Dutch Online Optics Colloquium

Recently, we applied wavefront shaping techniques to implement quantum-secure readout of a physical unclonable key for authentication [1] and communication [2]. Unfortunately, spatial wavefront shaping is unsuitable for long-distance use. We therefore propose to use wavefront shaping in the time domain using a single spatial mode. We show spatiotemporal control of a pulse through a complex medium. Furthermore, we investigate a secure communication method based on physical unclonable functions in the time domain.

[1] S.A. Goorden et al., Quantum-secure authentication of a physical unclonable key, Optica 1, 421-424 (2014).
[2] R. Uppu et al., Asymmetric cryptography with physical unclonable keys, Quantum Sci. Technol. 4, 045011 (2019).

Tetracene is a promising material for raising the quantum efficiency of solar cells above 100% thanks to a process known as singlet fission (SF). During SF one singlet exciton splits into two triplet excitons, doubling the number of photo-generated energy carriers. The optimal thickness of a SF layer in these solar cells is a trade-off between the absorbed photons and the number excitons reaching the interface, which is determined by the exciton diffusion length. Therefore, it is very important to understand (and even improve) exciton diffusion in tetracene in order to design the best possible SF solar cells.

We use two different microscopy techniques to probe the diffusion of singlet and triplet excitons in tetracene crystals. To study the bright, fast diffusing singlets (the hares), we use fluorescence imaging microscopy, while the dark triplets (tortoises) are studied by spatial resolved transient absorption microscopy. Surprisingly, the tortoises (triplet excitons) seem to diffuse faster than the hares (singlet excitons) that even appear to have a negative diffusion . We investigated the causes of this paradox and find that the origin lies in non-linear triplet-triplet annihilation.

Stochastic resonance is a phenomenon where noise plays a constructive role in signal transmission. It arises due to synchronization of random switching events in a bistable system with a weak periodic signal. Recently we demonstrated non-Markovian dynamics in an oil-filled laser-driven cavity [1]. The thermo-optic nonlinearity of the oil is associated with noninstantaneous effective photon-photon interactions. The deviation from a Markov approximation is characterized by the residence time distribution, which is no longer exponential on timescales comparable to the thermal relaxation time. This indicates level crossings which are correlated in time, a signature of non-Markovian dynamics. 

Here, we demonstrate the first observation of non-Markovian stochastic resonance in an oil-filled optical microcavity. The phenomenon is evidenced by a peak in the signal-to-noise ratio versus noise intensity. Furthermore, we study the role of the thermal relaxation time by means of simulations. We show that the frequency range for which the conditions for stochastic resonance hold is broadened as the thermal relaxation time increases.

[1] Z. Geng et al., PRL 124, 153603 (2020)

In this talk I will discuss a dispersive imaging method for trapped quantum gases based on digital off-axis holography. Both phase delay and intensity of the probe field are determined from the same image. Due to the heterodyne gain inherent to the holographic method, it is possible to retrieve the phase delay induced by the atoms at probe beam doses two orders of magnitude lower than phase-contrast imaging methods. Using the full field of the probe beam, we numerically correct for image defocusing. The talk is concluded by discussing some measurements which were previously considered impossible, but enabled by this new imaging method.

J. Smits, A. P. Mosk, and P. van der Straten, "Imaging trapped quantum gases by off-axis holography," Opt. Lett. 45, 981-984 (2020)

Moore's law is the observation that the number of transistors in a dense integrated circuit doubles about every two years. What happens with this is that structures in the circuits have reached extremely small dimensions, down to the few nanometer level. But to make this possible a tool is needed to “print” this circuit and build a network wish such small structures. The tool used nowadays is light. Focused light can reach a very small dimension. The smallest dimension we can reach with light, called “critical dimension” (CD), is proportional to its wavelength lambda . Needless to say then, the challenge is not only to generate light at such short wavelengths in a controlled way and in useful quantities, but also to be able to handle this light for printing. The semiconductor company ASML is using Extreme Ultra Violet (EUV) light at  wavelength to print nanostructures on substrates, which allows high volume manufacturing of chips with the smallest structures presently possible.

There are many ways of producing EUV light, including synchrotron radiation, free electron lasers, and high-harmonic generation. But chipmakers are using another technology: laser produced plasma sources (LPP). In particular, they produce plasma by shooting an infrared laser onto tiny tin droplets (few tens micron diameter). This plasma emits EUV light that is then collected and used for lithography (this word comes from “lithos” = stone + “graphein” = to write). The efficiency in this process is strongly dependent on the laser parameters. The illumination strategy affects the EUV yield, pollution in the droplet chamber (debris or ions generated from laser impact) and therefore the chip-making machine efficiency and lifetime.

Here in ARCNL (Advanced Research Center for Nano-Lithography) we have developed a laser that is capable of delivering infrared (1064 nm) laser light onto tin droplets in an extremely wide range of configurations. This is made possible by the capability of our system to shape laser temporal profiles at our will. We can have Gaussian and square top hat profiles with durations between 0.5 ns and 1.1 ms, and arbitrary temporal shapes with features as short as 0.25 ns. We can also have multiple-pulse configurations, where every pulse can by tuned independently in shape and energy. The system can deliver a maximum energy of 490 mJ for pulses from few ns total duration. For  pulses, we achieve a peak power of 0.6 GW, and an average power of 44 W when running at 100 Hz pulse repetition frequency. This paves the way to studies that are otherwise impossible (or require multiple lasers) on droplet deformation regimes, a key component in efficient EUV light generation.

In my talk I will show the laser system building blocks and how we perform this shaping, together with the effect on tin droplets of different laser configurations.

Circular dichroism (CD) is the differential absorption of left and right circular polarized light of chiral molecules and matter.

CD spectroscopy can be used to study stereochemistry of molecules and secondary structures of proteins.

However, CD signals are weak, which is currently restricting the optical measurements to ensembles of many molecules.

Furthermore all existing techniques do not measure CD absorption directly, but rely on transmission or extinction. We present a new optical technique, based on photothermal microscopy that directly measures absorption and therefore provides a direct measure of circular dichroism.

As a proof of principle, we studied single chiral plasmonic nanostructures with

CD resonances in the visible range, showing an unprecedented signal-to-noise ratio.

This sensitivity will allow the extension of circular dichroism to study individual nano-objects and molecules in low-concentration regime.

Recent results of our group identified the molybdenum impurity in SiC as a spin-active emitter near telecom wavelengths, suitable for quantum information applications [1,2]. We observed spin-relaxation times T₁ exceeding seconds at 2 K and coherence times T₂* of hundreds of nanoseconds. Together with this defect's bright optical transition near telecom wavelength these properties allow for efficient interfacing with neighboring quantum systems. The long T₁ lifetime is owed to the C_3v symmetry combined with strong spin-orbit coupling, which causes anisotropy in the g-factor, which in turn suppresses the interaction with other paramagnetic impurities. Studying the temperature dependence of T₁ enables us to identify localized vibrational modes as the main source of spin flips above 4 K, and confirms that transitions between different orbital states are largely spin-conserving. These results shed light on the role of spin-orbit coupling in mediating spin-lattice and spin-spin interactions of the electronic spin in this defect. They indicate highly favorable roles for particular symmetries of lattice sites in SiC. The conclusions obtained here are largely transferable to other transition-metal impurities in SiC, such as the vanadium color center, which emits at true telecom wavelength.

[1] T. Bosma, G. J. Lof, C. M. Gilardoni, O. V. Zwier, F. Hendriks, B. Magnusson, A. Ellison, A. G allstr om, I. G. Ivanov, N. Son, et al., npj Quantum Information 4, 1 (2018).

[2] C. M. Gilardoni, T. Bosma, D. van Hien, F. Hendriks, B. Magnusson, A. Ellison, I. G. Ivanov, N. Son, and C. H. van der Wal, arXiv preprint arXiv:1912.04612 (2019).

Introducing thin, light-weight and high efficiency photovoltaics will make solar cells more suitable to be integrated in urban landscapes or even small gadgets and would largely contribute to solving the global warming threat that we are facing today. Stacking of solar cells with different characteristic bandgaps is the most common strategy to surpass the Shockley-Queisser efficiency limit, but such tandem devices are typically heavy weight, rigid and costly. Thinning down of absorber materials is a good strategy to overcome these restrictions. However, nano- and micro-meter thicknesses come down to the expense of light absorption. An effective approach to tackle the absorption problem in thin materials is nanopatterning the absorbing layer.

In this work we introduce disordered hyperuniform designs as an effective way to control scattered light into particular range of angles (revealed as a ring in k-space of the reflected/transmitted light), with the aim to efficiently trap light in μm-thick Silicon (Si) cells. We first consider the –theoretical and experimental- case of a single Si solar cell, and thanks to an optimization algorithm, we show the highest light absorption in 1 μm-thick Si film to date. We also estimate conversion efficiency of realistic state-of-the-art devices with incorporated disordered hyperuniform designs, showing almost two-fold conversion efficiency increase for 1-μm Si device. Second, we incorporate a similar light trapping strategy in a tandem solar cell, by using a periodic GaAs nanowire array as a top cell. We introduce two waveguiding effects in GaAs NW-Si thin film architectures to explain the 4-fold light absorption in the Si ultrathin bottom cell for tailored geometries of the NW array. These results represent significant light trapping scheme that is obtained “for free” when using a nanostructured top cell.

Topological photonics has recently gained widespread interest due to its promise of robust transport of classical and quantum information and its potential for controlling light-matter interactions, promising e.g. topologically protected chiral spin networks [1]. In photonic analogues of different classes of topological insulators, specifically the quantum spin Hall effect (QSHE) and quantum valley Hall effect (QVHE), states propagating at the edge of a topologically nontrivial material exhibit a pseudospin that is uniquely linked to a given propagation direction. Such states were recently proposed and observed to exist in photonic crystals with suitable lattice symmetries [2-4]. They have been considered as promising components of chiral light-matter interfaces, which could link the spin of an optical emitter uniquely to a given edge state. Here, we experimentally explore the highly structured field distribution of these states with near- and far-field microscopy [5,6], characterizing their properties, including linear dispersion and low loss, as well as their robust transport. We show that the edge state pseudospin is encoded in unique circular far-field polarization, while the near-field spin texture is revealed to exhibit strong variations. This highlights that optical spin at the nanoscale is not as unambiguously determined in photonic topological insulators as it is in their electronic counterparts. Our experimental results can thus assist the design of topological systems with optimized spin-momentum locking for optical emitters.

[1] A. B. Khanikaev and G. Shvets,  Nat. Photonics 11, 763 (2017).

[2] L.-H. Wu and X. Hu, Phys. Rev. Lett. 114, 223901 (2015).

[3] S. Barik et al., Science 359, 666–668 (2018).

[4] M. A. Gorlach et al., Nat. Commun. 9, 909 (2018).

[5] N. Parappurath, F. Alpeggiani, L. Kuipers, and E. Verhagen, Sci. Adv. 6, eaaw4137 (2020).

[6] M. Burresi et al., Phys. Rev. Lett. 102, 033902 (2009).

Random scattering media in the diffusive regime provide a long light path and multipath interference in a compact area, resulting in strong dispersive properties which can be used for on-chip compressive spectrometry. However the performance suffers from the low light transmission through the diffusive medium. It has been found that there exist ‘open channels’ such that light with certain wavefronts can pass through the medium with high transmission. Here we show that a scattering structure can be designed so that open channels match target input-output channels, in order to maximize transmission while keeping the dispersive properties typical of random media. Specifically, we use inverse design to generate a scattering structure where the open channels match the output waveguides at desired wavelengths. For a proof of concept, we propose a 1×10 multiplexer covering a band of 500nm in the mid-infrared spectrum, with a footprint of only 9.4μm×14.4μm. We also show that filters with nearly arbitrary spectral response can be designed, enabling a new degree of freedom in on-chip spectrometer design, and we investigate the ultimate resolution limits of these structures. The structures can also be designed based on a simple topology consisting of circular holes with diameters from 200nm to 700nm etched in a dielectric slab, making them highly suited for fabrication.  With the help of compressive sensing, the proposed method represents an important tool in the quest towards integrated lab-on-a-chip spectroscopy.

The  transmission channels of a multiple scattering medium are the building  blocks for light transport in the medium and lie therefore at the heart of mesoscopic phenomena.  For samples of low dimensionless conductance, only few channels  contribute to light transport. This is reflected in the speckle  intensity statistics, which deviate from a negative exponential law. The deviation is stronger when the incident wave couples to fewer  transmission channels, as has been theoretically predicted [1,2] and  observed in microwave and optical experiments [3-6]. Since every channel  of the transmission operator (TO) of a medium is orthogonal by definition to the other channels, this raises the  research question as to what occurs to the dimensionless conductance  when an incident field couples to a single channel of the medium,  analogous to an Anderson-localized system that supports only one mode [6]. To be able to answer such a question experimentally,  the channels of the measured transmission matrix (TM), which is  necessarily a partial representation of the TO [7], must be an accurate  representation of the channels of the TO. It is therefore crucial that the largest possible fraction of the TM must be measured  precisely and accurately without inducing spurious correlations from the  optical system or the sampling method [8].

 
Here,  we explore the statistics of individual channels by measuring the  transmission matrices of scattering media in the diffusive regime, where the transport mean free path is  comparable to the wavelength of the incident light field. We observe  indications of a deviation from exponential statistics in the speckle  intensity of single channels.
 
[1] Nieuwenhuizen T. M., van Rossum M. C. W., Phys. Rev. Lett. 74 (1995)
[2] Kogan E., et al., Phys. Rev. B 48 (1993)
[3] Strudley T., et al., Nat. Phot. 7 (2013)
[4] Strudley T., et al., Opt. Lett. 39 (2014)
[5] Stoytchev M., Genack A. Z., Opt. Lett. 24 (1999)
[6] Peña A., et al., Nat. Comm. 5 (2014)
[7] Goetschy A., Stone A. D., Phys. Rev. Lett. 111 (2013)
[8] Pai P., et al., arXiv 1912.00933 (2019)

Soluble  biomarkers obtained from human bodily fluids can act as a diagnostic  tool for assessing the presence of a disease or monitoring its progression. Optical resonators are well suited for  detecting disease biomarkers since they have both a high sensitivity due  to a large evanescent field and high quality factors that allow  monitoring minute concentrations of analyte. This talk concerns the investigation of biomarker detection using optical  ring resonators on the rare-earth ion doped alumina (RE3+:Al2O3) photonic material platform. First, the development of integrated, undoped Al2O3 waveguides and ring resonators will be discussed, together with their demonstration for optical sensing. Then it will be shown how Yb3+ doped Al2O3 lasers can be used as optical sensors with better performance than their  undoped counterparts due to the narrow linewidth of the lasers.  Finally, it will be shown how mode-splitting in a ring resonator can be  used to obtain self-referenced sensor operation.