Ultrashort laser pulses are a fascinating tool for observing and manipulating atomic and molecular processes on their intrinsic time scales (femto- to attosecond time scale). In addition to the generation of femto- and attosecond laser pulses, the shaping of these pulses plays an important role today. The ability to tailor ultrashort laser pulses practically at will in amplitude, phase and polarisation forms the basis of coherent control, i.e. the control of ultrafast quantum dynamics (see Virtual Femto Laboratory Part 2), such as electronic excitations of atoms, the spatial alignment of molecules or the targeted breaking of molecular bonds.

This first part of the "Virtual Femto Laboratory" series of experiments provides the basics of a modern femtosecond laser laboratory in three simulation modules and introduces the theoretical description of ultrashort laser pulses. The first module is dedicated to the generation of such pulses in a typical Ti:Sa femtosecond oscillator. In the second module, the oscillator pulses are spectrally phase-modulated and the operation of a 4f Fourier transform pulse shaper is worked out. Finally, the third module is used to measure the shaped laser pulses using various characterisation methods, such as autocorrelation, spectral interference or spectrogram-based methods (FROG).

Keywords: Physical: laser, frequency comb, mode coupling, 4f setup, liquid crystal modulator, dispersion, Mach-Zehnder interferometer, autocorrelation, spectrometer, FROG (recommended for physics students)
Mathematical: Fourier transform.

Dr Tim Bayer, AG ULTRA

The experiment introduces the wavelength-selective manipulation of electromagnetic radiation using optical filters such as neutral density filters or short- or long-pass filters. The targeted adjustment of the refractive index and layer thickness of anti-reflective layers is analysed using a reflection measurement on a solar cell equipped with an anti-reflective layer. Interference effects in the optical transmission of thin semiconductor layers on glass are used to determine layer thicknesses of a few hundred nanometres with low error. An extensive characterisation of the optical semiconductor properties is achieved by deriving the spectral refractive index and the absorption coefficient with determination of the optical band gap of the semiconductor thin film.

The aim of the experiment is an in-depth study of the interaction of electromagnetic radiation with matter. The so-called Swanepoel method is learnt by studying recent scientific literature and applied to the students' own optical transmission measurements to determine the film thickness, refractive index, absorption coefficient and band gap of semiconductors.

The experiment is supervised in English.

Diksha Diksha; AG Raspe

The interaction of radiation with matter is studied using the absorption of light in semiconductors. The photogeneration of electrons and holes associated with absorption and the associated photocurrent form the basis for the functioning of many semiconductor components such as sensors or solar cells. In this experiment, some semiconductor parameters that are important for optoelectronic applications are discussed and experimentally determined using hydrogenated, amorphous silicon thin films. A spectrophotometer is used to measure the spectral transmission of the silicon samples. This allows the layer thickness, the spectral absorption coefficient, the spectral refractive index and the band gap of the absorber to be determined. With regard to the electronic properties, the position of the Fermi energy is determined from the measurement of the electrical dark current. Evaluation data from the above-mentioned optical transmission are subsequently used to determine the photoconductivity and the product of the mobility and lifetime for the majority charge carriers from the measurement of the photocurrent. The steady-state photocarrier grating method utilises the electronic transport properties due to the inhomogeneous carrier generation to derive the mobility-lifetime product of the minority carriers. These mobility-lifetime products correlate with the defect density of the sample and can thus be discussed as a quality criterion for the investigated semiconductor sample.

Test dates: 17.02.26 / 18.02.26 and 19.02.26 / 20.02.26

Rudi Brüggemann

Absorption spectra of molecules generally have a characteristic band structure, which is composed of the electronic transitions known from atoms as well as a superimposed vibrational and rotational structure. A corresponding description of the band spectra can be derived from the theory of molecular physics (Born-Oppenheimer approximation, Franck-Condon principle, ...).

In this practical experiment, characteristic quantities of the iodine molecule are to be determined using absorption spectra. For this purpose, the basics of optical spectroscopy will first be familiarised. After characterising the measurement set-up, temperature-dependent absorption spectra of an iodine gas will be recorded. Subsequently, selected lines will be assigned to the corresponding transitions and specific quantities such as the dissociation energy will be determined.

The experiment will be supervised in English.

Katem Mitkong, AG UNO

By interacting with light, matter can be brought into an electronically excited state by absorbing a photon. Upon relaxation to the ground state, a photon can in turn be emitted, whereby this is referred to as photoluminescence. The lifetime of the excited state can be between a few 100ps to ns (fluorescence) or even ms to hours (phosphorescence). In the practical experiment, the dye oxazine is analysed in various solvents using absorption and fluorescence spectroscopy. Time correlated single photon counting (TCSPC) will also be used to measure the fluorescence lifetime and to determine depolarisation times using polarisation-dependent investigations.

Antonietta de Sio, AG UNO

Particle Image Velocimetry (PIV) is an optical measurement method for the non-contact detection of fluid flows. It enables the quantitative investigation of fluid mechanical processes without disturbing them. This experiment is intended to provide an introduction to the methodology of PIV and demonstrate its possible applications, which have become increasingly important in recent years.
In the experiment, an optical setup is created to illustrate the basic properties of particle image velocimetry. The experiment begins with the characterisation of the Nd:YAG laser used. The laser beam is then shaped so that it can be used for PIV measurements. For a simple, first application, scattering particles are introduced into the previously formed light section and a laminar flow is simulated, which is analysed using discrete cross-correlation and the accuracy of the PIV method is determined. Finally, a water channel is used to characterise the wake of cylinders using PIV.

Simon Meckelnborg AG TWIST

The cup anemometer is the most frequently used sensor for measuring atmospheric wind speeds. Since its invention in 1846, many changes and optimisations have been made to the anemometer in order to improve the response behaviour in turbulent flows. Due to its construction, however, the anemometer will always overestimate the speed when the wind drops rapidly - this effect is called "over-speeding".

In this experiment, a cup anemometer is to be constructed using simple means. In addition to the shape of the cup, the symmetry of the structure is fundamental to the behaviour of the anemometer. These elements of the anemometer are to be systematically varied and characterised in the wind tunnel. A calibration is to be carried out for all configurations in order to subsequently discuss the differences. In addition, the behaviour in the event of rapid changes in speed is to be recorded and linked to the structure. Based on the basic equations of motion, theoretically expected results are to be compared with the measurement results. This requires an understanding of the forces and torques at work.

Michael Hölling, AG TWIST

In this experiment you will learn basic techniques of modern light-optical astronomy. The basic task is to determine the position, orbital parameters and brightness of minor planets. The selection of objects will be planned using planetarium software and the selected object will be observed during the semester. You will learn how to use robotically controlled telescopes in Chile and Tenerife to observe the celestial objects. The observations are not divided into two laboratory days of eight hours each, but run the entire semester with little weekly effort.

Prerequisite for the internship: Attendance of the astrophysics lecture

Compulsory introductory course on 30.10.2025: 10.00 to 12.00

Athleen Rietze, Vanessa Delfs, Matti Gehlen, Med. str. phys.

Polaritons are quasiparticles that arise from strong interactions between light and matter excitations (such as crystal vibrations, electron gas oscillations, magnetic spin waves, etc.). The research fields of photonics, optics and quantum computing have all identified polaritons as a common fundamental quantum system of particular significance. Given the currently rising interest in quantum computing and in optical information processing, polariton devices are also more and more often viewed in terms of their applicability as optical processing units.

In this experimental work, the students engage in the optical characterisation of a semiconductor polaritonic device. The sample under study is a monolithic microcavity, which can be thought of as a two-mirror assembly (a resonator) that confines the light within. An optically active two-dimensional quantum system is embedded into the centre of the cavity. This active material is a quantum well, which is made from alternating semiconductor layers (having different bandgaps). At very low temperatures (~10 Kelvin above absolute zero), a fundamental matter excitation, in particular, an exciton, can emerge in this quantum well. Excitons are Coulomb-bound electron-hole pairs that exhibit an optical transition dipole moment and thus can be optically accessed. When a strong electromagnetic field overlaps with the exciton, a new hybrid particle is formed - an exciton-polariton. The experimental task is to identify the manifestation of exciton-polaritons via low-temperature optical spectroscopy, which include photoluminescence and reflectivity probing of the sample.

The aim of the work is to give students an opportunity to dive into the world of modern photonics and experimental optics, to gain first experience in the handling of low-temperature equipment, and to gain some insights into current research interests of the polariton community. As much as the nature of its subject, polariton science is hybrid and consists of a wide range of topics such as: material science, photonics, classical and quantum optics, solid state and condensed matter physics. In this case, the subject offers a lot of appeal for anybody interested in the listed topics.

The experiment will be supervised in English.

Victor Mitryakhin AG Quantum Materials

Solid-state lasers are the "workhorses" of laser sources in industry, medicine and science. Applications include precision measurement, material ablation, welding, surgery, light-matter interactions and quantum control. In this experiment, a diode-pumped continuous wave laser resonator with an Nd:YAG crystal as the active medium is set up, adjusted and characterised from scratch. In preparation, various laser resonators are simulated using Gaussian optics and then realised experimentally. The characterisation includes the analysis of transverse electromagnetic modes (TEM) and the measurement of the efficiency. Finally, a non-linear frequency doubling is realised inside and outside the resonator. The aim of the experiment is to familiarise students with the fundamentals of laser technology and to directly implement the construction and characterisation of a laser resonator.

Lars Englert, AG ULTRA

LASER stands for Light Amplification by Stimulated Emission of Radiation. The basic principle of any laser is the stimulated emission of light, which causes light amplification while maintaining frequency, polarisation and phase. Modern semiconductor lasers, often referred to as laser diodes or diode lasers, are among the most efficient laser systems.

In this experiment, we will first familiarise ourselves with the basics of laser diodes. Then the characteristic current-voltage curve and the spectrum of a laser diode are to be determined. In addition, the temperature-dependent behaviour of the laser diode is to be investigated. The changes in the characteristic curves and spectra of the laser diode as a function of temperature will then be discussed.

The experiment will be supervised in English.

Juanmei Duan, AG UNO

Atomically thin semiconductor layers of transition metal dichalcogenides (TMDCs) have extraordinary optical properties that make them interesting as an active medium in nano-lasers, for example. A key feature is the transition from an indirect to a direct semiconductor, when the number of atomic layers in the material is gradually reduced to a single layer. In addition, the exciton binding energy increases many times over. The resulting physics of electrons and holes in two-dimensional systems can be excellently investigated by means of absorption measurements. The transition to a direct semiconductor for the monolayer results in a sharp increase in the photoluminescence quantum yield, which is directly detectable in the experiment.

The aim of the experiment is to produce atomically thin semiconductors by removing individual crystal layers ("mechanical exfoliation") in the experiment. Their exciton spectrum is then measured using high-resolution white light spectroscopy and photoluminescence.

This experiment offers students the opportunity to familiarise themselves with the basic optical properties of atomically thin semiconductors based on their experimental signatures. They learn how to produce these novel materials by exfoliation and can gain in-depth experience in handling optical experimental setups.

The experiment will be supervised in English.

Keywords: Semiconductor, TMDC, exfoliation, excitons, optical spectroscopy

Dr Martin Esmann, Dr Hangyon Shan, AG Quantum Materials

In this experiment a shoebox shaped scale model of a room will be used to explore and understand the main properties of the acoustics in a room. In a first part the modal structure of the empty room in form of standing waves will be calculated and determined experimentally for low frequencies. Secondly, the effect of modifying room acoustical properties will be explored. What is the effect of altering the boundary conditions of wave propagation by placing damping material on the rigid walls, of introducing an extra wall in the room, and of inserting damping material between the loudspeaker as source and the microphone as receiver? In a third part, the statistically describable room acoustical behaviour at high frequencies will be investigated and some statistical parameters that govern room acoustics at high frequencies will be measured.

Steven van de Par, Siegfried Gündert; AG Acoustics (Fac. VI)

Non-linear optics deals with the interaction of light with matter, which only occurs at particularly high field strengths. This violates the principle of superposition, as overlapping beams influence each other, which completely contradicts our everyday experience.

Wave packets of light can exchange energy and new frequency components are created by frequency doubling, sum or difference frequency generation.

Pulsed lasers can be used to bundle sufficient energy in time and space to achieve the intensities of GW/cm² to TW/cm² required for these effects.

In this experiment, various phenomena of non-linear optics are used to generate a colour-tunable beam in the visible spectral range from an infrared laser beam. In this way, (almost) all the colours of the rainbow are generated from light that is invisible to the human eye. Thanks to their colour tunability, the laser pulses generated in this way have become indispensable in today's laser research laboratories and complement other broadband light sources such as titanium-sapphire lasers.

You can learn to construct beam paths with mirrors and lenses, to vary intensities in a targeted manner and to superimpose beams precisely in order to excite non-linear processes efficiently. You will familiarise yourself with the central mechanisms used in current laser laboratories - from frequency mixing and self-phase modulation to dispersion and phase adjustment using birefringence.

Arvid Klösgen, Attosecond Microscopy Group

In the field of radiotherapy, linear accelerators are used to irradiate tumours. Each radiotherapy treatment is planned individually for each patient. The metrological verification of irradiation plans is of great importance.

As part of the virtual practical course, participants learn about radiation measurements on the accelerator with ionisation chambers, semiconductor detectors and ionisation chamber arrays. On the one hand, theoretical principles are learnt, and on the other, experiments are provided that can be followed online and put the theoretical principles into practice. Depth dose curves and dose profiles of photon fields of different sizes are measured and the absorbed dose to water is determined in accordance with DIN 6800-2. Furthermore, exemplary patient plans are verified metrologically.

As part of the practical course, students will gain an insight into the research focus of the Medical Radiation Physics working group and become familiar with typical questions from everyday clinical practice.

The practical course comprises 2 independent experiments which are carried out and assessed separately. The experiments are expected to be carried out within the first 2 weeks in March 2026. A corresponding preliminary discussion will be held in advance during the current winter semester.

Vanessa Delfs, Andreas Pflaum, AG Medical Radiation Physics (Faculty VI)

The block practical course takes place directly after the end of the lecture period of the winter semester (end of February / beginning of March) and comprises a total of 6 experimental days (corresponding to 3 experiments). It teaches the theory and practice of digital signal processing in acoustics and audio signal processing by means of experiments with computers and acoustic signals.

In the first week (5 experimental days), the basics such as analogue and digital signals, AD/DA conversion, spectral analysis and discrete Fourier transformation, convolution and digital filters are dealt with in practice. A seminar is held in the morning to teach the theory in discussion with the participants. In the afternoon, the material will be deepened by means of experiments on the computer.

Subsequently, projects are carried out in small groups, which are presented in the morning seminar (one experimental day per group). Possible project topics include adaptive filters, analysing non-stationary signals, data compression in digital systems, speech recognition, signal classification, blind source separation, perceptual audio coding and others.

Gerald Enzner, Stephan Ewert, Jörn Anemüller, (Simon Doclo, AG SIGPROC (Fak. VI))

The block practical course takes place in the middle/end of September and comprises a total of 6 trial days (corresponding to 3 trials). Contents are: Fundamentals and application of physics, psychophysics and neurosensorics, especially in hearing: fundamentals and methods of signal processing; anatomy, physiology, pathology and diagnostics of hearing; absolute and differential perception of sound; masking; signal detection theory; binaural hearing; speech intelligibility; acoustically evoked potentials; functional magnetic resonance imaging; otoacoustic emissions.

Stefan Uppenkamp, AG MEDI (Faculty VI)