In this work, we report on the fabrication of laser induced periodic surface structures (LIPSS) on stainless steel, using bursts of 200 fs sub-pulses at a wavelength of 1030 nm. A cascade of birefringent crystals was used to generate the bursts with tunable number of sub-pulses and intra-burst delays varying between 1.5 ps and 24 ps. Being such a delay shorter than the typical electron-lattice relaxation time in metals, the sub-pulses impinge on the sample surface when the material is still in a transient state after excitation from the first sub-pulse, thus allowing peculiar structures to be generated depending on the burst features. We obtained 1-D and 2-D periodic surface structures and investigated the influence of number of sub-pulses and polarization on their morphology. In particular, when bursts composed by all-aligned linearly polarized sub-pulses were used, 1-D LIPSS were obtained with different periodicity and depths depending on the number of sub-pulses. Bursts with crossed linear polarization or circular polarization sub-pulses produced 2-D LIPSS with morphology varying from triangular structures arranged in hexagonal lattice to pillar-like ordered or disordered structures depending on the bursts features. In most cases these structures exhibit a superhydrophobic behavior, as assessed by static contact angle measurements, which is achieved after a time of exposition to laboratory air. By XPS analysis we investigated the chemical variations occurring on the surfaces over this time.
KEYWORDS: Terahertz radiation, Near field, Near field optics, Quantum cascade lasers, Signal detection, Sensors, Spatial resolution, Mirrors, Interferometry, Nanostructures
Near-field imaging techniques at terahertz (THz) frequencies are severely restricted by diffraction. To date, different detection schemes have been developed, based either on sub-wavelength metallic apertures or on sharp metallic tips. However high-resolution THz imaging, so far, has been relying predominantly on detection techniques that require either an ultrafast laser or a cryogenically-cooled THz detector, at the expenses of a lack of sensitivity when high resolution levels are needed. Here, we demonstrate two novel near-field THz imaging techniques able to combine strongly sub-wavelength spatial resolution with highly sensitive amplitude and phase detection capability. The first technique exploits an interferometric optical setup based on a THz quantum cascade laser (QCL) and on a near-field probe nanodetector, operating at room temperature. By performing phase-sensitive imaging of THz intensity patterns we demonstrate the potential of our novel architecture for coherent imaging with sub-wavelength spatial resolution improved up to 17 μm. The second technique is a detector-less s-SNOM system, exploiting a THz QCL as source and detector simultaneously. This approach enables amplitude- and phase-sensitive imaging by self-mixing interferometry with spatial resolution of 60-70 nm.
Biodetection using electrolyte gated field effect transistors has been mainly correlated to charge modulated transduction. Therefore, such platforms are designed and studied for limited applications involving relatively small charged species and much care is taken in the operating conditions particularly pH and salt concentration (ionic strength). However, there are several reports suggesting that the device conductance can also be very sensitive towards variations in the capacitance coupling. Understanding the sensing mechanism is important for further exploitation of these promising sensors in broader range of applications. In this paper, we present a thorough and in depth study of a multilayer protein system coupled to an electrolyte gated transistor. It is demonstrated that detection associated to a binding event taking place at a distance of 30 nm far from the organic semiconductor-electrolyte interface is possible and the device conductance is dominated by Donnan’s capacitance of anchored biomolecules.
We will report on new developments of quartz-enhanced photo-acoustic (QEPAS) sensor employing THz quantum cascade laser (QCLs). The extension of the QEPAS technique in the THz range was made possible by the realization of custom made QTFs. With the aim to improve the QTF acousto-electric transduction efficiency, we designed and realized new QTF designs. A detailed analysis of the quality factor, the resonance frequency and the electrical resistance of custom quartz tuning forks (QTFs) with different geometrical parameters is reported. The custom QTFs were employed in QEPAS sensors using THz QCLs as the laser excitation sources and targeting CH3OH and H2S. Minimum detection limits of a few tens of ppb and normalized noise equivalent absorption factors down to 3.75×10-11 cm-1W/Hz½ were achieved.
A detailed analysis of the quality factor, the resonance frequency and the electrical resistance of custom quartz tuning forks (QTFs) having different geometrical parameters is reported. We implemented custom QTFs in a quartz enhanced photoacoustic sensor targeting water vapor detection and compared the fundamental and first overtone flexural modes gas sensor performances.
We report the successful realization of quartz-enhanced photo-acoustic (QEPAS) sensors employing quartz tuning forks (QTFs) with novel geometrical parameters. We investigated the influence of QTF sizes on the main resonator parameters, in order to identify the best design parameters optimizing the QTF figures of merit for optoacoustic gas sensing. To evaluate the QTF acousto-electric energy conversion efficiency, we operated the QEPAS sensors in the near- IR and selected water vapor as the target gas. QTFs are forced to resonate at both the fundamental and the first overtone vibrational mode frequencies. Our results shows that two QTF designs exhibit an higher quality factor (and consequently an higher QEPAS signal) when operating on the first overtone mode with respect to the fundamental one.
We report the first demonstration of single-mode laser beam delivery in hollow-core waveguides (HCWs) operating in the 3.7-7.3 μm spectral range. We investigated the transmission properties of HCWs with 200 μm bore diameter and internal coatings properly designed to enhance the spectral response in the spectral range of 3-12 μm. We achieved single mode output throughout the 3.7-7.3 μm range. We measured a coupling efficiency < 90% and transmission losses as low as 1 dB, when using a 15 cm-long fiber at 3.7 μm under optimized coupling conditions between input beam and HCW.
We report the first demonstration of a leak sensor based on a mid-IR quartz-enhanced photoacoustic (QEPAS) spectroscopic technique. A QEPAS sensor was integrated in a vacuum seal test station for mechatronic components. The laser source is a quantum cascade laser emitting at 10.56 μm, resonant with a strong absorption band of sulfur hexafluoride (SF6), which was selected as target gas for leak detection. The minimum detectable concentration of the QEPAS sensor is 6.9 ppb with an integration time of 1 s. This detection sensitivity allowed to measure SF6 leak flows as low as 3x10-5 standard cm3.
We study the dynamics of a Quantum Cascade Laser subject to strong optical feedback in the framework of the
Lang-Kobayashi model. In particular, we demonstrate that the continuous wave instability may lead to coherent
multimode oscillations that indicate spontaneous phase-locking among external cavity modes. We recently
predicted that this unique behavior is linked to the absence of relaxation oscillations in unipolar semiconductor
lasers, which are characterized by a fast carriers recombination time (class-A lasers). These theoretical evidences
may help understanding the mechanisms possibly leading to spontaneous mode-locking and pulse generation in
QCLs.
We report on the design and realization of custom quartz tuning forks with different geometries and sizes aimed to
improve the photoacoustic effect in quartz-enhanced photoacoustic (QEPAS) sensor systems. A detailed analysis of the
piezoelectric properties in terms of resonance frequencies, quality factors, gas damping was performed.
We report a theoretical and experimental study of laser coupling in hollow-core, fiber-optic waveguides with small-bore
diameters of d=200 μm. For the experiments we utilized three mid-infrared quantum cascade lasers with different
emission wavelength, which were coupled into the waveguides using lenses with focal lengths in the range 25-76 mm.
Measurements of the output beam profiles and propagation losses were obtained as a function of the coupling conditions.
With appropriate coupling parameters, single mode beam delivery can be obtained for all laser wavelengths, ranging
from λ ~ 5.4 to 10.5 μm.
We report on three different quartz enhanced photoacoustic (QEPAS) sensors operating in the near-IR, mid-IR and THz
spectral ranges, employing quartz tuning forks of different sizes and shapes. To test our sensors in the near-IR we used a
diode laser working at 2.7 μm, while in the mid-IR we employed a quantum cascade laser (QCL) operating at 7.9 μm,
fiber-coupled to the QEPAS cell. In the THz range we employed a QCL emitting at 2.95 THz. H2S absorption features
with line-strength up to 10-20 cm/mol were selected and QEPAS normalized noise-equivalent absorption in the 10-10
W•cm-1•Hz-1/2 range was achieved..
We demonstrate superresolution in position tracking sensing based on feedback interferometry in quantum cascade lasers
(QCLs). QCLs with optical feedback make highly compact sensors since they work as mixer oscillator and detector of
infrared radiation. Additionally, QCL continuous-wave emission remains stable at steady state in strong feedback
regimes, permitting to gain control on the nonlinearity of the QCL active medium. Here, nonlinear frequency mixing in a
QCL-based common-path interferometer is exploited to unveil object’s position with nanometer-scale resolution, far
beyond the intrinsic limit of half-wavelength. Experimental results are in excellent agreement with simulations based on
Lang-Kobayashi model encompassing multiple-target dynamics.
We report on a novel intracavity quartz enhanced photoacoustic (I-QEPAS) gas sensing technique taking advantage from
both the high Q-factor of standard tuning forks and the power build-up of a high-finesse optical resonator. This first
prototype employs a distributed feedback quantum cascade laser operating at 4.3 μm. CO2 has been selected as gas target. Preliminary results demonstrate an improved sensitivity, close to the cavity enhancement factor (500) times the optical coupling efficiency (about 0.5), with respect to standard QEPAS technique. The detection limit was pulled from 7
ppm (obtained with standard QEPAS) down to 32 ppb, corresponding to normalized noise-equivalent absorption in the
10-9 W•cm-1•Hz-1/2 range.
We report on an innovative quartz enhanced photoacoustic (QEPAS) gas sensor operating in the THz spectral range,
employing a custom quartz tuning fork (QTF) with the two prongs spaced by ~800 μm. To test our sensor we employed
a quantum cascade laser light source and selected a methanol rotational absorption line falling at 131.054 cm-1 (~3.93 THz), with line-strength S = 4.28•10-21 cm. The sensor operated at 10 Torr pressure on the QTF first flexion resonance
frequency at 4245 Hz. We achieved a QEPAS normalized noise-equivalent absorption of 2•10-10 W·cm-1•Hz-1/2 comparable with the best result of mid-IR QEPAS systems.
We demonstrated a sensing technique for in-line ablation rate detection using a quantum cascade laser (QCL) under external optical feedback. The design of the QCL-based diagnostic system allowed to monitor the voltage modulation at the laser terminals induced by fast dynamics in the ablation process. Real-time detection of the ablation front velocity as well as in-situ investigations of the surface temperature were provided. Experimental results on fast ablation rates per pulse correlate well with the theoretical prediction. The detection range was demonstrated to be limited only by the QCL-probe emission wavelength, which is scalable up to the THz spectral region.
We report on the experimental measurement of active region lattice (TL) and electronic temperatures (Te) in terahertz quantum cascade devices based on the phonon-photon-phonon scheme, by means of microprobe band-to-band photoluminescence spectroscopy. Three mesa devices, differing for doping region and number of quantum wells composing the active region, have been investigated. With device on, under band alignment for lasing condition, we
measured a difference (Te - TL) ~ 40 K much smaller than the typical value (Te - TL ~ 100 K) reported for resonantphonon THz QCLs.
We demonstrate a common-path optical interferometer based on a quantum-cascade laser (QCL), in which the QCL acts
both as source and detector of the infrared radiation. The collinear arms of the interferometer are terminated by a plastic
surface (acting as the beam splitter) and by a metallic one (acting as the mirror). The different reflectivity of the surfaces
allows for high contrast feedback-interferometry fringes exhibited on the laser-emitted power and revealed by voltage
compliance measurement at the QCL terminals. The displacement of each surface can be identified and measured with
sub wavelength resolution. The experimental results are in excellent agreement with the numerical simulations based on
the Lang-Kobayashi model for multiple cavities. Applications to microfluidics and resonant chemical detection can be
envisaged.
We will report here on the design and realization of optoacoustic sensors based on an external cavity QCL laser source
emitting at 10,54 μm, fiber-coupled with a QEPAS spectrophone module. SF6 has been selected as the target gas. Single
mode laser delivery through the prongs of the quartz tuning fork has been realized using a hollow waveguide fiber with
internal core size of 300 μm. The achieved sensitivity of the system was 50 part per trillion in 1 s corresponding to a
record for QEPAS normalized noise-equivalent absorption of 2,7•10-10 W•cm-1•Hz-1/2.
We consider a multi-transverse mode Vertical Cavity Surface Emitting Laser (VCSEL) subject to optical feedback.
When the field profile can be described in terms of few Gauss-Laguerre modes we show that the self-mixing
interferometric signal exhibits features peculiar of the spatial distribution and/or polarization state of the re-injected
field. Based on these results we provide both theoretically and experimentally the proof-of-principle of
an operational scheme for a sensor that can be used to simultaneously measure target translations along the
optical axis and target rotations in the orthogonal plane.
The recent development of ultrafast laser ablation technology in precision micromachining has dramatically increased
the demand for reliable and real-time detection systems to characterize the material removal process. In particular, the
laser percussion drilling of metals is lacking of non-invasive techniques able to monitor into the depth the spatial- and
time-dependent evolution all through the ablation process. To understand the physical interaction between bulk material
and high-energy light beam, accurate in-situ measurements of process parameters such as the penetration depth and the
removal rate are crucial. We report on direct real time measurements of the ablation front displacement and the removal
rate during ultrafast laser percussion drilling of metals by implementing a contactless sensing technique based on optical
feedback interferometry. High aspect ratio micro-holes were drilled onto steel plates with different thermal properties
(AISI 1095 and AISI 301) and Aluminum samples using 120-ps/110-kHz pulses delivered by a microchip laser fiber
amplifier. Percussion drilling experiments have been performed by coaxially aligning the diode laser probe beam with
the ablating laser. The displacement of the penetration front was instantaneously measured during the process with a
resolution of 0.41 μm by analyzing the sawtooth-like induced modulation of the interferometric signal out of the detector
system.
Direct real-time measurements of the penetration depth during laser micromachining has been demonstrated by
developing a novel ablation sensor based on laser diode feedback interferometry. Percussion drilling experiments have
been performed by focusing a 120-ps pulsed fiber laser onto metallic targets with different thermal conductivity. In-situ
monitoring of the material removal rate was achieved by coaxially aligning the beam probe with the ablating laser. The
displacement of the ablation front was revealed with sub-micrometric resolution by analyzing the sawtooth-like induced
modulation of the interferometric signal out of the detector system.
Imaging arrays of direct detectors in the 0.5-5 THz range are being experimentally developed. Terahertz active imaging
with amplitude-modulated quantum cascade lasers emitting at 2.5 and 4.4 THz performed by using an antenna-coupled
superconducting microbolometer. We then present two room-temperature terahertz detector technologies compatible
with monolithic arrays: i) GaAs Schottky diodes with air-bridge sub-micron anodes; ii) high electron mobility transistors
with sub-micron Schottky gate. Performances, requirements and fabrication costs of the different detector technologies
are compared.
We review the basic microscopic phenomena controlling the extraction of energy from the active regions of terahertz quantum cascade lasers. Using time-resolved microprobe photoluminescence, we measure the in-plane (v//) and cross-plane (v) heat transfer velocities and find a strong anisotropy, with v// slightly reduced by 30% and v reduced by a factor of 6.5 with respect to the corresponding bulk values. Combining this information with measurements of the cross-plane thermal conductivity enables us to estimate a phonon mean free path value L ~ 70 nm at 80 K, reduced by about one order of magnitude with respect to the bulk value in GaAs, but much larger than the average spacing between the interfaces in the active region. The latter finding is consistent with a heat transport model considering both thermal boundary resistance and phonon dispersion modification.
We will report on the design and realization of optoacoustic sensors based on commercial quantum cascade lasers for
environmental analysis applications. Different configurations will be described: i) sensors based on resonant
photoacoustic cells, a "standard" H cell and an innovative T-cell; ii) sensor based on quartz enhanced photoacoustic
spectroscopy. We will analyze the results obtained in the detection of nitric oxide.
We report on the measurement of both transverse and in-plane heat transfer speed in GaAs/AlGaAs quantum cascade
lasers via time-resolved micro-probe photoluminescence. We found approximately a one order of magnitude reduction
in the heat transverse speed with respect to bulk values that we ascribed to the heat interface boundary resistance. We
also compared the non equilibrium population of phonons via anti-Stokes Raman scattering in different active region
configurations.
The detection and quantification of trace gases is of great interest in a wide range of applications such as environmental
monitoring, industrial process control and medical diagnostics. In combination with quantum cascade lasers,
photoacoustic spectroscopy offers the advantage of high sensitivity (parts per billion detection limits), compact set-up,
fast time-response and simple optical alignment. We will report here on the design and fabrication of optoacoustic
sensors based on two different cell configurations to detect nitric oxide and formaldehyde.
We report on the development of an all-interferometric sensor based on the laser-self-mixing for the simultaneous
detection of multi-degrees-of-freedom displacement of a remote target. The prototype system consists only of a laser
head, equipped with 6 diode lasers and a properly designed reflective target. Information on a single degree-of-freedom
motion is extracted by the differential measurement of two linear displacements by means of two nominally identical
self-mixing interferometers. The sensor has been experimentally tested to measure yaw, pitch, roll, straightness and
flatness corrections over a continuous linear range of 1 m, with resolutions of 0.7 μm (longitudinal), 20 μm (straightness
and flatness), 0.001° (yaw and pitch) and 0.015 °(roll).
The development of a contactless sensor based on the Laser-Self-Mixing effect for the simultaneous measurement of
linear and transverse degrees-of-freedom (DOFs) of a moving target is described in this paper. The sensor is made of
three laser diodes with integrated monitor photodiodes, and a properly designed reflective target attached to the moving
object. The proposed technique exploits the differential measurement of linear displacements by two identical self-mixing
interferometers (SMIs) and makes the system more compact and easier to align with respect to traditional
interferometric systems, thus providing an effective low-cost motion control system. The feasibility of the proposed
sensor is experimentally demonstrated over a range of 1 m for linear motion and ± 6 mm for transverse displacements,
with resolutions of 0.7 μm and 20 μm, respectively.
A new fiber strain sensor based on the Laser-Self-Mixing effect in Distributed Feedback Laser diodes is presented.
Compared with existing fiber strain sensors based on Fiber Bragg Gratings, that are sensitive to local strain, our device
achieves comparable sensitivity (~ 1 με) distributed along the whole fiber length and requires a much simpler
electronics. The sensor is based on the interferometric principle of the laser-self-mixing in the moderate feedback
regime, whereby the fiber strain results in a variation of the optical path-length seen by the radiation reflected by the end
facet of the fiber itself. Increasing or decreasing strain recognition is directly provided by the sign of the sawtooth-like
fringes derivative.
We show that the internal quantum efficiency and the wall-plug efficiency of quantum cascade lasers (QCL) are
correlated with the hot-electron cooling associated with photon emission. The experimental procedure for the
assessment of these key device parameters is based on micro-probe photoluminescence (PL) that allows high resolution
measurements of the electronic and local lattice temperatures in operating QCLs. By using a terahertz QCL as a
prototype we demonstrate that the electronic distributions are Fermi-Dirac functions characterized by temperatures
significantly larger than the lattice one. The lattice temperature is in turn well above the one of the heat sink bath.
Combining the above observation with time-resolved PL experiments we assessed the characteristic time constants
controlling the heating and cooling processes of terahertz QCLs that are limited by the presence of a high density of
interfaces that causes phonon interference effects. The correlation between the above constants, the thermal diffusivities
and the diffusion lengths have been extracted from the comparison with the outcome of a transient heat diffusion model.
We report on the realization of a optoacoustic sensor based on mid-infrared quantum-cascade lasers (QCLs) for the detection of nitric oxide (NO) and formaldehyde (CH2O). A resonant photoacoustic cell equipped with 4 electret microphones was excited in its first longitudinal mode by the modulated laser light. A detection limit for of 300 parts in 109 (ppbv) for NO and 150 ppbv for CH2O is found, using distributed feedback QCLs operating in pulsed mode at 5.34 μm and 5.6 μm, respectively.
We report on the investigation of the electronic and thermal properties of high performance terahertz (THz) quantum cascade lasers (QCLs) based on a bound-to-continuum scheme and grown by molecular beam epitaxy (MBE) by a commercial provider. The local lattice temperatures, the subband electronic temperatures and the electron-lattice coupling constant have been extracted from the analysis of microprobe band-to-band photoluminescence spectra measured on devices operating in continuous wave (cw). We found that the electronic distributions in both the active region and the injector are thermalized and that all the subbands share the same electronic temperatures. The measured device thermal conductance C* = 4.6 W/(K x m) reflects the good thermal management of the investigated devices. The electron-lattice coupling constant = 78.5 Kcm2/kA is a factor 1.65 higher than in mid-IR QCLs thus indicating the inefficient cooling of the electronic ensemble caused by the strong reduction of electron-LO phonons scattering channels.
We report on the experimental study of the structural, electronic and thermal properties of state-of-art Sb-based quantum-cascade lasers (QCLs) operating in the range 4.3-4.9 µm. This information has been obtained by investigating the active region band-to-band photoluminescence signals, detected by means of an GaInAs-array detector. This technique allowed to probe the spatial distribution of conduction electrons as a function of the applied voltage and to correlate the quantum design of devices with their thermal performance. We demonstrate that electron transport in QCLs based on Sb-ternary barriers may be insufficient, thus affecting the tunneling of electrons and the electronic recycling and cascading scheme. Finally, we present the first measurement of the electronic and lattice temperatures and the electron-lattice coupling in Sb-based QCLs based on a quaternary-alloy. We extracted the thermal resistance (RL = 8.9 K/W) and the electrical power dependence of the electronic temperature (Re = 11.7 K/W) of Ga0.47In0.53As/Al0.62Ga0.38As1-xSbx structures operating at 4.9 µm, in the lattice temperature range 50 K - 80 K. The corresponding electron-lattice coupling constant ( = 10.8 Kcm2/kA) reflects the reduction of the electron-leakage channels associated with the use of a high conduction band-offset.
We report on the experimental study of the electronic and thermal properties in state of art Sb-based quantum-cascade lasers (QCLs) operating in the range 4.3-4.9 &mgr;m. This information has been obtained by investigating the band-to-band photoluminescence signals, detected by means of an InGaAs-array detector. This technique allowed to probe the spatial distribution of conduction electrons as a function of the applied voltage and to correlate the quantum design of devices with their thermal performance. We demonstrate that electron transport in these structures may be insufficient, thus affecting the tunneling of electrons and the electronic recycling and cascading scheme. Finally, we present the first measurement of the electronic and lattice temperatures and of the electron-lattice coupling in Sb-based QCLs based on a
quaternary-alloy. We extracted the thermal resistance (RL = 9.6 K/W) and the electrical power dependence of the
electronic temperature (Re = 12.5 K/W) of Ga0.47In0.53As/Al0.62Ga0.38As1-xSbx structures operating at 4.9 &mgr;m, in the lattice temperature range 60 K - 90 K. The corresponding electron-lattice coupling &agr;= 9.5 Kcm2/kA) reflects the efficient electronic cooling via optical phonon emission. The experimental normalized thermal resistance RL* = 3.9 Kxcm/W
demonstrates the beneficial use of quaternary thicker barriers in terms of device thermal management.
We report on the fabrication of THz quantum cascade lasers (QCLs) based on different optical waveguide
configurations and compare the thermal properties of THz devices fabricated with metal-metal optical waveguides based
on Au/Au or In/Au wafer bonding. In particular, we show how the careful choice of the metal sequence used for the
reactive bonding may lead to a considerable improvement of the device thermal performance. This information was
obtained from the analysis of microprobe band-to-band photoluminescence spectra measured on devices operating in
continuous wave (cw). The experimental normalized thermal resistances (RL*), show that the use of Au/Au wafer
bonding optimizes the heat dissipation. An extensive comparison with a set of surface-plasmon based THz QCLs,
demonstrate that the use of metal-metal wafer bonding can allow cw operation at progressively higher temperatures.
Finally, we present the experimental results obtained on a bound-to-continuum QCLs (2.84 THz) emitting 77 mW peak
power at 4K, fabricated from an MBE wafers acquired by a commercial provider.
We compare the electrical power dependence of the lattice temperature and the electronic temperature of THz quantum cascade lasers (QCLs) operating in the range 2.5- 3.8 THz and based on a resonant-phonon and bound-to-continnum quantum design. This analysis is performed by means of microprobe band-to-band photoluminescence experiments carried out on operating THz QCLs both below and above the lasing threshold. Thermalized non-equilibrium hot-electron distributions are found in both classes of QCLs. While in the case of bound-to-continuum devices a unique value of the electronic temperature is found in the active region minibands, in the case of resonant-phonon devices we found that the upper radiative state, in the lasing range, heats up to ~ 200 K, more than 100 K with respect to the ground state levels. From the measured thermal resistance and the power dependence of the ground state electronic temperature we obtain in the case of resonant-phonon structures a value of the electron-lattice energy relaxation rate comparable with that typical of mid-infrared QCLs, in the case of resonant-phonon structures and a value ~ 50 times higher in bound-to-continuum devices.
A recent study initiated by the European Space Agency aimed at identifying the most promising technologies to significantly improve on the generation of coherent electromagnetic radiation in the THz regime. The desired improvements include, amongst others, higher output powers and efficiencies at increasingly higher frequencies, wider tunability and miniaturization. The baseline technologies considered revolve around Photomixing and novel laser based technologies compared to all electronic techniques. Some of the most significant findings will be presented together with technological developments and experimental results selected for medium to short term development. These technologies include advanced p-i-n photomixer with superlattice structures and, THz quantum cascade lasers. Recent results achieved in these fields will be put into the potential perspective for the respective technology in the future.
It has been proposed that the temperature of the active layer in VCSELs could be inferred from the wavelength shift of the laser line. However, in VCSELs the laser emission wavelength is primarily selected by the cavity resonance, instead of the material gain peak. Hence the shift of the laser emission only provides an estimate of the temperature averaged over the whole cavity volume. We present a non-invasive microprobe technique for the temperature mapping of operating VCSELs, based on the analysis of the spontaneous electroluminescence emission transmitted through the DBR mirrors. While the sample is temperature stabilized and held onto a xy piezo stage, it is scanned across with an optical microscope (achieving ~2 um spatial resolution). The signal is spectrally resolved and analyzed by a CCD. By comparing the spectra taken under cw and pulsed current injection, the temperature contribution to the emission lineshape can be extracted straightforwardly. We demonstrate the capability of the proposed technique by mapping the temperature rise of a broad area proton implanted oxide VCSEL. Our results clearly demonstrate that the temperature rise is not uniform across the device cross-section, in contrast to the uniform temperature distribution measured by the laser wavelength shift method.
Thermal effects are of fundamental importance in the cw operation of both gain- and index-guided VCSELs. At today, the actual temperature of operating devices is usually estimated from the spectral shift of the laser emission. This method only probes the temperature distribution averaged over the whole cavity volume and cannot provide spatially resolved information across the VCSEL cross- section. One single experiment has been performed to locally measure the temperature distribution in VCSEL by using a thermal scanning microscope. However, such technique required the cleaving and re-processing of the device. We present a new non-invasive technique to map the temperature of operating VCSELs that can be used to test devices at the wafer level. The method is based on the analysis of the spontaneous emission transmitted through the DBR mirrors. While the sample is temperature stabilized and held onto a xy piezo stage, it is scanned across with an optical microscope (achieving 1 micrometers spatial resolution). The signal is spectrally resolved and analysed by a CCD. By comparing the spectra taken under cw and pulsed current injection, the temperature contribution to the emission lineshape can be extracted straightforwardly. We demonstrate this technique by mapping the temperature rise of a broad area proton implanted VCSEL.
A novel injector design allows to increase the peak optical power of quantum cascade (QC) lasers with GaInAs/AlInAs superlattice active regions. At wavelengths of 8.4 micrometers peak powers of 2.2 W per facet, corresponding to 88 mW/stage, have been measured at operating temperatures less than 80 K. The laser output power grows steadily above threshold with a slope efficiency of 160 mW/A up to currents 6 times larger than the threshold one, displaying the widest reported dynamic range without any sign of gain saturation. In order to measure the facet temperature profile and the active region thermal resistance, a micro-probe band-to-band photoluminescence technique is developed and tested on GaAs/Al0.33Ga0.67As and GaInAs/AlInAs/InP three-wells QC lasers. Comparison between substrate-side and epilayer-side mounted devices shows the superior thermal dissipation capacity of the latter and explains their better performance with respect to threshold current and maximum operating temperature.
Photoacoustic and photoluminescence studies on CdS1-xSex nanocrystals grown in a transparent borosilicate glass matrix are reported. Our analysis points out the important role played by the Coulomb interaction in quantum dots whose radii range around the exciton Bohr radius: (1) strong polar coupling to the lattice; (2) observation of optical transitions with different radial quantum numbers in photoacoustic spectra. Photoacoustic spectroscopy has allowed the direct observation of several low and high index states. The experimental results have been shown to be in good agreement with the effective mass- approximation calculations in which the strain effects, due to the hydrostatic compression of the glass matrix on semiconductor nanocrystals, and size dependent dielectric constant have been taken into account.
Absorption, photoluminescence, and Raman scattering studies of CdSxSe1-x nanocrystals, as a function of the crystallite mean radius, are reported. By properly considering the competing effects of spatial localization and strain, the chemical composition and the lattice contraction are assessed. Two different techniques have been used to analyze the electron-phonon coupling. Photoluminescence and absorption studies allow us to measure the Huang-Rhys parameter. The size dependence of the Frohlich electron-LO phonon interaction has been extracted, for the first time, from the polarization analysis of the Raman scattering. Our results unambiguously show that, in the investigated nanocrystal size range, the electron-phonon coupling monotonically increases at decreasing the nanocrystal radius. The observed behavior is in agreement with the latest calculations.
C. Corvasce, Vincenzo Spagnolo, Gaetano Scamarcio, M. Lugara, F. Adduci, Michele Ferrara, Michele Sibilano, Sergio Pellegrino, Massimo del Giudice, M. Re
A study of facet degradation of InGaAs quantum well lasers is reported. We tune up a Raman and photoluminescence micro-probe technique for determining the crystal structure and the temperature profile of the cladding layer, in steps of approximately 1 micrometer, with a temperature resolution better than 1 degree Kelvin. The cladding layer composition and cross- section temperature profile have been monitored during operation. A clear correlation between the facet degradation and the type of protective coating is found.
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