Measurement of Fine Structure and Spin Interaction of Quantum Materials through TriVista High Resolution Spectral Measurement System

background
The Jörg Debus team from the Technical University of Dortmund in Germany is dedicated to researching optical quantum information processing and quantum sensing in materials with potential applications. The team mainly studies the fine structure of materials under light fields, such as quantum dots, quantum effects of two-dimensional materials, semiconductor defects in diamonds, and rare earth ion quantum wells. For quantum information processing of light driven spin electrons, coherent spin manipulation using ultra short pulse lasers requires understanding the fine structure of excitons, especially the g-factor of electrons and holes: it defines the frequency of quantum bits. In addition to the spin level structure, the interaction between confined charge carriers is also crucial, and spin relaxation can limit the processing of quantum information.

Recently, the Debus team conducted measurement experiments on the energy and spin structure of nitrogen vacancies in diamond crystals. Due to its unique electronic confinement, electron spin exhibits an ultra long coherence time at room temperature, exceeding a few seconds, making it highly suitable for applications such as quantum information and quantum sensing. Therefore, it is very important to understand the fine structure of energy levels corresponding to different spin states in the magnetic field and the interaction mechanism of charge carriers in materials. The Debus team uses spectroscopy to measure these characteristics and uses spectroscopy to analyze these fine structures.

In addition to photoluminescence spectroscopy, the Debus team also used another technique, spin reversal Raman scattering, which is similar to ordinary Raman scattering, but the initial and final states of the material have different spin characteristics. The position where the spin reversal signal is detected will be offset from the spectral position of the excitation light by the energy difference of the spin state. Spin reversal Raman scattering can not only be used to measure spin energy levels, but also to prepare charge carriers confined to specific spin state quantum dots. The scattering mechanism helps to identify the spin interaction between electrons and holes. Moreover, most of the experiments in the laboratory are conducted in a low-temperature magnetic field, which can accurately control the energy and polarization of the excitation light.

TriVista settings for spin reversal Raman scattering detection
The Raman spectrum in the above figure shows the spin reversal signal of electrons in InGaAs/GaAs quantum dots under a magnetic field of 8 T and a temperature of 6 K, excited at 1.39 eV (892 nm), and detected using a liquid nitrogen cooled Spec-10 CCD camera.

challenge
However, Debus Laboratory's research is not just focused on one material, but on a wide range of materials. The spectral system needs to adapt to changes in signal wavelength. By using lasers with different excitation wavelengths or tunable lasers, sufficient spectral power can be obtained to analyze the fine structure and interaction details of spin states regulated by an external magnetic field.

There are many difficulties in analyzing the fine structure of semiconductor quantum dots, for example, small changes in the size or shape of quantum dots can cause non-uniform broadening caused by energy level distribution. By tuning the excitation wavelength to resonate with specific quantum dots, signals from other quantum dots in the sample will be suppressed, reducing spectral broadening. However, the laser will be located near the spectrum of the detection signal. The signal in resonance spin reversal Raman scattering is also the same, with only a slight shift of a few meV (a few wavenumbers) between it and the excited laser line.

Performing spectral measurements near laser lines is extremely challenging. The intensity of elastic scattered light is often much stronger than the signal, which can cause great interference to the detection of weak signals on the detector. Before testing, a filter must be used to reduce the laser intensity. The filter needs to accurately filter out the laser line and measure the signal near the laser line, and changing the excitation wavelength requires the use or purchase of corresponding filters.

Solution
The Debus team utilized the TriVista TR555 three-level spectral system, which not only achieved high resolution and strong suppression of stray light (for signals near laser lines), but also adapted to constantly changing experimental requirements such as different materials, excitation and detection wavelengths. In addition, the intensity of spin reversal Raman scattering signals is relatively low, and it is necessary to improve the efficiency of optical devices and the sensitivity of detectors.

The TriVista allows us to perform challenging optical spectra copy with high resolution as close as a few 100 μ EV (0.8 cm-1) from the examination laser line.

The TriVista system consists of three spectrometers, with a spectral resolution of up to 300% compared to a single stage. Another level 3 mode of TriVista allows for signal recording near the laser line 5cm-1 (0.62 meV). In this working mode, the first two stages are connected together in some way, acting as signal bandpass filters dispersed by the third stage spectrum. In addition, the Debus team sometimes uses single point detectors (such as PMT) for detection, which is suitable for experiments that do not require CCD detection.

The TriVista system can adapt to the constantly changing experimental requirements of the laboratory, and can detect lasers or signals in any wavelength range from ultraviolet to infrared without the need for additional filters corresponding to different wavelengths.

The TriVista system can also operate up to 4 signal output ports (one for the first and second stages, and two for the third stage), and apart from the combination mode mentioned above, each stage of the system can operate independently of each other.

The Debus team uses different detectors to detect visible and infrared signals at different output ports, and also conducts nanosecond resolution time-resolved measurements using ICCD, such as PIMAX.

The TriVista system has the ability to suppress high resolution and stray light from ultraviolet to near-infrared bands, and has multiple effective detection and operation options, meeting the various needs and requirements of the Debus team in the quantum material research process.

Source: Sohu