Iceblink and The Use of Supercontinuum Lasers for Angle-Resolved Spectroscopy: New Polaritonic Studies

Condensed matter studies are one of the cutting-edge fields of experimentation with the potential to achieve major advances in materials science. In this field, the study of polaritons is crucial to understand the quantum electrodynamics of complex chemical systems.

The contribution of polaritons to chemical scenarios has allowed for the tuning of chemical reactivity, enhancement of energy transfer between molecules, formation of Bose-Einstein condensates at room temperature, as well as the development of qubits for quantum applications [1]. Yet further comprehension on the photophysical characteristics of polaritons is still essential.

One strategy consists of the coupling between the electromagnetic modes of an optical cavity and the electronic states of condensed matter systems such as multiple molecules [2] or colloidal nanocrystals [3].

In this scope, researchers at Rochester University have proposed the coupling of high quality-factor Fabry-Pérot optical cavities with 2D cadmium selenide (CdSe) nanoplatelets (NPLs) as an alternative to control exciton-polariton photophysics.

 

Towards Photophysical studies:  Cavity Controlled Upconversion in CdSe Nanoplatelet Polaritons.

Polaritons are the result of the close interaction between light and a magnetic dipole or exciton. They can be generated by placing a quantum well into an optical resonator. When the quantum well is hit by light at a certain wavelength, this light interacts with its electronic structure, generating an electron vacancy with positive charge and an electron that orbits it with negative charge. This pair of charges is called an exciton [4].

The difference of charge between the electron and the vacancy eventually reconvenes, liberating energy in the form of light with the same wavelength of the one which hit the quantum well in the first place. And due to this quantum well being in an optical resonator, the emitted light reflects and hits the well again restarting the process.

Even though polaritons are made of the interaction between light and excitons, they are treated as a quasiparticle, with two hybrid light-matter eigenstates corresponding to the upper polariton (UP), with higher energy and a lower polariton (LP) with lower energy.

In this particular study, Prof Todd Krauss´ group from Rochester University leveraged strong light-matter interactions to control the excited state dynamics of colloidal CdSe nanoplatelets (NPLs) coupled to a Fabry-Pérot optical cavity. They synthesized the Monolayer CdSe Nanostructure, fabricated the microcavity and studied the photophysical properties of their sample by means of Angle-Resolved Spectroscopy and TCSPC Lifetime measurements.

As mentioned above, they made use of high Quality (Q)-factor FP optical cavity. The Q factor refers to the photon loss from the cavity, whose value was tunned by adding SiO2 spacers to it, altering the dynamics of polaritons and leading to interesting results [Figure 1].

Figure 1 - Fabry - Pérot Microcavity [1].
Figure 1 – Fabry – Pérot Microcavity [1].

Furthermore, in order to determinate the effects of modifying the Q coefficient of the cavity, the investigators used Angle Resolved Spectroscopy, a technique that measures the energy and momenta of the electrons in the sample.

 

Iceblink Supercontinuum Laser for the Optical Characterization of Polaritons with Angle Resolved Spectroscopy.

In this technique, the sample is hit by light and ejects an electron. By measuring the angle of emission and kinetic energy of that electron, the investigators can see the differences in the population of upper and lower polaritons because of their different energy states.

The set up used for this technique was composed by a light source (a laser that changed depending on the measurement needed), an objective that focused the light into the sample, a dichroic mirror (because the emission of the sample was taken from the same objective) and a spectrometer to measure the electron characteristics [Figure 2].

Figure 2 – Set up for Angle Resolved Spectroscopy [1]
Figure 2 – Set up for Angle Resolved Spectroscopy [1]

As is common in this type of techniques, depending on the light source that is being used, you can see different aspects of the sample. In this particular experiment many light sources were used in different samples to ensure a complete comprehension of the effects of changing the Q coefficient of the cavity; but among them, the most versatile was the Iceblink Supercontinuum laser. This platform was used with its band pass filter (called Boreal) to perform measurements in the 515 – 540 nm band at a repetition rate of 2.5 MHz. The capacity of this platform of changing not only the wavelength but also the repetition rate gives the investigators the chance to adjust the most suitable time scale for polaritonic measurements.

This kind of precise adjustments make the results more accurate and bring the possibility of using the same light source for different applications, like lifetime measurements, avoiding the necessity of buying new equipment. This is due to the long spectral range of the Iceblink that starts in 450nm and ends in 2300nm whit the possibility of selecting the wavelength in the visible range and the ability to choose the repetition rate between 1, 2.5, 5, 10, 20 and 40 MHz.

 

A Promising Future for the Use of Polaritons.

As the Q-factor was increased, they observed significant population of the upper polariton (UP) state, exemplified by the rare observation of substantial UP photoluminescence (PL) at room temperature. In fact, with low-energy excitation at the lower polariton (LP), which is not absorbed by uncoupled nanoplatelets, they observed upconverted photoluminescence emission from the UP branch due to efficient exchange of population between the LP, UP, and the reservoir of dark states present in collectively coupled polaritonic systems.

Figure 3 shows the observations made for Reflectance (left) and Photoluminescence (right) for two different Q coefficients. As can be seen when the Q coefficient is higher, the Upper Polariton Branch (UPB) is more visible indicating a grater population of upper polaritons. This increment in UP lead to a rare photoluminescence at room temperature.

 

Figure 3 – Observations [1]
Figure 3 – Observations [1]

The work sheds light on the photo physics of nanocrystal-based exciton-polariton systems. It also paves the way for practical polariton photochemistry platforms and all the potential benefits it can bring.