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Femtosecond Measurement Techniques

Working Group 2.54


Femtosecond Measurement Techniques

The Working Group Femtosecond Measurement Techniques focuses on the development of time-resolved optoelectronic measurement techniques for electric fields in the GHz and THz frequency range. Based on such optoelectronic measurements, the working group offers a calibration service for the time response of sampling oscilloscopes with a nominal bandwidth of up to 100 GHz. Furthermore, the group pursues basic research on carrier and current dynamics in semiconductors. Such studies might prove useful for the future characterization of electrical high-frequency components.

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Development of optoelectronic time-resolved techniques

In information processing and communication technologies, the data rates of electronic devices have been constantly increasing in recent years. In order to keep pace with this development, national metrology institutes, such as PTB, have to introduce new techniques for the traceable measurement of the transfer characteristics and intrinsic time responses of high-speed electronic devices. For this purpose optoelectronic measurement techniques based on femtosecond optics are well suited. The working group Femtosecond Measurement Techniques develops such novel measurement methods.

See publications for selected articles on this topic.

Basic research on carrier and current dynamics

For the characterization of highest-frequency components it is desirable to produce ultrashort current pulses, the temporal form of which can be varied arbitrarily. Hitherto methods to produce current pulses of a few 100 fs in length are based on a combination of electronic and optical procedures that do not allow a variation. The Femtosecond Measurement Techniques group investigates the generation of ultrashort current pulses by means of solely optical methods. With these methods it is, in principal, possible to modify the shape of these current pulses.

At PTB special semiconductor nanostructures were produced. These nanostructures are excited with short optical pulses taking certain symmetry conditions into account. By exploiting non-linear optical processes an electrical current is created in the semiconductor. In this process the charge carriers are not accelerated in an existing electric field as would be for a normal electrical current.

The pulses are measured via the simultaneously generated electromagnetic radiation: the pulses produce a polarization variation which acts as a source for electromagnetic radiation emitted into free space. Due to the ultrashort optical excitation the current pulses and radiated electromagnetic pulses are merely a few 100 fs in duration. Such short pulses contain frequency components of several THz which is why they are usually called THz pulses. The temporal shape of the emitted THz pulses is measured using electro-optic sampling methods.

In addition to possible interesting applications, this THz method is also employed for the investigation of light-matter interaction. In particular interesting effects of carrier and current dynamics in semiconductors are studied.

See publications for selected articles on this topic.

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Calibration of the time response of sampling oscilloscopes

High-speed sampling oscilloscopes are an important tool for the development of ultrafast electronic circuits in the information and communication industry. The intrinsic rise time of these oscilloscopes is very short, which allows the user to display ultrafast electrical transients. However, the oscilloscope rise time is finite and may distort the measured transients. In order to correct the distortion, the user needs to know the time response of the oscilloscope. For its measurement, methods are required with an even better time resolution. Therefore, PTB has developed an optoelectronic method that allows for the traceable measurement of the time response of sampling oscilloscopes.

For this purpose, ~1 ps short voltage pulses are generated on a coplanar waveguide with a photoconductive semiconductor switch, which is excited by 100 fs laser pulses. The voltage pulses are coupled into the oscilloscope via a microwave probe. Electro-optic sampling allows for the measurement of the voltage pulses on the waveguide with 300 fs time resolution. From such experimental data, the distortion that the voltage pulses experience on their way towards the oscilloscope can be calculated. In turn, the shape of the voltage pulses at the oscilloscope input can be computed. Deconvolution of the measured oscilloscope trace with the known input pulse yields the impulse response and transfer function of the oscilloscope.

Currently, the working group offers a calibration service for all 70 GHz und 100 GHz sampling oscilloscopes. 

See publications for selected articles on this topic.

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Press releases

Opens external link in new windowTime-resolved measurement of the anomalous velocity 12/2015

Movement of charge carriers perpendicular to an electric driving field in a solid state system detected for the first time with sub-picosecond time resolution

Opens external link in new windowVector network analysis using lasers 11/2015

Femtosecond lasers enable precise, cost-effective high-frequency measurements and could replace standard electrical devices in the future

Opens external link in new windowOptically steerable terahertz source 06/2013

Selected optical excitation of electron flows in semiconductors allows the controlled spatial orientation of electromagnetic radiation

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