Milton L. Peabody

Laser-based mid-infrared reflectance imaging of biological tissues

Mid-infrared (MIR) (3-12 um) spectral imaging is a power analytical tool, but difficult in the back-reflectance mode for in-vivo diagnostics. Feasibility of MIR back-reflectance imaging is demonstrated using MIR semiconductor lasers. Transmittance through 500-microm thick films of water and blood showed a capability to resolve more than 6-OD signal dynamic range. Reflectance scanning imaging through a 150-microm thick film of blood showed negligible scattering effect, indicating the feasibility of optical coherent imaging. The result of coherent imaging of a plant leaf shows a MIR sub-surface image that would not be visible in white light. With two wavelengths, a similar result for a chicken skin subcutaneous tissue at different focal depths was obtained, showing blood vessels beneath a lipid layer. These results suggest that advanced multilaser wavelength systems in the fingerprint spectral region can be a useful tool for in-vivo spectral imaging in biomedical research and diagnostic applications.

Optimized second-harmonic generation in quantum cascade lasers

Optimized second-harmonic generation (SHG) in quantum cascade (QC) lasers with specially designed active regions is reported. Nonlinear optical cascades of resonantly coupled intersubband transitions with giant second-order nonlinearities were integrated with each QC-laser active region. QC lasers with three-coupled quantum-well (QW) active regions showed up to 2 /spl mu/W of SHG light at 3.75 /spl mu/m wavelength at a fundamental peak power and wavelength of 1 W and 7.5 /spl mu/m, respectively. These lasers resulted in an external linear-to-nonlinear conversion efficiency of up to 1 /spl mu/W/W/sup 2/. An improved 2-QW active region design at fundamental and SHG wavelengths of 9.1 and 4.55 /spl mu/m, respectively, resulted in a 100-fold improved external linear-to-nonlinear power conversion efficiency, i.e. up to 100 /spl mu/W/W/sup 2/. Full theoretical treatment of nonlinear light generation in QC lasers is given, and excellent agreement with the experimental results is obtained. For the best structure, a second-order nonlinear susceptibility of 4.7/spl times/10/sup -5/ esu (2/spl times/10/sup 4/pm/V) is calculated, about two orders of magnitude above conventional nonlinear optical materials and bulk III-V semiconductors.

Improvement of second-harmonic generation in quantum-cascade lasers with true phase matching

About a 100-fold improvement of the second-harmonic generation in a quantum-cascade laser with integrated optical nonlinearity was obtained by including phase-matching considerations in the design of the deep-etched ridge waveguide. The waveguide layer structure was optimized to minimize the phase mismatch of the zero-order mode of the fundamental light with the second-order transverse mode of the second-harmonic light. Exact phase matching is made possible by the faster decrease of the modal refractive index of the fundamental light with decreasing ridge width relative to the refractive index of the second-harmonic light. Up to 240 μW of the second-harmonic power and a nonlinear power conversion efficiency of up to 36 mW/W2 were achieved.

Optimization of broadband quantum cascade lasers for continuous wave operation

We have optimized the design of the broadband quantum cascade laser for cw operation. The improved design leads to a gain ripple of only about 4 cm−1 over more than a 0.5-μm spectral range. Simultaneous cw emission at several wavelengths spanning the range between 6.7 and 7.4 μm has been achieved in a temperature interval from 20 to 77 K.

Third harmonic generation in a Quantum Cascade laser with monolithically integrated resonant optical nonlinearity

We present third harmonic generation from an InGaAs/AlInAs Quantum Cascade laser based on a three-well diagonal transition active region with an integrated third-order nonlinear oscillator. The device displays pump radiation at lambda ~ 11.1 mum and third order nonlinear light generation at lambda ~ 3.7 mum as well as second harmonic generation at lambda ~ 5.4 mum.

The SCattering with Angular Limitation in Projection Electron-Beam Lithography (SCALPEL) System

A SCALPEL\circledR (SCattering with Angular Limitation in Projection Electron-beam Lithography) proof-of-concept lithography system, comprising a tool, a reticle and a resist, has been designed to address the critical issues that must be investigated to determine if this approach is viable as a practical lithographic technology.

Temperature dependence and single-mode tuning behavior of second-harmonic generation in quantum cascade lasers

Second-harmonic generation (SHG) is reported in quantum cascade (QC) lasers with active regions that also support nonlinear cascades with large second order nonlinear susceptibility. SHG has been measured from 10 up to 250 K heat sink temperature, with about 1 μW of nonlinear power at 10 K and about 50 nW at 250 K. Single-mode and tunable SHG at 3.5 μm wavelength has been measured from single-mode QC distributed feedback lasers operating at the fundamental pump wavelength of 7.0 μm. Thermal tuning results in a tuning rate for the SHG emission of ∼0.2 nm/K for temperatures above ∼100 K.

Preliminary results from a prototype projection electron‐beam stepper‐scattering with angular limitation projection electron beam lithography proof‐of‐concept system

We have designed and constructed a proof‐of‐concept projection electron beam lithography system based on the scattering with angular limitation projection electron beam lithography principle. In this system, a thin membrane mask is used in a 4:1 reduction projection system at 100 keV. Image contrast is formed by scattering in the mask and subsequent aperturing of the scattered electrons in the back focal plane of the projection system. We have employed a step‐and‐scan architecture which uses continuously moving mask and wafer stages to trace out the full pattern. The electron beam can thus be kept small (1×1 mm in our case) which greatly simplifies the design of the electron optical system. In addition, the membrane areas can be kept small in linear dimension in one direction, minimizing in‐plane pattern distortions. Our system will be constructed in two stages. In the first stage, the mask stage is static and the wafer stage operates in step‐and‐repeat mode. This initial version of the system allows for ...

Stability of pulse emission and enhancement of intracavity second-harmonic generation in self-mode-locked quantum cascade lasers

We report the observation of stable pulse emission and enhancement of intracavity second-harmonic generation (SHG) in self-mode-locked quantum cascade (QC) lasers. Down-conversion of the detector signal by heterodyning with an RF signal allows the direct observation of the pulsed laser emission in the time domain and reveals a stable train of pulses characteristic of mode-locked lasers. The onset of self-mode locking in QC lasers with built-in optical nonlinearity results in a significant increase of the SHG signal. A pulse duration of /spl sim/12 ps is estimated from the measured increase of the SHG signal in pulsed emission compared to the power expected for the SHG signal in CW emission. This value is in good agreement with the pulse duration deduced from the optical spectral width.

Space-charge effects in projection electron-beam lithography: Results from the SCALPEL proof-of-lithography system

In projection electron-beam systems resolution and throughput are linked through electron–electron interactions collectively referred to as space-charge effects. Hence, a detailed understanding of these effects is essential to optimizing the lithographic performance of a projection electron-beam lithography system. Although many models have been developed to describe one or more of the various aspects of the Coulomb interactions that occur in the beam, there is minimal experimental data available. We have performed a series of experimental measurements in the scattering with angular limitation projection electron-beam lithography (SCALPEL) proof-of-lithography system to characterize the space-charge effects for such an optical configuration. The results of those measurements have been compared to a combination of computer simulations and analytical models. The agreement between the models and experiments was good, within the limits of experimental error. We determined the exponent in the dependence of blu...