Opto-electronics combines optical and electronic material and device properties to detect, generate and control light. Opto-electronics covers a wide range of applications including LEDs, laser diodes, photodiodes and solar cells. On the one hand, electrical control of an opto-electronic device results in an optical output as in the case of LEDs. On the other hand, optically exciting a device produces an electronic signal response such as the one obtained from a photodiode. Opto-electronic devices are semiconductor-based and mainly work on the principle of direct electron-photon conversion: the semiconductor's energy gap sets the energy scale of the device's optical excitation.
Importantly, the energy gap can be tuned and engineered for a particular application. In the case of graphene-based devices such as the one considered in the figure, which are highly transparent and provide good energy tunability, the bandgap can be induced by reducing the device's dimension, as with nano-ribbons and quantum dots, or by using double-layer graphene, for which the energy gap is on the order of 0.25 eV.
Measurement Strategies
The figure depicts an experiment where a graphene-based device is optically excited by laser light, and the current output of the photodetector or the voltage drop along the device is subsequently measured. This type of measurement takes place in a noisy environment which is even more challenging when the signal is transduced from the optical to the electrical domain or vice versa. Modulating the laser light excitation and monitoring the electrical response with a lock-in amplifier reduce the deleterious impact of noise. Indeed, the light excitation produces a response that is often very small and is thus optimally measured thanks to a lock-in amplifier with low input noise. This kind of measurement must be performed in an optically clean environment; it can take place at room temperature, or at low temperatures with specially designed cryostats for electro-optical applications. To make sure that the light modulation frequency and the lock-in reference frequency are the same, the laser can be controlled by using the lock-in reference output or by providing the reference of the laser source to the lock-in amplifier. In the setup shown in the figure, the device's electrical properties are tuned by applying the DC offset to the back-gate electrode.
The modulation frequency mainly depends on the device's mobility, on its optical properties and on the surrounding noise. The laser light can be modulated with a chopper that sets the measurement frequency in the few kHz range, ensuring that low-frequency noise does not affect the measurements. Devices such as photodetectors work at a few hundred kHz, whereas materials with high mobilities operate in the MHz range. This makes the MFLI Lock-in Amplifier an excellent choice with its operating range up to 500 kHz and its possible frequency extension to 5 MHz.
The Benefits of Choosing Zurich Instruments
Thanks to its many field-upgradeable options, a single MFLI Lock-in Amplifier allows you to perform all measurements discussed above.
Reduce the complexity of your setup and save time: you can achieve simultaneous optical and electrical device control by adding the MF-MD Multi-Demodulator and MF-MOD AM/FM Modulation options to one MFLI. Together, these two options enable you to measure the inter-modulation of electrical and optical signals at one or two inputs and at two internal or external reference frequencies.
To characterize the semiconductor junctions used in opto-electronic devices without tinkering with your setup and gain insight into the junction properties, upgrade your lock-in amplifier with the MF-IA Impedance Analyzer option. In this way you won't need to switch devices between the material characterization step and the device performance step.
You can complete your device characterization with concurrent noise measurements of the optical and electrical outputs thanks to the functionality of the MF-DIG Digitizer option and without the need for attaching a VNA or other device to your setup.
Take full control of your measurement results with the included LabOne software, which is designed to provide a complete overview of time- and frequency-domain signal analysis and features tools such as the Scope, a real-time data Plotter, a Spectrum Analyzer and the DAQ module for multi-parameter data acquisition.
