"Bubble optics: Upward emerging ray paths", Applied Optics (Appl. Opt.), Vol.60(29), 9188-9199, M. Selmke, J.A. Lock, 2021.

A study on the reflection by floating bubbles and the associated caustics based on numerical and analytical methods.

"Bubble Optics: Leonardo’s cross revisited: Part 1, numerical methods", Applied Optics (Appl. Opt.), Vol.60(21), 6213-6225, M. Selmke, J.A. Lock, 2021.

A study based on numerical ray tracing of floating bubbles and their assiciated surprisingly rich light patterns (including caustics) below them.

The article provides the Cover image for the July 20th 2021 issue of the tri-monthly Applied Optics journal.

Caption: "Large bubble floating on water, illuminated obliquely by sunlight late in the afternoon. Various caustics appear and change with the water depth to form a multi-component light pattern known as Leonardo’s cross. For details, see the three-part paper by Selmke and Lock, pp. 6213–6242."

link to OSA's tweet.

The article was highlighted by the Editor of the journal (Applied Optics) as an "Editor's Pick".

According to the journal, this serves to highlight articles with excellent scientific quality which are representative of the work taking place in a specific field.

"Bubble Optics: Leonardo’s cross revisited: Part 2, paraxial analytical methods", Applied Optics (Appl. Opt.), Vol.60(21), 6226-6234, J.A. Lock, M. Selmke, 2021.

Study of approximations to the bubble refraction problem (astroid component of Leonardo's cross) using paraxial analytical methods.

"Bubble Optics: Leonardo’s cross revisited: Part 3, non-paraxial analytical methods", Applied Optics (Appl. Opt.), Vol.60(21), 6235-6242, J.A. Lock, M. Selmke, 2021.

Study of approximations to the bubble refraction problem (astroid component of Leonardo's cross) as well as one of the magically matching extra cusp using non-paraxial analytical methods.

"An optical n-body gravitational lens analogy", American Journal of Physics (Am. J. Phys.) Vol. 89, 11-20, 2021. preprint: arXiv:1911.03921 [physics.ed-ph]

A new type of multi-component gravitational lens experiment using liquid menisci around small objects. The article includes a brief review of the history of gravitational lens analogies in classical (refractive) optics. 

A figure of the article was chosen for the cover image of the January 2021 issue in which the article is published.

The figure shows the fundamental idea underlying the article.

"Ground rainbows: A cautionary tale for authors and editors alike", Physics Teacher (Phys. Teach.) Vol. 58, 372, 2020.

A comment on literature review, twitter and ground rainbows.

"Optical Caustics of Multiple Objects in Water: Two Vertical Rods and Normally Incident Light", Applied Optics (Appl. Opt.), Vol.59(26), 7981-7993, M. Selmke, J.A. Lock, 2020.

In investigation, theoretical and experimental, of the caustic metamorphosis of the caustic associated with two close-by water surface perturbations. E.g. two vertical rods standing in water or two bubbles.

"Bubble optics", Applied Optics (Appl. Opt.), Vol. 59(1), 45-58, M. Selmke, 2020.

preprint: arXiv:1909.04401 [physics.optics]

The manuscript describes the optical properties (in GO) of floating bubbles, including their relation to common axicons, the shadow sausage effect, gravitational lensing and other natural caustics with a similar unfolded axial line caustic (astroid shape).

correction: p.5, Section 6, the magnification M misses a factor (1/n). It should read correctly: M=-d_i/d_o*(1/n).

related: "Optics & Photonics News": Image of the week 19-09-16 / OSA-OPN tweet / OSA facebook post / instagram post.

The article was highlighted by the Editor of the journal (Applied Optics) as an "Editor's Pick".

According to the journal, this serves to highlight articles with excellent scientific quality which are representative of the work taking place in a specific field.

OSA Publishing Twitter Post

"Halo in the box: a macroscopic crystal arrangement to project mosaic halos", Applied Optics (Appl. Optics), Vol. 57(29), 8614-8623, M. Selmke and S. Selmke, 2018.

Similar to the "rainbox in the box" (see below), this paper describes in detail the construction and experiments with a Parry-halo box built from tiny hexagonal glass prisms. The article also studies virtual halo boxes (orientation classes: Parry, column, plate, Lowitz, random) via raytracing experiments.

The article provides the Cover image for the October 10th 2018 issue of the tri-monthly Applied Optics journal.

Caption: "Artificial cirrus cloud, consisting of 45 hexagonal glass prisms arranged in a 3D volume,

used to project mosaic Parry halos upon illumination. For details, see Selmke and Selmke, pp. 8614–8623."

link to OSA's tweet.

"Wine glass caustic and halo analogies", Applied Optics (Appl. Optics), Vol. 57(19), 5259-5267, M. Selmke, 2018.

An investigation of a particular hour-glass-shaped wine glass caustic using geometrical optics. The study provides results using an analytical description (using analogies to certain halos: Parry arc and circumzenithal arc) as well as raytracing parameter studies using Blender.

The article was highlighted by the Editor of the journal (Applied Optics) as an "Editor's Pick".

According to the journal, this serves to highlight articles with excellent scientific quality which are representative of the work taking place in a specific field.

link to OSA's tweet.

"Theory for controlling individual self-propelled micro-swimmers by photon nudging I: directed transport", Phys. Chem. Chem. Phys. (PCCP), Vol. 20, 10502-10520, M. Selmke, U. Khadka, A. Bregulla, F. Chichos and H. Yang, 2018.

This article is the first part of a two-part foundational theory contribution to the stochastic localization and voyaging scheme called "Photon-Nudging". Part I focuses on the time-statistics and derives mean velocities under the stochastic driving. Optimization is performed for the time of travel to a distant target location.

"Theory for controlling individual self-propelled micro-swimmers by photon nudging II: confinement", Phys. Chem. Chem. Phys. (PCCP), Vol. 20, 10521-10532, M. Selmke, U. Khadka, A. Bregulla, F. Chichos and H. Yang, 2018.

This article is the second part of a two-part foundational theory contribution to the stochastic localization and voyaging scheme called "Photon-Nudging". Part II focuses on the spatial statistics of a localized swimmer in a mean-field model. Optimization is performed for the localization precision around a target location.

"The rainbow in the box", American Journal of Physics (Am. J. Phys.), Vol. 86(4), 316-318, M. Selmke and S. Selmke, 2018.

We describe a novel demonstration experiment highlighting the collective-scattering and observer-specific aspects of the rainbow phenomena. Throughout a volume of a small box we randomly placed 50 acrylic spheres on black carbon fibre pillars. Under illumination of parallel white light and defocused observation under an angle of about 24° (the primary rainbow caustic angle for acrylic glass) segments of the artificial mosaic rainbow appear.

"A Note on the history of gravity tunnels", American Journal of Physics (Am. J. Phys.), Vol. 86(2), 153, M. Selmke, 2018.

A short comment on the legacy of gravity tunnels research / thought-experiments in the light of missing historical references in several published articles in the Am. J. Phys.

"Revisiting the round bottom flask rainbow experiment", American Journal of Physics (Am. J. Phys.), Vol. 86(1), 14-21, M. Selmke and S. Selmke, 2018.

This article describes in detail the phenomena connected with a common rainbow demonstration experiment, the so-called Florence(-flask) rainbow. Wall thickness related effects are treated as well as the rainbow from a water glass under inclined illumination.

A figure of the article was chosen for the Cover image of the January 2018 issue in which the article is published.

The figure shows a photograph of the Florence flask experiment we did. Visible is Alexander's bright band (yes, bright),

as well as the splitting of the primary rainbow order due to the flask wall.

"Thermal Diffusivities Studied by Single-Particle Photothermal Deflection Microscopy", ACS Photonics, A. Heber, M. Selmke, and F. Cichos, 2017.

This article discusses the application of the Photothermal Rutherford Deflection technique to the measurement of thermal diffusivities. In particular, deflection resonances caused by the pseudo-wave of the nano-scopic refractive index lens are probed as predicted by the theory outlined earlier in "Photothermal single particle microscopy using a single laser beam", Applied Physics Letters (Appl. Phys. Lett.).

"Artificial circumzenithal and circumhorizontal arcs", American Journal of Physics (Am. J. Phys.), Vol. 85(8), 575-581, M. Selmke and S. Selmke, 2017.

(preprint: arXiv.org > Physics > Atmospheric and Oceanic Physics > arXiv:1608.08664)

We describe a simple set of experiments which use a glass of water only. We show that they reproduce the circumzenithal, circumhorizontal and suncave Parry arcs, see the physics at home section. Although the experiment itself has been known at least since 1920 (see "Gilbert light experiments for boys", p. 98, Experiment No. 94), or the numerous google search results for "glass water table rainbow", it has often been put in the wrong context of a rainbow. We re-derive the main characteristics of the observed natural and artificial phenomena.

Leonardo Da Vinci also described this experiment: Anatomical Drawings at Winsor, 0118-r (ca. 1485 - 1510-15 / accessible in the e-Leo archive of the Biblioteca Leonardiana of Vinci).

A figure of the article was chosen for the Cover image of the August Issue in which the article is published.

The figure shows a photograph of a natural circumzenithal arc ice halo and the recreation via an illuminated glass of water.

(also shared on Am. J. Phys.' Facebook page)

"Complex artificial halos for the classroom", American Journal of Physics (Am. J. Phys.), Vol. 84(7), 561-564, M. Selmke and S. Selmke, 2016.

The article describes the construction and operation of a small modular halo machine (see physics at home section) and how to use it to generate artificial Lowitz-, tangential /circumscribed, Parry halos and (sub-)parhelias / the parhelic circle. Finally, a method of generating complex composite displays is described, and how to emulate the sky sphere. (Note: we used a 20mW blue laser diode)

see also the BoredPanda.com article for many more pictures.

"The physics of the photothermal detection of single absorbing nano-objects: a review", ArXiv, M. Selmke and F. Cichos, 2015. (to be submitted)

A comprehensive review of the existing models on the photothermal signal generation mechanism, including new results on the signal modification above interfaces.

"Artificial Halos", American Journal of Physics (Am. J. Phys.), Vol. 83(9), 751-760, M. Selmke, 2015.

The article gives an introduction to ice halos and describes various experiments which artificially generate certain halos using a glass prism commercially available through Edmund Optics. Artificial Parhelic Circles and various Parhelia along with responsible ray paths are discussed in detail. The article also describes an artificial 22° ring halo machine (a random reorientation device based on an Arduino microcontroller and 3 steppermotors, see below).

A figure of the article was chosen for the Cover image of the September issue in which the article is published. The image shows an illuminated  transparent acrylic juggle ball with its corresponding caustic and an illuminated hexagonal BK7 glass prism with vividly colored columns of light emerging from it.

The halo articles were covered in a University Press article. It has been reproduced on the Faculty homepage, and various other news sources online such as myscience.de, innovations-report.de, informationsdienst Wissenschaft, pro-physik.de. The article appeared in the Leipziger Volkszeitung / LVZ, issue 01/09/2015, page 15, see article in the LVZ online archive. It also  appeared printed in the magazine "Liebigstraße aktuell", issue 18/2015.

"Hot Brownian motion and photophoretic self-propulsion", diffusion-fundamentals.org Vol. 23(1), 1-19, R. Schachoff, M. Selmke, A. Bregulla, F. Cichos, D. Rings, D. Chakraborty, K. Kroy, K. Günther, A. Henning-Knechtel, E. Sperling, M. Mertig, 2015.

Article summarizing the progress within the SFG Research Unit FOR 877, "From Local Constraints to Macroscopic Transport". It covers hot Brownian motion, photothermal correlation spectroscopy and microscopy as well as artificial swimmers, amongst other topics.

"Thermal diffusivity measured with a single plasmonic nanoparticle", Physical Chemistry Chemical Physics (PCCP), accepted, A. Heber, M. Selmke, F. Cichos, 2015.

This article presents measurements of thermal diffusivities of liquids and solids based upon frequency-dependent photothermal microscopy. See also [Appl. Phys. Lett. 105, 0135011, M. Selmke et al., 2014].

"Mie scattering by a refractive 1/r inhomogeneity: electromagnetic scattering by the infinite Coulomb-like scatterer", Journal of Quantitative Spectroscopy & Radiative Transfer (JQSRT, special issue on accompanying the LIP2014 conference), 162, 175-183, M. Selmke, 2015.

This is a theoretical study of the plane wave scattering characteristics for weak electromagnetic scattering by a 1/r refractive index inhomogeneity. Shaped beams are shown to resolve the divergences known from Coulomb scattering.

"Comment on: "Optimal detection angle in sub-diffraction resolution photothermal microscopy: application for high sensitivity imaging of biological tissues"", Optics Express (Opt. Express) Vol. 23, Issue 5, p. 6747-6750, M. Selmke and F. Cichos, 2015 (republished: Virtual Journal for Biomedical Optics, Vol. 10, Issue 4, 2015)

This comment points out several fundamental misconceptions in the theoretical treatment of the photothermal signal as done in the article [Opt. Express, Vol. 22, Issue 16, p. 18833–18842, Miyazaki et al.,2014]. 

The reply to the comment, [Opt. Express, Miyazaki et al., Vol. 23, Issue 5, p. 6751-6753, 2015], suggests that the weakness of the scattering resolves the inconsistencies. This is not accurate and misses the points raised in the original comment.

"Photothermal single particle microscopy using a single laser beam", Applied Physics Letters (Appl. Phys. Lett.), Vol. 105, 0135011, M. Selmke, A. Heber, M. Braun, F. Cichos, 2014

The article introduces a variant of photothermal microscopy for which only a single laser beam is necessary. The method rests on a detailed understanding of the frequency-dependence and phase of the signal.

"Energy-Redistribution Signitarures in Transmission microscopy of Rayleigh and Mie particles", Journal of the Optical Society of America A (JOSA A), Vol. 31, No. 11, pp 2370-2384, M. Selmke and F. Cichos, 2014 (republished in the Virtual Journal for Biomedical Optics, Vol. 9, Issue 13, 2014)

This article describes in detail the interaction of a focused beam with a spherical particle of arbitrary size and refractive index. Particularly, the transmitted power is considered. The special case of small Rayleigh-paticles gives an extension of the optical theorem and provides useful expressions for photothermal microscopy and high frequencies.

"Metal nanoparticle based all-optical photothermal modulator", ACS Nano, Vol. 8, No. 2, pp 1893-1898, A. Heber, M. Selmke, F. Cichos, 2013.

The article describes how the photothermal effect may be utilized to modulate light by light without non-linearities. Driving a liquid-crystalline first-order phase transition allows to modulate the probe beam effectively via the dissipative heating of a gold nanoparticle using a heating laser.

"Photothermal single particle Rutherford scattering microscopy", Physical Review Letters, (Phys. Rev. Lett.), Vol. 110, 103901, M. Selmke, F. Cichos, 2013.

The letter-format short article presents a new interpretation and detection scheme in transmission photothermal detection based on an optical analogon to quantum mechanical Coulomb / Rutherford scattering of matter wave-packets.

The article was promoted by the editors of the journal and carries the label "editors suggestion".

"Photonic Rutherford Scattering: A Classical and Quantum mechanical analogy in Ray- and Wave-optics", American Journal of Physics (Am. J. Phys.), Vol. 81, No. 6, pp 405-413, M. Selmke and F. Cichos, 2013.

This padagogical article explores the analogy between the optical scattering by a thermal lens and the particle scattering from a Coulomb potential. The analogy is discussed in the high and low energy limit.

An image from the supplementary media files to the article was chosen as the cover figure for the June 2013 issue of the Amercian Journal of Physics. 

The supplementary files are free and can be found uder "Article Objects (20)" → "Supplementary Files (EPAPS)"

I have defended my PhD thesis on Monday 7th October 2013, and received my PhD on October 14th. The title is 

"Photothermal Single Particle Detection in Theory & Experiments".

The thesis gives a complete account of the photothermal signal of single particles in transmission-type photothermal microscopes. It consistently combines various theoretical approaches and gives several applications and experimental proof-of-principles. You may find it:

published online, downloadable as a .pdf-file.

"Photothermal Signal Distribution Analysis (PhoSDA)", Physical Chemistry Chemical Physics (PCCP), Vol. 15, pp. 4250-4257, M. Selmke, R. Schachoff, M. Braun, F. Cichos, 2013. (advance article)

In this methodological article the signal statistics for the photothermal signal are discussed for solutions of absorbing nano-particles.

"Twin-Focus Photothermal Correlation Spectroscopy", RSC Advances (RSC Adv.), Vol. 3, pp. 394-400, M. Selmke, R. Schachoff, M. Braun, F. Cichos, 2013

A methodological article on the correlation spectroscopy of diffusing particles detected by photothermal microscopy. It specifically used the novel twin-focus split detection volume.

"Gaussian Beam Photothermal Single Particle Microscopy" (oa), Journal of the Optical Society of America A (JOSA A), Vol. 29, No.10, pp. 2237-2241, M. Selmke, M. Braun, F. Cichos, 2012.

A theoretical article developing a simple beam transformation formalism for Gaussian beams probing a thermal lens in photothermal microscopy. Analytical in nature, it allows the interpretation of the photothermal signal to originating from a lensing action.

"Nano-lens Diffraction around a Single Heated Nano Particle", Optics Express (Opt. Express), Vol. 20, No. 7, pp. 8055-8070, M. Selmke, M. Braun, F. Cichos, 2012, 

same article republished in the Virtual Journal for Biomedical Optics, Vol. 7, Iss. 5, 2012.

This article describes a simple model based on the solution to the paraxial Helmholtz equation (i.e. Kirchhoff-Fresnel diffraction) for a Gaussian beam scattered by a thermal lens. The photothermal signal is shown to be aperture dependent and invertable.

ERRATA: "Nano-lens Diffraction around a Single Heated Nano Particle: errata",Optics Express (Opt. Express), Vol. 21, No. 21, pp. 25344-25345

"Photothermal Single Particle Microscopy: Detection of a Nanolens",  ACS Nano, Vol. 6, No. 3, pp 2741–2749, M. Selmke, M. Braun, F. Cichos, 2012.

This article gives the first theoretical and quantitative description of the photothermal signal of single absorbing nano-particles. The thermal lens is demonstrated to act as a lens and is treated in a rigorous Lorenz-Mie scattering framework including focusing with aberrations.

"Temperature dependent Single Molecule Rotational Dynamics in PMA", Physical Chemistry Chemical Physics (PCCP), Vol. 13, 1849-1856, S. Adhikari, M. Selmke and F. Cichos, 2011.

This article gives, through experimental results compared to a new model, evidence for dynamical heterogeneity in polymers near the glass-transition temperature.

"Theory of Hot Brownian Motion", Soft Matter, Vol. 7, 3441-3452, D. Rings, M. Selmke, F. Cichos and K. Kroy, 2011.

This theoretical article gives the details behind the theory of hot Brownian motion.

"Hot Brownian Motion", Physical Review Letters (Phys. Rev. Lett.), 105, 090604, D. Rings, R. Schachoff, M. Selmke, F. Cichos, K. Kroy, 2010.

A letter-format short article introducing hot Brownian motion and the basic theoretical concepts describing it.

"Measuring Flow Profiles and Slip by Single Molecule Tracking Experiments in Thin Liquid Films", The Journal of Physical Chemistry C (J. Phys. Chem. C), Vol. 114 (10), pp 4479-4485,A. Schob, M. Pumpa, M. Selmke and F. Cichos, 2010.

In this article, the diffusion characteristics of single tracked particles in a periodically sheared thin film are discussed based on experimental results and compared to theory.

"Papas Pfannekuchen", Pnemologische Leckerbissen, pp 63-64, Markus Selmke, ca. 1993.

This is a pancake recipe.

• Illustrated article on BoredPanda about the Halo-Machine and the spherical projection screen experiments.

• Episode 5 of the BBC documentary series "World's Weirdest Events" featured the same BK7 glass crystal I have used for the two publications in Am. J. Phys. and Appl. Opt. (see list above). The segment starts at around 45 mins (watch it on youtube) into the hour-long episode and shows some footage of the 22° halo and sundogs. The presenter of the show, Chris Packham (see image on the right), uses the crystal to explain the refraction aspect of the phenomenon. Unfortunately, the sundog formation explanation is misleading / faulty and suggests the alignment of plate crystals in a layer. The correct explanation can be found on www.atoptics.co.uk.

• The halo articles were covered in a University Press article. It has been reproduced on the Faculty homepage, and various other news sources online such as myscience.de, innovations-report.de, informationsdienst Wissenschaft, pro-physik.de.

               The article appeared printed in the magazine "Liebigstraße aktuell", issue 18/2015, see image on the right.

     An article related to that press release was published in the local newspaper, i.e. the Leipziger Volkszeitung / LVZ, issue 01/09/2015, page 15,

          see article in the LVZ online archive. 

• My image of Kelvin Helmholtz instabilities was posted on the facebook page of the Nasa's Earth Observatory.