Advanced Optical Spectroscopy of Semiconductor Nanomaterials

This MSc level module focuses on advanced optical laser spectroscopy techniques and their application to probe the fundamental physical, electronic, vibrational and optical properties of semiconductors, novel heterointerfaces and quantum confined nanostructures. Modern laser systems are capable of generating intense, highly coherent electromagnetic fields that interact with the electrons in a solid. Such light-matter interactions give rise to a fascinating range of phenomena, ranging from incoherent responses such as stead-state and ultra-fast luminescence to coherent dynamical responses like four-wave mixing (FWM), optical pumping and multi-dimensional time-resolved spectroscopy having sub-picosecond temporal resolution. Besides facilitating the direct characterization of semiconductor materials, novel-heterointerfaces and nanoscale devices, these methods provide direct information on fundamental opto-electronic processes such as electron transfer, energy relaxation and thermalization, tunneling and transport dynamics and the interactions between electrons in the solid and diverse (e.g. vibrational, spin and magnetic) degrees of freedom in the nanoscale solids.  We will discuss both far-field optical spectroscopic methods, that operate over length scales beyond the diffraction limit, as well as nano-optical approaches capable of probing systems at the size of the electronic wavefunction. The aim of this module is to introduce MSc students to the state-of-the-art in optical spectroscopic methods as they are utilized in the condensed matter and semiconductor physics research communities. We will introduce the underlying physics of the various methods, describe how they are implemented experimentally in the lab and examine specific case studiesfrom the literature that have led to key breakthroughs in condensed matter and semiconductor physics. 

Specific topics will include:

Review of key-semiconductor materials and fundamental light-matter interactions (2 lectures)

Incoherent Optical Spectroscopy Methods (5 lectures)

• Tools of the trade (CW and ultrafast-lasers, photo-detectors, monochromators and interferometers, signal detection / processing, cryogenics)
• Photoluminescence
• Nanoscale optical microscopy
• Spectroscopy of single semiconductor nanostructures

Coherent (Non-Linear) Optical Spectroscopy (4 lectures)

• Luminescence vs Reflection / Transmission Spectroscopy
• Semiconductor Bloch Equations and Coherence Effects
• Strong Excitation Effects
• AC Stark Effect and Transient Spectral Oscillations
• Examples (FWM, Photon echo, Resonance Fluorescence)
• Decoherence and Phase Relaxation in NWs (exciton-exciton, e-X and exciton-phonon interactions
• Raman and Brillouin Scattering

Ultrafast Optical Methods (3 lectures)

• Regimes towards equilibrium (relaxation, thermalization and recombination)
• Pump-Probe Spectroscopy Methods
• Probing Exciton and Phonon Dynamics in Bulk, QWs and QDs
• Exciton Dynamics (Pico and Femtosecond Studies)
• Light-emission and optical interactions in nanoscale environments

After successful completion of the module the students are able to:

1.Understand the rationale underlying incoherent and coherent optical spectroscopic methods as applied to commonly studied semiconductor-based materials and their nanostructures.

2.Explain the physics underpinning light-matter interactions in semiconductors, including: (a) incoherent luminescence spectroscopy, (b) methods to focus optical fields to the nanoscale,  (c) super-resolution methods to probe individual nanostructures, (d) coherent optical spectroscopy and (e) time-resolved methods with sub-picosecond temporal resolution.   

3. Sketch typical experimental set-ups used to perform different types of incoherent and coherent optical spectroscopy experiments and explain the operational principles of the methods. 

4. Discuss the different regimes of incoherent spectroscopy and the related phenomena that occur at weak, intermediate and strong optical excitation levels.

5.Explain the working principles of Raman and Brillouin light scattering experiments and give examples

6. Design optical experiments to probe specific characteristics of nanostructures, interpret optical data and judge experimental data presented in the scientific literature

No preconditions in addition to the requirements for the Master’s program in Physics.

In the thematically structured lecture the learning content is presented. With cross references between different topics the universal concepts in physics are shown. In scientific discussions the students are involved to stimulate their analytic-physics intellectual power.

In the exercise the learning content is deepened and exercised using problem examples , calculations and laboratory visits to explore how different types of spectroscopic measurements are made. Thus the students are able to explain and apply the learned physics knowledge independently.

Blackboard Lecture plus PPT presentations and discussion of research publications.  

·      Semiconductor Optics, C. F. Klingshirn, Springer 1995, ISBN 978-3-642-28361-1

·     Luminescence Spectroscopy of Semiconductors, I. Pelant & J. Valenta, Oxford University Press 2012, ISBN 978-0-19-958833-6 

·      Ultrafast Spectroscopy of Semiconductors and Semiconductor Nanostructures, J. Shah, Springer 1999, ISBN- 978-3540642268

·      Spectroscopy of Semiconductors, W. Lu and F. Ying, Springer 2018. ISBN 978-3-319-94952-9

·      Principles of Nano-Optics, L. Novotny and B. Hecht , Cambridge University Press 2006, ISBN 978-0-521-83224-3

·      Laser Spectroscopy 2, Wolfgang Demtröder, Springer 2015, ISBN 978-3-662-44641-6

·  + Research Articles from the Literature

There will be an oral exam of 25 minutes duration. Therein the achievement of the competencies given in section learning outcome is tested exemplarily at least to the given cognition level using comprehension questions and sample calculations.

For example an assignment in the exam might be:

• What governs the spatial resolution of far-field & near-field optical experiments?
• Explain what happens when a coherent optical field with an energy below & above the bandgap propagates in a semiconductor?
• Describe the principles of a coherent four-wave-mixing experiment - what information can it give you about a material ?
• Here is a typical luminescence spectrum of a strongly excited indirect gap semiconductor at 20K - how do you expect it will change as you (a) increase the pumping density and (b) reduce the temperature ?
• Explain what the terms "Raman active" and "IR active" mean ?
• Describe the processes and timescales by which photo excited hot carriers relax and thermalise in a time resolved optical experiment