Dark state lasers

We propose a new type of laser resonator based on imaginary "energy-level splitting" (imaginary coupling, or quality factor Q splitting) in a pair of coupled microcavities. A particularly advantageous arrangement involves two microring cavities with different free-spectral ranges (FSRs) in a configuration wherein they are coupled by "far-field" interference in a shared radiation channel. A novel Vernier-like effect for laser resonators is designed where only one longitudinal resonant mode has a lower loss than the small signal gain and can achieve lasing while all other modes are suppressed. This configuration enables ultra-widely tunable single-frequency lasers based on either homogeneously or inhomogeneously broadened gain media. The concept is an alternative to the common external cavity configurations for achieving tunable single-mode operation in a laser. The proposed laser concept builds on a high-Q "dark state" that is established by radiative interference coupling and bears a direct analogy to parity-time (PT) symmetric Hamiltonians in optical systems. Variants of this concept should be extendable to parametric-gain based oscillators, enabling use of ultrabroadband parametric gain for widely tunable single-frequency light sources

The Character and Determinants of Corporate Capital Gains

This paper analyzes how corporate capital gains taxes affect the capital gain realization decisions of firms. The paper outlines the tax treatment of corporate capital gains, the consequent incentives for firms with gains and losses, the efficiency consequences of these taxes in the context of other taxes and capital market distortions, and the response of firms to these incentives. Despite receiving limited attention, corporate capital gain realizations have averaged 30 percent of individual capital gain realizations over the last fifty years and have increased dramatically in importance over the last decade. By 1999, the ratio of net long-term capital gains to income subject to tax was 21 percent and was distributed across a variety of industries suggesting the importance of realization behavior to corporate financing decisions. Time-series analysis of aggregate realization behavior demonstrates that corporate capital gains taxes impact realization behavior significantly. Similarly, an analysis of firm-level investment and property, plant, and equipment (PPE) disposal decisions and gain recognition behavior similarly suggests an important role for these taxes in determining when firms raise money by disposing of assets and realizing gains.

Taxes and Fringe Benefits Offered by Employers

Using cross-sectional data for blue and white collar workers for U.S. cities, we examine how the tax treatment of fringe benefits affects whether employers offer benefits. Differences in state-level income taxes cause variation across places in the tax incentives for fringe benefits. We find that employers respond to tax incentives to offer fringe benefits, especially to blue collar workers. The tax incentives affect both the probability of basic benefits, such as medical coverage, and more 'marginal' benefits, such as vision and dental coverage. Higher taxes also reduce the amount of explicit cost sharing for some benefits between employers and employees.

"Success Taxes," Entrepreneurial Entry, and Innovation

Interest in the role of entrepreneurial entry in innovation raises the question of the extent to which tax policy encourages or discourages entry. We find that, while the level of the marginal tax rate has a negative effect in entrepreneurial entry, the progressivity of the tax also discourages entrepreneurship, and significantly so for some groups of households. These effects are principally traceable to the upside' or success' convexity of the household tax schedule. Prospective entrants from a priori innovative industries and occupations are no less affected by the considerations we examine than other prospective entrants. In terms of destination-based industry and occupation measures of innovative entrepreneurs, we find mixed evidence on whether innovative entrepreneurs differ from the general population; the results for entrepreneurs moving to innovative industries suggest that they may be unaffected by tax convexity but the possible endogeneity of this measure of innovative entrepreneurs confounds interpreting this specification. Using education as a measure of potential for innovation, we find that tax convexity discourages entry into self-employment for people of all educational backgrounds. Overall, we find little evidence that the tax effects are focused simply on the employment changes of less skilled or less promising potential entrants.

The Distributional Effects of the Tax Treatment of Child Care Expenses

Tax relief for child care expenses, encompassing the Child Care Tax Credit and Dependent Care Assistance Plans, is the largest federal government program in the United States aimed at helping families with child care. We examine the distributional effects of these policies among families with children using both the National Child Care Survey and tax return data. Among families that use tax relief, the benefits average 1.24 percent of family income. Benefits as a percentage of income vary systematically over the income distribution. Despite being regressive at low income levels (mainly due to the credit being non-refundable), tax relief is progressively distributed over most of the income distribution with the ratio of benefits to income falling above the bottom quintile of the income distribution. The benefits of tax relief also vary among families with the same income depending on a family's structure and its labor market and child care choices.

What Can We Learn About the Sensitivity of Investment to Stock Prices with a Better Measure of Tobin's q?

This paper examines the responsiveness of investment to q (i.e., the ratio of a firm's market value to the replacement cost of its assets) using data on a unique type of firm: Real Estate Investment Trusts (REITs). For REITs, we have high quality estimates of the net asset value of the firm that we use to create relatively accurate measures of Tobin's q. In addition, REITs have institutional features that mitigate some of the complications faced by previous studies. We have three main results. First, there is little evidence of a statistical link between REIT investment and a traditional accounting-based measure of q. Second, REIT investment is highly sensitive to estimates of q that are based on analysts' appraisals of asset value. A REIT whose NAV-based q ratio rises from 1.0 to 1.1 will increase its assets by 4.3 percent in the next year. Third, the difference between the appraisal-based measure of q and the traditional accounting based measure typically increases with the age of the firm's assets and varies across types of properties. These results suggest that measurement error in q can lead to appreciable downward biases in investment sensitivities, even in an industry that seems to meet many of the assumptions in Tobin's original paper, but that Tobin's investment model performs well with a better measure of q.Investment; Tobin's q; Real Estate Investment Trusts

Tunable coupled-mode dispersion compensation and its application to on-chip resonant four-wave mixing

We propose and demonstrate localized mode coupling as a viable dispersion engineering technique for phase-matched resonant four-wave mixing (FWM). We demonstrate a dual-cavity resonant structure that employs coupling-induced frequency splitting at one of three resonances to compensate for cavity dispersion, enabling phase-matching. Coupling strength is controlled by thermal tuning of one cavity enabling active control of the resonant frequency-matching. In a fabricated silicon microresonator, we show an 8 dB enhancement of seeded FWM efficiency over the non-compensated state. The measured four-wave mixing has a peak wavelength conversion efficiency of -37.9 dB across a free spectral range (FSR) of 3.334 THz ($\sim$27 nm). Enabled by strong counteraction of dispersion, this FSR is, to our knowledge, the largest in silicon to demonstrate FWM to date. This form of mode-coupling-based, active dispersion compensation can be beneficial for many FWM-based devices including wavelength converters, parametric amplifiers, and widely detuned correlated photon-pair sources. Apart from compensating intrinsic dispersion, the proposed mechanism can alternatively be utilized in an otherwise dispersionless resonator to counteract the detuning effect of self- and cross-phase modulation on the pump resonance during FWM, thereby addressing a fundamental issue in the performance of light sources such as broadband optical frequency combs

The Effects of Progressive Income Taxation on Job Turnover

We examine whether the level of the income tax rate and the convexity of the income tax schedule affect job mobility, as measured by moving to a better job. While the predicted effect of the level of the tax rate is ambiguous, we predict that an increase in the convexity of the tax schedule decreases job search activity by taxing away some of the benefits of a successful job search. Using data from the Panel Study of Income Dynamics, we estimate that both higher tax rates and increased tax rate progressivity decrease the probability that a head of household will move to a better job during the coming year. Our estimates imply that a five-percentage-point reduction in the marginal tax rate increases the average probability of moving to a better job by 0.79 percentage points (a 8.0 percent increase in the turnover propensity) and that a onestandard- deviation in our measure of tax progressivity would increase this probability by 0.86 percentage points (a 8.7 percent increase in the turnover propensity).