Microwave pumped semiconductor superlattice parametric oscillator [Elektronische Ressource] : a new subterahertz and terahertz radiation source / vorgelegt von Benjamin Ingo Stahl

Sujets

• Physik

Informations

• Publié par: universitat_regensburg
• Publié le: 01 janvier 2008
• Nombre de lectures: 12
• Langue: English
• Poids de l'ouvrage: 1 Mo

Extrait

Microwave-pumped
semiconductor-superlattice parametric oscillator:
a new subterahertz and terahertz
radiation source
DISSERTATION
zur Erlangung des Doktorgrades der Naturwissenschaften (Dr. rer. nat.)
der Fakult¨at fu¨r Physik der Universit¨at Regensburg
vorgelegt von
Benjamin Ingo Stahl
Juni 2008Promotionsgesuch eingereicht am: 13.06.2008
Die Arbeit wurde angeleitet von: Prof. Dr. K. F. Renk
Pru¨fungsausschuss: Vorsitzender: Prof. Dr. G. Bali
1. Gutachter: Prof. Dr. K. F. Renk
2. Gutachter: Prof. Dr. W. Wegscheider
weiterer Pru¨fer: Prof. Dr. J. ZweckContents
Abstract 9
1 Introduction 13
2 Principle of the Superlattice Parametric Oscillator (SPO) 19
3 Theory 21
3.1 Electron transport in a semiconductor-superlattice . . . . . . . . . . . . 21
3.1.1 Superlattice structure and miniband formation . . . . . . . . . . 21
3.1.2 Electron motion in a static electric field . . . . . . . . . . . . . . 22
3.1.3 Electron motion in terahertz fields. . . . . . . . . . . . . . . . . 25
3.2 Theory of the SPO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2.1 Origin of gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2.2 Calculation of dynamic superlattice properties . . . . . . . . . . 28
3.3 Theoretical results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.3.1 Parametric gain . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.3.2 Optimum operation conditions of a subterahertz SPO . . . . . . 31
4 Experimental realization 37
4.1 SPO realization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.1.1 Quasiplanar SPO design . . . . . . . . . . . . . . . . . . . . . . 37
4.1.2 Modified SPO design with variable resonator length . . . . . . . 38
4.2 Superlattice structures . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.2.1 Quasiplanar superlattice device . . . . . . . . . . . . . . . . . . 39
4.2.2 Superlattice device with mesas for the modified SPO . . . . . . 40
4.3 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
55 Experimental results 45
5.1 SPO spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
5.2 Threshold behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
5.3 Feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
5.4 Power and conversion efficiency . . . . . . . . . . . . . . . . . . . . . . 48
5.5 Optimum output coupling . . . . . . . . . . . . . . . . . . . . . . . . . 49
5.6 Starting behavior with an external resonator . . . . . . . . . . . . . . . 49
5.7 Variation of the resonator length . . . . . . . . . . . . . . . . . . . . . 51
5.8 Broadband tunability . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
5.9 Monochromatic radiation source . . . . . . . . . . . . . . . . . . . . . . 52
5.10 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
6 Prospects of microwave-pumped superlattice THz radiation sources 55
6.1 Terahertz SPO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
6.1.1 Optimum operation conditions of a 3 THz SPO . . . . . . . . . 55
6.1.2 Maximum operation frequency . . . . . . . . . . . . . . . . . . . 57
6.1.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
6.2 SPO operation at higher harmonics . . . . . . . . . . . . . . . . . . . . 60
6.2.1 Gain at higher harmonics . . . . . . . . . . . . . . . . . . . . . 61
6.2.2 Double-resonance superlattice parametric quintupler . . . . . . 61
6.2.3 Proposal of a ninth harmonic double-resonance parametric oscil-
lator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
6.2.4 Increased seventh harmonic efficiency by double resonance . . . 63
6.2.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
7 Conclusion 67
8 Acknowledgment 69
9 Appendix: Comparison to a superlattice quintupler 71
Bibliography 81
678Abstract
The superlattice parametric oscillator (SPO), which was developed within the research
formydoctoralthesis, isdescribed. Theresearch ontheSPOcomprised anexperimen-
talandatheoreticalpart. TheSPO ispumpedwith microwave radiationandoscillates
at the third harmonic frequency of the pump frequency. A semiconductor superlattice
acts as active element giving rise to parametric gain for third harmonic radiation. A
resonator delivers the feedback necessary for parametric oscillation. The SPO works
at room temperature. We realized the SPO in a metal waveguide technique. When
pumped with radiation of a frequency near 100 GHz it generated third harmonic radi-
ation near 300 GHz. It was tunable over a broad frequency range. The SPO showed
characteristic behavior of an oscillator. The power (0.1 mW) of the third harmonic
radiation corresponded to a conversion efficiency of a few percent of the power (4 mW)
of the pump radiation. Parametric gain is based on miniband electron transport in a
superlattice. A theoretical analysis shows that gain can occur also at terahertz (THz)
frequencies. Terahertz gain should allow it to realize a terahertz-SPO.
The SPO consisted of a double-waveguide structure with a quasiplanar superlattice
device coupled by antennae to a third harmonic waveguide resonator and to a pump
waveguide. Two reflectors terminated the resonator: one reflector was formed by a
backshort, theotherone,apartialreflector,occurredasaconsequence ofanimpedance
mismatch between the output port of the resonator waveguide and a horn antenna.
Radiation was coupled out through the horn antenna. In a modified SPO design, the
resonator length could be varied by a movable backshort.
For the quasiplanar SPO and for the modified SPO, different types of GaAs/AlAs
superlattices were designed. The superlattice material was grown by molecular beam
epitaxy. Thesuperlatticedevices werestructuredbymeansofphotolithography,metal
vapor deposition, reactive ion etching and wet etching techniques.
The superlattice which was used in the quasiplanar SPO had 18 periods, each period
9Abstract
consisting of 14 monolayers GaAs and 4 monolayers AlAs, resulting in a miniband
width of 25 meV. Superlattice devices were prepared in a quasiplanar design with a
gold film bridge. For the modified SPO, superlattices with 60 periods (14 monolayers
GaAs, 2 monolayers AlAs) and a miniband width of 140 meV were used. Superlattice
devices with superlattice mesa structures (diameter 4 μm) were prepared.
The superlattice devices showed nonlinear, antisymmetric current-voltage characteris-
tics. A characteristic indicated an ohmic behavior of the superlattice for low voltages
and a peak current at a critical voltage. At higher voltages, the current decreased
with increasing voltage, displaying a negative differential mobility. From the current
voltage characteristics, the intraminiband electron relaxation time τ was estimated to
be about 150 fs for both superlattices.
In various experiments, the quasiplanar SPO, as well as the modified SPO, showed
properties which are characteristic for oscillators: feedback, threshold behavior, an in-
fluence of the resonator output coupling on the pump threshold , an optimum output
coupling andaninfluence of anexternal resonator, coupled to theSPO, onthestarting
behavior. In the frequency range that was investigated, the SPO was continuously
tunable from 225 GHz to 295 GHz.
The theoretical treatment of the SPO is based on a semiclassical single electron the-
ory. Using the miniband electron dispersion relation and the acceleration theorem, a
description of miniband electron transport under the influence of a time dependent
electric field is derived. For a time dependent electric field consisting of a pump field
and a third harmonic field, superlattice properties in the SPO (pump and third har-
monic mobility and resistance, harmonic power, conversion efficiency) are determined,
allowing to analyze the SPO mechanism.
The theoretical treatment shows that miniband transport in the SPO is strongly non-
linear. Thenonlinearitygivesrisetoparametricgainforthirdharmonicradiation. The
theory isinaccordance with theexperimental results. It explains theworking principle
of the SPO which produces radiation at subterahertz frequencies. If the pump power
exceeds a certain threshold pump power, third harmonic gain occurs. Gain is present
from zero amplitude of the third harmonic field on and increases with increasing am-
plitude, which enables theSPO tostart oscillating byitself. The theoreticalconversion
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