Raphael Schwanninger

Last Name

Schwanninger

First Name

Raphael

ORCID

0000-0001-7066-4644

Organisational unit

03974 - Leuthold, Juerg / Leuthold, Juerg

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Publications1 - 8 of 8

Engineering Graphene Phototransistors for High Dynamic Range Applications
Nashashibi, Shadi; Koepfli, Stefan M.; Schwanninger, Raphael; et al. (2024)
ACS Nano
Phototransistors are light-sensitive devices featuring a high dynamic range, low-light detection, and mechanisms to adapt to different ambient light conditions. These features are of interest for bioinspired applications such as artificial and restored vision. In this work, we report on a graphene-based phototransistor exploiting the photogating effect that features picowatt- to microwatt-level photodetection, a dynamic range covering six orders of magnitude from 7 to 10(7) lux, and a responsivity of up to 4.7 x 10(3) A/W. The proposed device offers the highest dynamic range and lowest optical power detected compared to the state of the art in interfacial photogating and further operates air stably. These results have been achieved by a combination of multiple developments. For example, by optimizing the geometry of our devices with respect to the graphene channel aspect ratio and by introducing a semitransparent top-gate electrode, we report a factor 20-30 improvement in responsivity over unoptimized reference devices. Furthermore, we use a built-in dynamic range compression based on a partial logarithmic optical power dependence in combination with control of responsivity. These features enable adaptation to changing lighting conditions and support high dynamic range operation, similar to what is known in human visual perception. The enhanced performance of our devices therefore holds potential for bioinspired applications, such as retinal implants.
Metasurface Colloidal Quantum Dot Photodetectors
Dordevic, Nikola; Schwanninger, Raphael; Yarema, Maksym; et al. (2022)
ACS Photonics
Efficient photodetectors that can be easily engineered for a specific spectral window are of high interest. Here, we report on the design, fabrication, and characterization of metasurface-enhanced photodetectors and photodiodes using colloidal quantum dots. We fabricate photoconductors optimized for the wavelength range around 1550 nm featuring responsivities of up to 8000 A/W with low noise equivalent powers on the order of tens of pW/Hz. Further, we produce photodiodes with responsivities of ∼5 mA/W that offer faster responses (14 μs rise time). The high responsivities are due to the metasurface, which increases the absorption by a factor of 10 compared to that of a quantum dot thin film of the same thickness and a structure that enables a photo-gain. We introduce a metasurface to detect either of the two orthogonal polarizations. The fabricated photoconductors operate at low voltages (1-5 V), making them compatible with the complementary metal-oxide-semiconductor (CMOS) read-out circuitry.
Metamaterial graphene photodetector with bandwidth exceeding 500 gigahertz
Koepfli, Stefan M.; Baumann, Michael; Koyaz, Yesim; et al. (2023)
Science
Although graphene has met many of its initially predicted optoelectronic, thermal, and mechanical properties, photodetectors with large spectral bandwidths and extremely high frequency responses remain outstanding. In this work, we demonstrate a >500 gigahertz, flat-frequency response, graphene-based photodetector that operates under ambient conditions across a 200-nanometer-wide spectral band with center wavelengths adaptable from <1400 to >4200 nanometers. Our detector combines graphene with metamaterial perfect absorbers with direct illumination from a single-mode fiber, which breaks with the conventional miniaturization of photodetectors on an integrated photonic platform. This design allows for much higher optical powers while still allowing record-high bandwidths and data rates. Our results demonstrate that graphene photodetectors can outperform conventional technologies in terms of speed, bandwidth, and operation across a large spectral range.
Graphene Photogating Devices for Retinal Implants
Nashashibi, Shadi; Koepfli, Stefan M.; Schwanninger, Raphael; et al. (2024)
Technical Digest Series ~ CLEO 2024
We demonstrate a graphene-based phototransistor array with a dynamic range of six orders of magnitude starting from 7 lux. These devices feature a biomimetic logarithmic power dependence and are thus of interest for retinal implants.
Advancements in Mid-Infrared Colloidal Quantum Dot Photodetectors: Optimization, Metamaterials, and Fabrication
Schwanninger, Raphael (2024)
Photodetectors operating in the mid-infrared spectral wavelength regime are crucial for numerous technologies and scientific advancements. They are integral to a wide array of applications, including biomedical imaging, environmental monitoring, thermal imaging, spectroscopy, and defense systems. However, developing photodetectors that are both highly sensitive and cost-effective remains a significant challenge. Therefore, this doctoral thesis explores the use of low-cost absorber materials in combination with performance enhancement schemes to overcome these limitations. The cost-efficient material used in this thesis are colloidal quantum dots. These zero-dimensional nanometer-sized crystalline materials can be synthesized in solution and then deposited on various substrates using simple methods. Furthermore, their tunable absorption spectrum, achieved by altering their size, makes them ideal for a wide range of applications. This makes them an ideal candidate to reduce cost. However, photodetectors employing these materials often exhibit poor responsivities and detectivities. Thus, narrow-band resonant metamaterials were used to overcome these performance shortcomings. They were designed to increase the light absorption while simultaneously improving charge extraction properties and reducing noise. The first photodetectors presented in this work were fabricated using lead selenide and lead sulfide colloidal quantum dots. A method was developed to sinter these quantum dots into a solid layer, resulting in improved charge transport properties and an extended absorption spectra to longer wavelengths. This sintered lead selenide layer was then covered by an additional, also sintered, lead sulfide layer to form a heterostructure. By stacking these layers, it was possible to enhance the photo-gain in the lead selenide layer and double the photoresponse. In the final step, this layer stack was combined with a metallic metamaterial perfect absorber. This enabled narrow-band tunable absorption enhancement with peak absorptivity’s reaching 98%. As a result, the responsivity was increased up to twenty-fold, reaching 375 A/W and 4 A/W at wavelengths of 2710 nm and 4250 nm, respectively. The second type of photodetector presented in this thesis utilizes mercury telluride colloidal quantum dots as an absorber layer. These colloidal quantum dots were also combined with metamaterials, which were systematically improved using electro-optical simulations. This enabled the optimization of the metamaterial to increase the responsivity while simultaneously decreasing the noise spectral current density. Specifically, this was achieved by enhancing the photogenerated charge carrier collection efficiency and reducing the active material volume without compromising near-unity absorption. The metamaterial optimization process started with a common disc resonator design. In a first step straight contacts were placed in between the disc resonators and then wrapped around them. Finally, the disc resonators and contacts were merged forming a narrow slot metamaterial. This design optimization process resulted in an approximate 13-fold increase in responsivity and a 345-fold increase in detectivity. The final metamaterial design achieves a responsivity of 16.2 A/W and a detectivity of 6×108 Jones at a wavelength of 2710 nm. This analysis provides a pathway to significantly improve the responsivity and noise characteristics of photodetectors based on cost-efficient cQDs.
Highly Responsive Mid-Infrared Metamaterial Enhanced Heterostructure Photodetector Formed out of Sintered PbSe/PbS Colloidal Quantum Dots
Schwanninger, Raphael; Koepfli, Stefan M.; Yarema, Olesya; et al. (2023)
ACS Applied Materials & Interfaces
Efficient and simple-to-fabricate light detectors in the mid infrared (MIR) spectral range are of great importance for various applications in existing and emerging technologies. Here, we demonstrate compact and efficient photodetectors operating at room temperature in a wavelength range of 2710–4250 nm with responsivities as high as 375 and 4 A/W. Key to the high performance is the combination of a sintered colloidal quantum dot (CQD) lead selenide (PbSe) and lead sulfide (PbS) heterojunction photoconductor with a metallic metasurface perfect absorber. The combination of this photoconductor stack with the metallic metasurface perfect absorber provides an overall ∼20-fold increase of the responsivity compared against reference sintered PbSe photoconductors. More precisely, the introduction of a PbSe/PbS heterojunction increases the responsivity by a factor of ∼2 and the metallic metasurface enhances the responsivity by an order of magnitude. The metasurface not only enhances the light–matter interaction but also acts as an electrode to the detector. Furthermore, fabrication of our devices relies on simple and inexpensive methods. This is in contrast to most of the currently available (state-of-the-art) MIR photodetectors that rely on rather expensive as well as nontrivial fabrication technologies that often require cooling for efficient operation.
Measuring Dielectric and Electro-optic Responses of Thin Films using Plasmonic Devices
Winiger, Joel; Keller, Killian; Gjini, Patrik; et al. (2024)
Optics Express
This paper introduces a simple method for the measurement of the relative permittivity and the Pockels coefficient of electro-optic (EO) materials in a waveguide up to sub-THz frequencies. By miniaturizing the device and making use of plasmonics, the complexities of traditional methods are mitigated. This work elaborates the fabrication tolerance and simplicity of the method, and highlights its applicability to various materials, substrates and configurations. The method is showcased using drop-casted perovskite barium titanate (BaTiO3, BTO) nano-particle thin-films and it has previously been used to measure epitaxial thin film BTO. In this work we show the effective relative permittivity of drop casted BTO to be εeff ∼ 30 at 200 MHz, dropping to ∼ 18 at 67 GHz and similarly, the effective Pockels coefficient was found to be reff ∼ 16 at 350 MHz and ∼ 8 at 70 GHz. These values are a factor > 50 below the values found for thin film BTO. Yet, the fact that the method can be applied to such different samples and Pockels strengths gives testimony to its versatility and sensitivity.
Metamaterial Engineering for Superior HgTe cQD Photodetector Performance
Schwanninger, Raphael; Nashashibi, Shadi; Yarema, Olesya; et al. (2024)
Advanced Optical Materials
Highly responsive, low noise, and inexpensive photodetectors that operate in the mid-infrared (MIR) wavelength regime are in high demand for applications ranging from fundamental science to large scale industries. However, simultaneously achieving all this in one device architecture is very challenging. In this work, mercury telluride (HgTe) colloidal quantum dot (cQD) based photodetectors are systematically improved by the introduction of new metamaterial designs. The new designs are found by utilizing simulations. Thereby the structures are optimized to increase the responsivity and simultaneously decrease the noise spectral current density. This is achieved by focusing on improving the photogenerated charge carrier collection efficiency while reducing the active material volume without altering the near unity absorption. A standard metamaterial perfect absorber architecture based on disc resonators is used as a starting point for the optimization process. By optimizing the carrier extraction through contact engineering, resulting in a narrow slot metamaterial, an overall ≈13-fold responsivity and ≈345-fold detectivity increase is achieved. The final metamaterial design reaches a responsivity of 16.2 A W⁻¹ and detectivity of 6×10⁸ Jones at a wavelength of 2710 nm. The analysis therefore provides a route to improve the responsivity and noise characteristics of mid-infrared photodetectors based on cost-efficient colloidal quantum dots.

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Raphael Schwanninger

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