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In this paper we describe the design and performance of nonlinear quantum cascade laser sources for near-infrared and terahertz applications. Our devices are based on monolithically integrated mid-infrared quantum cascade lasers and passive nonlinear structures which provide a giant nonlinear response for the pumping frequency. Such design concept can be applied for both short-wavelength and long-wavelength generation. In our work, short-wavelength devices were based on the concept of second-harmonic generation whereas long-wavelength devices utilized difference-frequency generation. With this approach we demonstrate room-temperature operation down to 2.7 m for near-infrared devices and up to 70 m at 210 K for terahertz devices. The performance of our nonlinear devices is very sensitive to the resonance condition for the pump and nonlinearity. We demonstrate that, once resonant, nonlinear powers in the mW range are available.

The near-normal incidence reflectances for a ternary system composed of 75% H_2SO_4, 10% HNO_3, and 15% H_2O and for a 10% aqueous solution of HNO_3 have been measured at ambient temperature over the spectral range of 5000 to 500 cm^−1. A Kramers–Kronig phase-shift analysis of the measured spectral reflectance has been used to determine the real and imaginary components of the index of refraction for each sample. The ternary-system spectrum has been compared with the spectrum of 10% HNO_3 and also with the spectrum of 75% H_2SO_4 obtained by Palmer and Williams. [ Appl. Opt.14, 208 ( 1975)]. These results may prove useful for studies concerning aerosol formation of this ternary system in the stratosphere.

Several esters of 4-methylumbelliferone and 2-naphthol were synthesized and examined as possible spectrofluorimetric titrants for bovine α-chymotrypsin, trypsin, thrombin, Factor Xa and for subtilisin Novo. 4-Methylumbelliferyl p-guanidinobenzoate hydrochloride (MUGB) is a satisfactory titrant for α- and β-trypsin, thrombin and Factor Xa and 4-methylumbelliferyl p-(NNN-trimethylammonium)cinnamate (MUTMAC) is a good titrant for α-chymotrypsin. The amount of enzyme used for spectrofluorimetric titration is 0.02–3.00nmol and the amount of 4-methylumbelliferone liberated is independent of the concentration of titrant and stoicheiometrically equal to the amount of enzyme used. Results obtained with MUGB and MUTMAC have been checked by spectrophotometric titration with p′-nitrophenyl p-guanidinobenzoate hydrochloride and p-nitrophenyl N2-acetyl-N1-benzylcarbazate respectively. p-Nitrophenyl N2-acetyl-N1-(9-anthrylmethyl)carbazate has been synthesized; it did not react with α-chymotrypsin. A satisfactory spectrofluorimetric titrant for subtilisin Novo was not discovered.

We discuss the design and performance of quantum cascade laser sources based on intra-cavity second harmonic generation operating in at wavelengths shorter than 3.7m. A passive heterostructure tailored for giant optical nonlinearity is integrated on top of an active region and patterned for quasi-phasematching. We demonstrate operation of 3.6m, 3.0m, and 2.6m devices based on lattice-matched and strain-compensated InGaAs/AlInAs/InP materials. Threshold current densities of typical devices with nonlinear sections are only 10-20% higher than that of the reference lasers without the nonlinear section. Our best devices have threshold current density of 2.2kA/cm2and provide approximately 35W of second-harmonic output at 2.95m at room temperature. The second-harmonic conversion efficiency is approximately 100W/W2. Up to two orders of magnitude higher conversion efficiencies are expected in fully-optimized devices.