Raphael Brechbühler

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Brechbühler

First Name

Raphael

ORCID

0000-0001-7498-9729

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Publications 1 - 10 of 13

Analysis of Breath-related Volatile Organic Compounds with Laser Absorption Spectroscopy
Selakovic, Milos; Brechbühler, Raphael; Scheidegger, Philipp; et al. (2023)
Chimia
Single-Pulse Measurement of Orbital Angular Momentum Generated by Microring Lasers
Keitel, Robert; le Feber, Boris; Dettlaff, Krispin; et al. (2021)
ACS Nano
Optical beams with helical phase fronts carry orbital angular momentum (OAM). To exploit this property in integrated photonics, micrometer-scale devices that generate beams with well-defined OAM are needed. Consequently, lasers based on microring resonators decorated with azimuthal grating elements have been investigated. However, future development of such devices requires better methods to determine their OAM, as current approaches are challenging to implement and interpret. If a simple and more sensitive technique were available, OAM microring lasers could be better understood and further improved. In particular, despite most devices being pulsed, their OAM output has been assumed to be constant. OAM fluctuations, which are detrimental for applications, need to be quantified. Here, we fabricate quantum-dot microring lasers and demonstrate a simple measurement method that can straightforwardly determine the magnitude and sign of the OAM down to the level of individual laser pulses. We exploit a Fourier microscope with a cylindrical lens and then investigate three types of microring lasers: with circular symmetry, with “blazed” grating elements, and with unidirectional rotational modes. Our results confirm that previous measurement techniques obscured key details about the OAM generation. For example, while time-averaged OAM from our unidirectional laser is very similar to our blazed grating device, single-pulse measurements show that detrimental effects of mode competition are almost entirely suppressed in the former. Nevertheless, even in this case, the OAM output exhibits shot-to-shot fluctuations. Thus, our approach reveals important details in the underlying device operation that can aid in the improvement of micrometer-scale sources with pure OAM output.
Microsecond Blinking Events in the Fluorescence of Colloidal Quantum Dots Revealed by Correlation Analysis on Preselected Photons
Rabouw, Freddy T.; Antolinez, Felipe; Brechbühler, Raphael; et al. (2019)
The Journal of Physical Chemistry Letters
Nearly all colloidal quantum dots, when measured at the single-emitter level, exhibit fluorescence “blinking”. However, despite over 20 years of research on this phenomenon, its microscopic origins are still debated. One reason is a gap in available experimental information, specifically for dynamics at short (submillisecond) time scales. Here, we use photon-correlation analysis to investigate microsecond blinking events in individual quantum dots. While the strongly distributed kinetics of blinking normally makes such events difficult to study, we show that they can be analyzed by excluding photons emitted during long bright or dark periods. Moreover, we find that submillisecond blinking events are more common than one might expect from extrapolating the power-law blinking statistics observed on longer (millisecond) time scales. This result provides important experimental data for developing a microscopic understanding of blinking. More generally, our method offers a simple strategy for analyzing microsecond switching dynamics in the fluorescence of quantum emitters.
Optical Fourier surfaces
Lassaline, Nolan; Brechbühler, Raphael; Vonk, Sander J.W.; et al. (2020)
Nature
Fluctuations in the Photoluminescence Excitation Spectra of Individual Semiconductor Nanocrystals
Keitel, Robert C.; Brechbühler, Raphael; Cocina, Ario; et al. (2024)
The Journal of Physical Chemistry Letters
Most single quantum emitters display non-steady emission properties. Models that explain this effect have primarily relied on photoluminescence measurements that reveal variations in intensity, wavelength, and excited-state lifetime. While photoluminescence excitation spectroscopy could provide complementary information, existing experimental methods cannot collect spectra before individual emitters change in intensity (blink) or wavelength (spectrally diffuse). Here, we present an experimental approach that circumvents such issues, allowing the collection of excitation spectra from individual emitters. Using rapid modulation of the excitation wavelength, we collect and classify excitation spectra from individual CdSe/CdS/ZnS core/shell/shell quantum dots. The spectra, along with simultaneous time-correlated single-photon counting, reveal two separate emission-reduction mechanisms caused by charging and trapping, respectively. During bright emission periods, we also observe a correlation between emission red-shifts and the increased oscillator strength of higher excited states. Quantum-mechanical modeling indicates that diffusion of charges in the vicinity of an emitter polarizes the exciton and transfers the oscillator strength to higher-energy transitions.
Rapid Detection of Volatile Organic Compounds by Switch-Scan Tuning of Vernier Quantum-Cascade Lasers
Brechbühler, Raphael; Selakovic, Milos; Scheidegger, Philipp; et al. (2023)
Analytical Chemistry
Volatile organic compounds (VOCs) exhibit typically broad and mutually overlapping ro-vibrational absorption fingerprints. This complexity has so far limited the applicability of laser-based spectroscopy for VOC measurements in complex gas matrices. Here, we exploit a Vernier-type quantum-cascade laser (QCL) as an electrically tunable multiwavelength source for selective and sensitive VOC analysis. This emerging class of lasers provides access to several spectral windows by discrete Vernier tuning (“switching”) and continuous coverage within these windows (“scanning”). We present a versatile driving technique that efficiently combines the two tuning mechanisms. Applied to our Vernier QCL, it enables the rapid acquisition (within 360 ms) of high-resolution spectra from six individual spectral windows, distributed over a wide range from 1063 to 1102 cm-1. Gaining access to the broad absorption envelopes of VOCs at multiple frequencies, along with their superimposed fine structure, which are especially pronounced at a reduced sample pressure, offers completely new opportunities in VOC analysis. The potential of this approach is assessed in a direct-laser-absorption setup with acetaldehyde, ethanol, and methanol as benchmark compounds with significant spectral overlaps. A measurement precision of 1-10 ppb is obtained after integration for 10 s at amount fractions below 10 ppm, and excellent linearity is found over at least 3 orders of magnitude. Combined with our dedicated spectral fitting algorithm, we demonstrate highly selective multicompound analyses with less than 3.5% relative expanded uncertainty, even in the presence of a 40× excess of an interfering compound with complete spectral overlap.
Diffraction of Light from Optical Fourier Surfaces
Glauser, Yannik M.; Maris, J.J. Erik; Brechbühler, Raphael; et al. (2025)
ACS Photonics
Diffractive surfaces shape optical wavefronts for applications in spectroscopy, high-speed communication, and imaging. The performance of these structures is primarily determined by how precisely they can be patterned. Fabrication constraints commonly lead to square-shaped, "binary" profiles that contain unwanted spatial frequencies that contaminate the diffraction. Recently, "wavy" surfaces (known as optical Fourier surfaces, OFSs) have been introduced that include only the desired spatial frequencies. However, the optical performance and reliability of these structures have not yet been experimentally tested with respect to models and simulations. Such a quantitative investigation could also provide previously unobtainable information about the diffraction process from the most fundamental diffractive surfaces-sinusoidally pure profiles. Here, we produce and study two classes of reflective OFSs: (i) single-sinusoidal profiles of varying depth and (ii) double-sinusoidal profiles with varying relative phase. After refining our fabrication procedure to obtain larger and deeper OFSs at higher yields, we find that the measured optical responses from our OFSs agree quantitatively with full electrodynamic simulations. In contrast, our measurements diverge from analytical scalar diffraction models routinely used by researchers to describe diffraction. Overall, our results confirm that OFSs provide a precise and powerful platform for Fourier-spectrum engineering, satisfying the growing demand for intricately patterned interfaces for applications in holography, augmented reality, and optical computing.
Role of Gain in Fabry−Pérot Surface Plasmon Polariton Lasers
Aellen, Marianne; Rossinelli, Aurelio; Keitel, Robert; et al. (2022)
ACS Photonics
Plasmonic lasers generate strongly confined electromagnetic fields over a narrow range of wavelengths. This is potentially useful for enhancing nonlinear effects, sensing chemical species, and providing on-chip sources of plasmons. By placing a semiconductor gain layer near a metallic interface with a gap layer in between, plasmonic lasers have been demonstrated. However, the role of gain in this common design has been understudied, leading to suboptimal choices. Here, we examine planar metallic lasers and explore the effect of gain on the lasing behavior. We print semiconductor nanoplatelets as a gain layer of controllable thickness onto alumina-coated silver films with integrated planar Fabry–Pérot cavities. Lasing behavior is then monitored with spectrally and polarization-resolved far-field imaging. The results are compared with a theoretical waveguide model and a rate-equation model, which consider both plasmonic and photonic modes and explicitly include losses and gain. We find that the nature of the lasing mode is dictated by the gain-layer thickness and, contrary to conventional wisdom, a gap layer with a high refractive index can be advantageous for plasmonic lasing in planar Fabry–Pérot cavities. Our rate-equation model also reveals a regime where plasmonic and photonic modes compete in an unintuitive way, potentially useful for facile, active mode switching. These results can guide future design of metallic lasers and could lead to on-chip lasers with controlled photonic and plasmonic output.
Compact Plasmonic Distributed-Feedback Lasers as Dark Sources of Surface Plasmon Polaritons
Brechbühler, Raphael; Vonk, Sander J.W.; Aellen, Marianne; et al. (2021)
ACS Nano
Plasmonic modes in optical cavities can be amplified through stimulated emission. Using this effect, plasmonic lasers can potentially provide chip-integrated sources of coherent surface plasmon polaritons (SPPs). However, while plasmonic lasers have been experimentally demonstrated, they have not generated propagating plasmons as their primary output signal. Instead, plasmonic lasers typically involve significant emission of free-space photons that are intentionally outcoupled from the cavity by Bragg diffraction or that leak from reflector edges due to uncontrolled scattering. Here, we report a simple cavity design that allows for straightforward extraction of the lasing mode as SPPs while minimizing photon leakage. We achieve plasmonic lasing in 10-μm-long distributed-feedback cavities consisting of a Ag surface periodically patterned with ridges coated by a thin layer of colloidal semiconductor nanoplatelets as the gain material. The diffraction to free-space photons from cavities designed with second-order feedback allows a direct experimental examination of the lasing-mode profile in real- and momentum-space, in good agreement with coupled-wave theory. In contrast, we demonstrate that first-order-feedback cavities remain “dark” above the lasing threshold and the output signal leaves the cavity as propagating SPPs, highlighting the potential of such lasers as on-chip sources of plasmons.
Two-Dimensional Drexhage Experiment for Electric- and Magnetic-Dipole Sources on Plasmonic Interfaces
Brechbühler, Raphael; Rabouw, Freddy T.; Rohner, Patrik; et al. (2018)
Physical Review Letters

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Raphael Brechbühler

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