Title: Type-II thin film superconductors studied by terahertz radiation Author: RNDr. Roman Tesař Department: Department of Low Temperature Physics Supervisor: prof. RNDr. Ladislav Skrbek, DrSc. Consultant: RNDr. Jan Koláček, CSc. Abstract: Utilization of type-II superconductors for future practical applications such as fluxonics requires detailed knowledge of their physical properties, espe- cially at high frequencies within the THz spectral region. We have investigated interactions of thin-film NbN samples deposited on Si substrate and of a high quality epitaxial film of the NbN superconductor grown on a birefringent R-cut sapphire substrate with monochromatic linearly polarized laser beam both below and above the critical temperature Tc. For photon energies lower than the optical gap, detailed measurements of transmission in zero field provide BCS-like tem- perature curves with a pronounced peak below Tc which disappears as the energy of incident radiation is increased above the gap. In externally applied magnetic fields up to 10 T oriented perpendicularly to the sample, i.e., in the Faraday exper- imental geometry, the temperature behavior of transmission is modified because the gap is suppressed and vanishes at the upper critical field and, additionally, the presence of quantized vortices changes the shape...

<p> The optical properties of high temperature superconductors have long been of interest to condensed matter physicists. The majority of the research has concentrated on the steady-state properties of cuprates. Optical excitation of cuprate superconductors provides the valuable opportunity to study the dynamics of the superconducting state via the evolution of the superconducting condensate and excited quasiparticles. Terahertz (THz) spectroscopy is particularly attractive for the study of thin film cuprates, as the energies available in broad spectrum THz pulses lie below the maximum superconducting gap values. Optical pump THz probe spectroscopy utilizes a high energy infrared pulse to destroy the condensate and excite quasiparticles states out of equilibrium. The THz probe is capable of spectrally resolving the sample's temporal response to the optical perturbation. The direct measurement of both the amplitude and phase of the electric fields associated with the transmitted THz radiation allow for the calculation of both the real and imaginary parts of the conductivity. This offers the tantalizing potential of untangling the condensate's recovery from quasi-particle dynamics.</p> <p> The focus of this thesis will be upon the long timescale dynamics of YBa2Cu3O7-δ (YBCO) thin films. It has generally been believed that the condensate is fully recovered after a few picoseconds, and that the recovery of these films at longer timescales is essentially a thermal process although perhaps slowed by a phonon bottleneck. However, we will show spectral evidence that this picture cannot fully explain long lived dynamics in YBCO thin films. Specifically we see a suppression of the low frequency components of the optical conductivity. This anomaly is consistent with the formation of spatial inhomogeneity in the superconducting fraction, which likely arises from a non-uniform formation of the condensate across the film. The role of local inhomogeneity in the condensate and its effect on the conductivity of the thin film will be discussed. Evidence of intrinsic inhomogeneities in YBCO films may prove useful to the theoretical understanding of condensate dynamics in the cuprates. The spectral response of three doping levels, from optimally doped to underdoped YBCO, will also be shown, with a brief discussion of the normal state dynamics in underdoped films and the possible sensitivity of THz radiation to pseudogap dynamics.</p> / Thesis / Doctor of Philosophy (PhD)

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THz spinduliuotės generavimas iš puslaidininkių paviršiaus turi didelį potencialą puslaidininkių fizikinėms savybėms tirti. Šis darbas skiriamas puslaidininkių tyrimams generuojant THz impulsus iš jų paviršių, apšviestų femtosekundiniais lazerio impulsais. THz spinduliuotė iš puslaidininkių paviršių gali būti generuojama dėl visos eilės fizikinių mechanizmų: paviršinio lauko ekranavimo, foto-Demberio efekto, optinio lyginimo, elektriniu lauku indukuoto optinio lyginimo, plazminių svyravimų, koherentinių fononų ir plazmonų. Tiriant THz spinduliuotės generacijos mechanizmus galima išmatuoti daug svarbių puslaidininkių parametrų, tokių kaip lūžio rodiklis, judris, krūvininkų gyvavimo trukmė, aukštesniųjų laidumo slėnių padėtys. Darbo metu tirti THz spinduliuotės generacijos puslaidininkio paviršiuje mechanizmai keičiant žadinimo sąlygas: aplinkos temperatūrą, magnetinį lauką, žadinančio lazerio bangos ilgį ir intensyvumą, bei impulso trukmę. Ištyrus visą eilę įvairių puslaidininkių nustatyta, kad geriausias THz spinduliuotės emiteris žadinant 800 nm bangos ilgio spinduliuote yra p-InAs. Pirmą kartą THz žadinimo spektroskopijos metodu tiesiogiai išmatuoti tarpslėniniai atstumai InxGa1-xAs , InAs ir InSb bandiniuose. / Generation of terahertz radiation from semiconductor surfaces has great potential for investigation of physical properties of semiconductors. This work focuses on the semiconductor research when generating terahertz pulses from a variety of semiconductor surfaces. THz radiation from semiconductor surfaces can be generated on a whole range of physical mechanisms: the surface field screening, photo-Dember effect, the optical rectification, electric field induced optical rectification, plasma oscillations, coherent phonons and plasmons. A number of important semiconductor parameters such as refractive index, mobility, carrier relaxation time and higher conductivity valley positions can be measured using THz generation from semiconductor surface technique. In this work THz radiation generation mechanisms were investigated when changing excitation conditions: ambient temperature, magnetic field, laser wavelength and intensity, pulse duration. After tests with variety different semiconductors it was found that p-InAs is the best surface emitter when excitation laser wavelength is 800 nm. Using THz excitation spectroscopy the intervalley distances were measured directly, for the first time, in two InxGa1-xAs, InAs and InSb samples.

Generation of terahertz radiation from semiconductor surfaces has great potential for investigation of physical properties of semiconductors. This work focuses on the semiconductor research when generating terahertz pulses from a variety of semiconductor surfaces. THz radiation from semiconductor surfaces can be generated on a whole range of physical mechanisms: the surface field screening, photo-Dember effect, the optical rectification, electric field induced optical rectification, plasma oscillations, coherent phonons and plasmons. A number of important semiconductor parameters such as refractive index, mobility, carrier relaxation time and higher conductivity valley positions can be measured using THz generation from semiconductor surface technique. In this work THz radiation generation mechanisms were investigated when changing excitation conditions: ambient temperature, magnetic field, laser wavelength and intensity, pulse duration. After tests with variety different semiconductors it was found that p-InAs is the best surface emitter when excitation laser wavelength is 800 nm. Using THz excitation spectroscopy the intervalley distances were measured directly, for the first time, in two InxGa1-xAs, InAs and InSb samples. / THz spinduliuotės generavimas iš puslaidininkių paviršiaus turi didelį potencialą puslaidininkių fizikinėms savybėms tirti. Šis darbas skiriamas puslaidininkių tyrimams generuojant THz impulsus iš jų paviršių, apšviestų femtosekundiniais lazerio impulsais. THz spinduliuotė iš puslaidininkių paviršių gali būti generuojama dėl visos eilės fizikinių mechanizmų: paviršinio lauko ekranavimo, foto-Demberio efekto, optinio lyginimo, elektriniu lauku indukuoto optinio lyginimo, plazminių svyravimų, koherentinių fononų ir plazmonų. Tiriant THz spinduliuotės generacijos mechanizmus galima išmatuoti daug svarbių puslaidininkių parametrų, tokių kaip lūžio rodiklis, judris, krūvininkų gyvavimo trukmė, aukštesniųjų laidumo slėnių padėtys. Darbo metu tirti THz spinduliuotės generacijos puslaidininkio paviršiuje mechanizmai keičiant žadinimo sąlygas: aplinkos temperatūrą, magnetinį lauką, žadinančio lazerio bangos ilgį ir intensyvumą, bei impulso trukmę. Ištyrus visą eilę įvairių puslaidininkių nustatyta, kad geriausias THz spinduliuotės emiteris žadinant 800 nm bangos ilgio spinduliuote yra p-InAs. Pirmą kartą THz žadinimo spektroskopijos metodu tiesiogiai išmatuoti tarpslėniniai atstumai InxGa1-xAs , InAs ir InSb bandiniuose.

Low-loss waveguides with minimal dispersion are desired throughout the electromagnetic spectrum. These properties are difficult to achieve in the Terahertz (THz) region due to material and geometric constraints. This thesis focuses on the design, fabrication, and testing of waveguide-based devices using two promising technologies: the free-space metallic-slit waveguide (MSWG) and the coplanar strip (CPS) waveguide on a thin (1 um) commercial silicon nitride membrane. The work presented here differs from standard THz waveguide research which commonly uses the field radiated by a photoconductive antenna (THz optics) for excitation and detection. To improve upon system integration, a focus is placed on planar waveguide devices without refractive THz elements. Three main waveguide devices are investigated. First, an edge-coupled MSWG-based linear tapered slot antenna (LTSA) was used for THz-Time Domain Spectroscopy (TDS). This device functions as an alternative to a standard photoconductive switch coupled to a silicon lens and maintains comparable performance. Next an edge-coupled tapered MSWG was investigated. The MSWG conductor separation was increased to a low-loss configuration where the field propagated for 24 mm, after which the conductors were tapered to focus the field onto the receiving active region where a THz-bandwidth pulse was detected. Finally a CPS waveguide was fabricated on a thin silicon nitride membrane where a THz-bandwidth pulse was detected after propagating for 10 mm. The active regions for this device were fabricated using a unique method. This method results in the creation of thousands of small (40 um x 20 um) active regions (from a 4 mm x 4 mm host substrate) which can be placed anywhere for THz excitation and detection. The small active regions in conjunction with the CPS waveguide on the silicon nitride membrane provide an excellent platform for THz system testing. A single membrane can host many THz circuits which can be made ``active" by the placement of a few thin-film photoconductive devices. Main potential future applications include waveguide-based spectroscopy and coherent THz-bandwidth circuit analysis. / Graduate

In this thesis, we describe the development of a time-domain terahertz (THz) spectrometer driven by two different laser sources: an Er-doped femtosecond fiber laser and a mode-locked Ti:Sapphire laser. The resulting THz electromagnetic radiation was generated and detected using photoconductive antenna detection methods in both systems. In these experiments we characterized the THz power output for both the fiber laser driven system and the Ti:Sapphire laser driven system. Emphasis is given throughout this thesis on understanding the working principles behind time-domain terahertz spectroscopy, applications of THz radiation and terahertz generation as well as terahertz detection methods. We calculated the THz power output using two different methods. By using the &ldquo / Hertzian Dipole&rdquo / method we estimated the generated THz power after the generation photoconductive antenna. Using this method, we showed that the v generated power is on the order of milliwatts, which is far larger than the expected power typical for these systems. The second, &ldquo / Open-Circuit Voltage&rdquo / method, allowed us to calculate the received power on the detection photoconductive antenna. Using this method we were able to show that the THz power generated and detected in these systems is on the order of microwatts. For the mode-locked fiber laser driven spectrometer we obtained on average a ~ 4 ps (0.25 THz) pulse length which corresponded to an average power in the range of 71.8 nW - 70.54 &amp / #956 / W on a dipole antenna with a 6 &amp / #956 / m dipole gap and 44 &amp / #956 / m dipole length. Using the mode-locked Ti:Sapphire laser driven spectrometer we observed a ~ 2 ps (0.5 THz) pulse length and average power in the range of 0.54 nW &ndash / 5.12 &amp / #956 / W on a different dipole antenna with a 5 &amp / #956 / m gap and 40 &amp / #956 / m dipole length. Since these values agree with expected values for these systems we believe the &ldquo / Open-Circuit Voltage&rdquo / method is appropriate when trying to calculate the THz power.

Electromagnetic radiation in the frequency range between about 100 GHz and 5 THz can be used for spectroscopy and microscopy, but it is also promising for security screening and even wireless communication. In the present thesis a planar photoconducting large-area THz radiation source is presented. The device exhibits outstanding properties, in particular high THz field strength and generation efficiency and large spectral bandwidth with short THz pulse length. The THz emission is based on acceleration and deceleration of photoexcited carriers in semiconductor substrates. A metallic interdigitated structure at the surface of semi-insulating GaAs provides the electrodes of an Auston switch. In a biased structure photoexcited charge carriers are accelerated. Hence electromagnetic waves are emitted. An appropriately structured second metallization, electrically isolated from the electrodes, prevents destructive interference of the emitted waves. The structure investigated here combines several advantages of different conventional photoconducting THz sources. First, it provides high electric acceleration fields at moderate voltages owing to the small electrode separation. Second, the large active area in the mm2 range allows excitation by large optical powers of some mW. Optical excitation with near-infrared femtosecond lasers is possible with repetition rates in the GHz range. The presented results point out the excellent characteristics regarding the emitted THz field strength, average power, spectral properties, and easy handling of the interdigitated structure in comparison to various conventional emitter structures. Various modifications of the semiconductor substrate and the optimum excitation conditions were investigated. In the second part of this thesis the dynamic conductivity of GaAs/AlxGa1-xAs superlattices in an applied static electric field was investigated with time-resolved THz spectroscopy. The original goal was to explore whether the predicted effect of gain of electromagnetic radiation at THz frequencies is present in such structures. Superlattice samples were grown according to the experimental requirements, which include high specific resistance and sufficient THz transparency. The characterization of the superlattices by Fourier transform infrared spectroscopy and photoluminescence spectroscopy confirms the pronounced miniband properties of the bandstructure. Furthermore indications of Bloch oscillations were found by transport measurements. However, we could not measure a change of the dynamic conductivity when the electric field was toggled. Specific reasons for this and related experiments of other groups are discussed. / Elektromagnetische Strahlung im Frequenzbereich zwischen etwa 100 GHz und 5 THz wird für verschiedene Anwendungen wie Spektroskopie und Mikroskopie genutzt, kann aber auch für Sicherheitstechnik oder sogar Datenübertragung interessant sein. In der hier vorgestellten Forschungsarbeit wird eine großflächige fotoleitende THz-Strahlungsquelle beschrieben, die sich durch eine große THz-Feldstärke und große spektrale Bandbreite auszeichnet. Die THz-Emission basiert auf der Beschleunigung und Verzögerung fotogenerierter Ladungen in Halbleitersubstraten. Eine metallische Interdigitalstruktur auf der Oberfläche von semi-isolierendem GaAs bildet die Elektroden eines Fotoschalters. Ist an diese Struktur eine Spannung angeschlossen, werden optisch generierte Ladungsträger beschleunigt und strahlen elektromagnetische Wellen ab. Eine geeignet strukturierte und isolierte zweite Metallisierung verhindert destruktive Interferenzen der abgestrahlten Wellen. Die vorgeschlagene Struktur vereinigt dabei die Vorteile verschiedener herkömmlicher fotoleitender THz-Quellen. Einerseits ermöglicht der kleine Elektrodenabstand große elektrische Felder zur Beschleunigung fotogenerierter Ladungen schon bei moderaten Spannungen. Andererseits kann die große aktive Fläche von einigen mm2 mit großen optischen Leistungen im Bereich einiger mW angeregt werden. Die optische Anregung mit Nahinfrarot-Femtosekunden- Lasern kann mit Wiederholraten bis in den GHz-Bereich geschehen. Bedingt durch die Eigenschaften der Anregungspulse entstehen kurze spektral breite THz-Pulse. Die vorliegenden Ergebnisse verdeutlichen die hervorragenden Eigenschaften der Interdigitalstruktur im Vergleich zu verschiedenen herkömmlichen Geometrien bezüglich der Feldstärke der abgestrahlten Wellen, der mittleren Leistung und der spektralen Eigenschaften. Dabei ist die Struktur sehr einfach zu handhaben. Es wurden verschiedene Modifikationen des Substrates und die optimalen Bedingungen der optischen Anregung untersucht. Der zweite Teil dieser Arbeit behandelt die Erforschung der dynamische Leitfähigkeit von GaAs/AlxGa1-xAs-Übergittern in Abhängigkeit von einem elektrischen Feld mit Hilfe der zeitaufgelösten THz-Spektroskopie. Es sollte geklärt werden, ob der vorhergesagte Effekt der Verstärkung elektro-magnetischer Strahlung in solchen Strukturen möglich ist. Dazu wurden Übergitterproben gemäß den experimentellen Anforderungen hergestellt. Zu den Vorgaben gehört ein hoher spezifischer Widerstand und ausreichende Transparenz im THz-Bereich. Die Charakterisierung der Übergitter mit Fotolumineszenz- und Fourier-Transformations-IR-Spektroskopie bestätigte die ausgeprägten Minibandeigenschaften der Bandstruktur. Hinweise auf Bloch-Oszillationen wurden durch Ladungstransportmessungen gefunden. Dennoch war eine Änderung der dynamischen Leitfähigkeit beim Schalten des elektrischen Feldes nicht messbar. Gründe dafür und ähnliche Experimente anderer Gruppen werden diskutiert.

Disertacijos darbo tikslas buvo sukurti ir ištirti puslaidininkinius terahercinių (THz) impulsų emiterius ir detektorius, skirtus sistemoms, naudojančioms 1 μm bangos ilgio femtosekundinę lazerinę spinduliuotę. THz impulsų generavimo ir detektavimo sistema, kurios optoelektroninius puslaidininkinius komponentus aktyvuoja femtosekundiniai lazerio impulsai, yra plačiai taikoma terahercinėje laikinės srities spektroskopijoje. Tradiciškai tokiose sistemose naudojami Ti:safyre femtosekundiniai lazeriai, kurių spinduliuotės bangos ilgis yra ~800 nm. Šios sistemos nėra patogios dėl jų matmenų, nes lazeriai turi sudėtingą kelių pakopų kaupinimo sistemą. Pastaruoju metu THz impulsų generavimui vis dažniau naudojami femtosekundiniai kietakūniai ir šviesolaidiniai lazeriai, kurių spinduliuotės bangos ilgis patenka į artimosios IR spinduliuotės sritį. Tačiau šios sistemos vis dar neturi tinkamos medžiagos fotolaidiems elementams gaminti, kurie būtų žadinami 1 – 1,55 µm bangos ilgio lazeriais. Tokios medžiagos, visų pirmą, turi būti jautrios optinei spinduliuotei, o jų draustinės energijos tarpas turi atitikti žadinamos spinduliuotės fotonų energiją, be to sluoksniai turi pasižymėti didele tamsine varža bei labai trumpomis krūvininkų gyvavimo trukmėmis (~ 1 ps). Šioje disertacijoje yra pateikiami THz impulsų generavimo panaudojus puslaidininkių paviršius ir fotolaidžias antenas rezultatai, žadinant 1 µm bangos ilgio femtosekundiniais lazerio impulsais. / The aim of dissertation was to develop and explore the semiconductor material terahertz (THz) pulse emitters, for Terahertz time–domain spectroscopy (THz–TDS) systems using a 1 μm wavelength femtosecond laser radiation. THz pulse generation and detection using optoelectronic semiconductor components in THz–TDS excited by femtosecond laser pulses become these days a powerful experimental technique. Traditionally, mode-locked Ti:sapphire lasers emitting at the wavelengths ~800 nm are used. However Ti:sapphire lasers require many-stage optical pumping arrangement, the system is quite bulky and complicated. The solution could be the lasers emitting in 1 – 1.55 µm, which can be directly pumped by diode laser bars. Recently, several compact, efficient and cost-effective solid-state and fiber laser systems that generate femtosecond pulses at near-infrared wavelengths have been developed and employed for activating THz–TDS systems. The main obstacle of these systems is the lack of material with appropriate bandgap, high dark resistivity and short (~ ps) carrier lifetimes.