High performance monolithic, broadly tunable mid-infrared quantum cascade lasers

Quantum cascade lasers are unipolar semiconductor lasers based on intersubband transitions in heterostructures. These lasers, which have demonstrated continuous wave operation at room temperature in the mid-infrared spectral range, are well suited for the realization of compact, ultra-sensitive, trace-gas sensors based on absorption spectroscopy. Up to now, only distributed feedback (DFB) single-mode devices have been used for such applications. DFB quantum cascade lasers have proven to be effective for gas sensing, but their relatively narrow tuning range, smaller or equal to about 1% of the wavelength, makes them not very versatile and limits their usefulness for spectroscopic investigations. In this thesis we developed broadly tunable external cavity quantum cascade lasers. The main advantage of these sources compared with DFBs is their broader tuning range, which is limited only by the spectral bandwidth of the gain element. We particularly studied broad gain bandwidth active regions based on bound-to-continuum designs. With that kind of active region, we have demonstrated a tuning range equal to 15% of the center wavelength at ~ 10 m, which was three times broader than the best values reported in the literature at that time, as well as good performance in pulsed mode at room temperature. Using a strain-compensated bound-to-continuum design emitting near 5.2 m, we have demonstrated for the first time continuous-wave operation of an external cavity quantum cascade laser on a thermoelectric cooler. The tuning range was comparable to that of pulsed devices, but with a much better side-mode suppression ratio and a much narrower linewidth. This continuous-wave device has successfully been applied to the spectroscopy of nitric oxide in collaboration with Prof. Tittel's Laser Science Group at Rice University. High resolution absorption spectra of that gas could be acquired over a large wavelength range. We also studied heterogeneous cascade structures, that is quantum cascade structures in which the various stages emit at several different wavelengths, as a way to increase further the tuning range of external cavity quantum cascade lasers. A pulsed laser tunable from 8.2 to 10.4 m, that is over 24% of the center wavelength has been realized using an active region composed of two substacks of bound-to-continuum designs. We conclude on the feasibility of room temperature, continuous-wave, broadly-tunable external cavity quantum cascade lasers in the mid-infrared spectral range. These devices will be useful for the analysis of multi-component gases, for high resolution spectroscopic investigations in fundamental science, and for the detection of heavy molecules with broad absorption features which DFB quantum cascade lasers cannot scan entirely.

Mass spectrometry using matrix-assisted laser desorption/ionization (MALDI) technique is one of the most widely used method to analyze proteins in biological research fields. However, it is difficult to analyze insoluble proteins which have important roles in researches on disease mechanisms or in developments of drugs by using ultraviolet (UV) lasers which have commonly been used for MALDI. Recently, a significant improvement in MALDI process of insoluble proteins using a combination of a UV nitrogen laser and a tunable mid-infrared (MIR) free electron laser (FEL) was reported. Since the FEL is a very large and expensive equipment, we have developed a tabletop laser source which can generate both UV and tunable MIR lasers. A tunable MIR laser (5.5–10 µm) was obtained by difference frequency generation (DFG) between a Nd:YAG and a tunable Cr:forsterite lasers using two AgGaS 2 crystals. The MIR laser can generate pulses with an energy of up to 1.4 mJ at a repetition rate of 10 Hz. A UV laser was obtained by third harmonic generation of a Nd:YAG laser splitted from that used for DFG. A time interval between the UV and the MIR laser pulses can be adjusted with a variable optical delay. Keywords: difference frequency generation, tunable mid-infrared laser, third harmonic generation, mass spectrometry, matrix-assisted laser desorption/ionization

The mid-infrared band of 3~5 μm wavelength is a very important atmospheric window. The mid-infrared lasers are widely applied in laser countermeasure, laser illumination and trace gas detection. At present, the mid-infrared laser sources mainly include solid-state optical parametric oscillation lasers, fiber lasers, mid-infrared supercontinuum spectrum laser and mid-infrared semiconductor lasers, i.e. quantum cascade lasers. In these lasers, quantum cascade laser is the only one that can realize the conversion from electricity to light. In this paper, the method of incoherent beam combination of mid-infrared semiconductor lasers is studied. Two lasers are combined in a common aperture by using a single polarizer based on the polarization characteristics of the output laser of quantum cascade laser. Results show that the incoherent power superposition of mid-infrared quantum cascade lasers can be achieved by polarization beam combining, and the beam combining efficiency is not less than 90%. The farfield divergence angle is about 5 mrad, which is consistent with the farfield divergence angle of the two sub-beams.

In this paper, we review the recent progress in broadly tunable single-mode mid-infrared quantum cascade lasers (QCLs). With a brief introduction on various applications of broadly tunable single-mode mid-infrared QCLs, we first categorize these lasers based on their configurations and tuning schemes. Then recent progress in each scheme is investigated, together with detailed discussions. Furthermore, the pros and cons of different approaches are compared, and the opportunities and challenges for future developments from our point of view are listed. Last but not least, potential new directions and prospects for obtaining broadly tunable single-mode characteristics are also provided. It is foreseeable that, with the development of high performance broadly tunable single-mode mid-infrared QCLs, more applications could be exploited in the near future with new phenomena to be discovered.

We present the recent development of high performance compact THz sources based on intracavity nonlinear frequency mixing in mid-infrared quantum cascade lasers. Significant performance improvements of our THz sources with respect to the continuous wave THz power output, monolithic THz tuning, and widely frequency are achieved by systematic optimization of the device's active region, waveguide design, and chip bonding strategy. Room temperature continuous wave THz power of more than 10 μW at 3.4 THz is demonstrated at room temperature. Monolithic THz tuning of a chip-based THz source from 2.6 to 4.2 THz with power up to 0.1 mW is achieved. Surface emission from the substrate via a diffraction grating with THz power up to 0.5 mW is also obtained. The developing characteristics show the potential for these THz sources as local oscillators for many astronomical and medical applications.

We present a semi-automated point-of-care (POC) sensor approach for the simultaneous and reagent-free determination of clinically relevant parameters in blood plasma. The portable sensor system performed direct mid-infrared (MIR) transmission measurements of blood plasma samples using a broadly tunable external-cavity quantum cascade laser source with high spectral power density. This enabled the use of a flow cell with a long path length (165μm) which resulted in high signal-to-noise ratios and a rugged system, insensitive to clogging. Multivariate calibration models were built using well established Partial-Least-Squares (PLS) regression analysis. Selection of spectral pre-processing procedures was optimized by an automated evaluation algorithm. Several analytes, including glucose, lactate, triglycerides, cholesterol, total protein as well as albumin, were successfully quantified in routinely taken blood plasma samples from 67 critically ill patients. Although relying on a spectral range from 1030cm(-1) to 1230cm(-1), which is optimal for glucose and lactate but rather unusual for protein analysis, it was possible to selectively determine the albumin and total protein concentrations with sufficient accuracy for POC application.

A rugged, mid-infrared (IR) CO laser absorption diagnostic has been developed to monitor the amount of CO produced by a high-pressure CH4-O2 combustor test rig operating at supercritical CO2 conditions (30 MPa and 1150°C). The laser system operates at the fundamental absorption band, ν″ = 0, R(12), of CO near 4.5 μm. The mid-IR diagnostic was constructed from a tunable quantum cascade laser (QCL), an absorption cell with two window ports for monitoring CO exhaust concentration, and two IR photodetectors. Temperature and pressure sensors were mounted near the absorption cell to monitor exhaust flow conditions, and the operational wavelength of the laser was determined by a calibration process using a known mixture of CO and N2. Environmental conditions at the remote outdoor test facility posed significant difficulties in the data acquisition process for the IR diagnostic. Fluctuating environmental temperatures proved to be problematic when operating cryogenic photodetectors and stabilizing a QCL designed to operate with an internal temperature of −15°C. Improvements to the IR system included elimination of problematic stagnation regions via a new absorption cell design and an increase in the CO detection limit. During steady state conditions, the mid-IR diagnostic measured the CO concentration to within ± 80.6 ppm. The IR diagnostic was shown to have superior CO detection response time and the ability to resolve features not detected by other CO detector counterparts.

After more than 20 years of development, quantum cascade lasers (QCLs) have become the leading laser source emitting in the mid-infrared spectrum region between 3 and 12μm, where CBRN and explosives have strong absorption signatures. The Center for Quantum Devices at Northwestern University currently holds the record for both the highest room temperature continuous wave output power of 5.1 W and the most efficient QCL of 53% wall plug efficiency operating at cryogenic temperature. This book chapter presents our latest development results on mid-infrared QCLs with wavelength tuning and high power beam steering capabilities, which significantly enhances QCL’s suitability for remote spectroscopy and sensing applications.

A monolithic tunable single mode terahertz quantum cascade laser source based on intra-cavity difference frequency generation with a tuning range of ~580 GHz is demonstrated at room temperature.

Room-temperature terahertz (THz) quantum cascade laser sources with intracavity difference-frequency nonlinear mixing are electrically pumped monolithic semiconductor laser sources operating in the 0.6–6 THz spectral range. We report widely tunable, low-frequency THz quantum cascade laser sources using a lens-coupled Cherenkov waveguide scheme. Based on a watt-class high-power, λ ∼ 13.7 μm quantum cascade laser, the monolithic THz source is strongly coupled with a high-resistivity silicon lens, which causes a major increase in the THz coupling efficiency and demonstrates significant performance improvements. A room-temperature 1.5 THz device produces a 0.2 mW peak output power with a high-quality beam pattern. Improved THz outcoupling efficiency using the lens-coupled scheme enabled the demonstration of a high-performance external-cavity semiconductor THz source that is tunable from 420 GHz to 2 THz. The external-cavity, lens-coupled device configuration can technically be assembled into a butterfly-style package for a thumb-sized, widely frequency tunable THz semiconductor source.

We present an aspheric collimating lens for mid-infrared (4{14  m) quantum cascade lasers. The lenses wereetched into silicon by an inductively coupled plasma reactive ion etching system on wafer level. The highrefractive index of silicon reduces the height of the lens pro le resulting in a simple element working at highnumerical aperture (up to 0.82). Wafer level processes enable the fabrication of about 5000 lenses in parallel.Such cost-e ective collimating lens is a step towards the adoption of quantum cascade lasers for all its potentialapplications.Keywords: Optical collimation, mid-infrared micro-optics, quantum cascade laser, reactive ion etching, massproduction 1. INTRODUCTION All chemical species including organic compounds exhibit resonances with their fundamental ro-vibrational modesat frequencies in the mid-infrared spectral range. This range is of great interest for chemical sensing for theenvironment or for health. 1 The mid-infrared technology include small and highly tunable laser sources: thequantum cascade lasers.

We have recently demonstrated a continuous wave molecular laser emission based on ammonia optically pumped by a quantum cascade laser. We report here new results obtained with an optimized laser prototype pumped by a tunable external cavity quantum cascade laser. New laser lines with high output power are demonstrated at frequencies between 1 and 5.5 THz.

The need for highly performing and stable methods for mid-IR molecular sensing and metrology pushes towards the development of more and more compact and robust systems. Among the innovative solutions aimed at answering the need for stable mid-IR references are crystalline microresonators, which have recently shown excellent capabilities for frequency stabilization and linewidth narrowing of quantum cascade lasers with compact setups. In this work, we report on the first system for mid-IR high-resolution spectroscopy based on a quantum cascade laser locked to a CaF microresonator. Electronic locking narrows the laser linewidth by one order of magnitude and guarantees good stability over long timescales, allowing, at the same time, an easy way for finely tuning the laser frequency over the molecular absorption line. Improvements in terms of resolution and frequency stability of the source are demonstrated by direct sub-Doppler recording of a molecular line.

Infrared active stand-off detection techniques often employ high power tunable quantum cascade lasers (QCLs) for target illumination. Due to the distances involved, any fluctuation of the laser beam direction and/or beam profile is amplified at the sample position. If not accounted for, this leads to diminished performance (both sensitivity and selectivity) of the detection technique as a direct result of uncertainties in laser irradiance at each imaged pixel of the sample. This is especially true for detection approaches which illuminate a relatively small footprint at the target since the laser beam profile spatial fluctuations are often comparable to the (focused) laser spot size. Also, there is often a necessary trade-off between high output QCL power and beam quality. Therefore, precise characterization of the laser beam profile and direction as a function of laser properties (tuning wavelength, current and operating mode: pulsed or CW) is imperative. We present detailed measurements of beam profiles, beam wander and power fluctuations and their reproducibility as function of laser wavelength and stand-off distance for a commercially available tunable quantum cascade laser. We present strategies for improving beam quality by compensating for fluctuations using a motorized mirror and a pair of motorized lenses. We also investigate QCL mode hops and how they affect laser beam properties at the sample. Detailed mode-hop stability maps were measured.

We report on a standoff chemical detection system using widely tunable external-cavity quantum cascade lasers (ECQCLs) to illuminate target surfaces in the mid infrared (λ = 7.4 – 10.5 μm). Hyperspectral images (hypercubes) are acquired by synchronously operating the EC-QCLs with a LN 2 -cooled HgCdTe camera. The use of rapidly tunable lasers and a high-frame-rate camera enables the capture of hypercubes with 128 x 128 pixels and >100 wavelengths in 2 area at 5-m standoff. Raw hypercubes are post-processed to generate a hypercube that represents the surface reflectance relative to that of a diffuse reflectance standard. Results will be shown for liquids (e.g., silicone oil) and solid particles (e.g., caffeine, acetaminophen) on a variety of surfaces (e.g., aluminum, plastic, glass). Signature spectra are obtained for particulate loadings of RDX on glass of 2 .