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The integration of mid-IR lasers with Si-based platforms is needed for the development of smart sensor grids. Here we review our recent results on laser diodes (LDs), interband-cascade lasers (ICLs) and quantum-cascade lasers (QCLs), all grown on on-axis (001) Si substrates and covering emission wavelengths from 2 to 10 µm. In addition, we will demonstrate that etching facets is a viable route toward cavity definition either on plain wafers or recessed Si wafers.

We report new developments of quantum cascade lasers (QCL), made of InAs/AlSb, operating in the long wavelength mid-infrared range from 10 to 20 µm. Fabry-Perot lasers operated in the CW regime up to a wavelength of 18 µm at 40°C, and up to 21 µm on Peltier cooler, with threshold current densities below 1 kA/cm². Distributed feedback QCL are also fabricated using a metal grating on the top of the laser ridges. They demonstrated single frequency operation in CW regime at room temperature. These lasers are suitable for new high resolution spectroscopy applications in this almost unexplored spectral range.

Turn-on delay is measured to be much longer and its pump-current dependence different from theoretical predictions for mid-infrared InP- and InAs-based quantum-cascade lasers. Experiment qualitatively agrees with numerical simulations of rate-equations under non-zero rise-time pumping.

In the present paper, we investigate a special type of optical coupling between two electrically pumped semiconductor whispering gallery mode lasers. We fabricate microdisk lasers with a conjunction between the laser cavities, making the coupling stronger. Our experimental results combined with numerical calculations demonstrate a variety of mode families that exist in such systems. Furthermore, we confirm the generation of resonance modes that exist only under simultaneous pumping conditions. These modes are attributed to the collective mode family, which can lead to a single mode operation.

International audience; We present InAs/AlSb quantum cascade lasers (QCLs) monolithically integrated on an on-axis (001) Si substrate. The lasers emit near 8 μm with threshold current densities of 0.92-0.95 kA/cm 2 at 300 K for 3.6-mm-long devices and operate in pulsed mode up to 410 K. QCLs of the same design grown for comparison on a native InAs substrate demonstrated a threshold current density of 0.75 kA/cm 2 and the same maximum operating temperature. The low threshold current density of the QCLs grown on Si makes them suitable for photonic integrated sensor implementation.

Indium arsenide (InAs)-based quantum cascade lasers (QCLs) operating close to 25 µm are reported. Thick n-doped InAs cladding layers were used for optical confinement. Fabry–Perot lasers exhibited a threshold current density as low as 1.1 kA/cm2 in pulsed mode at 80 K. The lasers operated in this regime up to 240 K, where the threshold current density increased up to 1.7 kA/cm2. These are the lowest threshold current densities for QCLs operating above 20 µm.

We report a study of quantum cascade lasers (QCL), made of InAs and AlSb, operating in the long wavelength mid-infrared range. Lasers based on a design with slightly diagonal intersubband transitions demonstrated improved performances as compared to the previously reported devices employing a vertical scheme. The fabricated lasers operated in the CW regime at 40°C up to a wavelength of 18 μm.

In order to improve the maximum operating temperature of THz quantum cascade lasers (QCLs) we propose to modify the three well resonant phonon design by increasing the energy of the separation between the bottom transition levels and the ground states, which is usually adjusted to the LO-phonon energy in GaAs. The fabricated devices based on this design with non-resonant depopulation of the bottom transition level operated in pulsed mode up to 190K, which is the best-reported performance for THz QCLs with Al0.15Ga0.85As barriers based on an Au-Au double-metal waveguide. The performed analysis showed that the achieved improvement could be explained by the proposed design modification.In order to improve the maximum operating temperature of THz quantum cascade lasers (QCLs) we propose to modify the three well resonant phonon design by increasing the energy of the separation between the bottom transition levels and the ground states, which is usually adjusted to the LO-phonon energy in GaAs. The fabricated devices based on this design with non-resonant depopulation of the bottom transition level operated in pulsed mode up to 190K, which is the best-reported performance for THz QCLs with Al0.15Ga0.85As barriers based on an Au-Au double-metal waveguide. The performed analysis showed that the achieved improvement could be explained by the proposed design modification.

We demonstrate the experimental measurements of the turn-on delay in InAs-and InP-based quantum cascade lasers operating under pulsed electrical pumping with non-zero rise time. We report the unexpected delay time dependence on the pumping current density, that does not agree with available theoretical predictions. Performed numerical simulation of the model based on rate equations is only in qualitative, but not quantitative, agreement with our observations.

International audience; We propose an efficient optically actuated THz modulator based on an ultrathin epsilon-near-zero (ENZ) slab photogenerated in an InAs semiconductor. We experimentally demonstrate a modulation depth of 90% at 1 THz obtained with a continuous laser at irradiation lower than 10 W cm À2. Beyond the strong attenuation of the THz transmission provided by the ENZ absorption effect, we also report a broadband modulation of the THz waves from 1 to 10 THz. In addition, our experimental results show that the cutoff frequency of 3 dB attains 2 MHz in the dynamic modulation regime.

The InAs/AlSb quantum cascade lasers (QCLs) operating ∼11 µm in the continuous wave (cw) regime at room temperature are reported. The lasers with cleaved Fabry–Perot resonators demonstrated pulsed threshold current densities below 1 kA/cm2 at 300 K and operated in a cw up to 320 K that is the maximum temperature of such regime of operation for InAs-based QCLs. Distributed feedback lasers fabricated from the same wafer exhibited single frequency emission near 11 µm and operated in the cw regime up to 295 K.

Usually electromagnetic modes inside a laser resonator are a matter of the theoretical studies. In a sense we manage "to have a look into a whispering gallery mode (WGM) resonator" and observe how the resonator modes arrange in reality. The picture occurs to be quite different from the commonly used Bessel modes in a disk resonator. A chance to explore optical modes inside a resonator appears in a WGM laser with a cleaved cavity. The flat laser facet gives an opportunity to study both far and near field patterns formed by different modes. In this research we use a high resolution technique of detection of laser emission based on an atomic force microscope, which allowed us to visualize even high Q modes normally sealed inside the resonator. This information was completed with spatially resolved emission spectra and far-field patterns measured using an infrared camera. The analysis of the obtained results using both wave and geometrical optics approaches and finite elements simulations showed that emission of the studied devices is governed by a few low order optical modes experiencing a small number of reflections from the resonator walls. These modes can be considered as counter propagating Gaussian beams and their interference at the laser facet was also observed in the experiment. This work showed that, contrary to conventional ridge or surface emitting lasers, in such deformed disk resonators outputs of different optical modes are spatially separated and can be studied individually along the cleaved facet of the laser.

We review the recent achievements in the epitaxial integration of III-Sb-based semiconductor lasers on Si substrates. Much work has been done on substrate preparation and III-Sb on Si nucleation, which allowed demonstrating GaInAsSb/AlGaAsSb quantum-well laser diodes operating in continuous wave above room temperature in the 1.5–2.3 μm range and InAs/AlSb quantum cascade lasers emitting near 11 μm operating up to 400 K. Although work remains to be done, in particular toward crystal defect density reduction, the wide wavelength range at reach with the III-Sb technology opens the way to the development of a large portfolio of mid-IR sensing systems.

We experimentally and theoretically examined the mode structure of half-disk lasers fabricated by cleaving whispering gallery mode (WGM)-disk lasers. The WGM lasers with a radii R of 50–150 μm emitting near 2.2 μm were fabricated using the GaSb-based quantum well structure [1]. The cleavage usually passes not through the centre of the disk with the deviation δ/R up to 20 % (Fig. 1a). Recently we have demonstrated that in the half-disk lasers the output beam is formed by optical modes located at a distance of 5–20 μm from the disk periphery [2], whereas in the full disk the light propagates along the disk wall, much closer to its edges.

International audience; We report low threshold continuous wave (CW) operation of InAs/AlSb quantum cascade lasers emitting around 20 lm. The laser cavity was made of a plasmon-enhanced dielectric waveguide with cladding layers made of n-InAs. The devices exhibited a threshold current density of 1.38 kA/cm 2 at the maximum temperature of CW operation of 240 K and an output power of 14 mW/facet at T ¼ 80 K. In pulsed mode, the lasers operated up to 380 K with a threshold current density of 1.15 kA/cm 2 at room temperature.

International audience; The extension of the available spectral range covered by quantum cascade lasers (QCL) would allow one to address new molecular spectroscopy applications, in particular in the long wavelength domain of the mid-infrared. We report in this paper the realization of distributed feedback (DFB) QCLs, made of InAs and AlSb, that demonstrated a continuous wave (CW) and a single mode emission at a wavelength of 17.7 µm, with output powers in the mW range. This is the longest wavelength for DFB QCLs, and for any QCLs or semiconductor lasers in general, operating in a CW at room temperature.

International audience; We report low threshold InAs/AlSb quantum cascade lasers emitting near 15 micron. The devices are based on a vertical design similar to those employed previously in far infrared InAs-based QCLs, whereas the doping level of the active core is considerably decreased. The lasers exhibit a threshold current density as low as 730 A/cm(2) in pulsed mode at room temperature and can operate in this regime up to 410K. The continuous wave regime of operation has been achieved in these devices at temperatures up to 20 degrees C. The cw regime is demonstrated for InAs-based QCLs for the first time at room temperature.

By means of atomic force microscopy (AFM) the spatial mapping of the laser intensity was performed on the cleavage of the whispering gallery modes (WGM) half-disk laser. The study was carried out in the near- and far-field regime. It showed a strong spatial divergence of different modes in the laser.

We demonstrate the effect of infrared (IR) illumination on mechanical resonance frequency of scanning probe microscope (SPM) probe. The analytical estimations permit to relate the observed effect with the SPM-probe heating by IR-radiation. This effect can be used for the SPM mapping of IR-radiation spatial distribution on the emitting surfaces. The proposed method demonstrates high sensitivity to the infrared radiation and lateral resolution in the sub-micron range. It was applied to visualize complex spatial structure of lasing modes on the mirror of whispering-gallery mode GaSb/GaAlAsSb/GaInAsSb laser.

International audience; We review the current state of the InAs/AlSb quantum cascade laser (QCL) technology. These materials have brought significant progress in short wavelength QCL due to the high-conduction band offset. The first QCLs emitting below 3 mu m have been demonstrated in this system. The short wavelength limit of QCL operation has further been moved down to 2.6 mu m. This system is also well suited for far-infrared QCLs because high intersubband gain can be achieved at low transition energies due to the small electron effective mass in InAs. Room temperature operation has been demonstrated in InAs-based QCLs to a wavelength of 21 mu m, for the first time for any semiconductor laser emitting above 16 mu m. Both short-and long-wavelength single-frequency distributed feedback InAs/AlSb QCLs have been realized, as well as widely tunable external cavity sources for the 3-3.5 mu m spectral range.

We have observed the interference of rays in a whispering gallery mode (WGM) semiconductor diode laser with a half-disk cavity. The interference leads to the formation of about 20 lobes in the radiation directivity pattern in the far field zone. Each of these lobes consists of even narrower components. It is established that the number of narrow lobes is determined by the coherent counterphase rays originating from peripheral points of the half-disk cavity mirror. The wide lobes are related to adjacent coherent radiation sources, the presence of which is confirmed by the near field pattern.

Frequency tuning in a whispering gallery mode (WGM) semiconductor laser (λ = 2.35 µm) with a sector (half-disk) cavity has been studied. Pumping by current pulses with a duration of up to 1.2 µs is accompanied by smooth tuning of the main WGM toward longer wavelengths by 30 A at room temperature, which is 1.4 times the intermode distance.

We demonstrate passive mode locking of a GaSb-based diode laser emitting at 2.1 μm. The active region of the studied device consists in two 10-nm-thick GaInSbAs/GaAlSbAs quantum wells. Passive mode locking has been achieved in a two-section laser with one of the sections used as a saturable absorber. A microwave signal at 20.6 GHz, measured in the electrical circuit of the absorber, corresponds to the fundamental photon round-trip frequency in the laser resonator. The linewidth of this signal as low as ∼10 kHz has been observed at certain operating conditions, indicating low phase noise mode-locked operation.

An operating semiconductor laser has been studied using a scanning probe microscope. A shift of the resonance frequency of probe that is due to its heating by laser radiation has been analyzed. The observed shift is proportional to the absorbed radiation and can be used to measure the laser near field or its output power. A periodical dependence of the measured signal has been observed as a function of distance between the probe and the surface of the laser due to the interference of the outgoing and cantilever-reflected waves. Due to the multiple reflections resulting in the interference, the light absorption by the probe cantilever is greatly enhanced compared with a single pass case. Interaction of infrared emission of a diode laser with different probes has been studied.

We demonstrate the high temperature operation, up to 80°C, of quantum cascade lasers emitting at a wavelength of 20 µm. The lasers are based on the InAs/AlSb materials and take benefit of a low loss plasmon-enhanced dielectric waveguide. The waveguide consists of doped InAs cladding layers and low-doped InAs spacers. For 2.9-mm-long devices, the threshold current density is 4.3 kA/cmsup2/supand the measured peak output power is 7 mW at room temperature. The cavity length dependence of the threshold currents also indicates that very large optical gain is achieved and effectively overcome the strong free carrier absorption.

Since the first demonstration of the quantum cascade laser (QCL) in 1994, three main material families have been used. Each of these three materials best suits a specific wavelength range, due to differences in electronic effective mass, conduction band offset and index of refraction. The material of choice for mid-infrared (mid-IR) applications is InGaAs/AlInAs, with excellent performances demonstrated from λ=3.5 μm to λ=10 μm and beyond. The GaAs/AlGaAs system is the preferred material for THz applications (65 μm max ) of almost 200K have been achieved. Finally, InAs-based QCLs were originally developed for short infrared wavelengths (λ ∼ 2.6–3.5 μm)1, benefiting of the large conduction band-offset. Another advantage of the InAs/AlSb system is the low electronic effective mass in the InAs quantum wells (0.023 m 0 , where m 0 is the electron mass), which provides elevated optical gain2.

InAs-based quantum cascade lasers (QCLs) operating near 20 µm are demonstrated. A double metal waveguide fabricated by transfer of the laser active region onto a host InAs substrate has been employed in these devices. The threshold current density of 1.6 kA/cm2 at 80 K is lower than that of InP-based QCLs emitting in the same spectral region. The lasers operated in pulsed mode up to 170 K.

We demonstrate quantum cascade lasers in the InAs/AlSb material system which operate up to 333 K (in pulsed regime) at λ = 17.8 μm. They employ metal-metal optical waveguides and the threshold current density is 1.6 kA/cm2 at 78 K. We also report distributed-feedback devices obtained using the same laser material via a 1st-order Bragg grating inscribed in the sole top metallic contact. Spectral single mode operation with more than 20 dB side mode suppression ratio is achieved at a temperature of 300 K. Large wavelength tuning rates, of the order of 1.5 nm/K, are demonstrated. A wavelength coverage of 0.38 μm is achieved in single-mode regime over a temperature range of 255 K.

Novel scanning probe microscopy method of near field imaging of laser radiation is proposed. The method providing a submicron spatial resolution is based on detection of a shift of the probe resonance related to its heating by absorbed radiation. The method has been realized with a conventional silicon probe and has been employed for visualization of infrared emission from a half-disk semiconductor whispering gallery mode laser. Keywords— scanning probe microscopy; near field; laser radiation

International audience; Development of a reliable, selective, sensitive, technique for atmospheric trace gas concentrations monitoring is a critical challenge in science and engineering. Tunable single-frequency laser in the 2.3 to 3.3µm wavelength range, working in a continuous regime at room temperature can be used for absorption spectroscopy to identify and quantify several gases such as methane (greenhouse gases) and ethylene (food-processing) which are studied in the IES. We report here on the design and fabrication of 1st to 4th order distributed-feedback (DFB) antimonide-lasers diodes in the 2.3 to 3.3µm wavelength range. This process is applied to all studied structures grown by molecular beam epitaxy (MBE) on GaSb substrate. Electromagnetic modeling helps us to determine the Bragg grating period as well as the global geometry of the structure in order to optimize both modal discrimination and optical power of the lasing mode. The grating is defined by holographic lithography. Two DFB laser diode designs are proposed and investigated in parallel: -Side wall corrugation DFB: A corrugation on the lateral sides of the ridge waveguide is transferred by both wet and dry on a hard mask followed by a Cl2/N2 dry etching in the III-V heterostructure. -Buried DFB: The MBE growth is stopped at the top of the active region. Then the Bragg grating is etched by Ar sputtering . A MBE regrowth process is performed allowing the growth of the upper cladding layer. Next chemical etching of the mesa is done with fluoro-chromic acid. Si3N4 isolation and evaporation of ohmic contacts ends those processes. Finally we will show the results on the fabrication and characterization of the devices. This work is supported by the ANR NexCILAS international project, ANR MIDAS project, NUMEV labex and RENATECH national Network.

We report on the direct observation of an increase in the wavelength of output radiation during the pumping current pulse in a whispering gallery mode (WGM) semiconductor diode laser (2.35 μm) with a sector (half-disk) cavity. During the current pulse with a duration of up to 1.5 μs, the main WGM wavelength smoothly increases by 30 A at room temperature.

A whispering gallery mode (WGM) semiconductor laser with a convex disk cavity operating at room temperature in the middle-IR range (λ = 2.4 μm) has been created for the first time. The convex cavity was formed by etching in a specially selected CrO3-HF-H2O mixture. The room-temperature emission spectra have been measured. The laser generates WGMs at room temperature in a pulsed regime.

Whispering gallery modes (WGMs) have been observed in lasers with cut disk (half-and quarterdisk) cavities operating at room temperature in the 2.0–2.4 μm wavelength range. The possibility of WGM generation in a disk-sector cavity is theoretically studied. It is established that the intermode distance in a full-and half-disk cavity is the same, while that in the quarter-disk cavity is doubled. A new method for the radiation extraction from WGM lasers is proposed.

The use of optical tweezers (OTs) and spin angular momentum transfer to birefringent particles allows new mechanical measurements in systems where torque and rotation are relevant parameters at the single-molecule level. There is a growing interest in developing simple, fast, and inexpensive protocols to produce a large number of submicron scale cylinders of quartz, a positive uniaxial birefringent crystal, to be employed for such angular measurements in OTs. Here, we show that laser interference lithography, a method well known for its simplicity, fulfills these requirements and produces quartz cylindrical particles that we successfully use to apply and measure optical torque in the piconewton nm range in an optical torque wrench.

International audience; InAs/AlSb quantum cascade lasers emitting at 3.06 and 3.22μm at room temperature has been studied. The lasers with high reflection coating on back facets operated in pulse mode up to 400 and 423 K, respectively. The obtained results showedno dramatic performance degradation of the InAs-based QCLs with decreasing emission wave length down to 3μm

Tunable semiconductor lasers with controlled absorber based on GaInAsSb/GaAlAsSb quantum well heterostructure was fabricated and experimentally studied. The emission wavelength of these lasers shifts from 2.24 to 2.28 μm when the bias on the control contact decreases from the voltage equal to that on the main electrode to 0 V when the control contact is grounded. The principle of operation of such a device is given.

We used high hydrostatic pressure techniques to understand the deteriorating temperature performance with decreasing wavelength of short wavelength quantum cascade lasers. Influence of inter-valley scattering and distribution of the electron wave functions will be discussed.

To investigate carrier scattering processes in short wavelength InAs/AlSb quantum cascade lasers we carried out experimental and theoretical studies of the threshold current, I th , as a function of high hydrostatic pressure and temperature. Using the calculated pressure dependence of the optical phonon scattering current, I ph , and the estimated pressure dependence of leakage current, I leak , we show that carrier leakage from the upper laser levels into the indirect L-valley of the conduction band in InAs quantum wells is negligible in the 3.3 μm QCLs at RT leading to their superior temperature performance. In the shorter wavelength devices emitting at 2.9 μm, this loss mechanism is more important and accounts for up to 13% of I th at 190 K.

The existence of the whispering gallery modes in a sector disk resonator has been demonstrated theoretically and experimentally observed in a disk, half-disk, and quarter-disk semiconductor lasers. The half-disk and quarter-disk lasers are fabricated from 285μm diameter disks emitted near 2.13μm at room temperature with continuous wave threshold currents of 55–60mA. Mode spacing in emission spectra of the half-disk lasers was the same as for the whole disk and doubled for the quarter disk as expected from the theoretical analysis. Half-disk lasers emit two parallel beams from the cleaved facet, while the two output beams are perpendicular in quarter-disk devices.

InAs/AlSb distributed feedback quantum cascade lasers are presented. The lasers can operate in the single frequency regime at 3.34-3.38 mum in the 0-100degC temperature range in pulse mode. The wavelength tuning rate of the lasers is 0.27 nm/K and continuous tuning range up to 10 nm can be achieved.

We show that by decreasing the lasing wavelength of InAs/AlSb-based quantum cascade lasers below 3mum, carrier leakage into the L-minimum of conduction band is responsible for their temperature sensitivity and limits their maximum operating temperature.

We report on recent progress in the development of short wavelength InAs/AlSb QCLs: a new short wavelength limit of 2.75 mum and high power, high temperature pulsed operation above 3 mum are demonstrated.

DOI: 10.1063/1.2769948The InAs/AlSb heterostructure has attracted consider-able interest since it provides unique physical properties. Thelow effective electron mass in InAs provides excellent mo-bility, long electron relaxation time, and a high intersubbandoptical matrix element, while the high conduction band off-set 2.1 eV at the -point of the Brillouin zone provides avery good electron confinement. These favorable propertieshave led to the adoption of this material system for the real-ization of high-performance quantum cascade lasers QCLsat the short wavelength limit of the midinfrared spectrum.Laser emission from InAs/AlSb QCLs has been demon-strated at wavelengths as short as 2.95 m, while spontane-ous intersubband emission from QC light emitting diodes hasbeen observed down to 2.5 m.

First quantum cascade lasers emitting below 3 mum are demonstrated. The lasers based on the InAs/AlSb material system emit at 2.95-2.97 mum at 83 K and operate up to room temperature.

We report quantum cascade lasers emitting at a wavelength near 3.3μm up to a temperature of 400K in pulsed mode. A fine tuning of the InAs∕AlSb active region design allowed to maintain efficient electron injection and to reduce thermal backfilling at high temperatures. The threshold current density is 3kA∕cm2 at 300K with characteristic temperature T0 of 175K. The lasers emitted up to 1W peak power in a single lateral mode at room temperature.

International audience; Quantum cascade lasers emitting below 3 μm are demonstrated. The lasers based on the InAs/AlSb material system emit at 2.95-2.97 μm in pulsed mode with threshold current densities near 3 kA/cm2 at 84 K and operate up to room temperature. No degradation has been observed in laser performances compared with InAs/AlSb quantum cascade lasers emitting at 3.14-3.35 μm. The obtained results show no influence of the L valley in InAs on operation of these short wavelength quantum cascade lasers

We report on the growth, fabrication, experimental study and application in an absorption gas setup of distributed feed-back antimonide diode lasers with buried grating. First, half laser structures were grown by molecular beam epitaxy on GaSb substrates and stopped at the top of the waveguide. A second order Bragg grating was then defined by interferometric lithography on the top of the structure and dry etched by Reactive Ion Etching. The grating was, afterwards, buried thanks to an epitaxial regrowth of the top cladding layer. Finally, the wafer was processed using standard photolithography and wet etched into 10 µm-wide laser ridges. A single frequency laser emission around 2.3 µm was recorded, a maximum output power of 25 mW and a total continuous tuning range reaching 4.2 nm at fixed temperature. A device has been used to detect methane gas and shows strong potential for gas spectroscopy. This process was also replicated for a target of 3 µm laser emission. These devices showed an output power of 2.5 mW and a SMSR of at least 23 dB, with a 2.5 nm continuous tuning range at fixed temperature.

International audience; We report on the modeling, growth, processing, characterization and integration in a gas detection setup of side wall corrugated distributed feed-back antimonide diode lasers emitting at 2.28 and 2.67 μm. The laser structures were grown by molecular beam epitaxy on GaSb substrate. Ridge lasers were fabricated from the grown wafers according to the following process: a second order Bragg grating was defined on the sides of the ridges by interferometric lithography, optical lithography and etched in a Cl-based inductively coupled plasma reactor. The devices exhibit a power reaching 40 mW, a side mode suppression ratio better than 28 dB and a tuning range of 3 nm at room temperature. One of these devices was successfully integrated in a tunable diode laser absorption spectroscopy setup, thus demonstrating that they are suitable for gas analysis.

International audience; The room temperature operation of InAs/AlSb quantum cascade lasers is reported. The structure, grown by molecular beam epitaxy on an InAs substrate, is based on a vertical transition design and a low loss n+-InAs plasmon enhanced waveguide. The lasers emitting near 4.5 μm operate in pulse regime up to 300 K. The threshold current density of 3.18-mm-long lasers is 1.5 kA/cm2 at 83 K and 9 kA/cm2 at 300 K.

International audience; Noncontact mode electrostatic force microscopy was employed to investigate cleaved sections of a GaSb-based quantum cascade laser structure. The technique enabled us to determine the surface potential distribution, at equilibrium and under applied external field, and to locate precisely the zone which is at the origin of failure in laser characteristics when a breakdown voltage is applied to the device