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Femtosecond Measurement Techniques and Nanomagnetism

Working Group 2.52

Profile

The Working Group Femtosecond Measurement Techniques and Nanomagnetism works 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 and photodiodes with a nominal bandwidth of up to 100 GHz.

The working group also does research and development for precise measurements of nanomagnetic quantities. The main fields of work in this context are measurements of magnetization dynamics, local stray field measurements, as well as measurements of magneto thermo electric properties of magnetic nanostructures.

Finally, basic research on carrier and current dynamics in semiconductors is persued. Such studies might prove useful for the future characterization of electrical high-frequency components.

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Research/Development

Development of time-resolved optoelectronic measurement 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.

Magnetic imaging

In the field of magnetic imaging we focus on the characterization of nano patterned magnetic structures by magnetic force microscope (MFM) supported by measurements based on SQUID susceptometry, magnetotransport and magneto optics. Our priorities are (i) the development of quantitative measurements for nano-scale field measurements and (ii) the use of MFM for the characterization of magnetic nanostructures and nano devices.

Magnetisation dynamics

An important question for the future enhancement of magnetic storage media is the maximum speed of with which digital data can be written. As this write operation is based on the magnetization reversal of a magnetic memory cell this question is directly related to a fundamental physical question: What is the physical ultra fast limit of magnetization reversal speed? In our present research project this question is addressed. A time resolved Opens internal link in current windowmagneto transport setup is used to detect the ferromagnetic Opens internal link in current windowprecession in magnetic memory devices. This allows us to obtain information about ultra fast Opens internal link in current windowswitching of the magnetization.

Time-resolved light-matter interaction

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 all of the topics mentioned above.

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Services

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.

Similar techniques are also used for the characterisation of the time response of ultrafast photodiodes. Currently, the working group offers a calibration service for sampling oscilloscopes and photodiodes with a nominal bandwidth of up to 100 GHz. 

 

See publications for selected articles on this topic.

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Information

Opens external link in new windowThermometer for nanocircuits 1/2018

Magnetic tunnel junctions enable absolute, time-resolved temperature measurements of nanocircuits

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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