Markus Walther - Profile on Academia.edu (original) (raw)
Papers by Markus Walther
Applied Physics Letters, 2013
We experimentally demonstrate intensity and phase modulation of terahertz radiation using activel... more We experimentally demonstrate intensity and phase modulation of terahertz radiation using actively controlled large-area planar metamaterial (metafilm) hybridized with a 12 m thick layer of a liquid crystal. Active control was introduced through in-plane electrical switching of the liquid crystal, which enabled us to achieve a reversible singlepass absolute transmission change of 20 % and a phase change of 40 deg at only 20 V.
Optical Terahertz Science and Technology, 2007
Terahertz (THz) time-domain spectroscopy is used to measure the complex conductivity of semiconti... more Terahertz (THz) time-domain spectroscopy is used to measure the complex conductivity of semicontinuous gold films in the spectral region 0.5-2.5 THz. The effects of the characteristic nano-structure of the films on their THz-conductivity are investigated.
Optics Express, 2009
We show that a resonant response with very high quality factors can be achieved in periodic metam... more We show that a resonant response with very high quality factors can be achieved in periodic metamaterials by radiatively coupling their structural elements. The coupling is mediated by lattice modes and can be efficiently controlled by tuning the lattice periodicity. Using a recently developed terahertz (THz) near-field imaging technique and conventional far-field spectroscopy together with numerical simulations we pinpoint the underlying mechanisms. In the strong coupling regimes we identify avoided crossings between the plasmonic eigenmodes and the diffractive lattice modes.
Journal of Molecular Liquids, 2011
Rattling the cage: Micro-to mesoscopic structure in liquids as simple as argon and as complicated... more Rattling the cage: Micro-to mesoscopic structure in liquids as simple as argon and as complicated as water. Journal of Molecular Liquids, 159 (1). pp. 2-8.
Journal of Applied Physics, 2011
Applied Physics Letters, 2008
Optics Letters, 2000
Using terahertz time-domain spectroscopy, we have measured the index of refraction and the absorp... more Using terahertz time-domain spectroscopy, we have measured the index of refraction and the absorption coeff icient of the organic ionic salt 4-N, N-dimethylamino-4 0-N 0-methyl-stilbazolium tosylate (DAST). This promising organic electro-optic material exhibits strong absorption and dispersion for frequencies above 1 THz at both room temperature and 83 K. No reduction in the absorption strength is observed when DAST is cooled, suggesting a single-phonon origin of the resonances. A simple vibration of the anion and cation of the salt is suggested as the origin of the exceptionally strong far-infrared absorption and the high-frequency electro-optic properties of DAST.
Optics Express, 2011
Using terahertz near-field imaging we experimentally investigate the resonant electromagnetic fie... more Using terahertz near-field imaging we experimentally investigate the resonant electromagnetic field distributions behind a split-ring resonator and its complementary structure with sub-wavelength spatial resolution. For the out-of-plane components we experimentally verify ...
Terahertz near-field imaging of dielectric resonators
Optics express, Jan 20, 2017
As an alternative to metallic resonators, dielectric resonators can increase radiation efficienci... more As an alternative to metallic resonators, dielectric resonators can increase radiation efficiencies of metasurfaces at terahertz frequencies. Such subwavelength resonators made from low-loss dielectric materials operate on the basis of oscillating displacement currents. For full control of electromagnetic waves, it is essential that dielectric resonators operate around their resonant modes. Thus, understanding the nature of these resonances is crucial towards design implementation. To this end, an array of silicon resonators on a quartz substrate is designed to operate in transmission at terahertz frequencies. The resonator dimensions are tailored to observe their low-order modes of resonance at 0.58 THz and 0.61 THz respectively. We employ a terahertz near-field imaging technique to measure the complex near-fields of this dielectric resonator array. This unique method allows direct experimental observation of the first two fundamental resonances.
We have applied a newly developed transient terahertz time- domain spectrometer to study the temp... more We have applied a newly developed transient terahertz time- domain spectrometer to study the temporal development of the dynamics of photogenerated carriers in semiconductor materials. The study presented here include semi-insulating (SI) and low-temperature-grown (LT) GaAs. By measuring the detailed shape of a subpicosecond electrical field pulse (THz pulse) transmitted through the sample at a time T after excitation with
Plasmonic focusing on metal and semiconductor disks under radially polarized terahertz illumination
2013 38th International Conference on Infrared Millimeter and Terahertz Waves, 2013
ABSTRACT Optimal focusing of surface plasmon polaritons in the center of a metal disc illuminated... more ABSTRACT Optimal focusing of surface plasmon polaritons in the center of a metal disc illuminated by radially polarized terahertz pulses is demonstrated. Due to the cylinder symmetrical structure surface plasmons can be excited along the entire circumference, which interfere constructively in the center of the disk forming a sharp frequency-depended focal spot. We map the field distribution on the disk by THz near-field microscopy and compare our result to numerical simulations. For comparison, behavior under linearly polarized THz illumination is characterized. Furthermore, first results of semiconducting plasmonic lenses are presented.
In this work we report two designs of subwavelength fibers packaged for practical terahertz wave ... more In this work we report two designs of subwavelength fibers packaged for practical terahertz wave guiding. We describe fabrication, modeling and characterization of microstructured polymer fibers featuring a subwavelength-size core suspended in the middle of a large porous outer cladding. This design allows convenient handling of the subwavelength fibers without distorting their modal profile. Additionally, the air-tight porous cladding serves as a natural enclosure for the fiber core, thus avoiding the need for a bulky external enclosure for humidity-purged atmosphere. Fibers of 5 mm and 3 mm in outer diameters with a 150 \mu m suspended solid core and a 900 \mu m suspended porous core respectively, were obtained by utilizing a combination of drilling and stacking techniques. Characterization of the fiber optical properties and the near-field imaging of the guided modes were performed using a terahertz near-field microscopy setup. Near-field imaging of the modal profiles at the fiber output confirmed the effectively single-mode behavior of such waveguides. The suspended core fibers exhibit broadband transmission from 0.10 THz to 0.27 THz (larger core), and from 0.25 THz to 0.51 THz (smaller core). Due to the large fraction of power that is guided in the holey cladding, fiber propagation losses as low as 0.02 cm-1 are demonstrated. Low-loss guidance combined with the core isolated from environmental perturbations make these all-dielectric fibers suitable for practical terahertz imaging and sensing applications.
Ultrabroadband perfect imaging in terahertz wire media using single-cycle pulses
Optica, 2016
Phys Med Biol, 2002
The far-infrared dielectric function of a wide range of organic molecules is dominated by vibrati... more The far-infrared dielectric function of a wide range of organic molecules is dominated by vibrations involving a substantial fraction of the atoms forming the molecule and motion associated with intermolecular hydrogen bond vibrations. Due to their collective nature such modes are highly sensitive to the intra-and intermolecular structure and thus provide a unique fingerprint of the conformational state of the molecule and effects of its environment. We demonstrate the use of terahertz time-domain spectroscopy (THz-TDS) for recording the far-infrared (0.5-4.0 THz) dielectric function of the four nucleobases and corresponding nucleosides forming the building blocks of deoxyribose nucleic acid (DNA). We observe numerous distinct spectral features with large differences between the molecules in both frequencydependent absorption coefficient and index of refraction. Assisted by results from density-functional calculations we interpret the origin of the observed resonances as vibrations of hydrogen bonds between the molecules.
Imaging electric and magnetic near-fields in terahertz metamaterials
CLEO/Europe - EQEC 2009 - European Conference on Lasers and Electro-Optics and the European Quantum Electronics Conference, 2009
ABSTRACT Metamaterial structures are investigated by a novel terahertz near-field imaging techniq... more ABSTRACT Metamaterial structures are investigated by a novel terahertz near-field imaging technique. Our approach maps the inplane electric and out-of-plane magnetic field vectors and thereby provides a detailed investigation of various metamaterial designs and their electromagnetic near-field response.
Metamaterials: Fundamentals and Applications II, 2009
ABSTRACT Experimental investigations of the microscopic electric and in particular the magnetic n... more ABSTRACT Experimental investigations of the microscopic electric and in particular the magnetic near-fields in metamaterials remain highly challenging and current studies rely mostly on numerical simulations to characterize their resonant microscopic behavior. Here, we present a terahertz imaging technique, which allows us to measure the amplitude, phase and polarization of the electric near-fields in the vicinity of the resonant structures in planar metamaterials. By our approach we are able to trace the electric field vectors close to the structures after their excitation on sub-ps time scales with sub-wavelength spatial resolution. From the measured in-plane electric vector fields we are able to reconstruct the out-of-plane magnetic field vectors. As a result we obtain a comprehensive microscopic picture of the electromagnetic response in metamaterials.
Modern spectroscopy on biological molecules : structure and bonding investigated by THz time-domain and transient phase-grating spectroscopy /
Since the first freely propagating THz pulses have been used to conduct spectroscopic investigati... more Since the first freely propagating THz pulses have been used to conduct spectroscopic investigations in the far-IR, THz time-domain spectroscopy (THz-TDS) has evolved into a versatile tool for the study of a variety of different sample materials. However its application to the investigation of biological molecules in this highly interesting spectral regime started only recently, and the work presented in this thesis represents one of the first investigations along this line. We present characteristic THz spectra of individual biomolecular systems in the crystalline and amorphous phase. In particular the correlation between the observed low-frequency modes and molecular structure and association was investigated. Our results show that the mode structure in the THz region represents a unique fingerprint for the molecular structure and conformation, e.g. allowing to distinguish related compounds, to discriminate between different isomers and to even observe structural phase-transitions...
Novel THz Metamaterial Designs: From Near- and Far-Field Coupling to High-Q Resonances
IEEE Transactions on Terahertz Science and Technology, 2013
Fiber Metamaterials: Recent Advances and Outlook
Workshop on Specialty Optical Fibers and their Applications, 2013
Dielectric response and collective dynamics of acetonitrile
Journal of Molecular Liquids, 2015
Applied Physics Letters, 2013
We experimentally demonstrate intensity and phase modulation of terahertz radiation using activel... more We experimentally demonstrate intensity and phase modulation of terahertz radiation using actively controlled large-area planar metamaterial (metafilm) hybridized with a 12 m thick layer of a liquid crystal. Active control was introduced through in-plane electrical switching of the liquid crystal, which enabled us to achieve a reversible singlepass absolute transmission change of 20 % and a phase change of 40 deg at only 20 V.
Optical Terahertz Science and Technology, 2007
Terahertz (THz) time-domain spectroscopy is used to measure the complex conductivity of semiconti... more Terahertz (THz) time-domain spectroscopy is used to measure the complex conductivity of semicontinuous gold films in the spectral region 0.5-2.5 THz. The effects of the characteristic nano-structure of the films on their THz-conductivity are investigated.
Optics Express, 2009
We show that a resonant response with very high quality factors can be achieved in periodic metam... more We show that a resonant response with very high quality factors can be achieved in periodic metamaterials by radiatively coupling their structural elements. The coupling is mediated by lattice modes and can be efficiently controlled by tuning the lattice periodicity. Using a recently developed terahertz (THz) near-field imaging technique and conventional far-field spectroscopy together with numerical simulations we pinpoint the underlying mechanisms. In the strong coupling regimes we identify avoided crossings between the plasmonic eigenmodes and the diffractive lattice modes.
Journal of Molecular Liquids, 2011
Rattling the cage: Micro-to mesoscopic structure in liquids as simple as argon and as complicated... more Rattling the cage: Micro-to mesoscopic structure in liquids as simple as argon and as complicated as water. Journal of Molecular Liquids, 159 (1). pp. 2-8.
Journal of Applied Physics, 2011
Applied Physics Letters, 2008
Optics Letters, 2000
Using terahertz time-domain spectroscopy, we have measured the index of refraction and the absorp... more Using terahertz time-domain spectroscopy, we have measured the index of refraction and the absorption coeff icient of the organic ionic salt 4-N, N-dimethylamino-4 0-N 0-methyl-stilbazolium tosylate (DAST). This promising organic electro-optic material exhibits strong absorption and dispersion for frequencies above 1 THz at both room temperature and 83 K. No reduction in the absorption strength is observed when DAST is cooled, suggesting a single-phonon origin of the resonances. A simple vibration of the anion and cation of the salt is suggested as the origin of the exceptionally strong far-infrared absorption and the high-frequency electro-optic properties of DAST.
Optics Express, 2011
Using terahertz near-field imaging we experimentally investigate the resonant electromagnetic fie... more Using terahertz near-field imaging we experimentally investigate the resonant electromagnetic field distributions behind a split-ring resonator and its complementary structure with sub-wavelength spatial resolution. For the out-of-plane components we experimentally verify ...
Terahertz near-field imaging of dielectric resonators
Optics express, Jan 20, 2017
As an alternative to metallic resonators, dielectric resonators can increase radiation efficienci... more As an alternative to metallic resonators, dielectric resonators can increase radiation efficiencies of metasurfaces at terahertz frequencies. Such subwavelength resonators made from low-loss dielectric materials operate on the basis of oscillating displacement currents. For full control of electromagnetic waves, it is essential that dielectric resonators operate around their resonant modes. Thus, understanding the nature of these resonances is crucial towards design implementation. To this end, an array of silicon resonators on a quartz substrate is designed to operate in transmission at terahertz frequencies. The resonator dimensions are tailored to observe their low-order modes of resonance at 0.58 THz and 0.61 THz respectively. We employ a terahertz near-field imaging technique to measure the complex near-fields of this dielectric resonator array. This unique method allows direct experimental observation of the first two fundamental resonances.
We have applied a newly developed transient terahertz time- domain spectrometer to study the temp... more We have applied a newly developed transient terahertz time- domain spectrometer to study the temporal development of the dynamics of photogenerated carriers in semiconductor materials. The study presented here include semi-insulating (SI) and low-temperature-grown (LT) GaAs. By measuring the detailed shape of a subpicosecond electrical field pulse (THz pulse) transmitted through the sample at a time T after excitation with
Plasmonic focusing on metal and semiconductor disks under radially polarized terahertz illumination
2013 38th International Conference on Infrared Millimeter and Terahertz Waves, 2013
ABSTRACT Optimal focusing of surface plasmon polaritons in the center of a metal disc illuminated... more ABSTRACT Optimal focusing of surface plasmon polaritons in the center of a metal disc illuminated by radially polarized terahertz pulses is demonstrated. Due to the cylinder symmetrical structure surface plasmons can be excited along the entire circumference, which interfere constructively in the center of the disk forming a sharp frequency-depended focal spot. We map the field distribution on the disk by THz near-field microscopy and compare our result to numerical simulations. For comparison, behavior under linearly polarized THz illumination is characterized. Furthermore, first results of semiconducting plasmonic lenses are presented.
In this work we report two designs of subwavelength fibers packaged for practical terahertz wave ... more In this work we report two designs of subwavelength fibers packaged for practical terahertz wave guiding. We describe fabrication, modeling and characterization of microstructured polymer fibers featuring a subwavelength-size core suspended in the middle of a large porous outer cladding. This design allows convenient handling of the subwavelength fibers without distorting their modal profile. Additionally, the air-tight porous cladding serves as a natural enclosure for the fiber core, thus avoiding the need for a bulky external enclosure for humidity-purged atmosphere. Fibers of 5 mm and 3 mm in outer diameters with a 150 \mu m suspended solid core and a 900 \mu m suspended porous core respectively, were obtained by utilizing a combination of drilling and stacking techniques. Characterization of the fiber optical properties and the near-field imaging of the guided modes were performed using a terahertz near-field microscopy setup. Near-field imaging of the modal profiles at the fiber output confirmed the effectively single-mode behavior of such waveguides. The suspended core fibers exhibit broadband transmission from 0.10 THz to 0.27 THz (larger core), and from 0.25 THz to 0.51 THz (smaller core). Due to the large fraction of power that is guided in the holey cladding, fiber propagation losses as low as 0.02 cm-1 are demonstrated. Low-loss guidance combined with the core isolated from environmental perturbations make these all-dielectric fibers suitable for practical terahertz imaging and sensing applications.
Ultrabroadband perfect imaging in terahertz wire media using single-cycle pulses
Optica, 2016
Phys Med Biol, 2002
The far-infrared dielectric function of a wide range of organic molecules is dominated by vibrati... more The far-infrared dielectric function of a wide range of organic molecules is dominated by vibrations involving a substantial fraction of the atoms forming the molecule and motion associated with intermolecular hydrogen bond vibrations. Due to their collective nature such modes are highly sensitive to the intra-and intermolecular structure and thus provide a unique fingerprint of the conformational state of the molecule and effects of its environment. We demonstrate the use of terahertz time-domain spectroscopy (THz-TDS) for recording the far-infrared (0.5-4.0 THz) dielectric function of the four nucleobases and corresponding nucleosides forming the building blocks of deoxyribose nucleic acid (DNA). We observe numerous distinct spectral features with large differences between the molecules in both frequencydependent absorption coefficient and index of refraction. Assisted by results from density-functional calculations we interpret the origin of the observed resonances as vibrations of hydrogen bonds between the molecules.
Imaging electric and magnetic near-fields in terahertz metamaterials
CLEO/Europe - EQEC 2009 - European Conference on Lasers and Electro-Optics and the European Quantum Electronics Conference, 2009
ABSTRACT Metamaterial structures are investigated by a novel terahertz near-field imaging techniq... more ABSTRACT Metamaterial structures are investigated by a novel terahertz near-field imaging technique. Our approach maps the inplane electric and out-of-plane magnetic field vectors and thereby provides a detailed investigation of various metamaterial designs and their electromagnetic near-field response.
Metamaterials: Fundamentals and Applications II, 2009
ABSTRACT Experimental investigations of the microscopic electric and in particular the magnetic n... more ABSTRACT Experimental investigations of the microscopic electric and in particular the magnetic near-fields in metamaterials remain highly challenging and current studies rely mostly on numerical simulations to characterize their resonant microscopic behavior. Here, we present a terahertz imaging technique, which allows us to measure the amplitude, phase and polarization of the electric near-fields in the vicinity of the resonant structures in planar metamaterials. By our approach we are able to trace the electric field vectors close to the structures after their excitation on sub-ps time scales with sub-wavelength spatial resolution. From the measured in-plane electric vector fields we are able to reconstruct the out-of-plane magnetic field vectors. As a result we obtain a comprehensive microscopic picture of the electromagnetic response in metamaterials.
Modern spectroscopy on biological molecules : structure and bonding investigated by THz time-domain and transient phase-grating spectroscopy /
Since the first freely propagating THz pulses have been used to conduct spectroscopic investigati... more Since the first freely propagating THz pulses have been used to conduct spectroscopic investigations in the far-IR, THz time-domain spectroscopy (THz-TDS) has evolved into a versatile tool for the study of a variety of different sample materials. However its application to the investigation of biological molecules in this highly interesting spectral regime started only recently, and the work presented in this thesis represents one of the first investigations along this line. We present characteristic THz spectra of individual biomolecular systems in the crystalline and amorphous phase. In particular the correlation between the observed low-frequency modes and molecular structure and association was investigated. Our results show that the mode structure in the THz region represents a unique fingerprint for the molecular structure and conformation, e.g. allowing to distinguish related compounds, to discriminate between different isomers and to even observe structural phase-transitions...
Novel THz Metamaterial Designs: From Near- and Far-Field Coupling to High-Q Resonances
IEEE Transactions on Terahertz Science and Technology, 2013
Fiber Metamaterials: Recent Advances and Outlook
Workshop on Specialty Optical Fibers and their Applications, 2013
Dielectric response and collective dynamics of acetonitrile
Journal of Molecular Liquids, 2015