very weak reflection on the surface of the film, the first term can be neglected and equation 13 can be simplified as:

Carrier-induced optical effects cause small changes in reflection coefficient R. Therefore, modulated reflectance, ∆R, can be expressed in terms of the complex index of refraction. Since ∆R is very small, it can be linearly approximated in terms of ∆n and ∆k as:

By using the Fresnel equation for reflectance at normal incidence, the differential reflectivity is given by

where,

In general, the real and imaginary parts of the refraction index, ∆n and ∆k are connected through Kramers-Kronig relations.

Since the extinction coefficient is directly proportional to the absorption coefficient, the change in the index of refraction, as a function of frequency, ∆n (ω), can also be expressed in terms of the change in absorption, ∆a (ω):

By using the Kramers-Kronig relations 17, one can obtain the change in reflectivity as a function of frequency in terms of the absorption as:

The relation between ∆n and the ∆a is essential for the interpretation of the differential reflectivity signal. If we assume the magnitude of n is much larger than k, in this case, A (n, k) >> B (n, k). In such instances, the photoinduced reflectance response is dominated by ∆n rather than ∆k, unlike photoinduced absorption which is dominated by ∆k. Therefore, in this case, the modulated reflectance ∆R/R can be simplified as:

Experimental setup

Figure 2 shows the schematic diagram of femtosecond pump-probe transient absorption spectrometer. The setup is comprised of five main parts, which are:

-Excitation state

-White light continuum module

-Delay-stage

-Spectrometer

-Processing software

Excitation Source: In most conventional pump-probe setup, excitation source feeds an optical parametric amplifier (OPA), which allows generating a wide range of pulses from deep-UV to MIR region. Pulses from the OPAs are used as a “pump” to promote a fraction of molecules to excited states. 

White Light Continuum Module

The probe pulse is generally white light. The continuum white light can be generated either by Sapphire crystal for Visible to NIR or CaF2 for Nir to MIR region. Laser pulse width, beam shape and power play a crucial role in WLC generation and its stability. Therefore, pulse width, beam shape and power were optimized by adjusting the compressor of the amplifier, iris and ND filter, respectively, to stabilize the WLC.

Delay Stage

A translation stage is used to generate temporal delay between pump and probe, which allows one to monitor the temporal dynamics of our system from a few 10s femtoseconds to a few nanoseconds.

Spectrometer

TA measurements can be performed using a monochromator coupled to a-Si photodiode, cooled PMT or a CCD camera.

Figure 2: Proposed experimental setup

Editor’s Notebook

Components for consideration

White Light Module

SIMTRUM offers WLC modules that can be easily mounted in your pump-probe setup. The white light continuum (WLC) is a third-order non-linear phenomenon was generated by tight focusing of the laser beam in a non-linear medium. IN our module, visible WLC is generated by focusing the 800 nm beam into a 2 mm thick sapphire crystal and Laser power is adjusted by ND filters and residual of 800 nm laser in WLC is blocked by a notch filter. The final output is a collimated white-light continuum. Our NIR module generates WLC by focusing NIR nm laser beam into a 4 mm thick yttrium aluminium garnet (YAG) crystal. The residue of NIR laser from the NIR WLC will be filtered by a short-pass Dichroic mirror.

Spatial Filter Module

SIMTRUM offers an innovative solution for generating perfect gaussian beam. Our spatial filter removes higher-order modes.

Second & Third Harmonic Generation

In our module, the SH is generated using a type-1 BBO crystal (phase-matching angle 29.20) while the TH is generated by the sum-frequency generation of 400 nm and 800 nm in a type-2 BBO crystal (phase-matching angle 55.50). A neutral density (ND) filters will be used for adjusting pump laser power.

Technical Tips

The probe beam is being used to detect the perturbation effect by the pump beam in the sample. It should be much weaker than the pump beam to avoid multi-photon/multistep excitation during probing.

The pump beam is chopped at a specific frequency (usually add number is chosen) and referenced to the lock-in amplifier. Optical nonlinearities effects (i.e. bimolecular processes) will be minimized by setting the pump power density to a value well below 100 µJ/cm2 per pulse.

Reflection geometry for opaque sample

Transient absorption measurements either in the transmission or reflection geometry can provide information on the dynamics of the nonlinear photoexcitation, which is fundamental to understanding the primary events taking place in the excited state. However, transient reflectance spectroscopy is more sensitive to the surface inhomogeneity. Besides, due to a limited penetration depth of the probe beam in the absorbing medium, the changes in the reflection signal due to the absorption modulation by the pump pulse are much smaller as compared to the change in the transmission signal. In reflection geometries, probe light is reflected off from sample, then collected and monitored using a spectrograph coupled with detector configuration with lock-in or CCD detection. Within this experimental configuration, the differential signal obtained by amplitude modulation of the pump beam effectively gauges the photoinduced transient reflectance. It should be taken in the account that interpretation of data for thin film samples in reflection geometry is more complicated due to double-pass backscattering at the interface of thin film and substrate.

Simtrum is active in the Photonics field and welcomes any emails for discussions, questions or implementation. We are based in Singapore (+8GMT) and available via Phone or Email. We are especially interested in customised solutions not commercially available.

Consultation

With over a combined 12 years of experience in the field, Simtrum’s R&D team has developed tens of spectroscopic setups with unique capabilities that are not available in commercial instruments. Our project team is focused on practical and elegant solutions. Please contact us If you need any enquiries about femtosecond transient absorption spectroscopy and setting up experiments.