Daniel Hofstetter | University of Neuchâtel (original) (raw)

Journal Papers by Daniel Hofstetter

Crystals, 2021

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Science, 2002

Continuous wave operation of quantum cascade lasers is reported up to a temperature of 312 kelvin... more Continuous wave operation of quantum cascade lasers is reported up to a temperature of 312 kelvin. The devices were fabricated as buried heterostructure lasers with high-reflection coatings on both laser facets, resulting in continuous wave operation with optical output power ranging from 17 milliwatts at 292 kelvin to 3 milliwatts at 312 kelvin, at an emission wavelength of 9.1 microme- ters. The results demonstrate the potential of quantum cascade lasers as con- tinuous wave mid-infrared light sources for high-resolution spectroscopy, chem- ical sensing applications, and free-space optical communication systems.

Superlattices and Microstructures, 2020

We present a GaN-based quantum-cascade device whose inter-subband emission shows strong electron-... more We present a GaN-based quantum-cascade device whose inter-subband emission shows strong electron-phonon interaction. To generate the luminescence, an external electrical field – which partially screened the internal polarization – had to be applied. In low intensity spectra, a pattern of secondary peaks occurs. Each side-peak is separated from its fundamental inter-subband transition by a characteristic phonon energy, which shifts with applied field at the same rate as the main transition. At high intensity, there exists resonance between the 92 meV LO-phonon and the vertical inter-subband transition. A strong electrical field of >1 MV cm􏰷1 reduced via QCSE the transition energy from 230 meV to 80 meV. Additionally, the low active region doping necessitated large operating voltages. Besides the emission of mid-infrared radiation, the elevated voltage generated lots of phonons. At an electrical field of 1.02 MV cm􏰷1, the frequency-shifted inter-subband luminescence became resonant with the LO-phonon. The effects of this resonance will be discussed.

IEEE Photonics Technology Letters, 2019

An indirect method to determine the internal polarization of a GaN/AlGaN-based quantum well is pr... more An indirect method to determine the internal polarization of a GaN/AlGaN-based quantum well is presented. The technique consists of carefully measuring the difference between the two lowest conduction subband levels under application of an electrical bias across the epitaxial layers. Due to a quantum confined Stark effect, the bound energy levels of the well show a strong frequency shift which depends on the size of the external field. Since this electric field was oriented oppositely to the internal polarization, partial screening of the latter occurred. By measuring these Stark shifts at three different fields, one was able to calculate the size of the internal polarization. Since the composition of the AlGaN barriers had previously been determined by X-ray diffraction, an extrapolated value of the internal polarization – as it would occur in a pure AlN/GaN interface – could be given. In agreement with the literature, a value of 720 MV/m was found.

In this article, we demonstrate closely spaced, monolithically integrated photodetectors in two l... more In this article, we demonstrate closely spaced, monolithically integrated photodetectors in two largely different wavelength ranges. The epitaxial structure of the devices was grown by plasma-assisted molecular-beam epitaxy on an AlN-on-sapphire template; it consists of a Si-doped AlGaN thin film, and a nearly strain compensated 40 period AlN/GaN superlattice with 1.0 nm thick GaN quantum wells and 2.0 nm thick AlN barriers. The entire structure is capped with an AlGaN cap layer. The superlattice acts as active region for the infrared detector, while the AlGaN buffer layer serves as active area for the ultraviolet detector. While the photovoltaic near-infrared detector has a center wavelength of 1.37 μm, the photoconductive ultraviolet detector has a long wavelength cutoff at 250 nm. The two detectors could be operated up to room temperature with reasonable sensitivities.

Closely spaced, monolithically integrated photodetectors in two largely different wavelengths ran... more Closely spaced, monolithically integrated photodetectors in two largely different wavelengths ranges are demonstrated. The device structure was grown by plasma-assisted molecular-beam epitaxy on an AlN-on-sapphire template, and it consists of a Si-doped AlGaN thin film, and a nearly strain compensated 40 period AlN/GaN superlattice with 1.0 nm-thick GaN quantum wells and 2.0 nm-thick AlN barriers. The entire structure is covered with an AlGaN cap. The superlattice constitutes the active region for the infrared detector, while the AlGaN buffer layer serves as active area for the ultraviolet detector. The photoconductive ultraviolet detector has a long wavelength cutoff at 250 nm, whereas the photovoltaic near-infrared detector has a centre wavelength of 1.37 µm.

In this paper, we give an overview of quantum cascade detector technology for the near-and mid-in... more In this paper, we give an overview of quantum cascade detector technology for the near-and mid-infrared wavelength range. Thanks to their photovoltaic operating principle, the most advanced quantum cascade detectors offer great opportunities in terms of high detection speed, reliable room temperature operation, and excellent Johnson noise limited detectivity. Besides some important features dealing with their fabrication and their general characteristics , we will also briefly present some possibilities for performance improvement. Elementary theoretical considerations adopted from photoconductive detectors confirm that optimization of such devices always involves various trade-offs.

We report on the molecular-beam epitaxial growth, fabrication and characterization of AlN/GaN pho... more We report on the molecular-beam epitaxial growth, fabrication and characterization of AlN/GaN photovoltaic quantum well infrared photodetectors operating at 1.55 µm. Devices display a spectrally-narrow photovoltaic response to p-polarized light in the near infrared at room temperature. We have analysed the effect of the growth temperature, quantum well thickness, and number of periods in the active region, concluding that responsivity is enhanced by growing at relatively low temperature and by increasing the number of quantum wells.

The effects on surface morphology, crystal quality, and growth rate of undoped AlN layers grown o... more The effects on surface morphology, crystal quality, and growth rate of undoped AlN layers grown on c-plane sapphire substrates due to the changes in growth parameters, such as V–III ratio, N2-to-H2 ratio, and growth temperature in LP-MOVPE are studied here. The optimized growth process resulted in an almost flat surface morphology with a significantly reduced number of hexagonal pits ð3 Â 10 7 cm À2 Þ and good crystalline quality having a rms value of roughness of 0.4 nm measured by atomic force microscopy and a high resolution X-ray diffraction (HRXRD) FWHM value for (0 0 0 2) reflection of 200 arcsecs. The threading dislocation density of the AlN layer is estimated approx. 10 9 cm À2 from cross-sectional transmission electron microscopy (TEM) measurements. Additionally, these optimized samples show a strong donor-bound exciton luminescence signal with a FWHM of 20 meV. Furthermore, small period AlN/ GaN superlattice structures with excellent uniformity were grown, as proven by our HRXRD and TEM studies.

We report on a prototype electro-optical modulator at telecommunication wavelength based on inter... more We report on a prototype electro-optical modulator at telecommunication wavelength based on intersubband (ISB) transitions in a short-period GaN/AlN superlattice (SL). The device has a vertical architecture resembling a nitride-based high-electron mobility transistor, whose barrier layer has been replaced by a 5 period SL. By applying an electrical field, we were able to influence the bandstructure and as a consequence to quench the ISB absorption peak originating from the SL.

We demonstrated a GaAs/AlGaAs-based far-infrared quantum well infrared photodetector at a wavelen... more We demonstrated a GaAs/AlGaAs-based far-infrared quantum well infrared photodetector at a wavelength of 84 µm. The relevant intersubband transition is slightly diagonal with a dipole matrix element of 3.0 nm. At 10 K, a responsivity of 8.6 mA/W and a detectivity of 510 7 cm Hz/W have been achieved; and successful detection up to a device temperature of 50 K has been observed. Being designed for zero bias operation, this device profits from a relatively low dark current and a good noise behavior.

Buried heterostructure quantum cascade lasers emitting at 5.64 µm are presented. Continuous-wave ... more Buried heterostructure quantum cascade lasers emitting at 5.64 µm are presented. Continuous-wave (CW) operation has been achieved at -30 °C for junction down mounted devices with both facets coated. A 750 µm-long laser exhibited 3 mW of CW power with a threshold current density of 5.4 kA/cm2.

We report spectral linewidth measurements of a 9.1-µm distributed-feedback quantum cascade laser ... more We report spectral linewidth measurements of a 9.1-µm distributed-feedback quantum cascade laser (QCL). The free-running QCL beam was mixed with a waveguide isotopic C18O2 laser onto a high-speed HgCdTe photomixer, and beat notes were recorded from a radio-frequency spectral analyzer. Beating was performed at two operating conditions, first near the QCL laser threshold (beating with the C18O2 R10 line) and then at a high injection current (beating with the C18O2 R8 line). Overall, beat note widths of 1.3 – 6.5 MHz were observed, which proves that a free-running QCL can have a short-term spectral width near 1 MHz.

We report a quantum-cascade laser monolithically integrated with an intracavity modulator which c... more We report a quantum-cascade laser monolithically integrated with an intracavity modulator which could be operated up to 1 GHz. In contrast to earlier approaches, where the radio frequency (RF) modulation signal was supplied to the entire cavity length of the laser structure, we drive only a relatively small 375 µm long section of the cavity. At the same time, a quasi-contiunous-wave signal was supplied to the remaining 1125 µm long section. This modulation scheme resulted in smaller parasitic capacitance effects than what we reported previously, and enabled us to work with lower RF voltages and currents.

Two different high performance quantum cascade distributed-feedback lasers with four quantum-well... more Two different high performance quantum cascade distributed-feedback lasers with four quantum-well-based active regions and InP top cladding layers are presented. The first device, which emitted at 9.5 µm, was mounted junction down in order to get high average powers of up to 71 mW at 30 °C and 30 mW at room temperature. The other device, which lased at 9.1 µm, was optimized for high pulsed operating temperatures and tested up to 150 °C at 1.5 % duty cycle. The emission of both lasers stayed single mode with more than 20-dB side-mode suppression ratio over the entire investigated power and temperature range.

Intersubband absorption measurements on two nominally undoped AlGaN/GaN-based high-electron-mobil... more Intersubband absorption measurements on two nominally undoped AlGaN/GaN-based high-electron-mobility transistors with different Al compositions in the barrier layer are presented. The first transistor with a barrier consisting of Al0.6Ga0.4N showed an absorption peak at 247 meV (1973 cm-1) with a full width at half maximum FWHM of 126 meV, while the second device utilizing an Al0.8Ga0.2N barrier had its peak at 306 meV (2447 cm-1) with a FWHM of 86 meV. Self-consistently computed potentials and intersubband transition energies showed good agreement with the experimental findings, and therefore confirmed previously published values for the internal piezoelectric field in such structures.

Recent advances in quantum-cascade (QC) laser active region design are reviewed. Based on a rate ... more Recent advances in quantum-cascade (QC) laser active region design are reviewed. Based on a rate equation model of the active region, we show why new gain regions based on a two-phonon resonance or a bound-to-continuum transition exhibit significantly better performance than the traditional design based on a three-quantum-well active region. Threshold current densities as low as 3 kA/cm2 at ≈300 K, operation with a peak power of 90 mW at 425 K, single-mode high-power operation up to temperatures above 330 K at 16 µm and continuous wave operation up to ≈311 K are demonstrated. QC lasers able to operate at high duty cycles (50%) on a Peltier cooler were used in a demonstration of a 300 MHz free-space optical link between two buildings separated by 350 m.

Continuous-wave operation of a terahertz quantum cascade laser is reported. Optical output powers... more Continuous-wave operation of a terahertz quantum cascade laser is reported. Optical output powers ranging from 410 µW at 10 K to 124 µW at 35 K, at an emission wavelength of 66 µm are obtained.

We present distributed feedback quantum cascade lasers at 965 cm-1 with a high average optical ou... more We present distributed feedback quantum cascade lasers at 965 cm-1 with a high average optical output power at temperatures of up to 60 °C. At a duty cycle of 3 %, the averaged maximal output power of a 55-µm-wide and 1.5-mm-long device at 30 °C was 13.6 mW; at 60 °C, the device emitted 2 mW. Corresponding peak optical powers of 450 mW at 30 °C and of 70 mW at 60 °C have been observed. Due to the lateral current injection, we achieved single-mode behavior in a slightly distorted zero-order lateral mode across the whole range of investigated temperatures and output powers. At room temperature, the threshold current density was on the order of 6.7 kA/cm2 ; the characteristic temperature T0 was, due to tuning of the Bragg resonance into the gain curve, rather high, namely 310 K.

Crystals, 2021

Science, 2002

Superlattices and Microstructures, 2020

IEEE Photonics Technology Letters, 2019

Published in M. Razeghi, L. Esaki, and K. von Klitzing, Eds., The Wonders of Nanotechnology: Quan... more Published in M. Razeghi, L. Esaki, and K. von Klitzing, Eds.,
The Wonders of Nanotechnology: Quantum and Optoelectronic Devices and Applications (chapter 12), 261-287, 2013

Topics in Applied Physics vol. 89, 2003

This chapter describes our results on distributed feedback quantum cascade lasers in the waveleng... more This chapter describes our results on distributed feedback quantum cascade lasers in the wavelength range around 5 µm and around 10 µm. We present two different gain region designs; one with three quantum wells and one with a double phonon resonance. Several fabrication techniques are also presented and analysed in terms of fabrication simplicity, performance, yield, and reliability. We will outline typical results for all devices and also show some interesting applications. In light of this, the chapter is organized as follows: We start with a brief introduction ; in Sect. 2, the advantages and drawbacks of the different gain regions are outlined; Sect. 3 deals with the fabrication technology which was required to build these lasers; in Sect. 4, we present the measurement results on the devices; and finally, Sect. 5 describes two examples of interesting applications in the fields of optical spectroscopy and optical data transmission. The chapter ends with a brief conclusion and an outlook.

The present invention provides an independently addressable, vertical cavity surface emitting las... more The present invention provides an independently addressable, vertical cavity surface emitting laser ("VCSEL") in the blue wavelength range of 390 to 430 nanometers. The gallium nitride-based laser structure is grown by selective area epitaxy and lateral mask overgrowth. By appropriate patterning of a dielectric mask on the gallium nitride layer on a sapphire substrate (202), areas in a second gallium nitride layer can have a low defect density upon which the remainder of the laser structure can be formed.

An independently addressable, edge emitting semiconductor laser (200) emitting light in the blue ... more An independently addressable, edge emitting semiconductor laser (200) emitting light in the blue wavelength range of 390 to 430 nanometers comprises a gallium nitride-based laser structure grown by selective area epitaxy and lateral mask overgrowth. By appropriate patterning of a dielectric mask (206) on the gallium nitride layer (204) on a sapphire substrate (202), areas in a second gallium nitride layer (210) can have a low defect density upon which the remainder of the laser structure (218 - 230) can be formed.

A distributed feedback structure includes a substrate material. An active layer has an alloy incl... more A distributed feedback structure includes a substrate material. An active layer has an alloy including at least one of aluminum, gallium, indium, and nitrogen. A first cladding, having an alloy including at least one of the aluminum, the gallium, the indium, and the nitrogen, is on a first side of the active layer. A second cladding, having an alloy including at least one of the aluminum, the gallium, the indium, and the nitrogen, is on a second side of the active layer. Periodic variations of refractive indices in at least one of the first and the second claddings provide a distributed optical feedback.

This invention relates to a multiple wavelength laser structure, and more particularly, to a mult... more This invention relates to a multiple wavelength laser structure, and more particularly, to a multiple wavelength laser array structure fabricated by flip-chip-bonding from laser structures on two different substrates. A side by side red/IR laser structure is flip-chip bonded to a blue laser structure to form a red/blue/IR hybrid integrated laser structure.

This invention describes a simplified fabrication method for infrared distributed feedback semic... more This invention describes a simplified fabrication method for infrared distributed feedback semiconductor lasers. The diffraction grating providing optical feedback is located on the top of the upper cladding layer - right at the surface of the entire layer stack. However, to avoid large absorption losses through the adjacent top metallization, these metal layers are present only on the shoulders of the ridge waveguide.

Group III-V nitride semiconductors are used as light emitters for optoelectronic devices. To prov... more Group III-V nitride semiconductors are used as light emitters for optoelectronic devices. To provide the desired range of bandgaps and band offsets in heterostructure devices, InGaN layers have to be grown. InGaN layers are difficult to grow because poor lattice mismatch between group III-V nitride alloys. Thus, a plurality of gratings or grooves are formed in the group III-V nitride layer in order to relieve strain between the group III-V nitride layer and the active region. The plurality of gratings allows segregation of In in a manner that optimizes the wavelength of light produced.

A side by side red/IR laser structure (156, 160) is flip chip bonded by solder balls (402, 404) t... more A side by side red/IR laser structure (156, 160) is flip chip bonded by solder balls (402, 404) to a blue laser structure (252) to form a red/blue/IR hybrid integrated laser structure (400).

Proceedings of the 29th International Symposium on Compound Semiconductors, IOP vol. 174

Effects of a strong perpendicular magnetic field on a terahertz quantum cascade (QC) laser are pr... more Effects of a strong perpendicular magnetic field on a terahertz quantum cascade (QC) laser are presented. A reduction of the threshold current density to 50% of the zero-field value together with an increase of the slope efficiency by a factor of 2.7 was observed. Experimental results suggest a strong influence of the intersubband Landau resonances in the extraction process of the laser structure. A new THz QC laser design that relies on this effect is presented.

Proceedings of the 29th International Symposium on Compound Semiconductors

Continuous Wave operation of Quantum Cascade Lasers (QCL) in the far-infrared emission is demonst... more Continuous Wave operation of Quantum Cascade Lasers (QCL) in the far-infrared emission is demonstrated for the first time. The devices designed using a three-quantum-well chirped-superlattice active region in a waveguide based on a signal interface plasmon and a buried contact, with high reflectivity backfacet coating, results in continuous wave operation with optical output power of 250 mW at 10 Kelvin, at an emission wavelength of 66 µm. The lasers work continuously up to 30 Kelvin.

Proceedings of the 29th International Symposium on Compound Semiconductors

Pulsed operation of zoo different designs with abrupt and graded interfaces in the active regi... more Pulsed operation of zoo different designs with abrupt and graded interfaces in the active region are compared. Junction up mounted devices are tested in pulsed mode at room temperature for both designs. Lasers based on the graded interface design exhibited a lower threshold current density but also a lower optical output power due to a higher operating voltage compared to the abrupt interface design. Junction down mounted graded interface devices which were fabricated as buried heterostructure lasers with high-reflection coatings on both facets could be operated in continuous wave up to a temperature of 268°K with an optical power of 0.5 mW.

Proceedings of SPIE vol. 6485

Room temperature, continuous wave (CW) operation of distributed feedback (DFB) quantum cascade la... more Room temperature, continuous wave (CW) operation of distributed feedback (DFB) quantum cascade lasers with widely spaced operation frequencies is reported. The relatively small temperature tuning range of a single device, smaller or equal to approximately 1 % of the wavelength, usually limits their efficiency for spectroscopic investigations. By using a bound-to-continuum active region to create a broad gain spectrum and monolithic integration of different DFB gratings, we achieved high-performance devices with single-mode emission between 7.7 and 8.3 m at a temperature of +30 °C. This frequency span corresponds to 8 % of the center frequency. The maximum CW operation temperature achieved was 63 °C at the gain center and as much as 35 °C and 45 °C, respectively, at the limits of the explored wavelength range.

Proc. PITTCON …

Page 1. 1418 NOVEL MID-INFRARED GAS SENSORS BASED ON HOLLOW WAVEGUIDES AND QUANTUM CASCADE LASERS... more Page 1. 1418 NOVEL MID-INFRARED GAS SENSORS BASED ON HOLLOW WAVEGUIDES AND QUANTUM CASCADE LASERS BORIS MIZAIKOFF1, CHAVALI SSS MURTHY1, MARTIN KRAFT1, VIKTOR PUSTOGOW1, ANTOINE ...

Femtosecond two-color pump-probe experiments in the mid-infrared give the first evidence for a di... more Femtosecond two-color pump-probe experiments in the mid-infrared give the first evidence for a distinct optical phonon progression in the linear and nonlinear intersubband absorption spectra of electrons in a GaN/AlGaN heterostructure. The optical lineshapes of electronic transitions in condensed matter reflect the ultrafast dynamics of the elementary excitations to which the electrons are coupled. Of particular interest for a broad range of phenomena is the coupling between electrons and nuclear motions, i.e., local vibrational modes and/or phonons. Coupling to underdamped nuclear motions gives rise to spectrally distinct sidebands in electronic spectra [1]. So far, the majority of investigations of electron-phonon coupling focused on interband transitions in solids. The influence of electron-phonon coupling on intraband transitions was mainly studied in the context of polaron physics [2]. Due to their strong electron-phonon coupling, nanostructures made from strongly polar materials like group-III nitrides are promising systems for the occurrence of intersubband (IS) phonon sidebands. Because they have large conduction band discontinuities and thus allow IS transitions at short wavelengths down to the telecommunication wavelength of 1.55 Mim [3], GaN/AlGaN multiple quantum wells have received much interest. Recently, IS absorption was also observed in GaN/AlGaN-based high-electron-mobility transistors [4]. Here, we show for the first time that linear and particularly nonlinear IS absorption spectra of electrons in a GaN/AlGaN heterostructure show a distinct vibrational progression due to strong polar coupling to optical phonons. In femtosecond two-color pump-probe experiments in the mid-infrared we observe during the temporal overlap of pump and probe pulses sharp spectral holes on different vibronic transitions spectrally separated by the LO phonon frequency. These spectral features wash out after excitation with a decay time of 80 fs due to spectral diffusion. The nonlinear transmission spectrum decays by IS scattering with a time constant of 380 fs.

Ultrafast electron dynamics in the inversion layer of an AlGaN/GaN transistor is studied in pumpp... more Ultrafast electron dynamics in the inversion layer of an AlGaN/GaN transistor is studied in pumpprobe experiments with 50-fs midinfrared pulses. el-e2 intersubband scattering and thermalization occur within 200 fs.

Femtosecond two-color pump-probe experiments on intersubband absorption spectra of electrons in a... more Femtosecond two-color pump-probe experiments on intersubband absorption spectra of electrons in a GaN/AlN superlattice show distinct spectral holes unraveling both the homogeneous broadening contribution and the underlying optical phonon progression. The optical line shapes of electronic transitions in condensed matter reflect the ultrafast dynamics of the elementary excitations to which the electrons are coupled. Of particular interest for a broad range of phenomena is the coupling between electrons and nuclear motions, i.e., local vibrational modes and/or phonons. Such coupling may give rise to spectrally distinct sidebands in electronic spectra [1]. We have shown recently that the linear and nonlinear IS absorption in GaN/AlGaN-based high-electron-mobility transistors exhibit a pronounced optical phonon progression [2], caused by the strong electron-phonon coupling in the group-III nitrides. Because they have large conduction band discontinuities and thus allow IS transitions at short wavelengths [3], GaN/AlGaN multiple quantum wells have received much interest. First femtosecond two-color pump-probe studies on such nanostructures provided IS relaxation times and valuable information on the heating/thermalization and cooling dynamics of the 2D electron gas [4]. Here, we show for the first time that the IS transitions of electrons in GaN/AlN superlattices experience a considerable homogeneous broadening of the order of 50 meV and couple strongly to LO phonons as observed through distinct sidebands in the nonlinear optical response. In femtosecond two-color pump-probe experiments we observe during the temporal overlap of pump and probe pulses sharp spectral holes on different vibronic transitions spectrally separated by the LO phonon frequency. We investigated a GaN/AlN superlattice (20 periods of 2.2 nm GaN:Si and of 2.2 nm AlN [5]), which had been processed into a prism-shaped sample [2]. The stationary IS absorption spectrum measured at room temperature is plotted in Fig. 1(b). The broad absorption band is due to transitions of electrons from the n = 1 to the n = 2 subband. In our time-resolved experiments, electrons are excited from the n = 1 to the n = 2 subband by bandwidth-limited 100-fs pump pulses, having different center frequencies within the IS absorption band. The nonlinear response is measured by independently tunable weak 50-fs probe pulses. Pump and probe pulses at a 1-kHz repetition rate are derived from the output of two synchronized optical parametric amplifiers by difference frequency mixing in 1-mm and 0.5-mm thick GaSe crystals, respectively [6]. We performed an extensive series of time-resolved experiments with pump and probe pulses at different frequencies within the IS absorption band. The most interesting transient spectra for pump photon energies of 660 meV and 756 meV are shown in Figs. 1 and 2, respectively. In each case we observe around time delay zero two pronounced peaks of nonlinear transmission (spectral holes) separated by the energy of the GaN longitudinal optical phonon (92 meV). Within 300 fs these spectral features gradually wash out and a weak 50 meV broad spectral hole around 650 meV remains for longer delays. The spectrally integrated nonlinear transmission spectrum [Figs. 1(c) and 2(c)] decays by IS scattering with a time constant of 150 fs. The transient spectra show that the electron-LO phonon coupling strongly influences the nonlinear IS absorption of the strongly polar GaN/AlN superlattice. Similar to the nonlinear IS absorption in GaN/AlGaN-based high-electron-mobility transistors [2], the large electron-LO phonon coupling in GaN (Fröhlich coupling constant α = 0.5) lead to a pronounced optical phonon progression—a clear manifestation of polaronic signatures in IS absorption spectra.

We report a strong performance improvement for 1.55 µm AlN/GaN-based intersubband photodetectors.... more We report a strong performance improvement for 1.55 µm AlN/GaN-based intersubband photodetectors. Thanks to the use of quantum dots instead of quantum wells, a factor of 60 could be gained in terms of maximum responsivity. In addition, this performance was achieved at a considerably higher temperature of 160 K instead of 80 K typical for quantum wells. For the responsivity of such detectors based on optical rectification, the excited state lifetime is the dominating quantity. Since the phonon bottleneck in quantum dots drastically increases this lifetime, we believe that this effect is responsible for the observed improvement.

The use of an optically non-linear effect in ultra-thin layers of a strongly piezo-and pyro-elect... more The use of an optically non-linear effect in ultra-thin layers of a strongly piezo-and pyro-electric semiconductor has resulted in the demonstration of a high-speed photo-detector for future multi-gbit/s data rate telecom applications. Currently used opto-electronic components for fiber-based optical data transmission are reaching their intrinsic limits. In particular, photodetectors on the receiver side start to feel the detrimental effects of the space charge region in the p-n-junction. It would therefore be desirable to have a new technology at hand which does not suffer from these limitations. Having to deal with photons at a precise energy, a solution taking advantage of a resonant absorption process and optical non-linearities would be ideal. Such a resonant process occurs for instance between the bound levels of a quantum well (QW) fabricated from ultra-thin semiconductor layers. This absorption process, which is known under the name intersubband (ISB) absorption, is not only very efficient but has the additional advantage of an ultra-fast recovery time. In the past, ISB absorption has been used in compound semiconductor systems like GaAs/AlGaAs or InGaAs/InAlAs for the fabrication of photoconductive quantum well infrared photo-detectors (QWIPs) in the mid-infrared (mid-IR) wavelength range. 1 In order to access shorter, near-IR wavelengths, a different class of materials with a considerably larger conduction band discontinuity must be employed. In our work, we therefore stack thin QW and barrier layers from nitride-semiconductors, more specifically GaN QWs and AlN barriers, on top of each other. This material combination offers a conduction band discontinuity of nearly 2 eV, which is sufficiently large to accommodate ISB transitions in the near-IR wavelength range. In order to avoid dark current noise, a photovoltaic detection mechanism has been chosen for this work. As figure 1 shows, it is based on the fact that electrons undergo a small lateral (i.e. in growth direction) displacement when being excited into the upper bound level of a QW. This displacement results in the formation of an electrical dipole which polarizes the surrounding material. Added up across the entire quantum well and repeated several times, this polarization shows up as an appreciable photoinduced voltage. This mechanism has been first described by Rosencher et al. in 1989 and is also called resonant optical rectification. 2 Our photodetector structures are epitaxially grown on c-face sapphire substrates using plasma enhanced molecular beam epitaxy. The active region consists of a 40 period superlattice with 1.5 nm thick Si-doped GaN QW layers and 1.5 nm thick undoped AlN barriers. This superlattice is deposited on a 1 micrometer thick AlN buffer and covered with a 100 nm AlN cap layer. Processed photodetectors are entirely planar and consist of a pair of evaporated metal contacts: a dark reference contact and an illuminated signal contact with a size of 100 m 2. The photovoltage produced by illumination is then measured between the reference and the signal contact. Due to the intersubband nature of the involved optical transition, a spectrally narrow response with a center wavelength of 1.55 m and a relative linewidth on the order of 10 % is seen. Although the maximum performance occurs at a temperature of 200 K, room temperature operation with a maximum response of 10 V/W could be observed as well. For high frequency testing, we illuminated the detector with a directly modulated 1.55 m laser diode coupled to an optical fiber. The detector signal was amplified with a single low-noise amplifier and measured in a spectrum analyzer. As figure 2 shows, the maximum frequency for which a signal could be seen was 2.94 GHz. 3 Since no special care was taken to reduce parasitic capacitance and impedance mismatch, this value does not constitute the intrinsic speed limit of this novel optoelectronic component. In addition, it shows that the wide bandgap semiconductor GaN is not only suitable for applications in the ultraviolet wavelength region, but has also a considerable potential as a material for future optical data transmission systems in the near-IR. In order to move on to next generation telecommunication components, we need among other things photodetectors which do not suffer from intrinsic limitations such as the

We present a 1.55 µm electro-optical modulator fabricated in the nitride material system. The bas... more We present a 1.55 µm electro-optical modulator fabricated in the nitride material system. The basic idea is to use intersubband transitions in narrow quantum wells (QWs) and an electric field-induced transfer of electrons between a 2 dimensional electron gas (2DEG) and the QWs on top. By applying voltages on the order of +-5 V, we demonstrated a change in absorbance by a factor of 3. Due to the extremely fast intersubband scattering times of below 1 ps, such devices are expected to be suitable for high-speed telecommunication applications.

We report on a fast 1.55 µm intersubband detector based on GaN/AlN superlattice architecture. The... more We report on a fast 1.55 µm intersubband detector based on GaN/AlN superlattice architecture. The device was tested using a commercial laser diode modulated at frequencies up to several GHz. The highest frequency for which the detector gave a reasonably high signal was 2.37 GHz. This result is a major step towards ultra-fast opto-electronic devices in the nitride material system.

We present quantum cascade detectors (QCDs) for mid-infrared sensing applications. Thanks to the ... more We present quantum cascade detectors (QCDs) for mid-infrared sensing applications. Thanks to the use of strain-compensated InGaAs/InAlAs layers on InP, we were able to push the short wavelength detection limit well below 4 µm. The good agreement between simulations and experiments proves that QCDs are coming closer to real world applications such as focal plane arrays.

Proceedings of IRS2

Introduction: There are various photodetector applications in the mid-infrared (MIR) spectral ran... more Introduction: There are various photodetector applications in the mid-infrared (MIR) spectral range, both in military and civil applications. Thermal imaging in the MIR is of general interest as the emission of a room temperature black body peaks at a wavelength of 10 µm. Cameras sensitive at this wavelength are thus commonly used in construction to detect thermal bridges, in fire protection to pinpoint pockets of embers, as well as in security and military applications such as missile detection. Dynamic area thermometry is used for breast cancer detection based on the detection of subtle temporal changes of the skin temperature [1]. Heat seeking of hot targets such as aircraft engine exhaust plumes also takes place in the MIR, namely between 3 µm and 8 µm. MIR spectroscopy is an important tool in chemistry and biology, as many molecules and atoms have specific absorption lines between 1 µm and 10 µm. Light detection at 17 µm is of interest for detection of cold interstellar molecular Hydrogen (H2) using heterodyne spectroscopy [2], as the shortest pure rotational line of H2 lies at 17.035 µm [3].
Intersubband (ISB) photodetectors detect in the MIR and are well suited for high speed operation in the GHz range. Due to their narrow linewidth, ISB detectors are a promising choice for molecular and atomar spectroscopy, where often only a small part of the spectrum is of interest. The high speed of ISB detectors makes them interesting for heterodyne spectroscopy in the MIR, which allows to distinguish spectrally close absorption lines. Another pecularity of ISB detectors is that their peak detection wavelength is to a large extent fixed by design and not by material. This allows to engineer narrowband ISB detectors across the whole MIR range.

Laser Focus World, Dec 11, 2002

Quantum-cascade lasers become photodetectors Because their mid-infrared emission transmits well... more Quantum-cascade lasers become photodetectors

Because their mid-infrared emission transmits well through the atmosphere, quantum-cascade (QC) lasers are being investigated for use in free-space optical communications. The traditional type of light sensor paired with such a laser is a mercury cadmium telluride interband photodetector; quantum-well infrared detectors (QWIPs) could also be used. But a detector fabricated with materials and structure similar to those of a QC laser would allow monolithic integration of light source and detector. Researchers at the Institute of Physics (Neuchâtel, Switzerland) are testing this approach by creating intersubband photodetectors that have QC-laser structures. Two devices were fabricated. One has a 5.3-µm two-photon-resonance structure, while the other is a 9.3-µm bound-to-transition laser. The former works at temperatures up to 325 K and has a responsivity of 120 µA/W at 2200 cm-1, while the latter works to 297 K and has a responsivity of 50 µA/W at 1330 cm-1. The absorption peak of these devices can be shifted by applying an external bias. Although the responsivity of these devices is small compared to QWIPs, their noise gain is also very small, a possible compensation.

Contact Daniel Hofstetter at daniel.hofstetter@unine.ch.

Proceedings SPIE 7608, 2010

Proceedings of SPIE 4651, 2002

Physics and applications of recent quantum cascade laser active region designs are discussed. Spe... more Physics and applications of recent quantum cascade laser active region designs are discussed. Specifically, the use of bound-to-continuum and two-phonon resonance active regions for high temperature, high duty cycle operation is reviewed. Threshold current densities ...

Proceedings of CLEO'01, 2001

We present an in-the-field optical data link using a high frequency-modulated Peltier-cooled quan... more We present an in-the-field optical data link using a high frequency-modulated Peltier-cooled quantum cascade laser operated at 50% duty cycle. A maximal transmission rate of 330 MHz with a signal-to-noise ratio of 3 was achieved.

2006 Digest of the LEOS Summer Topical Meetings, 2006

Proceedings of the ITQW

Presentation ITQW 2003

Research Project 2006 within NCCR Quantum Photonics

Quantum cascade detectors for the mid-IR wavelength range, 2006

Quantum cascade detectors for the mid-IR wavelength range D. Hofstetter, F.R. Giorgetta, E. Ba... more Quantum cascade detectors for the mid-IR wavelength range

D. Hofstetter, F.R. Giorgetta, E. Baumann, and M. Graf, University of Neuchâtel, Switzerland N. Hoyler, J. Faist, ETHZ, Zurich, Switzerland
Q. Yang, C. Manz, and K. Köhler, IAF, Freiburg i. Brsg, Germany

Key words:
- Principle of operation
- Electron transport
- Dominating noise
- QWIP
- External voltage bias
- Dark current noise
- QCD
- Internal potential ramp
- Johnson noise
- Intersubband transitions instead of interband transitions

Goals:
- Build photo voltaic intersubband detectors for the mid-infrared wavelength range
- Avoid dark current and therefore dark current noise
- Broadband detection possible
- Growth of 26 different active region periods,
- Try to cover technologically important wavelengths,
- Sensitivity between 4.7 and 7.4 µm
- Works up to 325 K

Basic characteristics:
- Nearly constant response up to 100 K
- Drop of responsivity by factor of 3 – 10 up to 300K
- Small spectral linewidth on the order of 5 – 10 %
- Small wavelength shift with temperature
- Performance of mid-IR QCDs
- Compare performance to theoretical maximum
- In some cases, performance is close to this maximum
- Especially around 5 – 10 µm, improvements will be necessary
- Increase device resistance at room temperature

Conclusions:
- Demonstrated working devices at very different wavelengths
- Further optimizations are on the way
- Broadband detection
- High speed operation up to 23 GHz 10-6

In dieser Arbeit wurde das Reaktionsgleichgewicht zwischen und Fettsäure-Monomeren und Fettsäure-... more In dieser Arbeit wurde das Reaktionsgleichgewicht zwischen und Fettsäure-Monomeren und Fettsäure-Dimeren untersucht. Ziel der Untersuchung war es, mit einer optischen Methode-der photoakustischen Spektroskopie-eine Grössenordnung für die Bindungsenergie von Dimer-Molekülen angeben zu können. Ein Vergleich des hier erzielten Wertes von etwa 15 kcal/mol mit Literaturwerten, die mit Hilfe anderer Methoden zustande gekommen sind, zeigt eine gute Uebereinstimmung.

A monolithically integrated optical displacement sensor in the GaAs/AlGaAs material system has be... more A monolithically integrated optical displacement sensor in the GaAs/AlGaAs material system has been developed, fabricated and characterized. The device was a double Michelson interferometer with an integrated light source, photodetectors, couplers, and phase shifters. A key point of the work was the development of a single growth-step distributed Bragg reflector laser which served as the light source of the interferometer circuit. Special attention was also directed at the establishment of a bandgap-engineering technique (vacancy-enhanced disordering) allowing the definition of absorbing areas for the pumped laser section and the photodetector and transparent areas for the waveguiding sections and the grating section of the laser. The combination of the vacancy-enhanced disordering and the laser process enabled the fabrication of a fully integrated optical Michelson interferometer with quadrature signal detection. The maximal measurement distance with this double Michelson interferometer was 25 cm, and a maximal resolution of 20 nm was seen. Although using relatively simple standard processes, complex optical functions could be realized on a single chip.

We report a strong performance improvement for 1.55-μm AIN-GaN-based intersubband photodetectors.... more We report a strong performance improvement for 1.55-μm AIN-GaN-based intersubband photodetectors. Thanks to the use of quantum dots (QDs) instead of quantum wells (QWs), a factor of 60 could be gained in terms of maximum responsivity. In addition, this performance was achieved at a considerably higher temperature of 160 K instead of 80 K as typically seen for QWs. The responsivity of these photodetectors, which are based on optical rectification, is strongly influenced by their excited state lifetime. We believe that a much longer electron lifetime in the upper QD states and an increased lateral electron displacement are responsible for the observed improvement.

IEEE Photonics Technology Letters

Appl Phys Lett, 1999

We present measurement results on a surface-emitting quantum-cascade distributed feedback laser e... more We present measurement results on a surface-emitting quantum-cascade distributed feedback laser emitting infrared radiation at 10.1 mum. The use of a second order grating enabled the laser to emit about 25% of its total optical power from the grating. The beam radiated from the grating was at a very low divergence angle of about 1°×14°. As already presented in a previous paper, we simplified the processing by using a lateral current injection scheme avoiding epitaxial regrowth. At 85 K, the laser emitted 210 and 60 mW of pulsed power from facet and grating, respectively; at room temperature, the corresponding numbers were 70 and 18 mW. Threshold current densities of 2.1 kA/cm2 at 85 K and 5.6 kA/cm2 at room temperature were observed. The device showed single mode behavior for the entire temperature range and all investigated power levels. In addition, a constant temperature tuning coefficient of 0.06 cm-1/K was seen.

Functional Photonic and Fiber Devices, 1996

Applied Physics Letters, 2014

ABSTRACT We present electro-modulated absorption and electro-luminescence measurements on chirped... more ABSTRACT We present electro-modulated absorption and electro-luminescence measurements on chirped AlGaN/GaN-based multi-quantum well inter-subband structures grown by metal-organic vapour phase epitaxy. The absorption signal is a TM-polarized, 70 meV wide feature centred at 230 meV. At medium injection current, a 58 meV wide luminescence peak corresponding to an inter-subband transition at 1450 cm−1 (180 meV) is observed. Under high injection current, we measured a 4 meV wide structure peaking at 92.5 meV in the luminescence spectrum. The energy location of this peak is exactly at the longitudinal optical phonon of GaN.

Physica E: Low-dimensional Systems and Nanostructures, 2000

Proceedings, IEEE Tenth International Conference on Terahertz Electronics, 2002

ABSTRACT We present a microscopic model, based on a density matrix formalism, which unifies the a... more ABSTRACT We present a microscopic model, based on a density matrix formalism, which unifies the apparently quite different gain mechanisms in intersubband transitions: the inversion gain of a quantum cascade laser and the Bloch gain in a periodic superlattice. The theory also provides an expression for the spontaneous emission in intersubband transitions, which accounts for the temperature-dependent asymmetry of measured luminescence spectra. While the luminescence profile is hardly altered, the peak gain position depends critically on the ratio of populations. Thus, the degree of population inversion is accessible by simultaneous observation of both signals. For these measurements, whispering gallery quantum cascade laser are particularly suitable as scattered laser light and luminescence are of comparable strength over a wide range of injected currents.

Novel In-Plane Semiconductor Lasers II, 2003

The utilization of visible laser diodes for laser printing is discussed. First, the characteristi... more The utilization of visible laser diodes for laser printing is discussed. First, the characteristics of a multiple- element array of single-mode, individually-addressed red (AlGaInP) laser diodes is described. The benefit of shorter- wavelength blue lasers is then evaluated. Finally, towards the realization of a blue laser diode, we describe results for AlGaInN and its heterostructures, which have been grown by OMVPE and characterized, including electrical injection and optical pumping of InGaN/AlGaN heterostructures.

physica status solidi (c), 2003

Physica E: Low- …, 2000

We present measurement results on high-power low-threshold quantum cascade-distributed feedback l... more We present measurement results on high-power low-threshold quantum cascade-distributed feedback lasers emitting infrared radiation at 10.16 μm. A lateral current injection scheme allowed the use of a strongly coupled surface grating without metal coverage and epitaxial ...

Applied Physics Letters, 1999

… status solidi (c), 2003